Critical Pressure of a Butterfly Motor Operated Valve

What size is the valve? Is it on/off or modulating? How long does it take to swing from open to closed and vice-versa? Is the disc fitted with "fingers"?

What is in the line? Temperature? Pressure?

What is downstream of the valve?

size of valves are 10 inch and 24 inch. They are on-off valve. Timing is 100 sec and 240 sec respectively for full open to full close and vice versa. Sorry I dont know what is finger in valve (can you please tell what is finger what is the purpose of finger) pleaseconsider no finger. Line is 10 inch & 24 inch respectively. Process fluid is hydrocarbon in liquid stage. Temperature is 65 deg celsius pressure is 1.87 bar and 11 bar respectively. These valves are at pump suction and discharge there are few pdt & pg.

Is the pump fitted with a variable speed device? What is the control protocol for pump start and stop according to the approved Control Philosophy document? Is there a downstream non-return valve in the line? How many pump sets are in parallel, and how many are operating at any time?

To what standard are the valves and pumps installed?

Please elaborate on <...hydrocarbon...>.

sir from this post i can understand one thing about you that you are very complicated man who does not know how to make things simple.

I had read few other comments given by you in other thread and all those are full of arrogance, pride and sarcastic and showing that you are Instrumentation God.

I tell you one things that may be you know many things but you dont know everything.

There are many people present in this blog who are far better than you dont take me otherwise but it is true.

Eagerly Waiting for 1 more comment which is full of arrogance, pride and sarcastic and showing that you are Instrumentation God.

Regards.

If this is an isolation valve to isolate the pump ,I wonder why a motor operated gate valve was not selected.

In hydrocarbon service I could only guess that you would not have any problems with the valve itself if it is line size valve, because the pressure drop would be minimal .A gate valve would have even less pressure drop.

Critical pressure and Vapor pressure are used in sizing throttling control valves.

In some other services like rich amines ,a butterfly valve in pump suction may cause cavitation in the pump if the NPSH is not available

Yes Valves are for isolation purpose. I dont know why Gate Valve is not selected may be because of tight shut off of butterfly valve. Pleas tell your view.

I am little bit confused that, when valve will be fully closed then outlet pressure will be zero. So before full shut off of valve there will be one stage when outlet pressure will go below the vapour pressure of fluid hence flashing will occur this will create bubbles so this will also effect the valve body. I think because this will remain for very less time thats why there is no significant problem this will also be the case during Control Valve. Can you please throw some light on this.

1 more doubt that for cavitation why critical pressure is required because critical pressure is only the vapour pressure at critical temperature and critical temperature is the temperature above which how much pressure is applied fluid shall not change its phase.

so I think it should be the vapour pressure at maximum operating pressure of fluid because vapour pressure changes with temperature directly, that should only be the criteria for flashing or Cavitation. Please reply.

Neither flashing nor cavitation can occur in a valve that is fully shut, because there will be no change in fluid pressure within the valve body. Both flashing and cavitation are also rare in cases when the valve is fully open, especially for valve types such as butterfly and ball because they are straight-through flow designs where there is little obstructing the flow which could cause dynamic pressures to change. Therefore, concerns about flashing or cavitation in an isolation (on/off service) valve would only be valid if the valve flashes or cavitates long enough during movement to accumulate damage over time.

You say in an earlier post that the valve takes 240 seconds to move from fully closed to fully open. That is a long time (4 minutes!), and so if the valve really does take that long to move then perhaps cavitation damage could be significant over many cycles of operation.

I think you are right about "critical pressure" being the wrong term for this application. What is important for calculating choked flow, flashing, and cavitation is "vapor pressure" at the temperature your fluid flows through the valve.

thankyou Tony Sir for clearing the doubt regarding the Cavitation in one line (first line) that cavitation in globe happens within the valve body and butterfly does not have such kind of shape where fluid can stay for longer time. thanks alot

Just to clarify: cavitation happens when the flowing fluid's pressure suddenly decreases due to increasing velocity through narrow portions of the valve trim or body, then if the pressure falls below the vapor pressure of that liquid the liquid "flashes" into vapor bubbles. As the liquid slows back down to pipeline velocity, it loses kinetic energy and gains potential energy (pressure) again, causing those vapor bubbles to condense back into liquid. The sudden condensation creates shock waves in the liquid, which we call cavitation.

A butterfly valve can indeed cavitate when in the partially-open (throttling) position. In fact, butterfly valves are particularly prone to this problem due to the fact that the fluid encounters sudden changes in velocity and pressure when flowing around the edges of the butterfly disk. It generally isn't a big problem in on/off service, though, because the valve should not spend enough time in the cavitating position to suffer much damage. If, however, that valve is very slow moving and/or it cycles a lot, all those episodes of cavitation can and will add up to permanent damage of the butterfly disk.

Outlet pressure would not go to zero ,because you have the static head downstream of the valve.

Outlet pressure would not go to zero ,because you have the static head downstream of the valve.

What if it's downhill after the valve and open ended?