sini4s
Mechanical
- May 14, 2012
- 9
How does one calculate opening /closing time in control butterfly valve & on -off butterfly valve
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opening/closing time
- Thread starter sini4s
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Opening and closing time are usually estimated or calculated for several purposes:
a) To avoid water hammer (liquid pressure peaks) that will damage equipment. The pressure peaks might under wrong circumstances (too abrupt closing) be well over allowed pressure limits for the equipment. Searching general on net and/or all forums here for keywoords 'water hammer' you will find several references to calculations and estimating methods.
b) To be well within suitable figures for the mechanical equipment (electrical, pneumatical or other orbiturators normal operating standards). Figueres after manufacturors recommandation, process consideration and for regulating valves to give well damped opearations. To high frequency regulations and abrupt movements to be avoided. 'As long as possible' or normal for equipment often sought rather than fastest.
c) To satisfy emergency shutdown valves need to shut down fast. Often for larger valves and water regulated in two steps: for instance 70-80% fast, rest slower to avoid water hammer. Mechanical: producers linits. Pipeline calculations as point a.
Model the system to determine the transient pressures occurring in the system. The time will depend upon the low, liquid, pipeline elevations and material.
Note that a butterfly valve could have 80% flow when only 20% open. This is a generalisation as it butterfly valves have different characteristics form onemanufacturer to another. The point is butterfly vavles are not good for control purposes. Often two speed opening is used.
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Note that a butterfly valve could have 80% flow when only 20% open. This is a generalisation as it butterfly valves have different characteristics form onemanufacturer to another. The point is butterfly vavles are not good for control purposes. Often two speed opening is used.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
If you are looking for the time it will take for an actuator to close/open a valve I would suggest that it is very difficult to model, especially the fail stroke of a spring return actuators.
That said you can do some empirical testing, to get an indication of the time it will take and use the Cv values of the various controls in the circuit as a guide. Basically the higher the Cv the faster the unit will operate!
However, and this was the impetus for writing, when considering fail safe actuator (spring return) generally the client will want the fail (spring) stroke to be optimised. To keep that stroke to a minimum (i.e. be quick) keep the air supply to a minimum! If the actuator is sized to work on 60 psi, only use 60 psi. If you have 100 psi of air in the cylinder it is doing nothing but adding time to the fail stroke; if the valve has happily operated with the 60 psi all you have done is add a useless mass of air which only has to be exhausted before the actuator can begin to react. More in not necessarily better!
Rant over.
That said you can do some empirical testing, to get an indication of the time it will take and use the Cv values of the various controls in the circuit as a guide. Basically the higher the Cv the faster the unit will operate!
However, and this was the impetus for writing, when considering fail safe actuator (spring return) generally the client will want the fail (spring) stroke to be optimised. To keep that stroke to a minimum (i.e. be quick) keep the air supply to a minimum! If the actuator is sized to work on 60 psi, only use 60 psi. If you have 100 psi of air in the cylinder it is doing nothing but adding time to the fail stroke; if the valve has happily operated with the 60 psi all you have done is add a useless mass of air which only has to be exhausted before the actuator can begin to react. More in not necessarily better!
Rant over.
PeterIgg,
If spring return actuator failure is required to have a slow operation fit a speed reducer to the SOV exhaust port.
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
If spring return actuator failure is required to have a slow operation fit a speed reducer to the SOV exhaust port.
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
Hi Stainer,
Noted, but you would be surprised at the number of times I have been asked: "Why doesn't this react more quickly?" when the first few seconds are spent just waiting for the excess pressure to evacuate from the cylinder! I don't know, maybe it is counter-intuitive to think that less supply pressure = more rapid response (and most of the spring return valves I have been involved with are on ESD service, and shutting, no matter what, is the criteria) but it has been a thorn in my heal for a while.
Noted, but you would be surprised at the number of times I have been asked: "Why doesn't this react more quickly?" when the first few seconds are spent just waiting for the excess pressure to evacuate from the cylinder! I don't know, maybe it is counter-intuitive to think that less supply pressure = more rapid response (and most of the spring return valves I have been involved with are on ESD service, and shutting, no matter what, is the criteria) but it has been a thorn in my heal for a while.
With ESD service one should not create a worse situation by effecting an ESD. Hence fast shutting is not always the criteria. No point is causing excess pressure in a system by closing a valve quickly. Opening closing times should be to protect the system
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
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