Turbine Bypass Valves

7/31/2019 Turbine Bypass Valves

1/6

Page 1 of 6

Weir Valves & Contro ls ExcellentEngineering

Solutions

TECHNI CAL BUL LETIN

Prepared By: Adrian Croft Issue Date: 01 /09 / 2005Reference: Pages: 6

Subject:Turbine Bypass Valves and their Application

Introduction

Turbine Bypass control valves have long

been associated with arduous processconditions. In controlling high temperaturesteam they are often required to open inseconds, while maintaining a high degreeof closure, often to Class V.

Under normal operation the turbine bypassvalve is used to reduce the pressure and

temperature of the superheated steambefore it enters the condenser. The valvesare critical to the safe operation of the

plant and must open very fast in the eventof a turbine failure.

The diagram below shows a typicalcombine cycle power plant. The location ofthe HP and LP turbine bypass valves canclearly be seen.


2/6

Technical Bulletin Continuation Sheet

Page 2 of 6

Brief History of Turbine Bypass Valves

Traditionally turbine bypass valves wereonly occasionally used on power plants.The potential to cause serious damage tothe condenser and other equipment nowmeans that it is unusual to build newplants without fitting this type of valve.

The first generation of turbine bypassvalves, simply bypassed the turbine on ashort term emergency basis until thesource of the steam could be isolated. Thedrive to reduce costs and to save space inmodern plants has meant that condenserdesigners have been required to makeinternal provisions for equipment such asfeed heaters and bled steam systems.Condensers and their associated systemswere identified as an ideal location to

make such savings by returning steamback into the overall cycle. In addition thecondenser was identified as the best placeto dump emergency and by-pass steamflows. These factors led to the growth inthe use and criticality of the turbine bypassvalves in the condensate system.

As the use of early turbine bypasssystems increased several problems wereencountered. Broadly speaking theseproblems fell into the following areas:

Mechanical failure of the internalpiping due to incorrect pipingdesign or thermal expansion.

Erosion of the internals of thecondenser due to the location ofthe discharge.

Tube damage from dischargedsteam due to incorrectly locatedlines.

Overheating of the condenserinternals due to improperlyconditioned steam.

Mechanical damage to thecondenser due to water hammerfrom improperly drained pipes.

Turbine damage due toimpingement from improperlydirected steam.

In the modern power station designers

have largely solved these problems by thecorrect design of the bypass system andturbine bypass valves.

Turbine Bypass Valve Applications

In a power plant using a fired boiler withre-heater, the high pressure (HP) bypassvalve controls the letdown of steam fromthe live steam line to the cold reheat line.The low pressure (LP) bypass valvecontrols the letdown of steam from the hot

reheat line into the condenser.

Where there is a none fired boiler or HeatRecovery Steam Boiler (HRSB) as used ina gas fired combined cycle plant then theHP valve either discharges into the coldreheat system or into the condenserdepending on the plant configuration. TheLP valves are a separate system and willdischarge into the condenser.

In power plants designed in accordance

with the German TRD codes, the HPvalves can also be used to provide overpressure protection for the superheaterand live steam lines. This is a recognisedpractise and it is also accepted that thedrum/superheater safety valves are notrequired.

In power plants designed in accordancewith the ASME codes, boiler drum andsuperheater safety valves are stillcompulsory, although it is becoming morerecognised that better use of the HPturbine bypass valves can minimise thecostly lifting of the safety valves in an


3/6


Page 3 of 6

overpressure situation. There is also anoise consideration in trying to prevent thesafety valves lifting.

Overall Design Principles

Fundamentally the turbine bypass valvefulfils two principle functions. Firstly as thesteam enters the valve pressure iscontrolled by a fully active variable orifice(the valve trim). Secondly, as the steampasses towards the valve outlet thenspraywater is injected into the valve todesuperheat the steam to the requiredtemperature.

The valves can be globe or angle designs,but the requirements of the turbine bypasssystem often mean that angle valves arepreferred. In recent years the bypasssystem design and the constraints of

space have often meant that angle valvesare laid on their sides so the valve isfunctioning in a horizontal motion.

When designing a turbine bypass systemthe engineer should ensure that the valveis located in a straight run of pipe after thedesuperheating section. This preventsbody wall erosion due to the impingementof water droplets.

Usually the high steam pressures and the

required pressure reduction at the valvedictates that the valve trim should be acage guided type. In a number of casesmulti stage trims are required. The cageguided trim, being a low recovery design,ensures that the levels of jet impingementand turbulence are controlled by the cage.The jet separation produced by the cageproduces a more stable downstream flowwhile reducing the acoustic efficiency ofthe power spectrum, ultimately reducingthe generated noise at the valve outlet.

Depending on the valve size, the requiredCv and the superior water pressure, water

injection can performed by either a fixedarea nozzle or variable orificedesuperheater. On a fixed areaarrangement water is injected into thesteam through a series of radial holes. Thespray water pressure is controlled by aseparate control valve. When a variableorifice desuperheater is used the water isinjected directly into the centre of the pipeensuring efficient atomisation of the waterinto the steam flow.

