TREE01312

MEASUREMENT AND CONTROL TECHNIQUES IN CEOTHERMAL POWER

PLANTS

J. F. WHITBECK

J. D. MILLER

R. H. DART

D. R. BREWER

January 1979

IDAHO OPERATIONS OFFICE UNDER CONTRACT EY-76-C-07-1570

Printed in the United States of America Available from

National Technical information Service U.S. Department of Commerce

5285 Port Royal Road Springfield, Virginia 22161

Price: Printed Copy $6.00; Microfiche $3.00

NOTICE

This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

W t.'

TREE- 131 2 D i s t r but ion Category:

. UC-66C r

MEASUREMENT AND CONTROL ., TECHNIQUES I N GEOTHERMAL - .

POWER PLANTS

$ 2 3. F. Whitbeck, EG&G Idaho, Inc: .-' f .

R. H. Dart, EG&G Idaho, Inc.

3. 0. M i l l e r , EG&G Idaho, Inc. . .

D. R. Brewer, Rogers Engineering

c ' ~EG&G Idaho, Inc.': 7

Idaho Fal ls , Idaho 83401 . ,

Pub1 i shed January 1979 -

PREPARED FOR THE . U.S. DEPARTMENT OF ENERGY - IDAHO OPERATIONS OFFICE-

UNDER CONTRACT NO. EY-76-5-02-4051

-

b+

i.J ACKNOWLEDGMENTS

t r i b u t e d information during preparation o f t h i s . report. I n p a r t i c u l a r the authors wish t o express t h e i r g ra t i tude t o

mpany f o r permission t o p r i n t the mater ia l which and 11, and Tables I1 and V, and t o acknowledge

Chi 1 ton Pub1 i sh ing

appears i n Figures the fo l low ing ind iv iduals and companies f o r t h e i r e f f o r t s and consul tat ion

whi 1 e preparing t h i s chapter:

-

Bechtel National Inc. T. R. Fick, A. N. Rogers, and A. F. Marks

The Ben H o l t Co. Ben Holt, A. J. L. Hutchinson, and Fletcher Leavenworth

Chevron O i l Co. A1 Cooper

Imperial Magma Co.

Tom Hinrichs, Cal Shepherd, and Mike Pierce

Lawrence Livermore Laboratory

Frank Locke

Pac i f i c Gas & E l e c t r i c Co.

Char1 i e Franks

Public Service o f New Mexico

A r t Martinez

Union O i l Co. A. 3. Chasteen, Frank Lemon, and W. .B. Blaik ie. *

Also the authors express

and Mr . Tom Hinrichs, "Imptiria f o r t h e i r e f fo r ts i n supplying

information which was included i n Sections 111-2.5 and 111-4.3.

Gatitude , t o Mr. Charl ie Franks, P.G.&E. Co., - - ,

id

i f

ABSTRACT b, 3 . The information contained in this report provided the background

and source material used in preparing the chapter of the Geothermal

This source book is being assembled and edited under the direction of Dr. J. Kesten of Brown University. and detailed information than could be included in the source book chapter and is being published for reference. examples o f instrumentation and control techniques currently being used in geothermal power plants. In addition, the basic guidelines and unique characteristics of instrumentation and control in geothermal systems, are presented.

* Source Book on instrumentation, measurement, and control techniques.

This report presents more complete

Included are detailed

This report addresses the instrumentation and control philosophy and the hardware involved in geothermal electric plants and their supply and injection systems.

systems. Sources of information for standard I&C practice are listed in the

The intent is to address €he unique 5 characteristics o f geothermal electric instrumentation and control (I&C)

Standard I&C practice is available in the general 1 iterature. -

Appendix.

The information contained in this report presents

(1) The philosophy o f I&C system design

(2) The development of the system, from power grid considerations through subsystem operation to specific system details

(3) Component selection and operating considerations.

3

CONTENTS

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . .

I1 . INSTRUMENTATION AND CONTROL SYSTEM DESIGN APPROACH . . . . 1 . 2 . GEOTHERMAL INSTRUMENTATION AND CONTROL

DATA AND CONTROL REQUIREMENTS . . . . . . . . . . . . DESIGN CONSIDERATIONS . . . . . . . . . . . . . . . . 2.1 Well and Plant D is t r i bu t i on . . . . . . . . . . . 2.2 Supplier/Plant In ter face . . . . . . . . . . . . 2.3 Operating Point Range . . . . . . . . . . . . . . 2.4 Environmental Constraints . . . . . . . . . . . .

3 . INSTRUMENTATION AND CONTROL SYSTEM DESIGN OPTIONS . . 3.1 Research Plant . . . . . . . . . . . . . . . . . 3.2 P i l o t Plant . . . . . . . . . . . . . . . . . . . 3.3 Commercial Plant . . . . . . . . . . . . . . . .

I11 . INSTRUMENTATION AND CONTROL SYSTEM DEVELOPMENT . . . . . . 1 . PLANT OPERATING MODES AND PARAMETERS . . . . . . . . .

1.1 Power Range Operation . . . . . . . . . . . . . . 1.2 Startup Operation . . . . . . . . . . . . . . . . 1.3 Upset Operation . . . . . . . . . . . . . . . . . 1.4 Bypass Operation . . . . . . . . . . . . . . . .

2 . STEAM PLANT OPERATION AND CONTROL . . . . . . . . . . 2.1 Configuration . . . . . . . . . . . . . . . . . . 2.2 Steam Supply System . . . . . . . . . . . . . . . 2.3 Coolant and System Condensate . . . . . . . . . . 2.4 Noncondensable Gas System . . . . . . . . . . . . 2.5 Geysers Steam Plants . . . . . . . . . . . . . .

3 . FLASH STEAM PLANT OPERATION AND CONTROL . . . . . . . 3.1 Configuration . . . . . . . . . . . . . . . . . . 3.2 Steam Supply System . . . . . . . . . . . . . . .

4 . BINARY PLANT OPERATION AND CONTROL.'-'. . . . . . . . . 4.1 Design and Operational Considerations . . . . . . 4.3 L iqu id Binary Systems . . . . . . . . . . . . . . 4.2 Flash Binary Systems . . . . . . . . . . . . . .

ii

iii

1 - 2

2

3

4

4 5 6

9

9

9 11 12 13

14

14 16 17 18 18

20

20 21

24

24 26 26

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f V

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5 . AUXILIARY SYSTEMS . . . . . . . . . . . . . . . . . . 49

5.1 Heat Rejection System . . . . . . . . . . . . . . 49 5.2 Aux i l i a ry Cooling System . . . . . . . . . . . . 50 5.3 Aux i l i a ry Heating System . . . . . . . . . . . . 51 5.4 Miscellaneous Aux i l i a ry Systems . . . . . . . . . . 51

I V . INSTRUMENTATION SYSTEMS . . . . . . . . . . . . . . . . . 52

1 . GENERAL SERVICE CONSIDERATIONS. . . . . . . . . . . . . 53

1.1 F l u i d Temperature . . . . . . . . . . . . . . . 53 1.2 Corrosion . . . . . . . . . . . . . . . . . . . 53 1.3 Deposition . . . . . . . . . . . . . . . . . . . 54 1.4 Operating Environment . . . . . . . . . . . . . . 54

2 . MEASUREMENT ELEMENTS . . . . . . . . . . . . . . . . . 55

2.1 Pressure Measurement . . . . . . . . . . . . . . 55 2.2 Temperature Measurement . . . . . . . . . . . . . 58 2.3 Flow Measurement . . . . . . . . . . . . . . . . 59 2.4 Level Measurement . . . . . . . . . . . . . . . . 64 2.5 Q u a l i t y Measurement . . . . . . . . . . . . . . . 67 2.6 Water Chemistry Monitors . . . . . . . . . . . . 67 2.7 Combustible Mixture Monitors . . . . . . . . . . 67 2.8 Addit ional Measurements . . . . . . . . . . . . . 68

3 . INSTALLATION REQUIREMENTS . . . . . . . . . . . . . . . 68

3.1 Codes and Standards . . . . . . . . . . . . . . . 68 3.2 Guidelines and Standard Practices . . . . . . . . 69

V . REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . 71

V I . APPEND I X- SE LECTED 8 I B LIOGRAPHY . . . . . . . . . . . . . . 72

FIGURES

1 . Typical steam p l a n t . . . . . . . . . . . . . . . . . . . . 15

2 . Geysers U n i t 11 contro l schematic . . . . . . . . . . . . . . . 19

3 . Dual f l a s h steam system . . . . . . . . . . . . . . . . . . 22 4 . L iqu id b inary system . . . . . . . . . . . . . . . . . . . 25

. . . . . . . . . . . . . . . . . . . . 5 . Flash b inary system 27

f . . U

u :

6.

7.

. 8a. 8

8b.

8c.

9.

10.

11.

I.

11.

111.

IV.

V.

VI.

Raft r i v e r dual b o i l e r b inary plant. . . . . . . . . . . 29

Raf t r i v e r s tar tup mapping . . . . . . . . . . . . . - . . 36

Turbine t r i p response - .valve posi t ion, ' speed, and' torque. 38

b4

e . I . .

Turbine t r i p response - pressure, leve l , and subcooling m a r g i n , . . . . . . . . . . . . . . . . . . . . . . . . 39

Turbine t r i p response - temperature and f lowrate . . . . . 40

Magmamax dual b inary cont ro l schematic . . . . . . . . . . 42

Pressure monitoring elements . . . . . . . . . . . . . . 56

Flow monitoring elements . . . . . . . . . . . . . . . . . 62

TABLES

Raf t River P lant S ta t i c Performance . . . . . . . . . . . 33

Character is t ics o f Pressure Measurement Devices . . . . . 57

Character ist ics o f Temperature Measurement Elements . . 60

Head Loss o r D i f f e r e n t i a l Pressure Flow Measurement.'. . . 61

Character ist ics o f Flowmeter Elements. . . . . . . . . : . 63

Character ist ics o f Level Measurement Elements. . . . . . . 66

'6, MEASUREMENT AND CONTROL

TECHNIQUES I N GEOTHERMAL '

POWER PLANTS

I. INTRODUCTION

Measurement and control techniques i n geothermal systems are t y p i c a l o f standard instrumentation and control (I&C) pract ices used i n the process and power industr ies. This repor t addresses the methods, systems, guidelines, and hardware unique t o geothermal e l e c t r i c f a c i l i t i e s . Standard I&C prac t ice i s avai lable i n the general l i t e r a t u r e . A selected bibl iography i s included i n the Appendix, l i s t i n g sources f o r more information on standard I&C pract ice.

11. INSTRUMENTATION AND CONTROL SYSTEM DESIGN APPROACH L -.. .

This Section presents an overview af’the desigh of a geothermal electric I&C system and points out the unique design considerations and options included in the design process.

1. DATA AND CONTROL REQUIREMENTS 1. DATA AND CONTROL REQUIREMENTS

The general considerations as to the number and character of panel mounted data channels for use in control, protection, and monitoring. are identical to those for any industrial process in that (a) experience gained in operating a pilot or experimental plant is utilized in de- veloping simp1 ified and improved designs for large scale commercial units and (b) technology advances are retrofitted. Both of these trends are typified in the evolution of the Geysers steam plants, as subsequently described. for control and protection are described in Section 111.

Specific subsystem and plant data requirements

Instrumentation accuracy beyond ordinary process instrumentation standards is justified only for metering applications, where leaseholder or supplier charges are affected, and for experimental plants where binary fluid properties and component performance are of interest.

Response requirements for monitoring and performance evaluation are not stringent, since these channels are o f interest in a static or quasistatic sense. data, monitoring data, and control device actuation are generally not stringent because of the 1 arge thermal masses dominating temperature responses. One exception to this rule is the rapid overspeed (less than one second) of the turbogenerator on loss of load. Both thermal and speed responses are illustrated in the simulated transient results given

Response and sampl ing rate requirements for control

4

,in Section 111. The response o f other p ro tec t ive functions and the consequences o f a f a u l t should be given consideration i n the select ion o f the t r i p levels.

2. GEOTHERMAL INSTRUMENTATION AND CONTROL DESIGN CONSIDERATIONS

. This Section summarizes those unique charac ter is t i cs o f geotherma e l e c t r i c p lants which inf luence the design o f the I & C system.

I . *

2.1 Wel l and Plant D i s t r i b u t i o n

Resource character ist ics, p lan t design, and economics d i c t a t e t h a t mu l t i p le plants, t y p i c a l l y 50 MW(e) each, be placed on a given resource, and t h a t each o f those p lants be supported by mu l t i p le supply and i n - j e c t i o n wells.

Each we1 1 t y p i c a l l y requires pressure, temperature, and f l o w i n - strumentation and manual o r automatic f low control. communicate i n both direct ions, w i t h the p l a n t o r control center, and may require e l e c t r i c power o r instrument a i r . . Analog communication, considering the distances involved, requires extensive signal processing and cabling. Signal mu l t ip lex ing o r complete d i g i t a l systems may there- fore be a t t rac t i ve .

Each we1 1 head must

Another consequence o f the distance between the wel ls and the p l a n t i s the water-hammer hazard f o r actuated valves i n l i q u i d service. Since rap id va lve-ac t ion i s general ly not required f o r cont ro l o r protect ion, the actuat ion speed should- be made low enough t o preclude water-hammer damage.

2.2 Suppl ier /Plant In te r face I .

Another unique cha rac te r i s t i c o f geothermal systems i s t h a t the suppl ier and the p l a n t operator are l i k e l y t o be t w o separate corporations. Control and communication interfaces must be considered i n the I & C system design.

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2.3 Operating Point Range

As presented i n Section 111-1, p l a n t operating parameters change w i t h var ia t ions i n ambient conditions, increased * fou l ing resistance, and resource degradation. the type o f p lan t and mode o f operation. Using the f l o a t i n g power concept w i t h a l i q u i d binary p lant , the "normal" value f o r an operating pipameter may vary, s ign i f i can t l y . The operator 's decision, as to whether operating parameters are w i th in the speci f ied l i m i t s , i s therefore made s i g n i f i c a n t l y more d i f f i c u l t .

