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Question:

Tough question about valves?

When you regulate flow with a valve in a fluid circuit, obviously the flow changes. But mathematically, it changes in function to what? the minimum cross-sectional area? I mean it has to have something to do with some integral of sth (since a valve can change the cross-sectional area of a portion of the tube with length of almost zero and the flow would still change dramatically). I have no idea how to work this through. Can you tell me what changes when you close a valve, which makes the total flow change?Please answer asprecisely as possible, this is for a college essay! thanks in advance!

Answer:

The shape of the internals of the valve determine the flow characteristics. Ball, butterfly and gate valves are usually only used for on/off operation and have very poor control abilities. Other valves like globe valves and flow control valves have specific flow characteristics. They can vary from a linear characteristic when flow is proportion of the percent of valve stem travel to others such as parabolic, quick opening etc.
The flow coefficient is a measurement usually given for a valve fully open, identified in the specs as the Cv. It basically tells you how much flow you'll get through the valve for a given pressure differential across it, or conversely how much your pressure will drop if you force a certain volume through it. Some valve types are effectively orifices (gate valves) or short lengths of pipe (non-full-port ball valves), others like solenoid types have more complicated internal paths the reduce flow because of the fluid having to change directions in addition to squeezing through orifices.
A valve reduces the cross-section of the pipe. .

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