fisher turbine bypass applications bulletin july 2002

7/30/2019 Fisher Turbine Bypass Applications Bulletin July 2002

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Turbine Bypass Condenser Dump

Applications

Figure 1. LP Turbine Bypass Condenser Dump Applications

Product Bulletin85.1:020July 2002 Turbine Bypass Applications


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Turbine Bypass ApplicationsProduct Bulletin

85.1:020July 2002

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W8425

Figure 2. Typical IP (HRH) / LP Turbine Bypass Dump to Condenser Arrangement

Dumping Steam into theCondenser

For some turbine bypass applications, in particularthe intermediate-pressure (also known as the HotReheat Bypass) and low-pressure turbines, it iscommon for the bypass steam to be dumped directlyinto the condenser. This application is moresensitive than other turbine bypass applicationsbecause the dump to condenser has a number ofelements that must be properly considered duringthe design stage.

The condenser is a very costly component of thesteam system that is sensitive to both pressure andheat. The responsibility of the turbine bypass valvesfor this application is to protect the condenser andassociated piping against excessive pressure,temperature, and enthalpy excursions during bypassoperation. Also, due to the limited space typicallyallowed for these applications, it is important that thecorrect strategy is implemented for controlling the

injected cooling water. A final factor that must be

considered is the noise that is generated by dumpinghigh-energy steam into a thin-walled, low pressurecondenser (or condenser duct) area that is prone tohigh levels of noise. These considerations varydepending on whether the condenser is awater-cooled design or an air-cooled model.

Condenser Dump Turbine Bypass

ApplicationThe typical pressure and temperature controlledbypassing of the IP/LP turbines to condenser isshown in figure 2. The turbine bypass valves in thisapplication are used to both control pressure in theboiler re-heat section and also protect the downstream equipment from the severe pressure andthermal transients. This application is also necessaryto prevent condensate losses during turbine tripsdue to the lifting of safety valves.


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Turbine Bypass Applications

Product Bulletin85.1:020July 2002

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Figure 3. Desuperheater Section of a Steam ConditioningValve

Steam ConditioningThe primary function of a turbine bypass valve issteam conditioning, which is defined as combining apressure reduction valve with a desuperheater tosimultaneously control or reduce both the pressureand temperature of steam. The key component tosteam conditioning in the IP / LP bypass tocondenser application is the water injection. Asteam-conditioning valve must have the capacity tospray enough water to cool all steam upon a turbinetrip and also spray it effectively so that it will result inrapid vaporization and mixing.

A second requirement of the steam conditioning is toprovide tight shutoff to prevent high temperaturesteam from leaking into the condenser. This can bedone either with the steam-conditioning valve or witha block valve.

Backpressure Spargers

The backpressure sparger is installed to providebackpressure for a turbine bypass valve, to limit

Figure 4. Spargers Designed to Spray Out the End toEliminate Steam Impinging Upon the Condenser Tubes

steam line velocity, to protect condenser tubes, andto reduce overall turbine bypass system noise. Thesparger can be specified in either the condenser orthe turbine bypass manufacturers scope of supply.In either case, it is highly recommended that thelayout and installation be coordinated by the twosuppliers for optimization and cost reduction of thesystem.

The sparger is custom designed to match the turbinebypass valve outlet steam piping and be insertedinto the condenser or turbine bypass duct. Thesparger is designed to provide the required drilledflow area to take the final pressure drop from theturbine bypass valve outlet to the condenser andminimize the pipe size based on the velocityresulting from the lower pressure. This pressuredrop across the sparger is optimized with the dropacross the turbine bypass valve to limit the totalinstalled cost of the steam conditioning valve,downstream piping, and the sparger.

Once the sparger flow area is determined, the nexttask is to determine the geometry available forinstallation. The sparger should be designed todistribute steam flow with respect to the condensersstructural layout and heat sensitive surfaces. Thisprevents the possible spray of the steam and waterdirectly onto the condenser tubes, thus extendingthe life and limiting maintenance on the condenser.Flow area is designed to either inject steam out ofthe end as shown in figure 4, or spray out the sidesas shown in figures 5 and 6.


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E0854

Water Cooled CondenserArrangement From Turbine

CondenserNeck

ConditionedSteam

Bypass Steam

To Water CooledCondenser

Spray Water

Figure 5. Low Pressure Bypass Condenser Dump with Water Cooled Condenser

E0855

Air Cooled Condenser

Arrangement

From Turbine

ConditionedSteam

Bypass Steam

To Air CooledCondenser

Spray Water

Figure 6. Low Pressure Bypass Condenser Dump with Air Cooled Condenser


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Figure 7. Steam Conditioning Valve Close-Coupled with the Condenser

Insertions / Installation

The IP/LP condenser dump steam-conditioningvalve is designed for an optional installation that isclose-coupled to the condenser as shown in figure 7.These installations do not allow for long distancesbetween the steam-conditioning valve and the

condenser for the cooling water to mix.

The structure of the condenser is a key element inthis stage of the design. After the drilled holes of thesparger are aligned to face away from the condensertubes, the next step is to find the proper installationdesign for the sparger. Insertion of the sparger in aninappropriate position could affect the turbineexhaust pattern, thus leading to decreased efficiencyfrom the turbine. Fisher offers both an insertionmodel sparger and also a branch connectionapproach to avoid wasting valuable space inside thecondenser duct. These options are shown in figures5 and 6.

Control Strategy

Because of this close-coupled installation,temperature sensor feedback control is not possible.