After the water injection point, turbinebypass valves are often fitted with baffleplates. These baffle plates help to mix thewater and the steam, by creating turbulentflow. They also help to reduce the noise

levels of the steam. The number of baffleplates, the position of the baffle plates andthe hole sizes in the plates depends on the

Turbine Bypass Valve & Spraywater

Control Valve Fitted at Didcot B Power

Station. The Valve is Mounted on its Side


4/6


Page 4 of 6

sizing calculations for the valve inquestion. All turbine bypass valves aretherefore unique and designed specificallyfor the application.

Pressure reducing and desuperheatingvalves are unique in that they are the onlytype of valve that can fulfil two controlfunctions, in controlling pressure andtemperature. The turbine bypass systemrelies on two independent measurementsof pressure and temperature to control thepressure letdown and the desuperheating.The locations of the sensing elements arecritical to achieving accurate processcontrol, and they should be situated awayfrom bends or sections of abnormalvelocity, and located in a straight run ofpipe. In cases where the sensing elementshave been improperly located, such as onbends, then water separation can take

place which leads to incorrect temperaturereadings.

One of the major differences between theUS and European market is that inEurope, turbine bypass systems arepredominantly actuated by hydraulicpower units and cylinders. In the UScombined cycle market, pneumaticactuation is preferred due to its relativelylow cost.

Required Design Features

The turbine bypass system requires thatthe turbine bypass valves are designedwith several special features.

Tight Shut Off Traditionally the turbinebypass valves were designed as atemporary solution for controllingemergency steam dumps into thecondenser. They used to be associatedwith an isolation valve which would isolate

the steam flow on a longer term basis. Toreduce the costs of the system design,designers have identified that control and

isolation can be performed by one valve.This now means that turbine bypassvalves are specified with Class V leakage.In view of the high temperaturesassociated with the steam, then this canprove problematic for the valve designer.The high temperature steam basicallymeans that resilient seals cannot be used,so the valve designer must rely on metalto metal sealing or some form of specialcarbon steel.

In the past many valve companies haveaddressed the problem of Class V seatingby using pilot balanced plugs. These have

often proved to be problematic due to thetransient effects of the steam flow whichhave caused the two plug heads to

Turbine Bypass Valves for Lost Pines

Power Station


5/6


Page 5 of 6

become unstable and chatter. This hascaused the valve to become unstable andhas ultimately induced hunting andvibration. Many companies have nowstopped supplying pilot balanced trims forturbine bypass applications.

Recent developments in turbine bypassvalve design has seen some of the majorcontrol valve companies release abalanced design valve with a specialcarbon seal to eliminate annular leakagebetween the plug and cage.

Valve Orientation - As previously stated,many turbine bypass valves are fitted ontheir side. This can cause problems interms of stability, seat leakage, and glandleakage. On large bypass valves theweight of the plug can be in excess of onetonne. This means when the valve is laid

on it side there is a huge load pushing theplug onto one side of the valve cage. Thisload can be significantly increased due tothe inlet pressure of the steam. Ultimatelythe valve design engineer must considerthe friction generated by the rubbing of theplug and cage due to the weight of theplug and the pressure exerted by the valveinlet pressure. The actuator size andstability can often be affected.

As a consequence of plug deflection, the

plug does not run completely true in thevalve. When the plug is seated this canmean that the plug does not fully contactthe seating face of the seat, hencecausing seat leakage.

Side loading of the plug can causeincreased leakage through the valvegland. This is because the stem is loadedagainst one side of the packing.

The valve design engineer must ensure

that the valves are substantially guidedwhen they are laid on their sides, andensure that the plug/cage clearance is

minimised to prevent deflection of the plugand stem.

Fast Stroke Speeds During emergencyconditions turbine bypass valves arerequired to open within seconds, whileunder modulating conditions the valvemust be stable and accurately respond tothe control signal relatively quickly. Usingelectro hydraulic actuators can be asolution for this requirement. In additionthe forces that can be generated by anelectro hydraulic actuator can bebeneficial. However, the relative cost of anelectro hydraulic actuator can often meanthat the preferred method of actuation is apneumatic piston.

As the sizes and stroke lengths of turbinebypass valves increase, then the relativeair volume to be ejected from the actuator

in is consequently increased. This canprove challenging for the instrumentengineer, especially during modulation.Large volume boosters and dump valvesmust be fitted, the size and quantityestablished by the required stroke speedof the valve.

To ensure maximum stability when fitting apneumatic piston actuator, the actuatorshould be a double acting design. Airpressure above and below the piston

regulated by the positioner, ensuresmaximum stability of the valve.

Conclusion

As discussed turbine bypass valves areunique in their design and application.There is no such thing as a valve.

When Class IV leakage is specified by theend user then the selection of a balanceddesign trim with carbon seals is preferred.

When class V leakage is specified by theend user then special consideration mustbe made of the trim. If the valve is small


6/6


Page 6 of 6

enough and a suitable actuator is availablethen an unbalanced trim is preferred. If anunbalanced valve is not suitable then thevalve trim design should be speciallyconsidered.

Instrument selection for fast stroke speedsfor both the modulating and quick openconditions should be carefully considered.

Documents

Turbine Bypass Valves