2.4 Envi.ronmenta1 Constraints

The magni tude-of these changes i s determined by

> -

" "

. - .

I n additi.oru t o the basic.plant.design, the l im i ta t i ons on noise leve l and emissions, a f fects . t rans ien t operatfon and control . Speci f ic inf luences o f hydrogen sulphide emission and noise l i m i t a t i o n s i n t rans ient operation o f the steam supply are discussed i n Section 111.

3. INSTRUMENTATION AND CONTROL SYSTEM DESIGN OPTIONS'

This Section describes the options in. system design which r e s u l t from a consideration a f t he -p lan t status (whether it i s a research, p i l o t , o r commercial p lant ) and o f the current t rend from analog t o d i g i t a l systems.

3.1 Research Plant .

A minimum-cost research plant, which would use ex is t i ng hardware and would replace the turb ine w i th a t h r o t t l e valve, could w e l l incorporate loca l and d i s t r i bu ted instrumentation, protect ion, and control . schedule and cost would be the most important.factors, p a r t i c u l a r l y - f o r a short term pro ject , the operator would manually provide most o f the contro l and p lan t protect ion

Since

he automatic contro l ;system would be a I . * . correspondingly small port ion. I , ,,- f

, L, \ X I

4

An analog system would t y p i c a l l y provide loca l ind icators and s t r i p -cha r t recorders. A current d i g i t a l approach would incorporate a data logger a t each location. This would provide, w i th adequate pro tec t ion from the f i e l d environment, a pe r iod i ca l l y sampled o r report-by-exception history.

3.2 P i l o t Plant

A more permanent and complex i n s t a l l a t i o n would j u s t i f y a central ized control system w i t h a much greater rel iance on automatic control.

The Magmamax p i l o t p lant , described i n d e t a i l i n Section 111, incorporates a dedicated signal l i n e f o r each data and control function, complete analog signal processing, s t r i p -cha r t recording, remote ind icators and an annunciator and scanner warning system. The shutdown chain i s t r iggered by any o f a number o f system f a u l t s o r by the operator. Both manual and automatic loops, based on analog c i r c u i t s , are extensively used f o r modulated cqntrol functions. This p l a n t i s an example o f a modern analog contro l system.

A d i g i t a l system f o r the same p l a n t would consist o f a cathode- ray-tube (CRT) d isplay module, a pushbutton keyboard, a control processor, a p r i n t e r , and remote stat ions.

The CRT would provide an automatic v isual d isplay o f a l l alarms. I n add i t ion it would permit the operator t o c a l l up any p l a n t information he might desire. It could provide a more f l e x i b l e display o f information than a s t r i p chart, p a r t i c u l a r l y since the reference far each channel could be computed and moved as a funct ion o f varying operating conditions.

-

The pushbutton keyboard would al low the operator t o converse w i t h the power p l a n t system. Through t h i s keyboard he would be able t o open o r close breakers, r a i s e o r lower equipment set points, review monitored breaker posi t ions o r metered values, c lear alarms, o r p r i n t information.

t -

5

L’ The p r i n t e r would automat ical ly p r i n t al-1 alarms and contro l act ions

performed by the operator. Display values could be p r in ted on a rou t ine basis o r upon operator request.

-

I n add i t ion t o the centra l processor, remote s tat ions gather data and contro l equipment i n t h e i r defined area. e l e c t r i c a l l y 1 inked t o the centra l cont ro l processor.

The remote s tat ions are

The cost of d i g i t a l systems i s now low enough t o be an,.at t ract ive

method of powerplant control . ._ :’ . *

With e i t h e r type o f cont ro l system, as long as the system operation i s not f u l l y understood, the operator s t i l l performs an essent ia l

3 .3 Commercial P lant

- . ’ ‘:

A f t e r development o f a p a r t i c u l a r process t o commercial status design considerations are operator and f&C system costs versus increased

p lan t a v a i l a b i l i t y . _ I

Plant operation can be automated t o various degrees. I n order o f increasing p l a n t a v a i l a b i l i t y t o the gr id , four degrees o f automation are

(1 ) Semiautomatic p lan ts w i t h a . rov ing operator

(2) Plants connected by an alarm system t o a d i s tan t controJ center

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(3) . Plants w i t h assigned loca l operators

(4) Plants equipped w i t h remote supervisory contro l and data acquis i t ion, i n add i t ion to-. rov ing operators.

.

( ) Semiautomatic Plants wi th a Roving Operator. I n t h i s case the - -

p lants would not have f u l l t ime operators and are unmanned except for 6, sequential v i s i t s by a rov ing operator. This type o f operation would be

6

hd appropriate f o r a reduced f i r s t cost design f o r which the power output o f an ind iv idua l u n i t would be o f minor importance t o the en t i re power system and i n which the loss o f a s ing le p l a n t ' s output could be to le ra ted u n t i l the downed u n i t could be brought back on l i ne .

An annunciator system w i th a " f i rs t - in-a larm" capab i l i t y , i s o f ten i n s t a l l e d i n t h i s type o f p lant. important i n troubleshooting a u n i t t h a t has been shutdown automatically.

This type o f p l a n t also commonly includes an automatic data logging system f o r selected parameters. This i s not a s t r i c t requirement, since a rov ing operator could take readings dur ing h i s v i s i t , but t h i s feature would a l low f o r important readings t o be recorded between the operator v i s i t s .

This type o f annunciator i s very

(2) Plants Connected by an Urgent and Ordinary Alarm System t o a Dis tant Control Center. This type o f system i s a var ia t ion o f an

operation w i t h semiautomatic control and a rov ing operator, as discussed above. t h a t an abnormal condi t ion i s known immediately and an operator can be dispatched t o t h a t p lant. This system provides the advantage t h a t the down t ime o f a p l a n t can be reduced or avoided due t o immediate knowledge o f an abnormal condit ion.

The p l a n t alarms are brought i n t o a centra l control area, so

An urgent alarm i s one which requires immediate a t ten t ion by an operator, such as a u n i t shutdown. An ordinary alarm i s one o f a lesser urgency i n which a monitored value i s out o f i t s normal range and should have the a t ten t ion o f an operator as soon as possible t o prevent more serious occurrences inc lud ing a u n i t t r i p .

The Pac i f i c Gas and E lec t r i c Company Geysers' i n s t a l l a t i o n began operation using the roving operator approach. Recently, however, PG&E has i n s t a l l e d a centra l alarm system t o minimize down time.

'6, (3) Plants w i t h Assigned Local Operators. I n t h i s case a t ra ined

operator would be assigned t o each p lan t a t a l l times. The presence o f

I

L trained personnel at the plant during both normal and abnormal conditions will result in a high degree of plant availability. This system normally contains a central control panel with control devices for operating selected plant equipment, data acquisition readouts, and alarms.

(4) Centralized Control of Mu1 tiple Plants. Relocation of individual plant control panels to a central control room permits a reduction in the number of operators and a coordination of plant operations. complete digital system, as previously described would be located in a

01 area. The control area could be located in a central dispatch

A

1

I I r, where operators control a power transmission system and various

types o f generating plants.

Certain infrequent functional operations , routine maintenance, and . component failures would still be attended to by roving operators.

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I 11. INSTRUMENTATION AND CONTROL SYSTEM DEVELOPMENT

This Section describes the development o f the I & C system inc lud ing the basic requirements imposed on a p lan t by i t s r o l e i n the power gr id,

the subsystem control requirements, and d e t a i l system and performance

descript ions f o r various types o f geothermal plants. covers power range, startup, and upset operation. Supporting systems

involved i n other less frequent and general ly manual operations, such as

f i l l i n g , draining, purging, venting, and blowdown are not 'discussed. The Section describes the I&C system f o r steam, flashed steam, f lash-

binary and l iqu id -b inary power plants. common t o d i f f e r e n t types o f p lants i s presented a t the end o f t h i s Section.

The discussion

A discussion o f a u x i l i a r y systems

1. PLANT OPERATING MODES AND PARAMETERS

This Section d e t a i l s those aspects o f p l a n t operation which are common t o a l l p l a n t cycles.

as power range, startup and upset are discussed.

1.1 Power Range Operation

The p r inc ipa l modes o f operation, defined

The most fundamental c l a s s i f i c a t i o n t o be considered i s t h a t o f the p lan t ' s r o l e i n the power gr id, i.e., base load 'a t o r near p l a n t capacity,

o r load-fol lowing i n which generation matches the t r a n s i e n t ' v a r i a t i o n o f power demand.

I n most cases the geothermal p l a n t output represents a small po r t i on o f the capacity o f a large power gr id.

considerations would require t h a t such geothermal plants be operated i n the base load condition.

geothermal p l a n t were iso la ted w i th i t s own load network.

Cost and operational *

Load fo l low ing would be required i f the

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Load contro l i s usual ly accomplished by modulating the vapor f low

(steam o r b ina ry - f l u id vapor) through the turbine, by moving a governor

valve o r other var iab le admission device a t the turb ine i n l e t . P a r t i a l c los ing o f t h i s valve reduces the turb ine i n l e t pressure and f low rate.

An addi t ional cont ro l element i s general ly employed t o contro l the

pressure i n the vapor generator o r steam main, as described below.

I n the base load appl icat ion the turbogenerator speed i s d ic ta ted by the g r i d system, and an operator changes the load demand t o

match the power output t o the power setpoint. Maximum p l a n t output i s

provided by a setpoint which equals o r exceeds the current capab i l i t y .

The generating capab i l i t y i s determined by the design p l a n t output less curtai lments (reductions due t o equipment out o f service, such as a pump not i n service).

I n the load-fol lowing appl icat ion, where the p lan t ex is ts alone o r has a s i g n i f i c a n t e f f e c t on the overa l l load generation, the system

frequency must be maintained by moving the t h r o t t l e valve t o contro l the

turbogenerator speed.

The maximum net power o f a geothermal p lan t would, i n general,

decl ine w i t h the cool ing and deplet ion a f the resource.

designs, notably the EPRI/SDGE Heber Plant, accommodate a cool ing

resource by changing the composition o f the binary f l u i d . f o r resource pumping would increase appreciably i f the f lowrate o f the geothermal f l u i d were increased t o o f f s e t the temperature decrease o f

the resource.

Some b inary

Power required

Addi t ional wel ls may also be necessary.

minor i n a

capabi 1 i ty

o f the coo

Maximum p l a n t output a lso var ies wi th the changes i n condenser

pressure caused by ambient temperature changes. The va r ia t i on i s r e l a t i v e l y

steam p lan t , because the condenser pressure i s l i m i t e d by the

o f the noncondensable gas removal system. As the temperature i n g water decreases, a lower condenser pressure i s established.

10

The a b i l i t y o f the noncondensable gas removal system t o remove the gases a t a lower pressure i s qu i te l imi ted.

exert t h e i r p a r t i a l pressure therefore l i m i t i n g the condenser pressure. A b inary p l a n t i s not subject t o the same l i m i t a t i o n , however, and the

r e s u l t i n g power va r ia t i on can be substantial as shown subsequently f o r

the Raft Rtver p i l o t power plant.

The gases remaining i n the condenser Lii 9

Both classes o f p lants show a reduction i n power output as fou l i ng deposits accumulate.

turbine, and condenser foul ing. due t o heat exchanger foul ing.

t o t h i s i s represented by a sawtooth curve; the power declines t o a

minimum as the fou l i ng bui lds up then peaks t o a maximum a f t e r shutdown and cleaning.

1.2 Startup Operation

The steam plants lose power due t o p ipe l ine, The binary plants lose power p r i n c i p a l l y

The common t rans ien t power va r ia t i on due

t This i s the most challenging o f a l l phases o f operation because o f the number o f subsystems simultaneously exercised, the off-design operating

leve ls o f the various components, and the wide ranging t rans ien t conditions.

The most obvious o f the hazards i s t h a t o f thermal stress.

throughout the geothermal system are subject t o design l i m i t a t i o n s on

heatup rate. .Wells general ly require a slow increase i n f low. speed w i t h which a wel l can be put on l i n e depends upon the temperature

o f the geothermal f l u i d a t the wellhead and the temperature o f the resource, and i s determined by temperature change l i m i t a t i o n s o f the

we1 1 pump ( p a r t i c u l a r l y a shaft-driven pump) o r the transmission systems.

Components

The

Generally each o f the l i q u i d loops, such as the brine, working f l u i d , and coolant, incorporates a pump which has a l i m i t e d range o f

the low f low range general ly requires addi t ional complications such as a

variable-speed e l e c t r i c o r turbine d r i ve or, f o r fixed-speed drives, a

bypass f 1 ow-control 1 oop.

* flow-to-speed r a t i o s f o r sustained operation. The slow t r a n s i t through

W

11

Turbine startup is not subject to the same temperature gradient' limitations as in fossil or nuclear plants, because of the substantially 1 ower vapor temperatures. The control .hardware and operating procedures are, however, similar. The acceleration to near synchronous speed is accomplished with the generator unloaded and with the governor or steam chest valves full open, to provide full admission for even heating and loading. A relatively small vapor flow is controlled by the speed controller and a separate throttle valve prior to synchronization. the throttle valve is full open, control is transferred to the governor valve. dictated by the grid ,frequency. v a h e provide load control as the load is increased to the desired leve

When

Connection to the grid results in fixed-speed operation, as The speed controller and the governor

P1 ant operation during pFant in the Sections which

1 .3 Upset Operation

turbine startup is detailed for each type of describe the individual plant types.

An upset condition is defined as an abnormal operating condition sensed by one or more of the plant protective devices or by an operator. Where system response to an upset does not depend on an operator reaction, it is accomplished by logic implemented in the plant protective system. That logic i s , of course, developed by a consideration of the consequences o f various upsets.

Upsets may result in the system responding by: curtailment, turbine shutdown, or total- shutdown.

.

A curtailment is a reduction in system output due to the failure of one of a set of parallel devices such as pumps and cooling tower fans.

operation continues at a reduced power output. .Tjw device is shutdown by automatic or operator response and-plant - . - _ . I

12

W

.

