Turbine Bypass Valve

7/31/2019 Turbine Bypass Valve

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WEIR CONTROL & CHOKE VALVES

Excellent solutions

providing optimum

performance for

all Turbine Bypass

applications

ExcellentPower & IndustrialSolutions

ExcellentEngineeringSolutions

Weir Turbine Bypass Valves & Desuperheaters


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www.weirpowerindustrial.comWeir Control & Choke valves Engineered valves for protection & process control2

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We have extensive references and a proven trackrecord in the supply of valves across a number ofkey industries.

Our valves are industry renowned brands,

each with an established reputation for qualityengineering and reliability.

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We can also perform gas, packing emission,cryogenic and advanced functional testing, aswell as seismic testing for nuclear applications.

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Our valve aftermarket solutions are based onour engineering heritage, applying our OEMknowledge and expertise to maintenance

strategies, life extension and upgrade projects.

Weir Control & Choke Valves provides a

wide range of control valves for the process

industry. These include severe service,

choke, desuperheating and turbine bypass

applications.

Our world-wide reputation is based

on engineering excellence applied to a

comprehensive range of specialist products

and effective customer support.Weir UK purpose built factory at Elland

ContentsTurbine bypass valve description 3

Reasons for Turbine bypass valve 4

Trim types 5

Plug seals 6

Outlet diffuser 6

Bonnet types 6

Water injection nozzles 7

Design options 8

Sizing and configuration 9

Weir Control & Choke Valves Weir Turbine Bypass Valve

Member of

Weir International, South Korea


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Weir Control & Choke valves Engineered valves for protection & process control 3www.weirpowerindustrial.com


The Turbine Bypass Valve

The turbine bypass valve is designed to bypass the steam cycle from the boiler

to the turbine and dump the steam directly into the condenser. The valve is

used on start-up and turbine trips.

The BV994 (Globe) and BV995 (Angle) range of turbine bypass valveshave been developed to handle the most severe process conditions

while maintaining valve stability, tight shut off, noise reduction and fast

operating speeds. The valves have a combined pressure letdown and steam

temperature reduction system. Water is injected through a series of nozzles

located towards the valve outlet.

Conditions usually associated with turbine bypass systems are high pressure

drop in the critical flow regime leading to sonic conditions across the valve

trim. Weir can offer a wide range of trims to eliminate the detrimental effects

of dropping the pressure. Available trims are:

Multi flow (single stage of pressure letdown)

Cascade (up to 5 stages of pressure letdown)

X-Stream (multi-stage pressure letdown)These trims are selected based on the valve sizing conditions but are selected

to handle the pressure drop without generating the detrimental by products

of dropping pressure such as vibration, erosion and high noise levels.

To maintain the station efficiency turbine bypass valves are usually specified

with a tight shut off (normally Class V). Depending on the valve size and the

actuation mechanism then the valves are designed with either a balanced or

unbalanced trim.

On balanced valves Weirs unique sealing mechanism ensures that Class V

closure can be maintained while ensuring actuation forces are minimised. The

valve is positionally controlled by either a pneumatic or hydraulic actuator

which ensures repeatable positional control.

Depending on the amount of water to be injected into the steam then a

selection of spray nozzle options can be offered. These nozzles ensure that the

water is effectively atomised to minimise the absorption time into the steam.

The turbine bypass valves are supplied with an associated spraywater valve

that is used to reduce the spray water pressure before injection into the

steam.

Standard Design Options

Pressure Ratings

ASME Class 150 to ASME Class 4500

Sizes

Inlet 40mm to 500mm (1 to 20)

Outlet 40mm to 1000mm (1 to 40)

Trim Options

Multiflow

Cascade

X-Stream

Actuation

Pneumaticdouble actingpiston

Electro hydraulic

BV995

Angle Turbine Bypass Valve

High pressure Turbine Bypass valve


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Weir Turbine Bypass ValveWeir Control & Choke Valves

Reasons for Turbine Bypass Valves

Turbine bypass valves are used to reduce thetemperature of steam by injection of water into thesteam flow. Where steam supply exists at a relativelyhigh temperature for driving the turbine, it needs

to contain a large amount of superheat to ensurethat dry steam is used. Any entrained water woulddamage the turbine blades. Steam is used for heattransfer which is a function that requires the steamto be cooled close to its saturation point. When theturbine bypass valve is used it must dump steaminto the condenser or cold re-heat system. Thereforeall superheat must be removed from the steam.

