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Valve critical flow (Gas and Liquid)

Valve critical flow (Gas and Liquid)

Valve critical flow (Gas and Liquid)


For liquid: Most of the equipment that I have seen operate in pressures where the DP will almost always yield critical flow. My question is, if the P1 and P2 across a liquid control or dump valve create critical flow and the downstream vessel can not be operated at a higher pressure to help reduce the DP, what are ways to help with that. Installation a mid-stage vessel will be very expensive, can one install midline control valve to reduce the pressure cut before the downstream vessel or may be glove valve or choke nipple to put back pressure on the upstream dump valve. What have you done in the past? I ask because the tendency for cavitation is present and trying to avoid it.

For gas: Is it correct to assume that the negative effect of critical flow on the gas side across a control valve is less severe than the liquid side? Also, what are some of the effect of flowing in a critical flow across gas control valves. Erosion? noise?

As much as possible, do it right the first time...

RE: Valve critical flow (Gas and Liquid)


There is only one way to prevent cavitation in a liquid flow scenario, and that is to stop the fluid from dropping below the vapor pressure. The best way to do this when the Pv is close to your P2 is to take the drop in multiple segments. Depending on the type of valve and the overall dP, will dictate how much below the P2 you will drop at the vena contracta. If you have a concern with cavitation I recommend you address it with your valve supplier as they will be able to recommend an appropriate solution. The option that you listed all seem viable in certain situations, but you should know that fixed solutions such as a positive choke and baffle or orifice plates will only help in a limited range of scenarios. If you have a wide range of flow rates, and operating pressures multiple adjustable valves, or a multistage vale may be your best option.

As to the detrimental effects of choked flow in a gas control valve the primary ones to lookout for are noise and vibration. The vibration is especially important around small branch connections, probes and injection quills.

RE: Valve critical flow (Gas and Liquid)

You just need to choose a different control valve. CV vendors are well aware of this and design the valve trim accordingly to essentially have multistage pressure drops within the valve and they also provide noise data for liquid and gas.

If you have a steady flow you can get multi orifice tubes which do the same thing.

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RE: Valve critical flow (Gas and Liquid)

For critical flow of gas across a control valve , other issues include noise and aerodynamic flutter or vibration of valve internals. The nice aspect of critical flow is that one can easily calculate the mass flowrate , knowing the inlet P, inlet T, molecular weight, and valve coeficients

For liquid valve, multi stage valves have been around for many decades, starting with the CCI self drag control valves in the 1960's and now including multistage valves from many vendors. What is lost in these valve designs is tight shutoff, so some other means of providing tight shut off may be needed.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

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