A tu rb ine shutdown i s d ic ta ted by a turbogenerator o r l i n e f a u l t

The remainder o f the p l a n t can continue operating i f the f a u l t

which requires separation from the g r i d and cessation o f turbine opera-

t ion.

can be cleared qu ick ly and i f the vapor can bypass the turbine, or , i f i n a steam plant, the vapor can be vented.

o f power generation. This allows rap id resumption

Turbine-only shutdown w i th continued p l a n t operation requires

external power f o r a u x i l i a r i e s unless an immediate resumption o f genera-

t i o n a t the s ta t i on -aux i l i a ry leve l can be accomplished. This i s common on conventional power plants i n Europe, because o f l i m i t e d t i e - l i n e capabi l i ty , and i s o f i n t e r e s t f o r U.S. geothermal plants w i t h high p a r a s i t i c losses.

The p l a n t must be t o t a l l y shutdown i f the f a u l t i s loca l and serious,

i f external f a u l t s pe rs i s t , o r i f turbine-only shutdown i s not possible.

A c r i t i c a l upset condi t ion i n any power system i s the loss o f

Vapor f l o w t o the generator load due t o separation from the gr id. tu rb ine must be stopped qu ick ly t o l i m i t the resu l tan t turbine overspeed.

I n Westinghouse's large-turbine design, f o r example, the turbine valves a t the main steam i n l e t t o the turbine are closed i n 0.10 t o 0.15 second

by venting the actuator o i l and al lowing rap id spring-driven closure.

A simulat ion o f a turbine t r i p w i t h vapor bypass i s described f o r the Raft River binary p l a n t below.

1.4 Bypass Operation

Plant operation and control , i n power range, startup, and upset

operation, may be improved by the add i t ion o f geothermal-supply o r tu rb ine bypasses.

independent startup and t e s t i n g o f the supply system.

around the tu rb ine allows s t a r t i n g and operating the p l a n t without

Bypassing geothermal f 1 u i d around the p lan t permits

Bypassing vapor

13

requ i r ing turb ine startup.

i t s own requirements. p lan t oper,ation by simply d i ve r t i ng excess f low around the turbine.

'al lows continued p l a n t operation i n the event o f a tu rb ine t r i p , 50 t h a t

normal operation can be quick ly restored i f the problem was external t o

the plant. p l a n t presentations which fol low.

The turb ine can be s ta r ted subject only t o The load can be cont ro l led wi thout d is tu rb ing

It

Bypass operations are discussed i n each o f the s p e c i f i c .

2. STEAM PLANT OPERATION AND CONTROL

2.1 Configuration

A p l a n t producing e l e c t r i c i t y from a geothermal steam resource, as

shown i n Figure 1, incorporates . four main process streams: coolant, condensate, and noncondensable gas. The f i gu re represents an

ea r l y Geysers design without hydrogen sulphide abatement.

f o r H2S abatement are addressed i n the fo l lowing discussion.

steam,

Modif icat ions

Steam flows from the producing wel ls through the supply l ines , The coolant stream i s scrubber, separator, turbine, and condenser.

brought t o a d i rect-contact condenser , ,And the mixed condensate-

coolant stream i s pumped t o the cool ing tower. Excess coolant i s pumped

from the cool ing tower and in jec ted i n t o the ground.

gases, inc lud ing H2S, are drawn from the condenser through two stages of steam-jet e jectors. The r e l a t i v e l y dry noncondensables, are t rea ted t o

remove the objectionable gases i ncl udi ng H2S, then vented.

from the e jectors i s returned t o the i n j e c t i o n well .

requires t h a t the complete f low o f coolant and condensate be t rea ted f o r

Noncondensable

The condensate

This process

gas removal. _ . . *

By changing t o a surface -condenser' and t reat4 ng the oncondensable

gas stream wi th the S t re t fo rd process 90% o f the H2S i s removed from the stream. Since only the condensate leaves the condenser w i t h dissolyed

L,

14

Steam Vent

I

I II .~ ~.~ .~.. . . .... . . .....

--- Coolant Noncondensable Gas -0- -- Condensate for Injection ---

.. - ..- - Supply

well

INEL-A-887 Injection

well

'0

Fig . 1 Typical steam plant .

LJ H2S, the amount o f water which must be t rea ted i s vas t l y decreased. Further abatement, t o approximately 95%, i s possible by t r e a t i n g the condensate before i t reaches the cool ing tower.

2.2 Steam Supply System

Before a geothermal power p l a n t can be started, the we l ls must be s ta r ted and brought up t o the normal f low o f clean steam. During t h i s per iod the steam flows through automatic pressure-control valves t o the atmosphere. When the p lan t i s brought up t o load .the pressure contro l valves w i l l automat ical ly close and the steam f low i s d i rected t o the turbine. The automatic pressure-control valves are backed up by -pressure- r e l i e f valves designed t o handle wel l f low i f the contro l valves f a i l t o open. I f the p l a n t i s t o be shutdown f o r a long per iod o f time, the we l ls w i l l be manually shut o f f . When the p lan t i s ready t o res ta r t , the we l ls w i l l be gradual ly brought up t o the design f lowrate, and vented which c lears them o f debris.

The i n i t i a l Geysers design responded t o the f low reduct ion r e s u l t i n g from a tu rb ine t r i p , by venting through pressure-control valves. outage continued f o r several hours, cross-connections w i t h other supply l i n e s were used t o d i v e r t the f low t o other plants. design, Unit 15, incorporates the capab i l i t y o f a 20% reduct ion i n wel l f low, i n order t o reduce noise and H2S emission without damaging the supply we l ls by a rap id shutin. Normal supply system shutdowns requi re over four hours t o prevent wel l damage.

I f the

A more recent p l a n t

Even though s i g n i f i c a n t condenser fou l i ng occurs the t i m e i n te rva l between shutdowns i s d ic ta ted by the fou l i ng o f the turb ine by par t icu la tes. This i n t e r v a l i s determined by how e f f i c i e n t l y the pa r t i cu la te scrubbers remove par t i cu la te .

A p l a n t t h a t i s fed dry steam from the wel ls i s not usual ly endangered by excessive moisture i n the steam. p ipe l ines i s drained o f f by steam traps. An i n - l i n e moisture separator

The condensate t h a t forms i n the

a

16

i s i n s t a l l e d i n the steam p ipe l i ne from the w e l l s before the l i n e enters the turbine bui ld ing; it co l l ec ts condensate t h a t may be coming through the pipel ine.

I

2.3 Coolant and Condensate

Coolant f l o w s from the cool ing tower basin t o the condenser by g r a v i t y o r by pumping. The coolant condenses the vapor f rom the tu rb ine and i s returned t o the cool ing tower.

I n a d i r e c t contact condenser, the condensate plus the t o t a l f l o w o f cool ing water must be pumped from the condenser basin t o the cool ing tower t o maintain a constant leve l i n the condenser basin. condenser i s o f the surface type, only the condensate must be pumped out t o maintain the level . I n both cases i t i s important t o maintain the condenser leve l w i t h i n l i m i t s . A high leve l w i l l reduce the condenser e f f i c i ency , whi le a low level w i l l cav i ta te the condensate pump.

I f the

The normal method o f c o n t r o l l i n g condenser basin leve l i s by t h r o t t l a f low-control valve, , in the discharge l i n e o f the coolant pumps, w i t h a signal f rom the leve l cont ro l ler . I n some designs a bypass valve permits re tu rn o f the condensate-pumb discharge back t o the condenser basin t o cont ro l leve l o r t o prevent pump overheating during low f l o w conditions.

The po r t i on o f the condensate t h a t i s not used as make-up i n the coo l ing tower w i l l overflow from the cooling-tower basin as blowdown i n t o a c o l l e c t i n g pond. To prevent t h i s from causing stream o r surface po l l u t i on , i t i s normally returned t o the acquifer v i a i n j e c t i o n wells. The we l ls -a re located so t h a t the cool re tu rn water w i l l not thermally degrade the producing geothermal wells. The operation o f the i n j e c t i o n pump i s cont ro l led by a leve l c o n t r o l l e r i n the pond. When the steam condensate i s i n s u f f i c i e n t o r unavailable f o r make up i t may be necessary t o add surface water t o the tower basin.

Cooling tower operation i s discussed i n Section, 111-5.2.

17

2.4 Noncondensable Gas System

The absolute pressure i n the condenser i s determined by the capabil o f the noncondensable gas removal system and by the heat- t ransfer capabi o f the cool ing system.

t Y i t y

Before turb ine startup, the noncondensable gas removal system i s s ta r ted t o evacuate a i r from the condenser and produce the low absolute pressure required t o operate the turb ine a t design condit ions. When the turb ine i s operating and steam i s f lowing i n t o the condenser, the vacuum i s cont ro l led by equipment capab i l i t y t o remove gases which exer t t h e i r p a r t i a l pressure i n the condenser.

The system shown i n Figure 1 requires a l eve l c o n t r o l l e r t o cont ro l the condensate discharge leve l o f the steam condenser.

2.5 Geysers Steam Plants

This sect ion presents a summary o f the contro l conf igurat ion f o r Geysers U n i t 11, which has two 55 MW turbines and a s ing le generator. Both turbine-exhaust flows t o a s ing le low leve l d i r e c t contact condenser. This i s the common conf igurat ion o f most o f the Geyers plants. l a t e r un i ts , cur ren t ly under construction, w i l l have surface type condensers

The

The steam, condensate, cool ant, and noncondensabl e f 1 ow conf igurat ion i s shown i n Figure 2. through a motor operated valve. A p a r a l l e l pressure equal iz ing valve i s used f o r warming the l i n e s and the turbines. i n t o two paths, each w i t h a s t ra iner , stop valve and swing check valve. One o f the stop valves i s pa ra l l e led by a 10-inch motor operated valve t o admit steam f o r i n i t i a l turb ine accelerat ion. Aux i l i a ry steam, f o r the gas ejectors, i s taken from the other l i ne . reconnected i n a header which supplies steam t o each turbine. tu rb ine en t ry l i n e has a b u t t e r f l y cont ro l valve. through the turbines and the exhaust flows t o the main condenser. Aux i l i a ry steam t o the f i r s t and second stage gas e jectors i s provided through a motor operated valve and a pressure contro l valve.

From the wel ls, steam flows i n t o the p l a n t

The f low i s then s p l i t

The two l i n e s are Each

The steam i s expanded

18

LCV 1 1005

INEL-A-10 143

- - - - - Non condensables

Fig. 2 Geysers U n i t 11 control schematic.

Cooli’ng water t o €he i n t e r - and after-condensers i s modulated by temperature control valves. L iqu id leve l i n the a f t e r condenser i s contrdl l e d by modulating the coolant re tu rn t o the main condenser. Coolant flows from the cool ing tower basin t o the main condenser by gravi ’ ty and vacuum drag. A f l o w contro l valve regulates the f lowrate accordimg t o condenser pressure. During the i n i t i a l po r t i on o f a startup t h e re la t i ’ve ly high condenser pressure bl’ocks the f l o w t o the t o p of the condenser. During t h i s period coolant f l o w i s d i rec ted through a s ta r tup dime and leve l control valve.

n t h e main condenser. When the condenser pressure has reached 14 t o 15 inches o f mercury the s ta r tup- l ine valve i s closed. The coolant

the rec i r cu la t i on l i n e from the pump. discharge header t o t h e condenser

The startup l i n e i s located a t a lower -

i s directed €0 the top o f the condenser and level con t ro l i s switetkd

The four condensate pumps are s tar ted i n sequence, Each pump-

the+% opened fwtly. li?ackfbw through a stopped pump {s preueltted by c-los-ipFs. the discharge valve. header t o the cool ing t&er b a s h i s used t o s t a r t ug the un%when freeZ.i&g coridi t i ons prevai 1. tower basin and proceeds v i a a w e i r t o a s e t t l i n g basin f o r u l t ima te r e i njecf ion.

schafge valve controls pump-di scharge pressure dur ing star€up and i s

A separate l i n e from the pump discharge

Excess condensate overflows the coo’li ng

3. FLASH, ST-UN PLANT OPERATION AND CONTROL

5. T Ca.nfi gurat ion

A f l a s h steam p l a n t d i f f e r s f rom a d i r e c t steam pTani i.n t h a t a l i q u i d or two-phase f l u i d f l o w s o r i s pumped t o the plant, where the vapor i s created i n a series o f f l a s h tanks. The steam supply t o the tu rb ine i s cleaner than t h a t o f a-direct-steam cycle. .The,brine discharge i s more sal ine and has a higher ra te o f f low, than the condensate discharged from the d i r e c t steam plant.

20

Figure 3 i l lustrates a typical two-stage flash supply system

-

21

As i n the direct steam plant, vapor can be bypassed around the .

turbine during turbine startup or turbine t r i p operation. In contrast w i t h the direct steam plant, however, the flash steam plant can bypass the liquid supply directly t o the injection system during plant startup.

'45(

The operation of control of turbine, condenser, cooling tower, and noncondensable gas system i s basically the same as for the direct steam cycle, and they will not be discussed further.

3.2 Steam Supply System

The discussion which follows i s ini t ia l ly directed towards a design w i t h o u t brine or turbine bypasses. Discussion of bypass system operation is deferred t o the end of Section 111.

w i t h o u t bypasses.

A mixture of hot liquid brine and steam flows from each well through a control valve t o the first-stage separator (flash drum). The pressure i n the drum is normally controlled a t primary steam pressure by operation of inlet-control valve PCV-1A. Steam flow through the flash drum and thence t o the turbine is controlled by the turbine-throttle valve. If the first-stage flash drum pressure cannot be controlled by the inlet- control valve PCV-lA, the ven t control valve PCV-1B w i l l open t o release steam t o the atmosphere. If PCV-16 cannot control pressure a t a level that is safe for the flasher, the pressure-safety valve PSV-1 will open t o release additional steam. I

The liquid level i n the first-stage flash drum must be maintained a t a level that wil1,prevent both steam flow in to the liquid line and l i q u i d flow i n t o the steam line. A level control sensor i n the flash drum, controls level-control valve LCV-lA, which allows l i q u i d brine t o flow i n t o the second-stage flash drum. If the flow rate t o the second- stage flash drum is not.sufficient, the level i n the first-stage flash drum will r ise, and the controller will then open drain-valve LCV-lB, which allows liquid brine t o flow t o the collection pond.

s

u

Production wells

To pond for pumping to injection

To injection wells

INEL-Ab876

Fig. 3 Dual f l ash steam system.

