Air flow to control valve 1

[MECE02]

I need to calculate the available flow rate of instrument air to a control valve. The tubing is 3/8" SS and the pressure is 80 psig minimum. I do not know the velocity of the air or the pressure drop in the line so I'm having a hard time applying Bernoulli's. What assumptions/method should I use?

 

[cloa]

The pressure of what- Inlet or outlet?

 

[JLSeagull]

Are you considering a control valve that is throttling air; or the air to the actuator and accessories?

 

[MECE02]

This is air supplied to the inlet of the actuator.

 

[MECE02]

Essentially I'm trying to calculate the velocity or flow rate of air in the tube. The pressure is supplied in the tubing at 80 psig and exits at 0 psig. The tubing is 15 feet in length. The inner diameter is 0.305 inches. It seems that all of the calculations I find require the velocity to be determined and then the pressure drop can be calculated. I think we should be able to calculate flow rate or velocity based on this information. Please help!

 

[Latexman]

You need to focus on the control valve actuator, not the tubing. The major resistance to flow is the actuator. The velocity and pressure drop in the tubing should be ~ 0. I recommend you gather the technical documentation on your control valve actuator and see if you can discover what you need in there. If that doesn't work out, call technical support at the vendor. They can help. If that doesn't work out, I may be able to dig out some rules of thumb for air usage by control valves. It ain't much; there are thousands of CVs in a big chemical plant.

Good luck,
Latexman

 

[MECE02]

Latexman, thanks for your help. I will call the vendor. However, for my piece of mind can someone help me determine what the flow rate is? This has now become a challenge to me, not just trying to get the answer. I really want to know how. I assume there are other situations where people need to know the maximum velocity or flow rate possible for an existing installation without knowing the pressure drop.

 

[zdas04]

The flow rate (and velocity) is varying over a really wide range. When the system first calls for air you have the maximum dP and very high flow rate for a few miliseconds. As the actuator fills, the dP goes down and the velocity with it. As pressure in the actuator approaches spring pressure the dP (and velocity) reaches a steady value for a few seconds. Finally, with the actuator at full travel the dP (and velocity) again becomes continuously variable until it reaches supply pressure (which probably isn't 80 psig, generally actuators expect 15-30 psig and people use a regulator to cut system pressure to those values).

If you calculate the initial velocity with max dP, you'll find a huge friction loss (which reduces the dP so you have to iterate on this step a few times). When you get a stable value, determine how much air got into the actuator in 50 mS at that velocity and recalculate the pressure in the actuator. With the new dP you can start over and calculate a new velocity. It is interesting to put a 100 Hz datalogger on the actuator to see the pressure traverse, it is impressive to see the real data match the theory.

A rigorous solution to this problem has eluded engineers throughout all time. You can get an answer that will work with the above process, but it is kind of a pain. I've done it when I was trying to determine if I had a problem with a slow-acting valve or if the latency really was that long. Once when I was doing this arithmetic I found that the valve really was taking far too long to operate and discovered that it had the wrong main spring in it. Replaced the spring and the valve worked as advertised.

David