comp air design guide

7/27/2019 Comp Air Design Guide

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DESIGN GUIDE

FOR

COMPRESSED AIR SYSTEM

PROJECT ENGINEERING MANAGEMENTNEW DELHI


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INDEX

S.NO DESCRIPTION PAGE NO.

1.0 AIM OF THE DESIGN GUIDE 2

2.0 SCOPE OF THE DESIGNGUIDE 2

3.0 SYSTEM DESCRIPTION 3

4.0 SIZING OF COMPRESSOR 4

5.0 TYPE OF COMPRESSORS 9

5.1 RECIPROCATING V/s SCREW COMPRESSORS 9

5.2 LUBRICATED V/S. NON-LUBRICATED 10

5.3 SINGLE V/S. MULTI STAGE 10

6.0 COMPRESSOR OUTLET PRESSURE 11

7.0 CAPACITY CONTROL 11

8.0 SIZING AND NO. OF AIR RECEIVERS 13

9.0 AIR DRYING PLANT 14

10.0 DISTRIBUTION PIPING AND JOINTS 15

11.0 DRAIN POINTS 16

12.0 VALVES 17

13.0 CODES AND STANDARDS 17

Shiv Pratap Singh 2 30/07/2013


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1.0 AIM OF THE DESIGN GUIDE

With the growth of automation in Power Engineering Instrumentation practice, Measurement

and process control has attained relevant importance in the design of systems. This control

system can be actuated either by pneumatic or electronic controllers. By far, the pneumatic

controllers are preferred for its simplicity and economy.

In addition to the compressed air being used for control (termed as Instrument Air - IA), it is

also used for purposes such as atomising fuel in a fuel gun, scavenging a fuel gun for

maintenance purposes power for driving pneumatic appliances (termed as Service Air-SA).

The aim of this design guide is to identify the compressed air requirement of major associated

equipment in a Thermal power station, Gas turbine plant and Combined cycle power plant

and to evolve guidelines for selection of design parameters of the compressed air system.



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2.0 SCOPE OF THE DESIGN GUIDE

This design guide with the different parameters of a compressed air system, which are

listed below: -

- Sizing and No. of air compressors.

- Types of Compressors.

- Compressor Outlet Pressure.

- Capacity Control of Compressors.

- Sizing and No. of air Receivers.

- Air Drying Plant.

- Drain Points.

- Valves

.

This design guide is discussed under the following heads:

- State of Art.

- Analysis & Recommendation.

State of art

This includes a brief discussion on the practices being followed specially by Indian

consultants, namely DCL and NTPC in the selection of the various design parameters

of a compressed air system. A station capacity of two (2) units of 210/500 MW each

has been considered while working out an example on compressor sizing.

Analysis & Recommendations:

The merits and demerits of the various prevalent practices that are controversial in

nature and discussed above under state of art are analysed. Based on this analysis

recommended practices to be adopted are made for implementation. It is suggested

that BHEL's view point in this matter are impressed upon the customer during

discussions with them.



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3.0 SYSTEM DESCRIPTION:

A compressed air plant comprises of the following:a) Air compressor

b) Compressor drive, which is generally an electric motor.

c) Inter and after coolers.

d) Air receiver

e) Air drying plant.

f) Distribution piping with valves, drain traps, etc.

The compressor is the heart of the plant which sucks ambient air through suction

filter-cum-silencer. The compressor can be horizontal or vertical, single or multistage,

reciprocating or screw and can be air or water cooled. The compressor is driven by an

electric motor either directly coupled or through `V' belts.

When ambient air is compressed in the compressor, enormous heat will be added to

the air due to compression. The resulting air temperature may be as high as 160C,

which, if directly used, will be harmful to the equipment. Hence in case of a

multistage compression, the compressed air is cooled in an intercooler and after final

compression, is cooled in an after cooler. The inter and after coolers can either be air

or water cooled and if water cooled, can either be horizontal or vertical.

Immediately next to the aftercooler, a moisture separator which can be integral part to

aftercooler or separate is provided to remove the condensed moisture from the cooled

compressed air. An automatic drain trap is provided at the bottom of the separator,

which drains out periodically, the collected water.