. I .

22

The steam pressure i n the second-stage flash drum i s controlled by releasing steam t o atmosphere when the pressure exceeds that required a t the turbine-throttle valve. A pressure-safety valve protects the flash drum from excessive pressure should the pressure control system fa i l . I t may be necessary t o close the low pressure turbine-stop valve i f the flash drum pressure becomes subatmospheric. The liquid level i n the second-stage flash drum must also be controlled t o prevent steam flow in to the l i q u i d disposal line and liquid brine t o flow i n the steam 1 i ne. brine t o be pumped t o the injection well.

Level control 1 er LIC-2 operates valve LCV-2, which a1 lows excess

23

Startup of the wells requires a period of time during which automatic pressure-control valves a t the well head allow the liquid t o flash i n t o steam. The steam is vented t o the atmosphere and the excess brine flows i n t o a pond. After the well operation becomes stable, the liquid brine and steam flow i s gradually shifted t o the high-pressure steam flash drum. The excess brine from the high-pressure flash drum flows t o the low-pressure flash drum. Steam is vented t o the atmosphere from both flash drums by automatic pressure-control valves. brought up t o load, the pressure-control valves will automatically close and a l l steam flows t o the turbine. The automatic pressure-control valves are backed up by pressure-relief valves designed t o handle well flow i f the control valves fa i l t o open.

When the plant is

If the plant is t o be shut down for a long period of time, the wells would be s h u t off manually. When the plant i s ready t o re- start, the wells will be gradually brought up t o flow t o clear them of debris, and t o control the heating rate of the well and of the piping system. With shorter shutdowns, the pressure-control valves a t the wellhead, vent steam as i n well startup.

Plant startup for a system w i t h a brine bypass would involve the manual or programmed closure of a flow-control valve i n the bypass line, w i t h brine admission t o the first-stage flash drum modified by the first-stage pressure controller. Turbine startup would involve increasing

, L the turbine-inlet flow while maintaining flash drum pressures by .controlling turbine-bypass flows. tr ip could be accommodated without s h u t t i n g the plant down or' venting to

i !

i 1 -

i I - the atmosphere. i

By opening the turbine bypass valves, turbine

I

I *

A flash steam system i s vulnerable to excessive moisture being fed into t h e turbine. To guard against this, the t u r b i n e is t r i p p e d , due to a signal from a level sensor when the l i q u i d level gets to the c r i t i ca l level i n any of the flash drums. Other alarms which may be provided on each flash drum are: low f lash drum level.

h i g h or low level f lash drum pressure and h i g h or

4. BINARY PLANT OPERATION AND CONTROL

4.1 Design and Operational Considerations

Figure 4 i l l u s t r a t e s a typical binary plant concept, where an intermediate working f l u i d i s vaporized, expanded through--a turbine, condensed, and pumped back through the loop, i n a closed cycle. The geothermal f l u i d i s used to heat the secondary working f luid.

One requirement o f binary systems i s mu1 t i p l e heat exchangers. Various types of heat exchangers can be used. The direct-contact heat exchanger i s one type currently being studied. Many different surface heat exchanger configurations have been studied. The s ignif icant d i s -

t inction from a controls standpoint is the boiler temperature re la t ive to the c r i t i ca l p o i n t of the binary f l u i d . subcrit ical boiler, i n which the l i q u i d level may be measured and con- t rol led by the feed control, matching the feed supply to the r a t e of vapor generation.

. level ; the feed control could be. based Dn boiler,exi-t temperature:

Figure 4 i l l u s t r a t e s a

A supercrit ical boiler, of course, has no l i q u i d . * .

The binary f l u i d boiler pressure, i n the variation shown i n the Figure 4 , i s controlled by slaving the turbine-bypass valve to the turbine-governor valve, i n order to provide a constant total vapor flow

- . -.. , _ _ . , <. . - - '

L

24

/

25

as the load varies. driving the turbine-bypass valve from a separate boiler-pressure controller or by modulating the geothermal f lu id flowrate. is discussed separately i n the sections describing the flash-binary and 1 iquid-binary systems.

Boiler pressure control may also be accomplished by

Geothermal f lu id control

As shown i n the figure, the working f lu id head r i s e is provided by a motor-driven pump. factory pump operation and a more constant head rise i n the s tar tup range. pump and a turbine-driven main feed pump. by modulating the flow of vapor t o the main feed pump turbine.

The bypass-flow control loop provides for sa t i s -

Another common arrangement incorporates a motor-driven boost Feed control would be provided

ki

._-

Surface type condenser and cooling tower operation are similar t o the operati on i n other types of geothermal plants previously di scussed.

4.2 Flash Binary Systems

Figure 5 shows a flash brine replacement for the supply system portion of the previous configuration. supply system i s similar t o the f lash steam plant control presented previously. f lash drum.

During low flow and startup condition the brine is flashed a t the wellhead, the vapor i s vented to the atmosphere, and the l i q u i d is pumped t o the injection well.

The control of t h e f lash binary

The l i q u i d level and pressure is maintained i n the f i r s t The l i q u i d level i s maintained i n the second f lash drum.

4.3 Liquid Binary Systems

4.3.1 General Design Considerations. Figure 4 is typical of a l i q u i d brine system. T h i s type of binary system uses the geothermal f lu id i n l iquid s t a t e , throughout the system; by making- certain the' pressure i s above the vapor pressure. I t i s also necessary, for brines w i t h high carbon dioxide content, t o keep pressures high enough t o prevent outgassing and the result ing deposition of calcium carbonate i n piping and equipment. Figure 4 shows the use of a pressure controller

26

U

.

1 I

27

bi and a contro l valve a t the i n j e c t i o n wellhead t o maintain the pressure on the geothermal f l u i d . wells, a t the p lant , and a t the i n j e c t i o n wells.

Pumps are t y p i c a l l y provided a t the supply

Figure 4 shows an arrangement which provides br ine i n l e t temperature contro l and a constant b r ine flow rate. r a t e t o minimize heat exchanger thermal stresses.

I t also provides a slow heatup

. The three contro l valves are dr iven by a s ing le c o n t r o l l e r prov id ing

a constant f low resistance whi le replacing some o f the hot incoming br ine with an equal amaunt o f colder p l a n t e x i t brine. i n j e c t i o n system i s s ta r ted and brought t o f u l l f low w i t h valves A and B open and valve C closed. The br ine boost pump i s s ta r ted and the design f l o w ra te i s establ ished through the heat exchangers whi le rec i r cu la t i ng co ld geothermal f l u i d through valve B. The con t ro l l e r adjusts valves A, B y and C y t o provide a mixed br ine temperature, matching a demand which i s ramped upwards t o the resource temperature. valves A and B and opens valve C u n t i l the bypasses are closed and a l l o f the br ine flows d i r e c t l y through the heat exchangers.

The supply and

This gradual ly closes

A much simpler arrangement may be used i f heat exchanger s tar tup l im i ta t i ons can be met by f low modulation. Valve B and the rec i r cu la t i on l i n e can be eliminated, valve A i s dr iven t o provide the required bypass and valve C i s dr iven t o provide the required heat exchanger flow.

4.3.2 Raft River Dual B o i l i n g Plant. The Raft River 5 MW p l a n t i n southern Idaho has been chosen t o i l l u s t r a t e binary system performance because the data i s avai lab le from the dynamic s imulat ion used t o develop the p l a n t cont ro ls and operating procedures. The informat ion and con- s iderat ions are t yp i ca l o f what might be developed f o r any b inary p l a n t

. . cont ro l system design.

(1) Configuration. Figure 6 i l l u s t r a t e s the conf igurat ion o f t h i s p lant . previously. The p lan t s intended t o demonstrate the technical f e a s i b i l i t y

This p lan t i s a va r ia t i on o f the l i q u i d b inary system discussed

t

28

C'

W

29

i , I of a dual -bo i l ing system u t i l i z i n g isobutane as a working f l u i d f o r

power generation f r o m a medium temperature (29OOF) geothermal f l u i d . I

The p lan t w i l l operate i n the base load, and power var ia t ions due t o changing operating conditions w i l l be accepted by the gr id . Turbine t h r o t t l i n g and bypass a l low t rans ien t load reduct ion wi thout adjustment o f b r ine o r coolant loops.

The br ine loop consists o f three supply w e l l s ( f o r s imp l i c i t y , the f i gu re shows only one) with check valves a t the junct ions w i t h the main supply l i ne . (control l e d heatup rate) o f the complete b r ine system.’ Pumped geothermal f l u i d f low i s established by s t a r t i n g the wel l pump and opening the manual cont ro l valve. p l i shed manual ly.

An ar tes ian bypass a t each we l l permits thermal condi t ion ing

Subsequent adjustment o f w e l l f l ow i s a lso accom-

Temperature contro l dur ing s tar tup i s provided a t the p l a n t by the A boost pump and the ganged three-valve arrangement discussed ea r l i e r .

hand balancing valve i s provided i n the f low path through the heat exchangers, t o ad just f o r var ia t ions i n heat exchanger performance and i n fou l i ng resistance.

This s tar tup procedure requires considerable external power t o d r ive the pumps, u n t i 1 the turbogenerator begins producing power. external power requirements could be reduced by s t a r t i n g the p l a n t a f t e r a s ing le supply wel l i s established. This would requi re addi t ional hardware t o d r ive an addi t ional balancing valve t h a t would match the rec i r cu la t i on pressure drop t o the bypass pressure drop, i n order t o permit the three valve system t o operate a t a su i tab le pressure d is t r - ibu t ion for the lower b r ine f lowrate.

Peak

I n an urgent upset condit ion, the bypass l i n e provides a rap id and redundant means o f stopping heat i n p u t - t o the working f l u i d loop. the major i t y o f the b r ine p ip ing is- low cost asbestos-cement w i t h a l i m i t e d capab i l i t y f o r rap id temperature change, the i n j e c t i o n l i n e

Since

,

30

t

cannot accept the sudden change from normal plant exit brine at 15OOF to supply brine at 29OoF. divert the flow to the pond, and the injection and supply pumps are shut down when the exit brine temperature rapidly changes.

The three way valve is therefore sequenced to

Each of two injection wells has a control valve at the pump discharge to automatically maintain the pump inlet pressure above the outgassing level.

The working fluid loop uses isobutane and a configuration identical to that shown in Figure 4 except for the parallel high and low pressure paths, which were provided to increase cycle efficiency, and the preheater bypass lines. flashing at the boiler inlets, to compensate for heat exchanger over- performance, and to adjust for variations in fouling,

These bypass lines are manually controlled to prevent

The boiler feed valves are, relative to the basic configuration discussed ,earlier, moved downstream to the boiler inlets. individual boiler level control. While this increases the vapor pressure at the control valves it ensures that cavitation will occur in these valves. The pressure drops are relatively small and cavitation resistant valve trims are provided to minimize erosion.

This provides

The coolant loop provides a constant cooling water flowrate and an isobutane condensing pressure varying with the ambient temperature. Cooling tower basin level is controlled by varying the treated brine makeup flow. ,

(2) Simulation. Transient simulation of the plant and the supply and injection system was accomplished using two specifically developed codes.

~

The thermohydraulic system behavior, neglecting acoustic phenomena, was simulated using the Time and Frequency (TAF) problem solver code c13 and a CDC 7600 computer. The acoustic behavior was studied using the

31

L C5MP CodeCP1. generated using the TAF code.

The fo l lowing discussion presents some o f the analysis

The TAF mathematical model o f the system incorporates the fol lowing:

I (1) Approximations t o Star1 data f o r isobutane I proper t i es

(2) Estimated o r vendor data f o r pump, turbine, and cool ing tower charac ter is t i cs

~

j l . (3) Deta i led lumped parameter models o f the condenser and the

heat exchanger

(4) Flow equi l ibr ium throughout the various process loops

(5) Mass and energy balances f o r uniform two-phase cont ro l volumes f o r the isobutane spaces i n the b o i l e r s and the -

~ condenser , I

I

(6) Proport ional p lus i n teg ra l mode contro l 1 ers.

The model accepts var iab le inputs f o r b r ine i n l e t and e x i t condit ions, pump speeds, fou l ing, ambient condit ions, t r i m valve posi t ions, and contro l 1 e r demands.

Table I summarizes the e f f e c t o f ambient a i r condi t ions and fou l i ng on p lan t performance. and co ld condit ions. condi t ion 1% o f the t ime , and goes below the co ld condi t ion 1% o f the time. A discontinuous curve must be used t o connect the data a t the three points, because the tower a i r f low i s reduced from 2255 t o 1500 cfm a t the co ld condit ion. .This was done t o keep the cold'water leaving the coo l ing tower a t l eas t 3 9 O F t o prevent c ing a t the worst locat ion.

The three data points represent the hot, median, Ambient temperature, exceeds the value o f the hot

32

t.

