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8/14/2019 Lucius Learning Series - MOV Seismic Weak Link.pdf
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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
2010, Lucius Pitkin, Inc.
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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.
Lucius Learning Series: MOV Seismic Weak Link2010, Lucius Pitkin, Inc. All Rights Reserved
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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.
Lucius Learning Series: MOV Seismic Weak Link2010, Lucius Pitkin, Inc. All Rights Reserved
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Torque arm. 66
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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.
Lucius Learning Series: MOV Seismic Weak Link2010, Lucius Pitkin, Inc. All Rights Reserved
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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