The air, thus compressed and cooled, is stored in an air receiver before distribution.

The receiver, in addition to storage, also dampens the pulsations in air and will

condense and drain as much moisture as possible through auto/manual drain traps

provided.

From the air receiver, service air is taken to the various consumer points directly

whereas the instrument air is taken through an Air drying plant. The purpose of the



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Air drying plant is to remove the moisture from the air, which otherwise gets

condensed on the working parts of the actuators and hampers the equipment. Now a

days to maintain the interchangability of air in case of emergency is envisaged and

subsequently the service air is also made moisture free. The air is dried to a dew

temperature of - 40o C.

Air Drying Plant generally used in power station are of following type

1. Reactivated Blower Type

2. Heat of Compression

3. Heatless Dryer Type

4. Refrigerated type

.



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4.0 SIZING OF COMPRESSOR.

The practice of sizing of on I-A compressor for a station capacity of two units (2x210 MW) isto consider the total continuous & intermittent requirement for each unit. Simultaneity factor

of 0.4 is considered for intermittent requirement. In addition to this a margin of 25% for

leakage, wear & tear and contingency is taken for sizing the compresses.

Service air requirement is generally less than Instrument air requirement, but compressor of

service air is select same as that of instrument air keeping in mind the contingency

requirement in case of emergency.

Analysis & Recommendation:

For the 2x100%/200 MW unites, it is recommended to provide capacity compressors, so that

one will act as a standby. Reciprocating v/s screw compressors. In power plants, generally

positive displacement compressors are installed for compressed air system because of large

flows and high pressure characteristics.

Wherever possible, it is advisable to provide same capacity compressors for IA & SA duties

for interchangeability and advantage of common spares.

Various Consumption Points in Power Plant: -

Typical Instrument Air Requirement for 500 MW

(Continuous and Intermittent)

S.NO DESCRIPTION Eqpt.

per

boiler

Cont./

Inter

QTY

Inter(per

opn.)

QTY Cont

(NM3/ min)

Total QTY

Cont (NM3/

min.)

1 Burner Tilt P/C 4 Cont - 0,22 0,88

2 Sec. air damper 88 Cont - 0,086 6,88

3 I/P converter for SADC 22 Cont - 0,006 0,12

4 Scanner air emer. Damper 1 Inter 0,018 - -

5 Scanner air fan dis. Damper 2 Inter 0,018 - -

6 Seal air fan dis. Damper 2 Inter 1,018 - -

7 Seal air filter DP controller 2 Cont. - 0,03 0,06

8 Hot air regulating dampers 10 Cont - 0,09 0,81

9 Cold air regulating dampers 10 Cont - 0,09 0,81

10 Hot air shutoff gate 10 Inter 0,018 - -

11 Cold air shut off gate 10 Inter 0,02 - -



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12 LO trip valve 1 Inter 0,015 - -