TABLE I

RAFT RIVER PLANT STATIC PERFORMANCE

Low High Low High Low High Brine Cold Hot Tot Wet Dry Foul Low High Cond Boi l B o i l Cond Bo i l Bo i l Subcool Subcool E x i t Coolant Coolant Evap Is0 Net

Bulb Bulb % Bypass B ass Press Press Press TempLa3 Temp Temp Margin Margin Temp Temp Tzmp Flow Flow Power OF OF Design % 'I psia ps ia psia OF O F O F OF O F OF O F F lb/sec lb/sec MW(e)

E f fec t o f Preheater Bypass

65 92 DES 10 0 74.8 196.7 364.1 100.8 175.2 233*.8 5.3 5.3 149.7 73.2 93.0 38.1 245.5 2.46 15 0 74.6 193.7 363.2 100.6 174.0 233.5 7.4 5.7 150.2 73.2 92.9 37.9 243.8 2.42

E f fec t o f Ambient and Fouling w w

65 92 DES 10 0 74.8 196.7 364.1 100.8 175.2 233.8 5.3 5.3 149.7 73.2 93.0 38.1 245.5 2.46 lb% 73.1 196.2 395.0 99.3 175.0 242.2 8.0 8.8 140.5 73.4 94.3 39.7 258.1 2.96

38.4 47 DES 10 0 54.5 192.3 362.8 80.2 173.4 233..4 8.0 5.8 140.7 50.9 71.8 26.2 243.6 3.34 10% 53.3 192.5 394.3 78.8 173.4 242.0 10.2 9.0 130.8 51.2 73.4 27.8 256.6 3.87

8 8 DES 10 0 44.7 190.2 362.2 68.8 172.5 233.2 9.6 6.1 135.8 38.6 60.1 19.9 242.4 .3.82 10% 43.8 190.7 393.9 67.8 172.7 241.9 11.3 9.2 125.7 39.4 62.2 21.4 255.7 4.37

[a] Saturation temperature: leaving condit ion i s 4OF lower

Standard Conditions besign fou l ing - Brine 0.0015 hr-ft2-'F/Btu

Br ine i n l e t 288.6 lb/sec @ 290 F Makeup temp 75 F Coolant f low 1869 lb/sec

Working f l u i d 0.0005 Cool ant O.OOl0

The low-temperature preheater bypasses 10% o f the t o t a l flow, and the high preheater bypass i s closed.

a t both bo i l e rs which are 5.3OF subcooled a t the c r i t i c a l conditions o f

''hot" ambient temperature and design (maximum) foul ing. As shown i n the

table, power output i s r e l a t i v e l y insens i t i ve t o preheater bypass.

These set t ings y i e l d i n l e t condi t ions

The e f f e c t o f reduced ambient temperature i s to: lower condenser temperature and pressure, lower coolant temperature, lower b r ine e x i t

%emperature, lower evaporation rate, increase the low preheater subcooling, and increase the net power output.

Reduced fou l i ng increases high b o i l e r pressure and temperature, increases subcool ing, increases isobutane f low rate, increases net

power, and appreciably decreases br ine e x i t temperature.

The maximum range o f p l a n t power output, considering 1% temperature extremes and fou l i ng o f 10 and 100% o f the design value, i s 1.91 MW(e),

o r 78.0% o f the minimum power.

output t o vary wi th the ambient conditions are clear. The advantages o f a l lowing the power

It should be noted, i n summary, t h a t the contro ls operating over the above range o f f u l l power operation are

High and low pressure b o i l e r feed contro ls

Turbine power-boiler pressure contro l

Cooling tower basin leve l cont ro l

I n j e c t i o n pump i n l e t pressure contro l

Bu i ld ing heating/cool i ng and freeze pro tec t ion cont ro l

valves

Manual stepping o f cool ing tower fan speed ( f u l l , ha l f ,

zero) and d i rec t i on under i c i n g condit ions.

L

6.i

34

Further descr ip t ion of cool ing tower contro l i s given i n Section 111-5.2.

, -

(3) P lant Startup. As described e a r l i e r , the supply and i n j e c t i o n system i s brought t o f u l l f low and the temperature i s s tab i l i zed by using the p l a n t bypass. The isobutane loop s tar tup i s i n i t i a t e d by successive purges w i t h water and nitrogen. The loop i s then completely f i l l e d w i t h subcooled isobutane. c i r c u l a t i o n continues u n t i l the pump work has heated the f l u i d t o 13OoF and any remaining water has been removed by a dryer. drained t o the required inventory o f isobutane.

The isobutane pumps are s ta r ted and

The loop i s then

The br ine boost pumps are s ta r ted and the br ine temperature c o n t r o l l e r i s enabled w i t h an i n i t i a l demand o f 13OoF, thereby prov id ing a small b r ine supply t o the p lan t w i t h the near ly open rec i r cu la t i on and bypass valves reducing the mixed p lan t i n l e t temperature t o 130OF.

The tu rb ine inlet-bypass valve combinations -are manually control led, w i t h the bypass valves i n i t i a l l y closed, and turb ine f low blocked by the stop valves. l eve l cont ro ls are enabled, and the isobutane pumps restarted. The cool ing tower basin l eve l cont ro l i s then enabled, and the fans and the coolant pumps are started. The br ine temperature demand i s then ramped t o 29OoF.

The isobutane rec i rcu la t ion- f low contro l and the b o i l e r

Figure 7 shows the resu l t s o f a quasis tat ic mapping study made to. : determine the turbine-bypass valve sequencing dur ing the heatup t o f u l l operat ing condit ions. a t the b o i l e r i n le t s , and t o minimize the change i n s e n s i t i v i t y o f b r ine rec i r cu la t i on f low t o ' b r i n e 'mixed temperature over the e n t i r e startup. Both preheater bypasses were se t a t 15%.. The high pressure preheater- bypass i s necessary a t the co ld end o f the startup; i t w i l l be shut o f f a f t e r s tar tup i s comple The data i s f o r a l l heat t ransfer surfaces fouqed t o 10% o f the design maximum. D i f fe ren t bypass set t ings would be requi red i f fou l i ng were more severe.

The object ive was t o maintain a subcooled condi t ion

35

NOTE: The selected bypass valve schedule ( C v a l tnear funcf'idn of brine temper- ature) is shown as the dashed l ines. The solid lines represent constant C values for bypass valves a!! indicated.

a, C .L

fi 80 - 40 -

I . . I I I I I I \, 140 180 180 200 220 240 260 280

Mixed brine temperature (OF)

INEL-A-8878

c

F i g . 7 Raft River startup mapping.

36

i

Figure 7 shows the high pressure and low pressure b o i l e r i n l e t

subcooling and the br ine rec i r cu la t i on f low as functions o f mixed br ine temperature f o r several turbine-bypass-valve f low coef f ic ients . The

high and low pressure bypass-valve coe f f i c i en ts are made equal f o r the purposes o f t h i s study.

k,

The data show t h a t the subcooling a t both b o i l e r i n l e t s var ies

inverse ly w i t h the bypass-valve f low c o e f f i c i e n t as the br ine temperature

increased. A 1 inear (wi th mixed br ine temperature) bypass valve sequencing

w i t h a very small i n i t i a l value was selected f o r s imp l ic i t y . The r e s u l t i n g subcooling and br ine rec i r cu la t i on f low i s shown by the dotted l ines. The minimum subcooling values were 5.0 and 5.5OF f o r the high and low

pressure b o i l e r i n l e t s , respectively.

change t o mixed-brine-temperature change var ied by a fac to r o f 3.0 over the complete s tar tup range.

The r a t i o o f rec i rcu la t ion- f low

The tu rb ine would be s ta r ted a t the completion o f the p l a n t startup,

when the turbine-bypass valves are f u l l y open, the slaved tu rb ine - in le t valves are f u l l y closed, and the turbine-stop valves are closed. The

stop valves would be opened, the turb ine speed contro l enabled, and the speed demand ramped up t o the design leve l . The generator would be p u t

on l i n e , cont ro l t ransferred t o the load contro l mode, and the load

demand ramped t o , the desired p l a n t output.

(4) Upset Operation. Figures 8a through 8c' shows the simulat ion

o f a tu rb ine t r i p a t f u l l power.

(conservatively) by stepping the generator torque t o zero a t 1.0 second. A f t e r .a 0.2 second dead t ime , the turbine-stop valves (both stages) were

closed i n 0.5 second and the tu rb ine bypass valves opened i n 5.0,seconds.

The loss o f the generator was simulated

Figure 8a shows the tu rb ine speed increase from 8000 t o 8800 revolut ions

per minute and the subsequent decay.

per turbat ion i n l i q u i d leve ls i n the b o i l e r s and condenser, which i s due

t o blocking the vapor f low u n t i l the bypasses,open f u l l y and the feed

Figures 8a through 8c show the

z

37

- High feed --0-- Low feed -0- High throttle

i -...- High bypass I

~

I I

1

! I

50 100 750 200

- Turbine speed ----- Generator torque - -- Turbine torque

-

-- I I I -2500 +

Time (sec)

38

, .

400

300

f 200 tn tn 2 n

100

0

C .- g 6 E

I I I r

- High boiler Low boiler -.- Condenser '

----..I-

7-- ~~

I I I 50 100

' Time (sec) I50 200

I I -High boiler level

Low boiler level Condenser level

I

--am--

-*-

-

-

I I 50 100 150 200

Time (sec) INEL-A-10 271

\

Fig. 8b Turbine trip response -- pressure, level, and subcooling *mrgin.

3

- 39

!

I

!

I

- Brine exit --I-- Coolant irtlet -0- Coolant exit

LVV I

-0- --Do-

h 150

E 52 - IO0

0 Q) v) \

Y'-...- - High turbine -----High bypass

50 -- Low bypass -**.- Low feed --- High feed

I I n

50 100 150 200 Time (sec) INEL-A-10 270

Fig. 8c Turbine trip response -- temperature and flowrate.

I 40

6, contro ls re tu rn t o the design condit ion. r i s e i n the high bo i l e r , a 3.0 p s i r i s e i n the low bo i l e r , and a 7.0 p s i drop i n the condenser, the system i s r e l a t i v e l y undisturbed.

Except f o r a 7.0 p s i pressure

While the f low and pressure disturbances could be el iminated by prov id ing bypass-valve speeds equal t o those o f the turb ine stop valves, the t rans ien t response i 11 ust rated above i s considered acceptable.

4.3.3 Magmamax Dual Binary Plant. The Magmamax system i s presented because o f the i n te res t i ng cont ro l aspects o f the dual b inary cycle, the p a r t i c u l a r design options chosen, and t o show the de ta i led I&C design f o r a p l a n t cur ren t ly being constructed.

(1) Configuration. As shown i n Figure 9, the design incorporates

(a) A l i q u i d b r ine system w i t h a f low modulation p l a n t bypass

(b) A superc r i t i ca l isobutane loop, w i t h a tu rb ine dr iven feed pump, a tandem synchronous turbogenerator, and recuperation t o a separate propane loop

(c) A lower’temperature propane loop w i t h an induct ion turbogenerator, and

(d) A spray pond cool ing system with extensive management o f , cool ing capabi 1 i ty.

Another unique feature o f both isobutane and propane loops i s the use o f heat exchanger bypass f low t o cool the tu rb ine bypass flow, and thereby reducing condenser thermal shock.

41

P h)

. . . . . . . . _.. . .. ... . .

- Injection

wells (2)

Discharge to drain I canal lsobutane

condenser I I L--t--l

D V W l S

e,

c

.. *

(2) Geothermal "Brine C i rcu i t . The heat source f o r the Magmamax power p l a n t i s geothermal b r ine supplied by deepwell pumps from three separate wells. Since these wel ls are qu i te f a r apart (as much as 1/2 mile), and since the equipment yard and the heat exchange equipment i s remqte from these wel l s i tes, the geothermal b r ine i s piped from each we l l t o a common p o i n t where i t i s manifolded, and brought t o the p l a n t as a s ing le combined flow. Each o f these ind iv idua l wel l l i n e s contains a sand separator for removing any pa r t i cu la te matter which may f low from the we1 1. actuated l o c a l l y t o shut o f f we l l f low and stop the wel l pump i n the event o f p ipe l i ne rupture. ad just we l l supply quant i ty and w i l l be used f o r w e l l balancing purposes.

Valves VB1, VB2, and VB3 are production we1 1 shutof f valves,

These valves may a lso be used t o manually

The br ine i s supplied from the wel ls a t 36OOF and about 200 psia,

Flow quant i ty from each i n order t h a t i t might be pumped t o the p l a n t wi thout f lash ing and wi thout releasing any o f i t s dissolved gases. we l l w i l l be measured (probably near the wellhead) and a remote i nd i ca t i on o f these f lows w i l l be required. A t the p lant , the geothermal b r ine i s pressurized by the booster pump t o 270 p s i a t o prevent f lash ing i n the heat exchanger f i e l d . The t o t a l p lan t f low passes through valve VB4 i n t o the heat exchanger f i e l d (HXF) when the p l a n t i s i n f u l l operation. Valve VB4 i s a two-position i s o l a t i o n valve which i s used t o p ro tec t the heat exchanger i n the HXF from rap id thermal changes a t startup. w i l l be cont ro l led by HXF leaving water temperature, and i s bypassed by a small o r i f i c e f o r HXF warmup. b r ine had been b led through the f i e l d t o ra i se i t s temperature t o a preset value.

It

It would open only a f t e r enough hot

-

Valve VB8 i s a small bleed valve. It bypasses VB7 t o provide warmup flow contro l and it i s cont ro l led by a preset f low r a t e ramp-up. Valve VB7 i s the main f low contro l valve f o r the HXF, and i s con t ro l l ed . by f low rate, AT, o r isobutane knockout drum temperature, depending on whether contro l i s i n the s ta r t , intermediate, o r run mode. Valve VB9

43

i s the tlXF dllow .bypass valve. This valve i s pressure con t ro l l ed t o keep

dur ing sitavtup, and dur ipg load f l uc tua t i on t ransients. the c i r c u i t above sa tura t ion pressure, and wou1.d bypass t h e p l a n t c i r c u i t LJ

_ *

The heat exchanger f i e l d consists of eleven Iong-tube t rue counterfJaw Ten o f these are piped i n a ser ies-paral l*el ,&at ,exchangers.

used t o pr,eheat, b o i l , and .Superheat the isobutary ,working f l u l d . hangers i s '18 inches i n diameter, with 70 foot-1

Isobytane flows i n the tubes, and geothermal b r i ne f l o w s i n the shel l . The eleventh she l l i s a shorter exchanger used t o .bo i l and superheat the dual f l u i d cycle XDFC) propane working f l u i d a f t e r i t has been heated by

ust. Valves V85 and YB6 are u$ed co b r i n e floy through t h i s propane bo i ler-superheater.

tor the wat;?r 5ide o f the HXF i s abou; two minutes, and u t 153 ps i . A t $he HXF ex i t , gooled b r i

Each

Design th rwghput

the i n j e c t i p n pump .where it i s pressurized and sent t o the i n j e c t i o n B l l pre the shqtoff ya

' l led i n a manner similar t o YB1

ne $.low c i r c u i t is no t team power cycle, @ut uses the hydrocarbon as

i t s working' f lu id . heated dur ing i t s ser ies t rave l through s i x o f the 10 counterflow beat

Isobutane l i q u i d enter ing a t a nominal 9

t h e HXF.

a t 345O.F and 500 psia. Design isobutane f low i s 1.03 x 10 pounds per hour. A f te r passing through the knockout drum, the gas passes through a t r i p valve and then through the main turb ine contro l valve V11. This valve i s con t ro l led by a speed-load contro!ler, the speed signal comes from a magnetic pickup on the'main tu rb ine gear reducer output sha f t and the load signal provided by devices sensing generator e l e c t r i c a l output. con t ro l valve are required. A t startup, and p r i o r t~ synchronization, manual speed contro l w i l l be used t o b r i ng the turb ine t o i d l e speed. A f te r reaching i d l e speed, the turb ine accelerat ion w i l l be cont ro l led through automatic ramp-up by the governor.