The object of a desuperheater is therefore to removesome or nearly all of the superheat. The process ofadding water to steam to lower its temperature isquite a complex process due to the two stage flowthat is generated at the point of water injection.

Desuperheaters are therefore selected to: Reduce the steam temperature to protectdownstream equipment from excesstemperature.

To prevent superheat steam from reducing thepressure of the steam.

To improve the thermal efficiency of the heattransfer process by reducing the overall steamtemperature close to the saturation point.

In order for the temperature to be reduced and

controlled effectively several conditions must be

met. These include:

Efficient atomization of the cooling medium

A vapour velocity that promotes mixing

The correct cooling medium temperature

Pressure Reducing Trim

The ratio between inlet and outlet pressure for bothhigh and low pressure turbine bypass applicationscan be significant. The steam mass flow range canalso be significant, and these two factors suggestthat the steam pressure reduction must be done in anumber of stages. These stages must be active overthe complete flow range, unlike fixed baffle plates

which are optimised for one flow rate only.

Spraywater Injection Nozzles

It is usually important for the spraywater to be

evaporated within a very short distance from its

injection point. It is also critical to ensure that water

does not impinge on the valve body or pipe wall. It

is normal practice to locate the spraywater injection

nozzles downstream of all the valve pressure

reducing trim, and the use of atomising steam spray

nozzles is used for efficient water absobtion.

Seat Tightness

When Turbine Bypass Valves are discharging into

the condenser it is imperative that they have a tightshutoff to ensure that the partial vacuum existing

in the condenser is maintained. The tight shut off is

achieved with a Class V sealing system.

Valve Body design

The valve body design must take into

consideration the thermal cycling that will occur

during its lifetime. The flow path must be as

smooth as possible to ensure that noise and

vibration levels caused by turbulent flow areminimised.

As with most types of high pressure valve used

in the power industry, a pressure seal bonnet is

preferred at least for the HP Bypass Valves.

Materials are selected to ensure the valve body has

a minimum wall thickness. This minimises the risk

of a large temperature gradient across the valve.

Temperature gradients can cause thermal stress

in the body and result in cracking due to the rapid

changes in temperature.

ActuationOne of the major differences between the US

market and Europe is that in Europe Turbine

Bypass Systems are predominantly actuated by

Hydraulic Power Units and cylinders. In the US

Combined Cycle market Pneumatic systems are

preferred for cost reasons, although in the US Fossil

fired stations, hydraulic units are required due to

the higher actuation forces required.

In addition to the main bypass and spraywater

valves there are a number of other significant

elements that make up a Bypass system.

Hydraulic Power Units including PLC systems Pressure Safety Device

Steam Blowthrough Trim

Acid Cleaning Trim

Condenser Dump Tubes

Special Features

Warming Line

In order to prevent thermal stress on startup,

turbine bypass warming lines can be

incorporated to pass a small steam volume into

the downstream pipe.

Orientation

Turbine bypass valves are often required to lay

horizontal due. Stable seat guiding ensures

this can be achieved without special design

modifications.

Fully Accessible Trip and Diffuser

The valve incorporated a fully serviceable trim

and diffuser.


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Trim Design

Control valve trims for turbine bypass valves are selected

according to the valve process conditions. Cage designs are

described below.

Multiflow and Single Stage MultiflowThe Multiflow and Single Stage Multiflow trim gives a single

stage of pressure letdown. In this design the flow is broken up

into multiple jets by a number of radial holes in the valve cage.

On turbine bypass valves the flow is usually from outside to in so

that jet impingement and high turbulence levels are controlled

with the confines of the trim. Impingement of the jets within the

centre of the trim gives a more stable downstream flow, reduces

the effect of large scale separation and produces a smaller scale

turbulence structure in the valve outlet. This in turn leads to a

reduction in acoustic efficiency and changes the power spectrum

of the generated noise both of which contribute to an overall noise

reduction of 15 to 20dBA compared to contoured trim valves.