13 HO pressure control valve 1 Cont - 0,03 0,06

14 HO pressure control valve I/P

converter

1 Cont - 0,0025 0,005

15 HO temperature control valve 3 Cont - 0,03 0,18

16 HO temperature control valve I/P

converter

3 Cont. - 0,03 0,18

17 HO cooler temperature control valve 1 Cont. - 0,03 0,03

18 HO cooler temperature control valve

I/P converter

1 Cont. - 0,03 0,03

19 Atomising steam press. reducing

valve

1 Cont - 0,03 0,06

20 HO recirculation line trip valve 1 Inter 0,015 - -21 HO trip valve 1 Inter 0,015 - -

22 HO flow control valve 1 Cont - 0,03 0,06

23 Corner nozzle valves 68 Inter 0,03 - -

24 Ignitor advance - Retract mechanism 20 Inter 0,006 - -

25 SH spray control valve 4 Cont - 0,033 0,132

26 SH Block Valve 1 Inter 0,0174 -

27 RH block valve 1 Inter 0,0174 - -

28 RH spray control valve 4 Cont - 0,033 0,132

29 SB steam PRV 2 Cont - 0,033 0,066

30 AH DA Head valve 4 Inter 0,015 - -

31 SB steam drain temp. control valve 5 Cont - 0,03 0,18

32 LO pressure control valve 1 Cont - 0,03 0,06

33 LO pressure control valve I/P

converter

1 Cont - 0,0025 0,005

34 LO flow control valve 1 Cont - 0,09 0,18

35 Seal Air to mill discharge 10 Inter 0,09 - -

36 Seal Air to mill 10 Inter 0,09 - -

37 Seal Air to Feeder 10 Inter 0,09 - -

38 Feeder outlet gate 10 Inter 0,18 - -

39 Purge Meters 20 Cont - 0,028 0,504

40 AH outlet regulating damper 4 Cont - 0,22 0,88

41 Purge Air for Flue gas & mill

pressure tappings

90 Cont. 0.028 2.52

42 Boiler Frame Analysing System 36 Cont - 0,18 6,48

C & I Requirement ( EDN Bangalore )

1 IA for oprn. of control valve/ E-P linestor Cont. 4.688

2 IA for SO2/NOx probe purging Inter 0,283

PIPING CENTRE ( MADRAS )

1 Primary SCAPH steam controlvalve 2 Cont 0,05 0,1



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I/P Converter 2 Cont 0,16 0,32

2 Secondary SCAPH steam controlvalve

2 Cont 0,05 0,1


3 CBD level control 1 Cont 0,05 0,05

CBD I/P Converter 1 Cont 0,16 0,164 Primary Scaph drain level control 1 Cont 0,05 0,05


5 Secondary Scaph drain levelcontrol

1 Cont 0,05 0,05


BFP(Hyderabad)

1 BFP Recirculation ValvePneumatic Actuator

3 Cont. 0.066 0.198

Miscellaneous

1 Ash Handling Plant Cont 5

2 Mill Reject System Cont 1.25

3 DM Plant Cont 3

Typical Service Air Requirement for 500 MW

(Continuous and Intermittent)

S.NO DESCRIPTION No.. Per

boiler

Cont./

Inter/

startup

QTY per

eqpt

(NM3/hr)

Total QTY

per Boiler

(NM3

/ Hr)

1 Atomising air for LDO firing 4 During

Start

up

350 1400

2 Pulveriser Coal sampler 2 Inter 35 70

3 HEA Ignitor 20 Cont 27 540

4 Feeders bulls eye cleaning. 10 Inter 1 10

5 Furnace temp. probe 2 Inter 600 1200

6 Regenerative air heater air motor -

-primary 2 Emerg 405 810

- secondary 2 Emerg 405 8107 Regenerative air heater air blower -

-primary 2 Inter 640 1280

- secondary 2 Inter 640 1280

8 Air Heater fire sensing device 4 Cont. 1 4

9 Acoustic Pyrometer Cont - 408

10 Furnace Observation door

- Bleed Air 90 Cont. 2 180

- Air Curtain 4 Inter 20 ---

C & I Requirement ( EDN Bangalore )

1 Purging of transmitter lines in boiler linese Inter. 8.55



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Miscellaneous

1 Mill Reject System Cont. 5

CUSTOMER REQUIREMENT

TOTAL (Cont.)



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Notes:

1. Air consumption listed above is typical for any 2X 210/500 MW Thermal power station

with requirement varying from case to case.

2. Air requirement during initial start up & commissioning has not been consumption in the

consumption for sizing of compressor for e.g.,

For purging operation (generator at stand still)

For air leakage during commissioning

For pressure testing after terminal box welding

Atomising air for LDO firing

Regenerative air heater air motor

Regenerative air heater air blower

3. Acoustic Pyrometer requirement is considered in calculation of Service air requirement

4. Air requirement for Ash Handling plant and Mill reject system may be considered in the

sizing of compressor, in case customer asks for.

5. Coal Flow measurement requirement is also to be considered in calculation of instrument

air Requirement (Excluded in this case)

6. Customer Requirement is also to be taken into care.

7. For sizing of compressor capacity, we take a factor of 0.4 for intermittent requirement

with an overall margin of 25 % on the total air requirement. For e.g., In Bakreswar, we

have compressor of 15NM3/ min and for Simhadri 30 NM3/ min capacity.