It then s p l i t s i n t o two p a r a l l e l flqws, each gh a b o i l e r and a superheater before passing i n t o the

6

Seyeral modes o f operation o f the turbine-

Once synchronization i s

- 44

accomplished, the generator speed i s set by the g r i d l i n e frequency, and valve VI1 w i l l be used t o control only the load. Normally, t h i s valve w i l l be wide open and the p l a n t w i l l be running a t f u l l capacity, since it i s only a p i l o t operation. When i t i s necessary t o reduce the p l a n t output, a means o f se t t i ng and c o n t r o l l i n g VI1 i s required.

u ?

The main turbine s t a r t i n g bypass valve, V18, i s used t o bypass gas around the main turbine t o the condenser during the startup sequence and during any load change s t a b i l i z a t i o n periods. This valve would be under knockout drum pressure control , and would be closed during normal operation. The bypass desuperheater valve, VI9, would be modulated during periods o f bypass f l o w t o provide l i q u i d f o r cool ing t h i s bypass gas, since the condensers should not be subjected t o temperatures as high as may be encountered i n the main tu rb ine supply f 1 ow.

The main turbine i s a YORK 3338 tandem design w i t h dual three-stage uni ts , each u n i t exhausting i n t o the she l l side o f the isobutane recuperator. The tu rb ine dr ives an Allis-Chalmers 10,500 KW, 1200 rpm synchronous generator through a General E l e c t r i c reduction gear, a t a turbine speed o f 6391 rpm. Extract ion gas from the main tu rb ine flows through a f low measuring device (FI2) and then through the b o i l e r feed pump-turbine control-valve, V12, t o the b o i l e r feed pump-turbine (BFPT). The BFPT i s a YORK 226 two-stage turbine d r i v i n g a, United Centr i fugal Pump, three- stage hor izontal cent r i fugal pump through a Western reduction gear. VI2 i s con t ro l led by a governor, which senses both speed and knockout drum pressure. The speed signal comes from a magnetic pickup on the BFPT gear reducer. The BFPT speed w i l l be var ied during operation, t o contro l l i q u i d f low t o the HXF. A pressure con t ro l l e r , sensing b o i l e r feed pump. (BFP) discharge pressure , w i 11 override the knockout drum pressure signal. The ex t rac t ion _ _ f l o w contro l VI7 w i l l normally be closed during operation, but would be cont ro l led by a f low con t ro l l e r , and would open i f ex t rac t ion 'flow was t h r o t t l e d by VI2 t o the p o i n t where i t began t o a f f e c t main tu rb ine output. A pressure c o n t r o l l e r sensing ex t rac t ion l ine-pressure w i l l overr ide t h i s f low signal t o maintain the proper

'6

45

in ters tage pressure. The operational con$,rol 0;f VI7 must be f l e x i b enough t o a l low fu tu re rese t t i ng and "tuning," since t h i s vaJve w i l l

cont ro l mode w i l l depend upqn the operational e f f i c i enc ies o f some of

,-

1.J probably be used mainly for @proving the p lan t power outpyt.

the other p l a n t equjpment.

I t s . _

- Exhaust gas from the generator and feed pump tu rb f 1 ows thvough

the recuperator, where it gives up some of i t s heat t o the dual .f

cycle (DFC] by heating and s t a r t i n g l i q u i d propane -to Q o i l from the

b o i l e r feed puqp. The isobutane exhaust gas p

t o the ty9 ispbutane ndensers, where the pow re jected t o the cool ing water c i rcu i$ . This i passe5 t o the receiver, where i t is picked up booster pumps and passed on t o the BFP.

working f lu id t o HFX pressure. and bypass valves. These valves ass i s t

the flpw o f working f l y i d t o t he HFX, and ,prevent the ,6FP fr

The @fPT s t a r p valye, V16, w i l l be supply BF.?T gas during startup, and would p'ls_o sypply cond i t jpn where ex t rac t ion flow was i n s u f f i c i . The kwckout drum- b l owdown-val ve , V I S , under primary contro l o f knockout drum-1 eve1 , i s

used t o re tu rn any l i q u i d carryover from the HXF t o the receiver. would be i n use mainly dur ing the warmup and s tar tup sequences.

om ;he recuperator

The BFP then pressurizes the .

e BFPT governor, in c o n t r o l l i n g Valves V I 3 and Y14 are the BFP

ro t* led t o shutoff.

V I 5

(4) Propane .Ciycuit. The propane flow c i r c u i t i s very s im i la r i n

t o the isobutane c i r c u i t , prov id ing a !'bottoming cycle" t o ional he?t from the geothermal b r ine and a lso red

amount o f heat re jected t o the atmosphere through the cool ing

c i r c u i t .

Propane l i qu id , enter ing a t a nominal 77OF i s heated and p a r t i a l l y

bo i l ed i n the tube side o f the isobutane recyperator, ex t rac t ing heat

from the isobutane turb ine exhaust.

bo i ler-superheater where boi 1 i n 205OF and 460 psia.

It then passes i n t o the propane

s completed and the gas i s heat This second stage o f heating i s provided by geothermal

rc

6. 46

br ine, which passes. through the she l l s ide of,. the boiler-superheater a f t e r leav ing the isobutane boi lers . The br ine then flows on t o the

br

isobutane heaters. Design propane f low i s 274,400 pounds per hour. From the bo i ler-superheater, propane gas passes through the knockout drum, through a t r i p valve, and then through VP1, (DFC Turbine contro l valve). This valve i s con t ro l led by a governor, the speed signal being provided by a magnetic pickup on the turb ine and the load signal provided by devices sensing generator e l e c t r i c output. The turb ine a lso contains var iab le i n l e t nozzles t h a t are share-controlled by the governor. modes o f operation o f the DFC turb ine contro l are s im i la r t o those. ou t l ined f o r the main turbine, bu t are se t up t o handle an induct ion generator 1 oad.

The

Valve VP2 (DFC turb ine bypass valve) i s con t ro l led by propane knockout drum pressure. This valve would be used dur ing propane c i r c u i t s ta r tup t o provide turb ine bypass; i t would ass i s t i n f low c i r c u i t s t a b i l i z a t i o n dur ing any rap id load changes and i t would provide the capab i l i t y t o "tune" the "bottoming" cyc le f o r optimum dual cyc le operation. Valve VP4 (propane knockout drum blowdown valve) i s con t ro l led by a l eve l c o n t r o l l e r and i s used t o re tu rn any l i q u i d carryover from the heat ing vessels t o the propane receiver. This valve would be subject t o the same overr ide considerations dur ing the s tar tup mode t h a t the isobutane knockout drum blowdown valve contro l experiences. Valve VP5 (DFC turb ine bypass desuperheater valve) i s cont ro l l e d by bypass 1 ine gas temperature and protects the propane condensers from excessively high i n l e t tempera- tures dur ing bypass periods.

&

< .

r b i ne exhaust f 1 ows t o the propane. condenser, where it i s cooled and condensed by the cool ing water system. This l i q u i d *

drains t o the receiver, 'where 1 t i s taken by the motor-driven condensate pump and passed~ on t o the 'motor-driven b o i l e r feed pump. ' Valve VP3 (propane BFP t h r o t t l e valve) i s cont ro l l e d by knockout drum temperature, and i s bypassed by a f low o r i f i c e t o prevent the condensate and b o i l e r feed pumps from overheating by going t o shutof f .

47

(5) Cooling Water C i r cu i t . The cool ing water c i r c u i t for the Magmamax power p lan t i s s i m i l a r t o a standard spray pond system, but has same unique impkements. The sprays are o f the j e t spray type, and are mt atomized t o the-ex ten t t h a t they are i n conventional spray ponds. These j e t sprays u t i l i z e a g rav i t y f a l l t o provide cont ro l led stream breakup t o optimum drop size.

I l e d to’-provide operation counterflow w i t h the wind d i r e c t i o n f o r aptimum cooling. Spray cool ing i s on ly accomplished during off-peak periods when the atmospheric cool ing condi t ions are the most favorable.

The deep storage pond i s the reservoir fyom which a 25,000 gpm

The spray headers are d i rec t iona l -

vertfcal wet-pit type turbine pump c i r cu la tes water t o the condensers. This pump i s mounted on a specially-designed tower i n the pond. submerged po r t i on of the tower .contains a device which allows the pump

The

to draw water from the bottom (3Q feet deep) o r from ;he surface This two-pasitjon selector valve i s operated by a manually .

ar motor mounted above tk water surface. Valve positian i s on p lan t peer output requfrmetrts and water temperatures- from the circulbthg p u p through the tube s i d

propane C O ~ ~ ~ R S W (single pass), and Ohem it: splits, h a l f & o f each o f the two isobotane condensers. These condensers rge t o separate day storage pond-spray header c m b i nations.

The 12,500 gpm f lows pass directly i n t o day storage ponds dur ing a namitral &how peak load per iod (daytime), when the a i r we t buib tempera- ture i s the highest. This allows €he p lan t t o operate w i t h the l e a s t amount o f p a r a s i t i c power requirements, since none o f the spray pumps are operated during t h i s period, and the c i r c u l a t i n g pump i s drawing frk the deep, co ld water i n the main pond. the water leve l i! the day storage ponds r i s e

period, valves VW1, 2, 3, and 4 (hor i valves) are opened, and valves W5 and 6 (day storage pond f i l l valves)

, 9, and 10 (spray pond bypass valves) are used

During t h i s 8-hour per iod bout 5“ feet . t a l spray pump suction

a t the end

The horizontal spray pumps ‘are s tar ted and jet-spray cool ing

a

t o ad just spray header supply pressure, w i t h f u l l pressure being used when wind ve loc i t i es arerlow, and the pressure being reduced as wind ve loc i t i es increase. maximize cooling. basis. I n the meantime, v e r t i c a l wet-pipe.pumps i n the day storage ponds would begin t o t rans fer water by j e t sprays from these ponds. A l l spray-cooled water would be co l lec ted by the cool ing stream surface and would f low i n t o the deep storage pond.

These valves would modulate t o minimize d r i f t and Spray or ien ta t ion would a lso be cont ro l led on t h i s

It i s expected t h a t a l l water system valves ( inc lud ing those required f o r the makeup and blowdown c i r c u i t s ) would be operated from the contro l panel, bu t would be manually actuated. and pond leve l ind icat ions would be used t o es tab l i sh the proper operating p r o f i l e . Automatic contro l o f these functions could be added a t a l a t e r date.

Posi t ion spray header pressure

2 5 %

5. AUXILIARY SYSTEMS >

5.1 Heat Rejection System

The techniques f o r re jec t i ng heat t o the surroundings are, o f .

course, those i n common pract ice' i n the power industry. 'The options are: o r cool ing tower, and dry a i r cooling. The choice depends on a v a i l a b i l i t y , environmental, and economic considerations. Controls f o r the heat . r e j e c t i o n ,system can include, coolant .and a i r f lowrate contro l as >required. These are general ly manual controls. a cont ro l loop i s required t o modulate the makeup f low t o maintain a basin leve l .

once-through-cool i ng , evaporative cool i ng i n e i t h e r a spray pond

I n the case o f evaporative cool ing

Cold weather operation requires special a t ten t i on f o r an evaporative

. cool ing system. Cooling towers can be damaged by ic ing , which tends t o occur on the windward louvers. To prevent t h i s , a minimum bu lk f l u i d temperature o f 37 t o 40°F must be maintained, by reducing fan speed o r reversing fan d i rec t ion , t o a l low the warm water introduced a t the top o f the tower t o cascade over the exposed surface rather than the i n te rna l

49 "

port ions o f the tower. required t o a t t a i n these l o w coolant temperatures w i th a t y p i c a l mul t i - c e l l tower.

A s i g n i f i c a n t amount o f operator a t ten t i on i s

The Magmamax spray pond system discussed i n Section 111-4.3 i s of i n t e r e s t because i t provides an e f fec t i ve management of cool ing resources. It provides and stores water produced i n the cooler pa r t s o f the day. This cooler water i s used t o maximize pTant output during the peak load demand per iod dai ly .

5.2 Aux i l i a ry Cooling System

Cooling water i s required f o r cool ing a va r ie t y o f a u x i l i a r y equipment w i t h i n the plant. This includes the i n t e r - and after-condensers o f a noncandensabt e gas remowal system, the generator cool ing medi m* the lube o i l , the a i r compressors, and the p lan t a i r condi t ioning system-

The intercondenser and aftercondemer require coo l ing water ta

I n most systems t h i s flaw i s set manually, condense t h e vapor carryover from the condenser *and the steam from the steam-jet gas ejectors. since the amount o f steam t o be condensed i s reTat ive ly constant and independent o f load.