Further noise reductions in this style of valve can be achieved bydrilling smaller holes in the valve cage. This design is referred to as

Single Stage Multiflow.

Cascade

The cascade trim is used in applications which require up to 5

stages of pressure letdown. The cascade trim has been designed to

eliminate problems such as noise, vibration and erosion by staging

the pressure drop through a series of discrete pressure drop

stages. The cascade trim is manufactured to a close tolerances

and consists of a series of drilled sleeves. The number of sleeves

(pressure drop stages) required depends on the amount of

treatment required for a particular application. Each successive

sleeve has a number of radial holes and a carefully calculated

increase in flow area to ensure correct apportionment of the

pressure drop. The small radial jets pass through a tortuous path

resulting in high frictional and impingement losses. At the same

time the impingement of the jets onto the outer radial sleeves

control the shock wave formation which has a major influence on

the overall noise reduction.

X-Stream

The X-Stream trim is specifically designed for severe service

applications where detrimental valve factors such as noise, erosionand vibration need to be eliminated. Using a series of stacked discs,

multiple stages of pressure letdown are provided using a series of

complex flow paths. The flow path of the X-Stream is designed

with a series of columns which ensure a smooth flow pattern.

This ensures that if dirt/debris should enter the trim then it will be

washed through to the valve outlet.

On steam applications the X-Stream trim has a special jet control

row. This eliminates shock cells at the outlet of the trim. These

shock cells are responsible for high noise and vibration at the valve

outlet.

Further details of the X-Stream can be found in the X-Stream

trim brochure.


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Class V Sealing

In order to maintain a condenser vacuum and ensure

maximum power station efficiency turbine bypass

valves often require a tight shut off which is referred

to as Class V. Due to the high temperatures associated

with a turbine bypass valve conventional resilientseals cannot be used, therefore many valve suppliers

use pilot balanced trim designs. Depending on the

stability of the process flow, pilot balanced trims can

often generate trim instability and poor control.

Weirs solution is to use a conventional balanced

valve trim and install a metallic seal to prevent leakage

between the valve plug and cage. The seal is pressure

energised so that higher pressures across the valve

trim result in increased sealing load and therefore

tighter shut-offs.

Controlling the Outlet Steam Conditions

Due to the pressure drops associated with turbine

bypass valves then additional noise and velocity

control is often required in the outlet of the valve.

Existing turbine bypass valve suppliers often weld

diffusers into the valve outlet. This can result in

thermal cracking and when these diffusers require to

be serviced then the complete valve outlet section

must be completely removed.

In the Weir system trim components have been

designed for ease of service. All trim components are

clamped into the valve body. The seat ring extends

into the valve outlet section which contains the trim

outlet diffusers. These can incorporate a number ofbasket style diffusers. Diffusers are designed so that

steam expansion due to pressure drop is achieved in

the expanded outlet section resulting in lower steam

velocities. The location of the holes in the diffuser

results in minimum jetting against the walls of the

outlet section.

Stable Plug Guiding

To ensure the most stable trim the valve plug is

guided in the seat rather than in the valve cage. An

additional stage of pressure letdown is incorporated

on an articulated section of the plug nose which

ensures stable pressure letdown and thereforeelimination of pressure induced vibration.

Bonnet Designs

Bonnet designs are selected according to the pressure

rating, size and operating temperature of the valve.

On high temperature steam applications then a

normalizing bonnet is used to lift the valve packings

away from the main temperature gradient. In high

pressure and temperature applications a pressure

seal bonnet is used to simplify the bonnet assembly,

remove weight from the bonnet neck and to eliminate

the requirement for special torque equipment.

Outlet diffuser

Normalising bonnet Pressure seal bonnet


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Water Injection Options

BV984 Spring Loaded Nozzle

The BV984 nozzle gives high capacity water injection while

offering protection of the water system in case of loss ofwater supply. The superior pressure of the water system

lifts the spring loaded nozzle away from its seat which

results in a cone shape jet of water being injected into the

steam. The system requires a separate spray water control

valve to control the amount of water injection. This system

ensures a high rangeability while ensuring the water

droplet size is kept to a minimum. On loss of water pressure

the water system is protected against high temperatures

due to spring loading of the nozzle.