TO CONCLUDE THE FOLLOWING ARE RECOMMENDED:

1. The IA and S.A compressor shall be sized for the total of all continuous

requirements plus the intermittent requirement of one unit.

2. For each stage of power station two (2) 100% IA and two (2) 100% SA

compressors shall be proposed (one (1) working and one (1) standby).

4. 25% margin of compressor wear & tear, leakage etc., shall be added to the

requirement in all cases.

5. Identical and equal capacity compressors will be considered, wherever

possible.



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5.0 TYPE OF COMPRESSORS

5.1 Reciprocating V/s Screw compressor.5.1.1 State of art

(a) Reciprocating compressor - In this type of compressor, we generally have a

decrease in volume of air, resulting in increase of pressure by positive

displacement of piston. Due to reciprocating action of piston, gas (air)

compresses and pressure is increased.

(b) Screw compressor - Screw compressor to basically a twin Helical lobe

compressor, in which both lobes rotate simultaneously and compress the air/gas

in between. The advantage of the screw over reciprocating compressors, is the

economy with regard to

Lesser space requirement.

Lesser expensive foundation.

Lower creation time.

Lower maintenance cost.

5.1.2 Analysis and Recommendation

In present scenario consultants like NTPC and DCL are preferring Screw

compressors over reciprocating compressors due to aforesaid reasons.

5.2 Lubricated v/s Non-lubricated:

5.2.1 State of Art:

Instrument air compressors are required to be non-lubricated type since pressure of

oil is harmful and affects the performance of the diaphragm operated controllers. TheI.A. compressors are designed to be oil free by provision of Teflon rings instead of

the conventional oil rings.

5.2.2 Analysis & Recommendations:

It is essential that the instrument air compressors should be oil free type. In case of

service air, even though oil free air is not a must, still certain advantages as under will

be obtained if made oil free like Instrument air compressors.

a) The traces of oil carried over settles in the distribution piping and in course of

time may choke the lines.



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b) The inter-connection between the IA system & SA system is possible without the

necessity to include the oil separator, which increases the multiplicity of the IA

system.

c) In case of identical service and instrument air compressors, spare inventory is

reduced.

Hence it is recommended that the service air compressor also may be of non-

lubricated type.

5.3 Single stage vs. Multiple stage:

5.3.1 State of art

The pressure of say 8.0 kg/cm2 (g) can be achieved with a single stage compression

from atmospheric to 8.0 kg/cm2 (g) or compress it to an intermediate pressure of

about 4 kg/cm2(g) in the first stage, cool it, then compress it to 8.0 kg/cm2 (g) in the

second stage.

5.3.2 Analysis & Recommendation:

The advantage obtained in two stage compression is that because of the interstage

cooling, and the lesser compression ratio in both the stages, the temperature of air

attained will be about 140C, whereas in case of single stage compression the

temperature attained will be about 210C. Handling of air at lesser temperature has its

own advantages and results in better life and better performance of the machine.

Another advantage of two-stage compression is the lesser HP required at the shaft of

the compressor.

It is therefore recommended that a two-stage compressor be provided for both LA &

SA compressors with a discharge pressure greater than 7 kg/cm2 (g) except for very

small capacity compressors.



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6.0 COMPRESSOR OUTLET PRESSURE:

6.1 State of art:Most of the controllers and other consumer points of the compressed air system

require a pressure of 7 kg/cm2 (g) for their proper operation. Allowing a line pressure

drop of 0.5 kg/cm2 upto the farthest point (for 2 units) and a further pressure drop of

0.5 kg/cm2 in the air drying plant, after cooler, inter cooler, vales etc., discharge

pressure of the compressor is to be around 8.0 kg/cm2 (g).

6.2 Analysis & Recommendation:

It is recommended that both IA & SA compressor (s) shall have a discharge pressure

of 8 kg/cm2 (g).