The lube o i l temperature must be controTled i n order t o hold the o i l v i scos i t y i n the proper range. This i s usual ly done i n a shell and tube heat exchanger. Since geothermal condensate tends t o inciude finely suspended sol ids, i t i s desirable t o pass the cool ing water through the tube side o f a shel l and tube heat exchanger, a t a-constant ve loc i t y s u f f i c i e n t t o minimize deposit ion of solids in t h e t u k s . To maintain heat t rans fer capabi l i ty , the tubes may be rodded out i f they s t a r t t o become plugged. A bypass around the exchanger i s used t o control the o i l f l o w ra te through the exchanger and t o maintain the desired temperature of the t o t a l f l o w o f o i 1.

I The generator i s normally cooled by the f l o w o f a gas through i t s windings. This gas w i l l be e i the r a i r o r hydrogen, depending mainly on

b'

L

50

generator size. tubes mounted i n the generator gas cool ing stream. f low i s se t manually, t o maintain proper temperatures w i t h i n the generator when i t i s a t f u l l load.

The gas i s cooled by c i r c u l a t i n g cool ing water through U' The cool ing water

The a i r compressors and the a i r condi t ion ing system requi re cool i ng

water. Therefore, a closed f resh water cool ing system i s used. It I consists o f c i r c u l a t i n g pumps, piping, heat exchanger, surge tank, and t reated water. The f resh water i s cooled by cool ing tower water which f lows through the tube side o f the heat exchanger a t constant ve loc i ty , whi le the f low o f the t reated water i s con t ro l led t o maintain the tempera- t u r e o f the system.

_ $

5.3 Heating System

The geothermal supply may a lso be required t o provide space heating and freeze protect ion. The geothermal f l u i d f l ow 'through these systems

+ should be thermostat ica l ly cont ro l led.

J 5.4 Miscel laneous-Auxi l iary Systems '

Subsystems f o r f i l l i n g , draining, purging, venting, blowdown, f 1 are, i nstrument , and p l ant a i r , in- house power, and other m i nor subsystems are not discussed i n t h i s chapter because they are common t o standard process pract ice.

u

.

51

IV. INSTRUMENTATION SYSTEM

Mo5t measurement systems include three subsystems: transducer, signal conditioning, and action or results. transducer subsystem is general ly a measurement element which detects the physical variable and transforms this into a mechanical or elec- trieal signal. The signal conditioning subsystem modifies the direct measurement element signal by amplification, filtering, or other mod- ifhxitfons so that the output can be used by the actlm or results wbsystem. ~ontrol~, or acts on the physical variable being measured.

detector- The detector-

The action or results subsystem indicates, stores, records,

-,--

‘bpi

The detector-transducer subsystem or measurement element is the primary concern of the following Section. The remaiwder o f the measure- ment system i s cmon to any process system and i s adequately covered by the many -texts on instrumentation and controls.

The specificatton and selection o f measurement elements for use in geothermal systems requires a knowledge o f the operating range and loeation o f measurements. The expected end use o f the measurement and the necessary accuracy must also be known. This information is generally the result o f process control studies and test or experiment plans.

The various types of control systems have been examined in the

With this information the particular previous sections. Each control system requires certain types, range, and location o f measurements. instruments to accomplish the desired results can be selected.

The following discussion of primary measurement elements addresses three aspects o f this topic

(1) The service considerations common to a1 1 geothermal measurement elements

52

kd

,

I

(2) The accuracies, ranges', and responses avai lable f o r each type o f measurement' device

(3) And the applicable codes and standards i n measurement systems.

1. GENERAL SERVICE CONSIDERATIONS

The select ion o f measurement elements f o r use i n geothermal systems must consider f l u i d temperature, corrosion, deposition, and operating environment. This Section discusses each o f these considerations as they apply uniquely t o geothermal systems.

1.1 F1 u i d Temperature

. -

An instrument selected f o r use i n a geothermal system must be compatible w i t h the operating temperature range. The operating temperature may range from a low ambient temperature t o the maximum temperature o f the resource. The material5 o f construction must be able t o withstand these temperature extremes.

The accuracy and r e l i a b i l i t y o f some instrument elements i s a f fec ted by a temperature var iat ion. This inaccuracy may be unacceptable f o r c e r t a i n c r i t i c a l measurement. -Instruments which would be unaffected by the temperature * v a r i a t i o n are avai 1 ab1 e, and coul d very we1 1 be necessary f o r c e r t a i n appl icat ions.

1.2 Corrosion

Geothermal f 1 uids are general l y corrosive. Therefore, when a measurement device i s i n d i r e c t contact w i t h the geothermal f l u i d , the se lec t ion o f mater ia ls i s very important.

I . $

A t yp i ca l Bourdon-tube pressure gauge, f o r example, has a brass c

bourdon and i s connected t o the system w i th a brass nipple. A geothermal

L, f l u i d which contains hydrogen s u l f i d e ( H p S ) i s very corrosive t o copper

53

L and copper al loys; a more su i tab le material, such as aus ten i t i c stainless steel , may therefore be necessary.

Corrosion due to -d isso lved oxygen, pH, or ch lo r ide ions i s a lso . .

common t o some geothermal f lu ids . necessary t o determine the type and sever i ty o f corrosion a p a r t i c u l a r geothermal f l u i d w i l l haue. The resu l t s o f t es ts should be used when select ing measurement devices. must also be compatible with the system's secondary fluid.

1 . 3 Deposition

A materials t e s t program i s o f t e n

I n a binary system, measurement- devices

Most geothermal f l u i d s tend t o form deposits. Measurement devices i n d i r e c t contact w i t h the geothermal f l u i d may become plugged and. inoperable due t o deposition. Deposition i s caused by a pressure drop across a device, a disturbance o f the f l u i d f low o r an area o f stagnant f l u i d . Most devices commonly used are subject t o one o r more o f these conditions. Pa r t i cu la r a t ten t i on must therefore be given t o the type of device used. A method of ten used t o reduce the sever i ty o f the problem i s t o avoid d i r e c t contact w i t h the geotkrmal f l u i d by using a diaphragm seal between the element and the geothermal f l u i d .

Deposition may also a f f e c t the accuracy and response o f an element w i th t ime. resistance t o heat t rans fe r and causes inaccurate measurements. When t h i s happens the instrument should be taken out o f service and cleaned.

A deposit formation on a thermowell, f o r instance, has

1.4 Operating Environment

The problems associated with the environment i n which measurement devices are expected t o operate are due t o ambient temperatures and corrosive gases. exposed t o the ambient environment. t o withstand var iat ions i n ambient conditions without damage o performance.

Geothermal systems are t y p i c a l l y located outside, - Measurement devices should be able

%

I n some locations there i s a freezing problem. I n these locations the geothermal f l u i d should not be stagnant without some type L

54

W s

3

l

a

o f heating. inaccuracies. and i n s t a l l i n g a measurement device.

I n other locat ions extreme heat may cause malfunctions o r Both o f these condi t ions should be considered when se lect ing I

Hydrogen s u l f i d e (H2S) i n the environment is both corrosive and flammable. Copper and copper a l loys are p a r t i c u l a r l y susceptible t o corrosive at tack by hydrogen su l f ide. Copper contai n i ng devices can be enclosed i n an a i r t i g h t environment, o r the use o f copper can be avoided en t i re l y .

? -

Other gases commonly associated with geothermal systems, p a r t i c u l a r l y b inary systems, can be corrosive o r f 1 ammabl e. such as the i n s t a l l a t i o n o f explosion proof enclosures, must be taken t o avoid problems and hazards.

Appropriate measures,

_ _

2. MEASUREMENT ELEMENTS

e fo l low ing Section reviews the common types o f measurement elements i n commercial use. i n geothermal systems i s included f o r each type o f measurement.

2.1 Pressure Measurement

actual operating experience

\i

Pressure i s one o f the most important o f the measured and cont ro l led process variables. devices such as Bourdon tubes, diaphragms, o r bellows as the basic detector elements. These elements de f l ec t under pressure. This de f lec t ion moves a pointer , i n the case o f a gauge, o r creates an e l e c t r i c a l s ignal , i n the case o f a transducer.

Most commercial pressure elements use mechanical

A p i c t o r i a l summary o f the method o f operation o f various pressure- measurement elements i s shown i n Figure loE4? temperature range, accuracy, and other charac ter is t i cs o f commercially

The normal pressure and

avai lab le pressure-measurement elements are l i s t e d i n Table I1 c 51 .

55

. -

(lb.) Spml Source: Ametek

(Ic)Hclix Soune:Ametek

Fig. 10 Pressure monitoring elements.

56

1

TABLE I 1

CHARACTER1 STI CS OF PRESSURE MEASUREMENT DEVICES ,

,

b. The most significant problem which affects pressure measurement i n geothermal systems is deposition. appropriate materials functions well i n geothermal service u n t i l the

solved by using a seal element between the geothermal f l u i d and the gauge; however, the accuracy o f the gauge i s degraded w i t h time due t o the deposition d n the seal element. Th i s same approach, of isolating the gauge or element from the geothermal f l u i d , has been used successfully with other types -of pressure-measurement elements.

A Bourdon-tube pressure gauge made of

passageway -becomes plugged by deposits, This problem has been partially . . . ...-.

Because fIJids i n a pressute clement are mostfy stagnant another ern i n sme locations i s the possibility that the geo This can be resolved by heat tracing.

-. 1he.problem .of material selection for a1 1 elements i n direct .fpnta& w i t h the geothermal f l u i d is m m n t o all types o f measurement devices. The appropriate materials can be determined from the results o f a testing program.

In other plant systems, such as i n the secondary f lu id or t h e cooling water, the elements and accessory equipment must withs&d the operating environment, Th i s includes sm~ll amounts o f hydrogen sulfide (H2S’) which is very corrosive t o copper and copper alloys.

In binary systems w i t h flammable secondary f lu ids , the instrument enclosures must be o f the appropriate class as defined by the National Fire Protection Association.

2.2 Temperature Measurement

The measurement and control of temperature w i t h i n a geothermal’ system is of primary concern. generator and supply system operation, and many other functions depend on the measurement and control o f temperature.

Heat exchange, heat balances, turbine-

58

Most commercial temperature-measuring elements detect the change i n heat-affected physical properties l ike thermal expansion, radiation, or e lectr ical properties, ture . change.

These properties change w i t h a change i n tempera- This difference i s measured, and correlated to a temperature.

The accuracy, response, range, and relat ive cost of commercially available temperature-measurement elements, i s shown i n Table 111.

2.3 Flow Measurement

.

f

W

The measurement and control of f luid flow is important to the safe and ef f ic ien t operation of a geothermal system. the plant i n l e t from the supply system, i f this i s the basis-of payment for the-supply), extreme precision i s called for; i n other instances only crude measurements a re necessary.

In some cases ( a s a t

Flow measurement i s generally an inferred measuring of another variable such as pressure, pressure drop, velocity; flow disturbance, or other properties,-which change w i t h a change i n flowrate. most commonly indicated by head loss or different ia l pressure measurement. Three common devices - or i f i ce plate, flow nozzle, and venturi tube - cause a different ia l pressure by a flow obstruction. T h i s d i f ferent ia l is measured by a pressure or different ia l pressure gauge, and the flowrate can be derived from this measurement. Table I V summarizes the character is t ics of head-loss flow-measurement elements.

Flowrate is

Many other types of flow-measurement elements are available, and they have possible application I n geothermal service. A pictorial review of the principles of operation i s given i n Figure llC6’. A

-summary of flowmeter character is t ics is shown i n Table V 171 .

59

TABLE I11

CHARACTERISTICS OF TEMPERATURE MEASUREMENT ELEMENTS

L i qu i d- i n- F l u i d B ima ta l l i c E l e c t r i c glass Expansion S t r i p Resistance l e s Thermocouples The ouples Thermocouples

t hemometer Thermometer Thermometer Thermistor Type J Type K Type T -

TemDerature -200 t o -150 t o -100 t o -300 t o -150 t o -300 t o -300 t o -300 t o -300 t o Range 600 ( O F 1

1000 1000 1800 600 1600 1400 2300 700

Approximate 21

(% o f reading) Accuracy

- +2 - 9 . 1 - +o. 2 - +0.5. - +o. 75 +o. 75 - +o. 375 - E l e c t r i c E l e c t r i c output Mechanical Mechanical Mechanical Resistance Resistance E l e c t r i c E l e c t r i c

Type Change Change Current Current Current Current

Trans ient Poor Poor Poor F a i r t o Very Good Good Good Good Response dood Good

Shock and Good Good Good F a i r F a i r F a i r t o F a i r t o F a i r t o F a i r t o V i b r a t i o n ' Good Good Good Good S e n s i t i v i t y

cos t Low Low Low Readout Readout Low Low Low Low Rather Rather Expensive Expensive

Advantages Low cost Low cos t Re1 i ab1 e Most accurate Fast ' Highest Recom- Resists Linear Dependable Rugged f o r indust. o f a l l methods response Emf mended ox idat ion S e n s i t i v i t y

measurement very s tab le sinal 1 AT f o r re- f o r moist duci ng. atm. atm

Disadvantages Poor Lacks Lacks Addi t ional Small Protect P ro tec t Low r e s i s t Temperature Response accuracy s t a b i l i t y reactout range from f rom t o reduc- l i m i t e d .

i n g atm p r o t e c t poor response requi red s u l f u r Z % e , from ac id

s u l f u r

1

c *

Device

0 * “I

TABLE I V

HEAD LOSS. OR DIFFERENTIAL .PRESSURE FLOW MEAS

Reaui red . ”

1 Upstream F l u i d Head Loss ccuracy Piping Viscosity Relat ive Types , (46 Produced) (% Max. ) (Pipe Dia.) E f fec t cost Advantages Disadvantages

0r i . f i c e +1 ’ Concentric Liqu -

Eccentric - + 2 o r steam

10-30

10-30

Segmented 60- 100 - + 2.5 10-30

Quadrant 40-80 - + 1 20-50

0 9 -I

Flow Nozzle Liquid, gas, 30-70 - + 1.5 10-30 - o r steam 7

-

Venturi Tube l i qu id , gas, 10-20 o r steam

. L

O r i f i ce Large ’ * Low 1. Lowest cost 1. High head loss

‘ Large Low 2. Easi ly i n s t a l l e d -2. Traps o r replaced Suspended

sol i ds * Large Low ’ 3. Well established 3. Low capacity coe f f i c i en t

I Small Medium 4. Continued r e l i - 4. Reauires abi 1 i t y flanges

Flow Nozzle Small Medium t o high 1. Can be used w/o 1. Higher cost

2. Lower cost than 2. Higher head f 1 anges than o r i f i c e

Venturi loss than Venturi

3. Lower head loss 3. I n s t a l l a t i o n i s than o r i f i c e c r i t i c a l .

Very large ’ High , Venturi Tube

1. Lowest head loss 1. Highest eost

2. Largest and heaviest

3. Shortest upstream p ip ing required

4. Does no t obstruct f 1 ow o f suspend- ed so l ids

5, Does not require pipe flanges

6. Well established coe f f i c i en t

Har

Fig. l a Orifice plate some sippa

Fig. l b Orifice plate installed Sourn: Singer

Fig. 2a Trapezoidal weir

n

and rotor toppart Fig. 3 Turbine Source: L i

Fig. 5h Vortex shedding sourrr: ncphmentrteen

Fig. 5a Vortex precession Smner F i L Porlrrlsinper

lnld

Fig. 7 Positive displacement smnc: Singer

. - Fig. 8 SoniclUltrasonic

9b

Fig 9a Fluidic Swrcc YOOR PWduaS *

Fig . 11 F1 ow monitoring elements.

