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LuciusLucius Learnin SeriesLearnin Series

Motor Operated ValveMotor Operated ValveSe sm cSe sm c Weak L nk Analys sWeak L nk Analys s

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TypicalTypical Gate ValveGate Valve

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General Valve Com onentGeneral Valve Com onent

Stems Discs

Description (Description (Gates and Globes)

Outside screw, non-rotating, rising. Solid wedge.

Flexible wedge.

Outside screw, rising-

rotating,

Parallel.

Globe.

Two piece with coupling,-lower piece rising, non-

-

Disc lifters, ,

rising-rotating. - . Lifting arm.

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CategoriesCategories of Valves (Gates andof Valves (Gates and

Pressure retaining components Body.

Bonnet.

Bonnet bolts.

Disc (pressure boundary).

Pressure seal gasket retaining ring.

- Yoke.

Operator flange.

Yoke bolts.

Operator bolts.

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Disc Variations (Gates and GlobesDisc Variations (Gates and Globes)

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GateGate and Globe Valve Considerationsand Globe Valve ConsiderationsGate valves

-

head into the disc and then through body rails orseats.

.

Majority react out the stem torque through a torque

arm or stem key into a bushing in the yoke.

n a r s ng ro a ng s em, no orque reac on -however, torque losses as the stem rotates at thepacking, disc, and the stem nut adapter

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Gate and Globe Valve ConsiderationsGate and Globe Valve Considerations

The actuator reacts the appliedtorque/thrust back through the yoke

concon

structure.

On closing:

Stem is in compression.

Disc is in compression.

All other com onents are in tension.

On opening:

Stem is in tension.

Disc is in tension.

All other components are incom ression.

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GateGate Valve Typical Closing SequenceValve Typical Closing Sequence

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Gate Valve Typical Opening SequenceGate Valve Typical Opening Sequence

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PressurePressure Locking/ThermalLocking/Thermal

What is PL/TB?

ressure oc ng: pressure rappe e ween ex

wedge and bonnet causes differential pressureexceedin o erator thrust ca acit .

Thermal binding: caused by differential

expansion between the valve body and disc.

Susceptible valves:

. Valves essential to reliable plant operation in high

energy systems.

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Disc PinchingDisc Pinching

Disc pinching

Similar to thermal bindin .

In closed valve, pressure increases in valve body,causing body to expand. When disc seat areax , v ur r

seat. If the pressure should decrease, the body

reflexes causing the disc to pinch into the seat.

Susceptible valves:

Safety related gate valves with flex or solidwedge.

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DiscDisc Pinching: ProcessPinching: Process

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MOVMOV Calculation ConsiderationsCalculation Considerations-- Stroke TimeStroke Time Stroke Time Considerations

For rising Stem MOVs:

ro e me = em rave

RPM * Stem Lead

For quarter turn (Butterfly) MOVs

Stroke Time = HBC Ratio * 60 sec/min * OAR

RPM * 4

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-- Valve Weak LinkValve Weak Link

The valve weak link analysis is performed todetermine the maximum load the valve cane su ec e o.

the valve to determine the maximum load

the weakest component can safely sustain.

Loads are calculated for continuous

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SeismicSeismic -- Weak Link ReviewWeak Link Review-

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Weak Link Calculation MethodWeak Link Calculation Method

Stress - stress

Thrust =f (geometry)

where

Thrust = maximum thrust for evaluated component

Stressallowable = allowable stress for evaluated component

stressother = component stress due to pressure / gravity / seismic

f (geometry) = geometric relation between thrust and the componentstress that would be caused solely by thrust:

stresscomponent = Thrust x f (geometry)

Lucius Learning Series: MOV Seismic Weak Link2010, Lucius Pitkin, Inc. All Rights Reserved

2020