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7.0 CAPACITY CONTROL OF COMPRESSORS:

7.1 State of art:(A) Reciprocating Compressor

The capacity of a compressor can be controlled by two methods termed as `Dual

Control'.

Load/unload method

ON/OFF method

Load/unload method:

In this mode the compressor runs continuously but its capacity is controlled by

loading and unloading a set of cylinder by lifting the suction valve plates off their

seats, which is affected through an electric means. Once the desired pressure setting

in the pressure switch (s)/, these cylinders are loaded again on fall of pressure. The

Drawing `Scheme of Controlling Air Compressor' indicates a scheme where in the

capacity control is achieved in steps of 100%, 50% and 0% through the solenoid

valves SV1 & SV2 which are energised to open at set valves by means of pressure

switches installed on the outlet header of each compressor. This allows the air from

the air receivers to the unloading valves of the compressor thus unloading the

compressor 50% & 100% respectively. Similarly, the loading in steps of 50% &

100% is achieved when the solenoid valves SV1 & SV2 are closed successively.

On-off method

In this mode the compressor is started or stopped automatically depending on the air

pressure. The pressure switches mounted on each compressor outlet header give

signal to compressor motor starter thereby starting and stopping compressor motor.

(B) Screw Compressor

Capacity Control for Screw Compressor.

a) The capacity is regulated by throttling the air inlet and at the same time, opening a

blow off valve. Under load the blow-off valve is closed and the throttle valve fully

open. The unloading device is operated by a spring loaded automatic air relay of the



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same type as that used.

b) Slide valve control for screw compressor.

This type of control incorporates a third bars in the compressor casing, at the cusp of

the rotor bars. In this bars is positioned a slide valve at the discharge side of the rotor,

enabling recycling of the partially compressed air.

As the compressor is off-loaded, the valve moves progressively opening up parts,

which connect the compressed air back to inlet. This form of control is only suitable.

when the compressor is driven by a constant speed device such as an electric motor.

In case of screw compressor, we have normally 0-100% control system as compared

to reciprocating compressor where we have 0-50-100% capacity control.


In case of intermittent demand of compressed air the fall and rise in pressure will be

very rapid and in case of the ON/OFF mode the compressor motor is subject to

frequent start and stop which will naturally be harmful to the motor and cable and the

compressor will have reduced life in view of the frequent stress imposed on it.

The IS. No. IS-6206-1971 entitled Guide for Selection, installation and maintenance

of air compressor plants with operating pressures up to 10 Bars. states Automatic start

- stop operation of motor is also possible but its use shall depend upon the size of the

motor, the nature of the application, and local conditions of power supply.

The following are recommended.

a) `Dual Control' with the load/unload control being, `Main' and ON/OFF as

`Standby' shall be provided.

b) The Load/Unload system shall have both electric and mechanical means of

load/unload, the mechanical as a standby to the electrical one. In case of failure of

the pressure switch or the solenoid valve associated with the electrical system

(which is quite frequent), the air regulator path i.e. `Mechanical' control can be

opened manually through a selector, switch.



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c) Automatic changeover to `Mechanical' in case of failure of control supply is not

recommended and an alarm then will be initiated to indicate `Control supply

failure.

d) Loading & unloading in steps of 0%, 50% & 100% are only recommended.

e) The compressors in whatever mode of capacity control shall be able to start only

in unloaded condition.



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8.0 SIZING AND NO OF AIR RECEIVERS:

8.1 State Of Art:

When the boiler/turbine unit trips due to power failure the compressors also trip. But

however, compressed air is required for bringing down the essential control/trip

valves to their original position and also emergency running of regenerative air heater

air motor. These requirements are met from the air receiver.

.

8.2 Analysis and Recommendation:

The following are recommended:

i) The total live I.A. and S.A. receivers(s) capacity shall be sized based on storing

ten (10) minutes of total working Instrument air compressor (s) capacity or as per

customer requirement taking into account the resultant increase in volume of air

due to fall in pressure from receiver pressure of 8 kg/cm2 (g) which being the

minimum pressure at which most of the instruments can operate.

ii) Each compressor whether IA or SA shall have an individual receiver attached to

it for dampening the vibrations of compressed air. There shall be a common air

header after the receivers and no valve shall be provided on the inlet side to the

receivers.

iii) All receivers shall preferably be located outdoors, adjacent to the compressor

plant.



