Control Valve Sizing: deltaP increases/ decreases w/ increasing load?

W

Thread Starter

William

1) It is my understanding that if a control valve is installed downstream of a centrifugal
pump, then the following will occur for minimum flow: namely, the ACTUAL inlet pressure will be maximum and the ACTUAL pressure drop across the control valve will be maximum.

Similarily, the following would be true at maximum flow: namely, the ACTUAL inlet pressure will be minimum and the ACTUAL pressure drop across the control valve will be minimum.

In other words, for increasing load (flow), the ACTUAL pressure drop across the valve is
decreasing.

Question A:
Are the statements in (1) above correct?

Question B:
Will someone please explain to me why, in some situations, the pressure drop across a control valve would INCREASE with INCREASING load?? What are these situations?

Question C:
When a process engineer fills in my valve spec sheet with the MAX deltaP occurring at MAX flow
and the MIN deltaP occurring at MIN flow, how do I determine if this is the actual case or if
indeed this is not correct and in actual fact the MAX deltaP occurs at MIN flow and the MIN
deltaP occurring at MAX flow ????

Will some expert please please enlighten me. Thanking you profusely in anticipation.
 
P

Peter Nachtwey

> Question A:
> Are the statements in (1) above correct?

Yes. Although the valve DeltaP occurs when there is no flow.

Think of this as a voltage source with internal resistance, a resistive pot and a load resistor.

> Question B:
> Will someone please explain to me why, in
some situations, the pressure drop across a control valve would INCREASE with
INCREASING load?? What are these situations?
>

This may happen when the load resistor becomes small relative to the resistive pot. In hydraulic terms it takes very little DeltaP across the piston to move the mechanical load.
In electrical terms, then resistive load is shorted out or close to it.

> Question C:
> When a process engineer fills in my valve
spec sheet with the MAX deltaP occurring at MAX flow
> and the MIN deltaP occurring at MIN flow,
how do I determine if this is the actual case or if
> indeed this is not correct and in actual
fact the MAX deltaP occurs at MIN flow and the MIN
> deltaP occurring at MAX flow ????
>

Yes the pressure drop across a valve increases with flow. This is true when the load is determining how much fluid going through the valve but this is not usually the case. Usually the valve is reducing its DeltaP so that more of the DeltaP is across the piston causing it to accelerate.
 
A

Ajay Kasliwal

Purpose of valve is what ??, How valve is piped ??, Is it in the Spill back ???.

I can try to clerify you if you can provide more picture ??
 
K
> From: William <[email protected]>
>
> 1) It is my understanding that if a control valve is installed
> downstream of a centrifugal pump, then the following will occur for
> minimum flow: namely, the ACTUAL inlet pressure will be maximum and
> the ACTUAL pressure drop across the control valve will be maximum.
>
> Similarily, the following would be true at maximum flow: namely, the
> ACTUAL inlet pressure will be minimum and the ACTUAL pressure drop
> across the control valve will be minimum.
>
> In other words, for increasing load (flow), the ACTUAL pressure drop
> across the valve is decreasing.
>
> Question A:
> Are the statements in (1) above correct?

I think the answer is yes, but your statements seem a bit confusing to me. If the load consists of fixed, non-changing elements, e.g., lengths of pipe, and the pump is running at a fixed speed, then the only thing causing any change is the valve itself. Simplistically, to reduce the flow rate the valve must increase its resistance to flow, i.e., it must increase its pressure drop. I think this is consistent with your description.

> Question B:
> Will someone please explain to me why, in some situations, the
> pressure drop across a control valve would INCREASE with INCREASING
> load?? What are these situations?

Variable speed pump? Something changing in the load? From above I guess by increasing load you mean the load's flow rate is increasing; if the "control" valve position doesn't change but the load's resistance to flow decreases (somehow), then the valve would have a higher pressure drop. I don't see how the control valve itself could effect this change.

> Question C:
> When a process engineer fills in my valve spec sheet with the MAX
> deltaP occurring at MAX flow and the MIN deltaP occurring at MIN flow,
> how do I determine if this is the actual case or if indeed this is not
> correct and in actual fact the MAX deltaP occurs at MIN flow and the
> MIN deltaP occurring at MAX flow ????

There's no magic happening, so try to go back to basics. The pump must be operating somewhere on its performance curve, and the system will have its performance curve. The pump manufacturer should provide the former, and you'll need to either analyze, measure, or assume the latter. Toss in the control valve (and any balance valves) and see what happens.

--
Ken Irving <[email protected]>

 
J

Jaime Renovell

The pump supplies a constant power to the fluid so, the less flow the more pressure you get. If you want to control the pressure you must
close the valve more for a higher inlet pressure in order to get a higher dP and reach the set point. The process eng. gives you lower dP to
the minimum flow because he is thinking in a valve with the same % of opening to both flow rates (maximun and minimum). I hope this helps.
 
I don't know if this will help you but...

You are correct, as load inceases the REQUIRED pressure drop across the valve decreases. This is because the valve will always change position to match the required downstream pressure, as frictional losses increase the delta P demanded from the valve is less.

Hypothetically if the CV were to maintain a fixed position, yes as flow increases the delta P over the valve will increase, however the required
downstream pressure will then be too low, the valve control system will have failed to do it's job!

Typically when designing pumped systems where frictional loss/ operating pressures are relatively low, the CV will be spec'd for min pressure drop of 10 psi at maximum flow, this figure is sometimes lower.

In higher pressure drop systems the min delta P over the valve is usually a fixed percentage say 33% of frictional loss, providing this is higher than the 10 psi described above.

If you have come across the sitation where valve pressure drop has increased with increased flow, you should check your system parameters have not changed, ie supply/destination target pressure, if these have not changed you should check if pump impeller size has been changed or if a variable speed drive is in use.
 
Top