62

m W

TABLE V

CHARACTERISTICS OF FLOWMETER ELEMENTS

m m m

I I I

Lid As previously stated, the most common method o f f lowrate measurement uses an o r i f i c e p l a t e and d i f f e r e n t i a l pressure gauge. t rue i n geothermal systems.

This i s aTso Operating experience has shown t h a t i n some

he brine.chemistry and locat ion, o r i f i c e p la tes . - _ . . . - .

w i l l trap scale and b u i l d up a scale deposi t upstream. readings and w i l l eventual ly p lug the hole. o r i f i c e p l a t e i s damaged by erosion from part icu la tes. , however, and under most service Considerations an o r i f i c e

Th is causes Another p rob l

I

d

te Will give r e l i a b l e and accurate f lowrate measurement. It must f requent ly be checked t o determine i f the edges are rounded o r the diameter changed, due t a erosion o r scale formation.

Another method whlch has been .used wi th some success i s a p i t o t vers this requires very c lean service and idea l conditions. Qds which are i n use include u l t rasonic , vortex, magnetic, and

Each o f these can be gfven consideration when the

tube ; Other tu rb ine meters. condltfons, accuracy, pressure drop, o r scale formation w i l l not allow using a less exp

The measurement and contro l o f l i q u i d leve l i s f requent ly a primary contro l clement i n a geothermal system. geothermal system are separator leve l , condenser leve l , and cool ing tower leve l .

Typical appl icat ions i n a

the measurement of l i q u i d l eve l ca.n be a d i r e c t o r i n fe r red measurement. The methods commonly used are as follows:

Di rec t methods

(1) Gauge glass L .

(2) ' B a l l f l o a t

64

(3) Electric conductivity electrode bJ t (4) Light, sonic, or ultrasonic beams

Inferred methods

(1) Pressure gauge

(2) Differential pressure

(3) Physical or electrical property change

(4) .Radiation

(5) -Displacement transmitter

(6) Bubbler or purge.

The characteristics of these types of 1 iquid level measurement

,

't

elements are summarized in Table VI.

Level-measurement elements in contact with the geothermal fluid have two common problems: corrosion and deposition. Therefore, a measurement element which is disabled by either of these, such as a ball float, gauge glasses, or electrodes, must use a method to resolve this. In some cases this is not possible, as in any type of float device, the float will collect deposits and the accuracy of measurement will be degraded. In these cases sonic, ultrasonic, or pressure gauges can sometimes be used. In other "clean-water" *applications, such as in cool ing water or condensate, f 1 oat devices may perform sati sfactori ly.

In some cases two different types of measurement may be used, one for continuous monitoring and the other for high or low level switches.

i

65

" . ... " . . ~ -... ~

. . . . . .. ~ ~. ... . , .~ . ~~ . . .. ... . . .

TABLE V I

CHARACTERISTICS OF LEVEL MEASUREIIENT ELEMENTS

Mechanism Components Pr inc ip le o f Operation Advantages Disadvantages i

Gauge glass Hydrostatic Transparent tube connected Gauge acts as a manometer Simple, inexpensive, S low response, l i m i t e d

B a l l f l o a t b loa t n erna o r ex erna oa oa moves up an own, w i i m e inexpensive, d i r e c t Narrow range, corrosion

Conductivi ty t lectronic ktectrodes w i th switches Conducting l i q u i d completes S i m p l e , inexpensive, Ueposition, corrosion, electrode and detecting relays a c i r c u i t when level reaches unl imited range, accurate viscous f l u i d s

L igh t , sonic, t l ec t ron i c Transmitter and receiver, Beam sent f rom transmitter Kelet ive ly small, no moving txperlslve, ca l i b ra t i on u l t rasonic transducer, power supply i s re f lected t o receiver, parts, out o f process f l u i d , required f o r viscosity, beams t iming i s measured continuous level monitor, temperature, density,

Pressure Hydrostatic Pressure gauge and other Measured hydrostat ic head due t o sim le, inexpensive, standard ueposition, corrosion, OI g w e standard instrument l i q u i d leve l equqpment, unl imited range density sensitive, tempera-

D i f f e r e n t i a l Hydrostatic D i f f e ren t i a l pressure gauge, Level hydrostat ic head causes Simple, inexpensive, standard Deposition, corrosion, temp- '

pressure transducer, pneumatic system a d i f f e r e n t i a l pressure f rom top equipment, unl imited range erature, pressure, and d density 1 imited Fa1 ibrat ion, cleaning.

Property v1sc0s1 ty, Uevice t o measure a change Wh en the phase changes a t the Varies w i th the type of Uepends on known properties, . Change thermal, o r i n the property o f the gas l i q u i d leve l the measured meagurernent deposition, corrosion,. must

e l e c t r i c a l vs the l i q u i d property also changes i n contact w i th process property f l u i d

Radiation Kadiatioii Source, detector, transducer, Change i n level varies the Accurate, out o f process txpensive, safety attenuation ampli f ier, power supply quanti ty and in tens i t y o f f l u l d , large range precautions, .

radiat ion from source t o detector

t o vessel showing level accurate pressure, v isual , reading, deposition

:iEh mehhanica; o r inefImaEic ::ve: w i th motiondtdransmitti! o r deposition, must be i n contact w i th process f l u i d

L R h c a l act ion

electrode switch

accurate s a l i n i t y

components ture sensit ive

t o bottom o f vessel

4

2 .5 Quality Measurement W i The most widely used methods to measure quality are the sodium

tracer , e lec t r ica l ' conductivity, throt t l ing calorimeter, and gravimetric.

The thro t t l ing calorimeter determines the quality direct ly , whereas the other methods infer the quali ty from measuring the solid content of the steam. The most accurate method for continuous measurement i s the sodium tracer method. Conductivity methods a re useful w i t h i n a 1 imi ted range. The thro t t l ing calorimeter is not suitable for small quantit ies of carry-over. Gravimetric methods require large samples and do not detect carry-over peaks.

Other methods 'sometimes used are: capacitance measurement, piezo- /

e lec t r i c , various wave-absorption methods; however, these are not i n common operational use.

e 2.6 Mater Chemistry Monitors

Water chemistry monitoring, i n both the geothermal brine and the cooling water systems, is limited t o measurement of pH and conductivity. In some cases corrosivity and total disolved solids are inferred from other measurements.

These types of instruments a re commercially available, and the i r use i n geothermal service does no t involve unique problems. However, as previously discussed, corrosion, deposi t ton, and temperature, should be given consideration when selecting an instrument.

2.7 Combustible Mixture Monitors

d Combustible mixture monitors are necessary i n a binary system u s i n g -

a flammable secondary f luid, and where concentrations of hydrogen sulphide (H2S] or other gases a re possible. important for detecting process leaks and for ensuring tha t the i r con- centrations a re w i t h i n safe limits, below the lower explosive limit.

a

W

Measurement of combustible gases i s

67

k-' The most common method used f o r measuring combustible gas mixtures employs sel f-heated "hot w i re" detectors o f a high-temperature material such as platinum The hot. w i r e causes a combustible mixture t o burn.

imbalance. concentration of combustibles.

These are mounted i n a Wheatstone bridge' c i r c u i t . *

The combustion o f the mixture causes a temperature r i s e which resu l t s i n a resistance t

When t h e bridge i s brought t o a balance i t indicates t h e

Other methods include in f ra red absorbtion; thermoconductSvi ty, and ox i dat i on.

2.8 Addit ional Measurements

Other measurements that may be required include geophysica seismic monitoring, a i r qual.ity monitoring, and weather measurements: Devices for these measurements are avai lable and i n common use. The application, i n s t a l l a t i o n , and operation o f these instruments should f o l 1 ow the manufacturers ' suggested methods.

3. INSTALLATJON REQUIREblENTS

3.1 Codes and Standards

The fo l low ing codes and standards should be followed when selecting, i n s t a l l i n g , and operating measurement and control systems

(1) American Society o f Mechanical Engineers (ASME), Bo i l e r and Pressure Vessel Code, Section VIII, Div . 1

(2) Occupational Safety and Health Administrat ion (OSHA) Regulations

(3) National E l e c t r i c Code

(4) ASME Power Tes t Codes

7

E

GV

68

0 (5) ASME f low metering and other measurement standards

i

4

(6) American National Standards I n s t i t u t e (ANSI) standards

(7) I n s t i t u t e o f E l e c t r i c and Elect ron ic Engineers (IEEE) standards f o r instrumentation

(8) Instrument Society o f America ( ISA) Standards and Practices Manual

(9) National E l e c t r i c Manufacturers Association (NEMA) standards f o r e l e c t r i c a l equipment

(10) National F i r e Protect ion Association (NFPA) standards.

3.2 Guide1 ines and Standard Practices

d

W

I n add i t ion t o the appl icable codes and standards previously discussed, t h i s sect ion addresses those items which are used as general guidel ines f o r i n s t a l l a t i o n o f measurement devices i n geothermal systems.

The main ta inab i l i t y o f a measurement device i s o f p a r t i c u l a r concern i n geothermal systems. This i s due t o the po ten t i a l f o r corrosion o r deposit ion. considered i n the process of select ion and should be followed dur ing i n s t a l l a t i o n o f the device. I s o l a t i o n valves should be provided f o r devices such as pressure gauges, f o r example, which can become plugged by deposition. Also, bypasses are normally provided where a device i s

The r e l a t i v e ease o f maintenance o f a p a r t i c u l a r device i s

i n the f u l l process stream.

I n areas c r i t i c a l t o the operation, redundant instrumentation may be necessary. w i l l reduce the p o s s i b i l i t y o f plugging and freezing.

I n general, areas o f stagnation should be avoided. This

The loca t ion o f a device should be c a r e f u l l y selected t o ensure t h a t the desired process variable-measurement i s ac tua l l y being measured.

69

For example, a. pH meter,.in the.cool ing water system should be located a t

a po in t o f complete mixed condition, not i n areas o f concentration

ce l ls .

-iping- runs f o r ingtrumentatim should be as short-as possible, t o avoid damage due t o thermal expansion. + ,

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6.

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V. REFERENCES

T. E. Springer and 0. A. Farmer, "TAF - A Steady State, Frequency Response, and Time Response Simulation Program, I Fa1 1 J o i n t Computer Conference, San Francisco , Cal i f o r n i a, December, 1968.

In te rna t iona l Business Machines, Continuous System Model i n Program 111, In te rna t iona l Business Machines Corporation, Q975.

R. S. Deeds and K. E. Star l ing, THERPP-A Thermodynamic Properties Program, TREE- 1081 (May 1977).

John Hal 1, "A uide t o Pressure Monitoring Devices ," Chi 1 ton 's Instruments & E on t ro l Systems, - 51, 4 (Apr i l 1978) p 20. John Hal 1, "A Guide t o Pressure Monitoring Devices ," Chi 1 ton 's Instruments & Control Systems, - 51, 4 (Apr i l 1978) p 22.

John Hal l , "Flowmeters - Matching Appl icat ion and Devices,'' Chi1 ton 's Instruments & Control Systems, - 51, 2 (February 1978) p 18.

John Hal 1, ''Flowmeters - Matching Appl icat ion and Devices," Ch i l ton 's Instruments & Control Systems, - 51, 2 (February 1978) p 20.

-

71

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APPENDIX

SELECTED B I B LfOGRAPHY

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,APPENDIX

SELECTED BIBLIOGRAPHY

Douglas M. Considine, Process Instruments and Controls Handbook, 2nd edi t ion, New York: McGraw-Hill Book Company, Inc., 1974.

Howard P. Kallen, Handbook o f Instrumentation and Controls, New York: McGraw-Hill Book Company, Inc., 1961.

3. P. Holman, Experimental Methods f o r Engineers, 2nd edi t ion, New York: McGraw-Hill Book Company, Inc., 1971.

R. H. Perry and C. H. Chi l ton, Chemical Engineers' Handbook, 5 th ed i t ion, New York:

T. Baumeister and L. S. Marks, Standard Handbook f o r Mechanical Engineers, 7 th edi t ion, New Y o r k McGraw-Hill Book Company, Inc., 1967.

McGraw-Hill Book Company, Inc., 1973.

Steam - I t s Generation and Use, 38th ed i t ion, New York: 81 Wilcox Company, 1972.

The Babcock

. , - I . . 1 -

Engineer Data Book, 9 th ed i t ion, Tulsa, Oklahoma: Suppliers Association, 1972.

Gas Processors

1 . ' . . ngson, Handbook o f Control ves, 2nd edi t ion,

Pittsburgh, Pennsylvania: strument Society of America, 1976. - I

75

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