Cv rating question
Cv rating question
(OP)
Good day,
I have a small pressure manifold outfitted with a back pressure regulator to control downstream system pressure and relieve excess pump generated flow and pressure back to tank.
Pump flow rate = 15.8 gpm
Desired system pressure = 150 psi
Tubing size = 3/4"
BP valve size = 1/2" npt
BP Cv rating = 1.2
BP Pressure rating = 0 - 500 psi
The issue is the BP valve has no affect on the system pressure, at full open (max flow back to tank) the lowest pressure we can achieve is 210 psi (desired is 150).
If I back into the calculation and solve for Cv I get a value of 1.76, this is only 30% more than what is currently installed, it does not seem correct to me, do any of you believe this extra 30% will make that much difference and allow us to regulate down to 150 psi?
We have swapped out the valve with no affect, we have double checked the pump output and it is doing what it is sized to do - any suggestions or opinions are welcomed.
Dutch
I have a small pressure manifold outfitted with a back pressure regulator to control downstream system pressure and relieve excess pump generated flow and pressure back to tank.
Pump flow rate = 15.8 gpm
Desired system pressure = 150 psi
Tubing size = 3/4"
BP valve size = 1/2" npt
BP Cv rating = 1.2
BP Pressure rating = 0 - 500 psi
The issue is the BP valve has no affect on the system pressure, at full open (max flow back to tank) the lowest pressure we can achieve is 210 psi (desired is 150).
If I back into the calculation and solve for Cv I get a value of 1.76, this is only 30% more than what is currently installed, it does not seem correct to me, do any of you believe this extra 30% will make that much difference and allow us to regulate down to 150 psi?
We have swapped out the valve with no affect, we have double checked the pump output and it is doing what it is sized to do - any suggestions or opinions are welcomed.
Dutch





RE: Cv rating question
If water, dP = (Q/Cv)2
Q = 15.8 gpm
Cv = 1.2
dP = 173 psi (Nope, 150 psi isn't likely.)
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Cv rating question
It is important to understand we are not trying to push 15.8 gpm through the valve, this is really a relief valve so if we were allowing 5 gpm through the valve in your formula the dp would be equal to 17.36 psi but it is not, am I missing something obvious?
thanks for the quick response!
dutch
RE: Cv rating question
How did you get a Cv of 1.76?? This implies a flow much more than 5 gpm.
What type of pump is this - centrifugal or PD?
What type of valve?
This is a simple system so there seems to be something wrong in your data or description.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Cv rating question
If it's a constant 15.8 gpm from the pump and you had no forward flow so everything is going through this backpressure regulator, you would need about 215 psig to get the 15/8 gpm flow through the regulator just based on its Cv and assuming no back pressure.
If the regulator can't get the system pressure below 210 psig it sounds like the "process" or whatever is using part of the 15.8 gpm isn't taking much flow requiring most of the flow from the pump to be recirculated. If that flow is more than 13 gpm, the regulator won't have the capacity to pass enough flow to keep the inlet pressure at 150 psig.
Could the regulator have a wrong spring in it? Could what you are seeing be akin to "droop" in downstream pressure regulators? Do you have a curve showing how this requlator should control the upstream pressure versus flow?
http://www.documentation.emersonprocess.com/groups...
RE: Cv rating question
If the pump is a PD pump, then the "relief" has to flow the whole thing, and
dP = (Q/Cv)2s.g.
Q = 15.8 gpm
Cv = 1.2
s.g. = 1.25
dP = 217 psi (Nope, 150 psi isn't likely. Pretty dang close to the 210 psi, ain't it?)
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Cv rating question
RE: Cv rating question
Hopefully I can fill in some of the gaps for you with this explanation.
From above:
Pump flow rate = 15.8 gpm
Desired system pressure = 150 psi
Tubing size = 3/4"
BP valve size = 1/2" npt
BP Cv rating = 1.2
BP Pressure rating = 0 - 500 psi
Additional Info:
Pump type = multistage centrifugal (vertical)
The system pump most times is deadheading so the flow must go through bypass valve at pressure (150psi). We are not able to get this valve regulated below 210 psi (TD2K & Latexman - you are right on!). The valve supplier insists that this is correct but obviously not.
We have this design duplicated in other systems but they all operate at higher pressures (230psi) and there is a PD pump in all of the other designs... (not sure that that is important but worth mentioning).
My designer wants to be able to go lower than 150psi so for the calc I solved for 108 psi dp = ((15.8/1.7)^2)1.25 = 107.9
Does my math and reasoning work? If we get a valve with a Cv of 1.7 am I golden??!
RE: Cv rating question
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Cv rating question
Ps - I have always said the glass is twice as big as required...
Thanks again to all who participated in this response.
Dutch
RE: Cv rating question
< The valve supplier insists that this is correct but obviously not. >
Depends what they are looking at. If they are essentially saying "you need 210 psi inlet pressure to get 15.8 gpm through this valve", then they are right and this is why you are seeing a high inlet pressure. If they are saying that the valve is properly sized to pass 15.8 gpm at an inlet pressure of 150 psig, I disagree with them.
Yes, regulator with a larger Cv will fix the problem. The 1.7 Cv is the Cv you need. Get a regulator with a greater Cv so it's not running wide open, you might want to check the velocity through your tubing, I don't usually use it and don't have a feel for 15.8 gpm through a 3/4" tubing run.
RE: Cv rating question
RE: Cv rating question
RE: Cv rating question
Look at or post the pump curve. The fact the others are pd pumps is very significant. Those you get a constant flow rate at constant speed. Centrifugals you get a pressure within a band at a range of flows.
If your pump is "a bit oversized" you might not get to 150 this way. What you beefed to do is instal a pressure regulating valve in series with the pump discharge, but allowing it to deadhead for long periods is not a good idea.
A sketch of your system and the pump curve would help a lot
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Cv rating question
RE: Cv rating question
Simply a follow up message as I promised.
It took a few weeks however the replacement valve (2.0 Cv) was installed yesterday and we are now able to regulate below 200 psi, we have met our goal and actually exceeded it.
Thank you all for your help/direction and comments.
warm regards,
Dutch
RE: Cv rating question