control valve sizing question

[glowing]

I was wondering what the best practice is for this...

If I have a pump distributing a liquid reagent, I can determine P1 (upstream) via the pump curve. However, what does one normally do for P2?

I was recently talking to a vendor about a couple valve and had only the upstream pressure. One valve was essentially discharging directly into an open tank, one had a couple hundred feet of pipe with about 30 feat of static head.

I provided max/min/normal flow, temperature, SG, pump discharge pressure, pressure estimate at upstream face of valve (unchoked), line size.... I was asked for downstream pressure. Do you suppose he meant final discharge pressure on the line? I was at a bit of a loss here. It is the 1st time i have ever really had issues sizing out a valve.

Thanks

 

[ilan7]

Assume the vender need the design pressure drop accross the valve.
usually, pressure drop will be at 1 to 100 psi, when for liquid service. if the pump deliver more then 100 psi, and the liquid is flowing into an atmospheric tank (no backpressure, you might need to add an Orifice to take part of the pressure, limit the flow and keep the pressure at the line when the valve is open.
When you have few valves in paralel, you need to check your system and decude what pressure drop is required for each valve, taking into consideration what will happned to the system (pump) when each (or all) the valves are fully open.
ilan

 

[glowing]

Ahh, I see...he is worried about choked flow causing critical pressure due to the increased velocity via the fixed u/s pressure and a the potential for a higher dp at points. This reagent must have a lower critical pressure or something....I dont really ever run into the issue, but it makes sense. Thanks a bunch.

 

[sheiko]

Trying to answer your original questions and assuming:

P1: pressure at inlet flange of the valve
P2: pressure at outlet flange of the valve

Then knowing:

Pup: pump discharge´s pressure
Pdown: destination pressure (tank)
DPup: pressure drop between the pump´s discharge and the valve (static and dynamic)
DPdown: pressure drop between the valve and the destination (static and dynamic)

P1 = Pup - DPup
P2 = Pdown + DPdown

And, of course:

DP = P1 - P2 = Pup - Pdown - (DPup + DPdown)
DP: pressure drop across the valve

P.S: DP is also equal to the difference between the pump curve and the system curve.

Having said that, the vendor normally only requires either P1 and P2, or P1 and DP.

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