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9.0 AIR DRYING PLANT:

9.1 State of art:

The air Drying Plant is needed for providing moisture free compressed air which is

required for the instrument controls. The present day air drying plant are capable of

drying air to a dew point of -40C. This quality of air is generally acceptable to all

the instruments and controllers.

.

(a) Blower Regenerated No loss Type Drier - The regeneration air from centrifugal

blower at low pressure is allowed to pass through an electrical heater of pre-

determined rating. The hot air which has got very high moisture holding capacity

picks up the absorbed moisture from saturated desiccant bed and vented to

atmosphere.

(b) Heat of compression: -

(i) Full flow - Hot air from last stage compression is bed into the absorber vessel

for regeneration and then same air is cooled in a specially designed combination cooler. After

cooling; this air goes through the second absorber where the moisture goes absorbed in

activated bed and dry air goes out.

(ii) Split Flow: - The main compressed air stream passes through after cooler,

then into drying section, and finally out of the dryer into the dry compressed air net. All

moisture is removed through adsorption by the silica gel powder on the glass fibre based

paper drum. The regeneration airstream by passes the after cooler and in lead is shunted into

the regeneration section. The regenerated airflow is mixed with the main flow in the ejector

nozzle.

.


Earlier non-purged air flow, desiccant type, air drying plant with separate blower and

suction filter arrangement was recommended but know a days due to advent of screw

compressors in the compressed air system, Heat of compression type Air drying plant is being

asked by the customer.



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10.0 DISTRIBUTION PIPING & JOINTS:

10.1 State of Art:

The compressed air piping can either be of mild steel galvanised. It is an obvious fact

that galvanised pipes are superior to black pipes, because of its corrosion resistance. The cost

difference between a black and galvanised compressed air pipework, which are of small bore,

is very insignificant and hence galvanised piping for both IA and SA pipework are preferred.

.

To conclude the following are recommended:

a) Galvanised pipes for both IA & SA with screwed joints as per BS: 21:1973

only with Teflon tapes, if necessary.

b) For site threading a coat of zinc epoxy shall be applied both inside and

outside after due cleaning of the part.

c) The distribution piping shall be run over ground with drain points at suitablelocation.

11.0 DRAIN POINTS:

11.1 State of Art:

At convenient intervals (preferably 30 m) in the distribution pipework, drain points

are to be provided at operating level, this is with a view to drain out periodically, the water

collected in the pipework, this can be through a normally closed globe valve, which can be

opened manually at periodic intervals, or an automatic drain trap.

In case of auto drain traps, they get clogged due to dirt in course of time and hence do

not function properly. It, therefore, becomes necessary to provide manual bypass

arrangements.

11.2 Analysis & Recommendations:



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It is recommended to provide only Manual Drain Traps in compressed air system at,

accessible & visible locations at periodic intervals not exceeding 30 M at the lowest points.

The pipe should be given a fall of not less than 1 M in the direction of air flow.

However, in the compressor house where the piping is run in trenches, auto drain

traps with manual Bypass shall be provided.

12.0 VALVES:

12.1 State of Art:

The following types of valves are being used in the compressed air lines:

a) Gate valve

b) Globe valve

c) Plug valve

d) Ball valve

13.0 CODES AND STANDARDS:

1. IS- 2825/1969: Code for unfired pressure vessels.

2. IS- 4503/1967: Shell and Tube Type Heat Exchanger

3. IS- 5456/1985: Code of practice for testing of positive displacement type air

compressors and exhausters.

4. IS- 5727/1981: Glossary of terms relating to compressors and exhausters.

5. IS- 1239/1990: Mild steel tubes, tubular and other wrought steel fittings (Part - 1)

6. IS- 1239/1992: Mild steel tubes, tubular and other wrought steel fittings (Part - 2)

7. IS- 6206/1985: Guide for selection, installation and maintenance of air compressors/



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comp air design guide

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