Existing Control Valve Acceptable with new pumps?

[Engineer6512]

I am in the process of sizing a new pump to increase the flow rate in my system from about 850 gpm to 970 gpm. The system has an existing control valve, and I simply want to determine whether the existing valve will be acceptable at the new flow rate, or if I have to install a new one.

My thinking is that I will calculate the Cv at the new flow rate and use the manufacturer's chart to see how much the valve is open at the new flow rate. As long as it's still in the 50 - 80% range the existing valve should be ok. If it's outside this range, I should probably size and install a new valve.

So, I know my new flow rate, I have the valve Cv chart, the only unknwn left in the equation is what to use for the delta-P across the valve? The usual way of sizing a control valve is to take Pump discharge pressure and subtract losses and final required system pressure. The problem is that I need to know the delta-P of the Control Valve to calculate the discharge pressure required for my pumps! So do I just assume a delta-P for the CV (I read somewhere that 30% of Total system pressure drop is common), and go from there? Or am I missing something?

Some guidance here would be greatly appreciated.
Thanks.

 

[Yorkman]

Correct me if I'm wrong, but won't the presure drop change proportional to the square of the G.P.M.? If you can find the pressure drop acrossed the control valve at the original flow rate I think you would be able to reasonably calculate the pressure drop at the new rate.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[TD2K]

Yorkman is right on. The control valve doesn't 'take' a pressure drop, the system imposes a pressure drop on it.

Let's say the pump puts out 300 psig. Your control valve takes 50 psi with an outlet pressure of 250 psig. The flow goes to a vessel controlled at 150 psig so your line losses are 100 psi obviously. All assumed numbers for discussion.

You increase your flow from 800 gpm to 900 gpm. The pump discharge pressure decreases according to the pump curve. The line losses increase to about 126 psi so the outlet pressure on the control valve is now 276 psig. Using the discharge pressure from the pump and your calculated outlet pressure for the control valve, calculate the Cv required for the new flow. Compare that to the installed Cv to determine if the new flow is possible.

 

[Engineer6512]

Thanks for the replies.

So what I'll do is:
1. Figure out the current Control Valve dP based on current pump discharge, and friction losses, etc.
2. New control valve dP = (970/850)^2 * Current DP
3. Calculate new Cv and verify acceptable or not
4. Calculate pump discharge pressure for the new pump using new CV delta P value.

Anything I'm missing?

Thanks again.

 

[davefitz]

the new pump has a flow vs hd curve. For the new rated flow, access the pump curve to get the developed pump hed, add this to the pumpp inlet pressure, and you should end up with the pressure entering the new valve.

From past data of the downstream process, plot the process pressure vs process flow curve. It is usually some set pressure plus a variance that varies by the square of flowrate, but it can also be a fixed pressure or can increase linear with pressure in some processes. For the new flow, extrapolate the old presssure vs flow curve to get the pump outlet pressure.

The difference between these 2 pressures is roughly the new pressure drop across the new valve at the new flow rating.