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Control Valve operating principle

Control Valve operating principle

(OP)
Hi,
I'm confused with how the control valve decrease flow rate.
Control valve creates pressure drop which causes increase in velocity( based on Bernoulli equation), on the other hand, continuity equation refers that when area is decreased then the velocity should increase and flow rate will stay constant. If that so, i think, the control valve firsly decrease pressure and it increases velocity at upstream, then increasing area cause decrease in velocity ( the pressure will increase a little but will be lower than upstream). And as a result, the flow rate will decrease. Is that right? If not, what is the correct one?

RE: Control Valve operating principle

A control valve impacts flow because it creates an additional pressure drop and flow restriction in the system as it closes.

Up to certain critical flow limits, if the pressure upstream or downstream changed to allow for this increase in pressure drop then the flow change might be negated.

However most of the time the pressures stay more or less the same and hence the flow rate changes.

E.g. a tap / faucet is a simple control valve. You turn it on part way and get a certain flow. If your system pressure increases ( say someone else stopped using the supply) the flow rate goes up. To return to your original flow rate with the higher pressure, you close the valve a bit more.

you're confusing yourself in your OP. Continuity simply means that the flowrate entering the valve, the flowrate inside the valve and the flowrate on the exit are the same thing. What that flow rate is is impacted by the restriction in area and increased pressure required to flow the same flow through the valve

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RE: Control Valve operating principle

(OP)
LittleInch
Thanks for your reply,
Is that mean mass flow rate is constant all the time and only volume flow rate changes?
And the energy which need to be converted to velocity, is exerted by pressure drop, that's why there is no velocity increase and flow rate decreases?

RE: Control Valve operating principle

Not quite.

Mass flow is constant from inlet to outlet as is volumetric flow.

you seem to be confusing an ideal reduction in area idea ( as in Bernoulli) with a control valve where there is permanent loss of energy in the form of pressure, from one side to the other.

#within a control valve there will be a lower pressure than exists at the outlet because the pressure will partly recover as the velocity reduces, but the key part is that there is friction and there are energy and pressure losses. A control valve is NOT an ideal orifice / venturi. You really need to grasp this point or you won't able able to move on.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Control Valve operating principle

Another point is that when the control valve goes towards shut, mass flow rate does actually decrease--the continuity equation only applies to steady state flow, if you change something then the mass flow rate in the entire system changes. When you increase dP with a control valve the downstream pressure of the upstream line increases which reduces the potential energy available to drive flow and flow decreases.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Control Valve operating principle

Suppose you have a pipeline which is connected to a constant pressure system of 3 barg (upstream). The outlet of the pipeline is atmospheric (downstream) and in between is a control valve. This pressure difference of 3 bar is your driving force which will correspond to a certain flow rate. If you would now close the control valve more, part of the pressure drop of 3 bar is taken by this action so less flow is 'needed' to achieve this pressure drop. If you would open the valve more, the opposite happens.

This also indicates that the flow rate doesn't change when entering and leaving the valve. The only thing that happens is that the velocity increases in the throat of the valve as you have a smaller area there, this corresponds to the continuity equation. This increase in velocity also means a reduction in pressure, according to Bernoulli's principle, which could lead to choking and cavitation in the end.

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