deltaP across a control valve 2

[AndreChE]

Can anyone check my calculations?

I am using Q = Cv * SQRT (DP/SG); Q in US gal/min, DP in psi

I have the valve curve (linear): Cv=f(Valve opening)

To calculate the DP for my process flow I am using the inverted equation above. Since Cv is the flow of water(..) in US gal/min for 100% valve opening and 1 psi of DP.

In my process, Q=10m3/h = 44.03 US gal/min

Cv Valve opening(%) DP(psi)
3.56 10 105.54
11.1 30 10.86
26.7 70 1.88
34.9 100 1.10

Is this way of thinking correct? Can I calculate any DP for any valve opening according any flowrate knowing this valve curve?

Thanks,
AndreChE

 

[AndreChE]

...and SG=0.69....

 

[bchoate]

bchoate
Your post seems to indicate that you have a control valve and trim and you are trying to see what DP is created under process conditions. I have just a few simple comments that others may build upon later. If you are trying to choose a control valve for a process application, the following parameters are useful in that selection: process flow and temperature, upstream (CV) pressure, acceptable DP across the CV, and fluid physical properties. Typically one will know the flow range that the process is expected to have. Optimum control valve operation is obtained if the valve opening is between 20% and 70%. A C(v) is calculated from the DP, upstream pressure, flow, and physical properties. The valve trim is chosen for valve operation in the optimum range if possible. You mentioned linear trim in your post. Equal per centage trim is very often used and one should explore appropriate applications for linear and =% trims. Essentially one choses the CV for the amount of DP that is acceptable in their process.
Simple concepts; little detail. Bill Choate

 

[AndreChE]

Yes, it is an existing valve. I have to size the upstream pump. That's why I need to calculate the DP across the control valve in order to calculate the total head for the pump.

AndreChE

 

[Tremolo]

AndreChe:

Yes. That is typically the way we do it -- given the manufacturer Cv curve and a known flow, we invert the equation you provided and find the corresponding pressure drop.

 

[TD2K]

Your numbers are right.

Typically, for sizing a pump, I would take my max downstream pressure, plus line losses and equipment losses (at max flow) plus control valve dP to come up with the required pump discharge pressure. With the pump minimum suction pressure, that gives my pump dP and therefore head.

Control valve dP is commonly set in a lot of company's specs as 1/3 of the dynamic losses and/or a minimum value (10 or 15 psi are common values). In this case, the dynamic losses are the dP through your piping and equipment. The intent of the 1/3 rule is to ensure that you have room hydraulically to handle process swings.

For example, if you are taking 50 psi line losses and your flow has to increase by 10%, your line losses will increase to about 121% or another 10 psi. That 10 psi must be supplied by the control valve opening up. Otherwise, the valve goes fully open and you are flow limited. You definitely won't want to size your pump IMO for the existing control valve being 70% open or taking 1.88 psi. Essentially, the system has no effective control at that point.

For determing the turndown conditions on the control valve, once you know the pump head from the last set of calcs, you go through a similar exercise at low flow and minimum downstream pressure. That will give you the maximum dP at minimum flow that the control valve has to control.