BV985 Variable Area Nozzle

The BV985 spray nozzle is used on applications where high

rangeability is required with direct control across the spraynozzle. The BV985 is designed so that steam is injected into

the centre of the pipe resulting in minimum droplet contact

against the pipe walls. Water is injected across a series of 12

variable area nozzles which atomises the water into micro

fine droplets resulting in faster absorption rates and shorter

outlet steam pipe lengths. The amount of water injected

into the pipe is directly controlled by the valve plug which

is in turn controlled by the actuator. Where the pressure

differential between the cooling medium and the vapour

exceeds 60 bar, a two stage nozzle is available that extends

the available pressure drop range to 100 bar. The two stage

nozzle ensures erosion across the valve plug is maximised.

The BV985 unit is fitted into steam pipes greater than150mm (6).

BV986 Mini Desuperheater

The BV986 unit is a simplified spray nozzle where the spray

water is circulated around an internal gallery and then

injected into the steam through a series of radial holes.

The BV986 unit fits between conventional flanges and

is therefore easy to remove for service and maintenance

purposes. The unit is connected to a stand alone

spraywater valve which controls the amount of water being

injected into the vapour. The BV986 can also be directly

mounted onto the outlet of a pressure control valve so that

water is injected into the turbulent vapor flow at the outletof the valve. The BV986 is used on pipe sizes from 25mm

(1) and above.

BV988 Fixed Area Spray Nozzle

The BV988 unit offers good spraywater atomisation but

with separate spray water control. The valve is similar to

the BV985 unit but with all spraywater control removed

from the unit. A flanged or butt weld connection is used

to connect the spraywater which in turn is controlled via

a separate control valve. This allows for high technology

control valve trims to be used to eliminate the detrimental

effects of erosion sometimes created by high pressure

drops. The unit is also used in situations where spacearound the desuperheating system is limited.

BV984 Spring Loaded Nozzle

BV985 Variable Area Nozzle

BV986 Mini Desuperheater

BV988 Fixed Area Sparay Nozzle


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Special Applications

Pressure reducing and desuperheating units are often

specified and selected according to the application.

Customer requirements often require special designs

to accommodate unusual requirements. Selected

special designs are shown below.

Condenser Steam Dump

End User S2 Ruwais Power Company

Customer Samsung C & T

Project Shuweihat S2 IWPP U.A.E.

Product 4 off 1400mm Inlet x 1000mm Body x2700mm Outlet ANSI 150

Angles Control Valves c/w Electro-HydraulicActuators and Control Panels

Application LP Process Steam Dump toCondensator

Materials ASTM A216 WCB with 410 SS Trim Total Weight 15 metric tonnes each

Delivery completed in 28 weeks

Special Shape

Turbine bypass valves are often required to lie

horizontally due to the nature of steam systems in

a power station. This can cause premature wear on

seals and cause increased friction through the valve

trim. Producing a special body design with the flow

turned through 90 degrees in the horizontal plane

allows for the trim and actuator to be mounted

vertically in the pipe. This in turn ensures more stable

control through the full valve stroke range.

Actuation

There are two main types of actuators used on

turbine bypass valves, either pneumatic actuators or

hydraulic actuators. In both cases to ensure stable

control and protection of the turbine the actuators

are selected for fast stroke speeds often below 3

seconds for full stroke.

Hydraulic actuators have the advantage of delivering

a high thrust capability while maintaining stable

valve control.

Pneumatic systems are used where hydraulicsupplies are limited but where plant air is readily

available. Pneumatic systems are specified with

a high thrust piston actuator which is

controlled via a series of high capacity

instruments. Stable control is

achieved due to the responsiveness

of the instruments through the full

stroke of the valve.

