control valve question needs help

[anber]

I know that if we have a pump connected to a pipe which have a control valve ,as we change the opening of the valve , the resistance of the system decreases or increases and the pump moves on its curve and the flow rate from the pump itself changes , so the control valve controls the flowrate in indirect way as it changes the resistances in the pump way and the pump is the device that changes flowrate.... An enginner once told me that the control valve itself changes the flow rate , for example if we install a glope valve and open it at certain opening the flow rate upstream valve is 20 m3/h and the flowrate downstram the valve is 10m3/h ,, is what he said is right ? i guess that what enters the valve must get out of it , if 20m3/h enters the valve , the 20m3/h must get out of it otherwise where they goes ?

 

[LittleInch]

Amber,

You need to calm down. You have two posts here on the same thing plus another in the valve forum.

There ate two golden rules.

One is the conservation of mass
The second is the conservation of energy

The engineer you spoke to is wrong. The mass entering a valve is the same as that leaving it.

It is very difficult to separate pressure from flow. Each affects the other.

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

 

[anber]

Thank you for your response,
so it is right that the pump itself is what controls the flow rate with response to the resistance created by control valve ?

 

[LittleInch]

I got into trouble a few weeks ago by saying that pumps create pressure instead of flow, but reality is that they do both. A centrifugal pump essentially creates flow at a govern head which translates to pressure. A control valve can only create a pressure drop across it in proportion to the flow through it.

A pump curve stays at a high head and gradually decreases as flow increase. A system curve starts low and then increases as the flow increases. Where they meet is the flow you week get. A control valve acts as a vertical line between the two this moving the intersection punt left on the graph, I.E less flow.

A control valve cannot create flow.

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

 

[zdas04]

The pump capacity is related to the system resistance. The total resistance can be plotted as a system curve. The pump capacity can be plotted as a pump curve. Where the two lines cross is the capacity of the pump in that system. If you add or remove resistance, the system curve moves in or out and the crossing point changes. Saying which is controlling what is simply semantics. If I add resistance, the pump (for the same power input) must take a smaller volume to a higher pressure. Which controlled?

[bold]David Simpson, PE[/bold]
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

 

[anber]

Thank you littleInch so what i said was true and you clarified it in a perfect way ,,
Thank you zdas04

 

[BigInch]

No. It's an energy thing.

Available differential pressure nd resistance to motion (flow) in a path is what allows flow to occur. Differential pressure is potential energy available to drive flow. Resistance is energy consumed by friction during flow.

A pump increases available differential pressure (or head) across the path. Once again, that is the energy available to drive flow.

A valve increases or decreases the resistance to flow (head loss) within the path. Some of the total resistance is due to the friction of the flow against the pipe itself. Again, that is the energy used during flow.

Increasing head by adding pump energy or by increasing gravational elevation energy, or by reducing energy loss to friction during flow, by increasing pipe diameter, or by using smoother pipe, or by opening a valve more, will all tend to increase flow.

What makes a valve a flow control valve or a pressure control valve has nothing to do with the valve. That is decided simply by what controls the valve's open-close position, a pressure sensor, or a flow sensor.

If you are looking at (or somehow measuring) the flow coming out of the pipe and adjusting the valve by hand to allow more, or less flow, or constant flow, you are a "flow sensor" and have made a "flow control" valve.

If you have one hand in front of the hose nozzle and adjust a valve with the other to keep the water hitting your hand at constant force, then you are a "pressure sensor" and you have made a "pressure control" valve.

Can you see how you could make a water temperature control valve?
What about a level control valve?

Find what you like to do, earn a living at it, and then make your lifestyle fit your income. — Chuck Yeager

 

[IRstuff]

Unclear from your posting who said what. The valve on your shower controls flow by changing the size of the orifice through which the water has to flow.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[BigInch]

Yes, but what does changing the size of the orifice do? If available energy also changed, flow might remain constant, or even increase.

Decreasing the size of the orifice increases the energy consumption of the flow passing through the system, which, if available energy driving flow remaind constant, caused that flow to diminish.

Find what you like to do, earn a living at it, and then make your lifestyle fit your income. — Chuck Yeager