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Consider the following:

                               /‾‾ Valve --- Watering hose
Pump --- Pressure regulator ------ Valve --- Watering hose
                               \__ Valve --- Watering hose

A pump generating an arbitrary pressure and a pressure regulator to curb it at 1.5 bar. After the regulator we have three valves in parallel.

If I close one of the valves partially or completely will the volume flowing through the other two valves change? I hope that the pressure regulator would compensate closing/opening valves by keeping the pressure constant but I'm not sure.

Background is a watering solution in gardening. I aim to decouple several strands of watering hose so they can be adjusted independently.

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1 Answer

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I hope that the pressure regulator would compensate closing/opening valves by keeping the pressure constant but I'm not sure.

In the absence of confounding factors, this is the correct viewpoint.

In order for the pressure regulator to be "perfect", you would ideally have a large volume between the pressure regulator and the valves, and the valves would be spaced a good distance away from each other. In the current picture, the pressure regulator controls the pressure by controlling the flow from the pump. At least this is my understanding.

Non-ideal conditions will be seen when the interacting kinetic forces between the different valves are significant. In that case, the pressure that pressure-regulator sees can't exactly be called the back pressure behind each valve. If we treat the region up to the valve as a stagnant, constant pressure, region then there is no problem with your view. We would also require that the pressure-regulator performs its job perfectly, obviously.

note: I'm assuming the pressure-regulator controls the pressure with its own valve. The representation could be trying to imply that the pressure-regulator is just a sensor and the controller opens and closes one or more of the hose valves, but I don't think this is the system because the OP implies that the hose valves are manually controlled

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+ As long as the pressure regulator does its job and maintains a constant pressure going into each valve, that's all the valve will see. – Mike Dunlavey Mar 5 at 19:17
Your assumption is correct, only one simple (household installation kind) pressure regulator controlling it's own valve. – wupdiwup Mar 5 at 22:09
That pressure will be the same at each valve is intuitive, what I was not sure about is if volumetric flow of one valve will be influenced by the state of the other valves. I hoped this wasn't the case as the pressure regulator keeps the pressure going into the valves constant but according to user2018790's answer this is not the case. – wupdiwup Mar 5 at 22:18
@wupdiwup: There's only one thing a valve sees - the pressure drop across it. The only way flow through valves B and C can reduce the flow through A is by reducing the pressure that A sees. If the pressure regulator is doing its job, that cannot happen. You have to make sure the pressure regulator can keep up the pressure even with all three valves open. – Mike Dunlavey Mar 6 at 0:15
@MikeDunlavey: That's what my intuiton told me. Now I have a few explanations supporting and rejecting it, I guess I'll try an experiment and report back! – wupdiwup Mar 6 at 11:46

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