Condenser Steam Dump valve

Condenser Steam

Dump valve section

90 degree mounted

valve with vertical

actuator

Hydraulic actuation Fast operating pneumatic system


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Initial Sizing of Desuperheaters

Information required at enquiry stage

Initial calculations

Calculate the required flow of water WW, kg/hr (lb/hr), needed to

control the steam temperature at the outlet, by the heat balance

method.WW = WS (h1 - h2)

(h2 - hF)

where:

h1 = enthalpy of superheated steam at inlet

h2 = enthalpy of steam mixture at outlet

hF = enthalpy of spraywater at inlet

values in kJ/kg (Btu/lb)

Total outlet steam flowrate WM = WS + 2W kg/hr (lb/hr)

Sizing of low pressure pipeline

This is the recommended pipe size for BV985, BV986 and BV988

pipeline types, or the outlet size for the BV995 design for efficient

desuperheating.

The pipe is sized so that the steam velocity does not exceed 90m/s

(300ft/s) or, for BV984, BV985, BV986, BV988 types, fall below 4.5m/s

(14ft/s). The preferred velocity is 75m/s (250ft/s).

The minimum pipe diameter is calculated using the following formulae.

For BV984, BV985, BV986 and BV988 desuperheaters there is a selection

of standard trim sizes available

BV994 and BV995 units are often associated with outlet silencer

sections depending upon the ratio of inlet and outlet pressures and

the maximum permissible sound pressure. For these reasons each unit

receives individual considerations based upon customer requirements.

D = 18.8 mm or D = 0.225 in.

where:

WM = outlet steam flowrate kg/hr (lb/hr)

VS = outlet specific volume m3/kg (ft3/lb)

Velocity m/sec (ft/sec)

Location in pipework

The desuperheater should be installed so that the

spray nozzle is located at the steam inlet of the tube (if

supplied). A filter should be fitted in the spray water inlet

line to prevent ingress of dirt.

Pipe Joints

Owing to the severe expansion strains which may be

imposed on the joints when starting up it is essential that

all flange joint bolts are manufactured from high tensile

alloy steel irrespective of the steam pressure. These

remarks also apply to the water joint flanges which are

also subject to sudden temperature changes.

Drainage and drainage systems

Efficient drainage of the pipework following the

desuperheater is essential. To ensure that water cannot

accumulate at any point the pipe should be arrangedto fall in the direction of flow approximately 20mm per

metre (14 per foot) under actual working conditions

and be provided with an efficient large capacity trap

(10% of maximum flow to facilitate start-up and shut

down of plant) at the lowest point. To prevent the trap

becoming airbound the drain pipe should have ample

capacity to deal with the drainage and be fixed as near

to vertical as possible. There must be sufficient space

in the drain pipe for water to flow down and air to pass

up the pipe.

When starting up the plant it is advisable to open the

trap by-pass valve to deal with any excess water. If aby-pass valve is not fitted the trap should be inspected

to ensure that it is passing water and has not become

airbound. When the pipework has warmed through to

working temperature and a reasonable amount of steam

is flowing the drainage of water should practically cease

and the trap by-pass valve can then be closed.

Successful operation of a desuperheater depends to

a large extent on the injection of water being hot,

preferably near to the saturation temperature of the

steam to be cooled so that it is mainly the latent heat

which is extracted from the steam to evaporate the

injected water. This minimises the time of the suspension

of the water particles in the steam so that all the wateris evaporated and none falls to the inside walls of the

pipework. As mentioned below the pipes connecting the

water supply to the injection nozzle should be efficiently

lagged to minimise the loss of heat.

The water pressure and temperature should be no less

than the values originally specified at the enquiry/order

stage since these figures are used for design purposes

in sizing the injection nozzle. The pipes connecting

the water supply to the injection nozzle should be no

less in diameter than the water isolating valve flange

connections indicate.

Condensate supply should be free from debris and

effectively filtered to less than 0.25mm.

P1 Inlet Pressure Bara (Psia)

T1 Inlet temperature C (F)

P2 Required outlet pressure Bara (Psia)

T2 Required outlet temperature C (F)

PW Available spraywater pressure Bara (Psia)

TW Spraywater temperature C (F)

WS Maximum inlet steam flow kg/hr (lb/hr)

WM x VS

Velocity

WM x VS

Velocity


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NotesWeir Control & Choke Valves



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Weir Control & Choke Valves Notes



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