A more accurate method for this application is afeedforward control strategy. Other turbine bypassapplications may use a downstream temperaturesensor for feedback control to determine the exactamount of water that is required for desuperheating.This option is usually not possible due to the shortdistances after the spray nozzle in most plants and

the time it takes for the water to fully vaporize in thesteam due to the extreme volume of water addition(often in excess of 500 GPM).

Feedforward control is accomplished using analgorithm supplied by Fisher that is characterizedspecifically to the valve and sparger installed in theapplication. The algorithm is programmed into theDistributed Control System to provide an accuratecalculation of the spraywater that is required toreduce the steam enthalpy and temperature prior toadmittance into the condenser. The algorithm

requires input of upstream temperature and pressureas well as the position of the valve. Upstreamenthalpy and spraywater enthalpy are thendetermined using steam property tables within theDCS. The total spraywater required is calculatedfrom a heat balance using the final enthalpy into thecondenser.


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Figure 8. Shadowgraph Showing the Effects of Jet Reformation Using Three Fixed Holes and Variable Pressure Drops

Noise Control

The sparger is an essential element to noise controlfor high velocity dumps into low pressure, thin walledcondensers. Application noise limits vary dependingon the required level at condenser duct, bypassdownstream piping, or the plant fence line. Thesystem noise (valve, piping, sparger, andtransmission outside of the system) is an importantconsideration for the bypass design. The spargerdesign employs two methods to lower the noisegenerated by the bypass valve and noise generatedby flow into the condenser duct. First, by providingthe proper backpressure to the bypass valve,velocities in the bypass line are kept within anappropriate limit. Second, by separating the bypassflow into numerous jets through the sparger bymeans of an engineered hole pattern, the generatedfrequencies can be shifted out of the audible range.Selection of the correct spacing of flow exit holes iscrucial to eliminate jet interaction between theindividual flow streams. The prevention of jetrecombination keeps the peak frequency of thenoise spectrum above the audible range. Also, bymaintaining the higher frequencies, the amount ofenergy that is effective in exciting pipe vibrations isreduced. This results in reduced radiated noise. Asan illustration, the shadowgraphs in figure 8 showthe importance of maintaining jet separation. Notethe jets recombining as the pressure drop is

increased while the hole spacing is kept constant. Itis evident that the desired frequency shift present inthe 40 psi case has been lost in the higher dropcases, as the flow through the three holes hasrecombined to produce lower noise spectrum peakfrequencies. Consideration of space limitations iscrucial to the design of the spargers where thetradeoff between hole spacing and space limitationis concerned. Maximizing hole spacing provides themost noise reduction, but results in increasedsparger size that may exceed space limitations.Noise calculations can be provided for eachapplication and can include customized drilled holespacing patterns to allow for recovery and low noisegeneration.

The pressure drop ratio must be established tooptimize the hole spacing and assure that jetindependence will exist. High pressure drop ratioswill result in the jets recombining into one larger jetin the outlet of the sparger unless the proper holespacing is applied. This scenario is common for thebypass to condenser application since thecondenser pressure is typically about 3 inches of Hg(approximately 1 to 2 PSIA). The high pressure dropratios associated with spargers often require bafflesto be added to the sparger. The bafflesresponsibility is to diffuse the flow and reduce noiselevels by maintaining steam jet independence.


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For multiple bypass dumps to the same condenser,spargers must be separated by an appropriatespacing ratio. The Fisher noise research teamshould be consulted for the appropriate spacing inorder to assure that the steam jets exiting thespargers dont combine to create a higher level ofnoise. Recommended sparger spacing will varydepending on the range of operating conditions andthe geometry of the condenser duct.

Summary of Application

It has been common in North America over the pastdecades to supply water-cooled condensers withmost power plants. In recent years this trend has

changed to more frequent use of air-cooledcondensers. This change has been driven by thefact that the current sites for these plants dont haveaccess to the water source that is required tooperate a water-cooled condenser.

Now, more than ever, it is critical that this applicationis carefully engineered to assure protection of thecondenser duct or tubes, low noise levels, and lowtotal installed cost. It is important that the turbinebypass system is designed considering all variablesto assure smooth and continuous operation. Theseconsiderations should all be evaluated during thedesign stage as the engineering firms work closelywith the turbine bypass manufacturers. It is alsohighly recommended for the plant to consult with anexperienced equipment supplier who can providestartup service during this phase of plantcommissioning.

Note

Fisher does not assume responsibilityfor the selection, use, or maintenance

of any product. Responsibility forproper selection, use, andmaintenance of any Fisher productremains solely with the purchaser andend-user.


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FisherMarshalltown, Iowa 50158 USACernay 68700 FranceSao Paulo 05424 BrazilSingapore 128461

The contents of this publication are presented for informational purposes only, and while every effort has been made to ensure their accuracy,they are not to be construed as warranties or guarantees, express or implied, regarding the products or services described herein or their useor applicability. We reserve the right to modify or improve the designs or specifications of such products at any time without notice.

Fisher does not assume responsibility for the selection, use or maintenance of any product. Responsibility for proper selection, use andmaintenance of any Fisher product remains solely with the purchaser and end-user.

EFisher Controls International, Inc. 2001, 2002; All Rights Reserved Printed in USA

Fisher is a mark owned by Fisher Controls International, Inc., a business of Emerson Process Management. The Emerson logo is atrademark and service mark of Emerson Electric Co. All other marks are the property of their respective owners.

Emerson Process Management

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fisher turbine bypass applications bulletin july 2002

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