Cv rating question

[Dutch7]

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

 

[Latexman]

Is this water? What is the s.g. of the fluid?

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.

 

[Dutch7]

The fluid is not water, SG = 1.25
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

 

[LittleInch]

You need to draw this out so we can understand your system.

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.

 

[TD2K]

I'm with Big Inch, I'm a little puzzled by how your system operates.

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/public/documents/reference/d351798x012_05.pdf

 

[Latexman]

We need a sketch and to know the pump type, as Little Inch said.

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.

 

[georgeverghese]

Also, what is the vapor pressure of this liquid at the operating temp? Internal flashing of this liquid can also contribute to this loss in capacity - throat pressure in the valve may be a lot less than tank pressure.

 

[Dutch7]

Gentlemen - Thank you all for your responses and comments, I am grateful to receive so many.
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??!

 

[Latexman]

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Dutch7]

Thank you for the confirmation, the next largest valve has a Cv of 2.0 which will allow even more flow through = more adjustment.

Ps - I have always said the glass is twice as big as required...

Thanks again to all who participated in this response.
Dutch

 

[TD2K]

You must have a pretty steep pump curve so that the pressure from the pump rises sharply with relatively a small reduction in flow. Normally you would just back up on the pump curve and I wouldn't expect to see this much of a pressure increase.

< 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.

 

[Dutch7]

TD2K - I agree with your view(s), we might be a bit oversized on the pump - we will install a 2.0 Cv valve and report back in a few weeks.

 

[georgeverghese]

Also check if you have 150psi dp available for this backpressure relief valve with the source tank at some high level, and also account / include for frictional drop in the return line back to the tank.

 

[LittleInch]

The issue here might be your pump. To get 150 psi, you might need to flow a LOT more than 15.8 gpm.

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.

 

[georgeverghese]

Agreed, the duty point for the recycle valve should be based on the vendor recommended min continous flow for this multistage vertical turbine pump. Running this recycle on pressure wouldnt work well on a Q-h curve that is flat around the min flow recycle point - further complicated by the fact that the tank level will add on to this pump discharge pressure, so you wouldnt really know when you've hit min flow.

 

[Dutch7]

Hello all,
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

 

[TD2K]

Great to hear, glad we could help.