11 Replies Latest reply on Jun 22, 2017 10:33 AM by Joe Galliera

# Flow coefficient, Cv

Could anyone please tell about the flow coefficient determination in Valves?
The formula for the flow coefficient is:
Cv= Q/(p1-p2)^0.5
Is Cv unitless ? If yes,then the right hand side is not matching the left hand side dimensionally.Please explain this.
• ###### Flow coefficient, Cv
Cv is dimensionless.

The flow coefficient Cv is the volume (in US gallons) of water at 60°F that will flow per minute through a valve with a pressure drop of 1 psi across the valve.

The use of the flow coefficient offers a standard method of comparing valve capacities and sizing valves for specific applications that is widely accepted by industry. Most valve manufacturers offer lookup tables that show Cv, desired flow rate, and pressure drop to make it easier to select valves without having to do a lot of complex calculations.
• ###### Flow coefficient, Cv
The formula for the flow coefficient is:
Cv= Q/(p1-p2)^0.5
if you see the formula then you find that it is dimensionally not correct, may be the constant which has the value of 1 but the units for this constant is satisfy the equation dimensionally.
Please tell whether i am saying right or not
• ###### Flow coefficient, Cv
The formula mentioned by you holds true only for water whose specific gravity is 1. The formula is Cv= Q/{S.G/(p1-p2)}^0.5 where Q is the flow rate in GPM, SG is specific gravity and p1-p2 is the pressure drop across valve in psi. The coefficient is not dimensionless.
• ###### Flow coefficient, Cv
Here in Brazil,

We`re trying to reproduce the lab test in the flow simulation for an oil retention valve.

Have you been able to correctly set up this kind of analysis in FlowSimulation? If the answer is affirmative, could you tell me how did you do?

The bench test I`m talking is :
- Set up the valve in the hidarulic piping;
- Put two sensor to measure the pressure before (p1) and after (p2) the valve;
- Put an inlet flow/rate Q in galons/minute;
- Increase Q by adding more fluid to the system;
- when the delta between p1 and p2 (p1-p2) is equal 1 we`ve found out the Q for this valve (the lab told is that this flow shall be the Cv for this valve, but looking for whathever you`ve told me now, I`ll have to pick this calculations and the use in the equation mentioned above, right?)

For the CFD analysis we`ve tried (failed - wrong results):
- introduce the valve in a piping;
- try an inlet flow (guessed from the valve type table);
- in the outlet of the piping, used a enviroment pressure (in this case should I use a static pressure in the inlet? Which value should I use? Because the pressure values are very importante for the cv rate);
- create a equation goal to control the pressure drop (p1-p2)
- when the pressure drop is equal 1, we`ve got our CV (flui is water at 28 celcius degrees);

i guess for my boundary conditions it`s pretty wrong, because softawre is saying that my valve has a CV of almos 250, and the real CV is almost 320.

Could anyone help me?

(pretty urgent this case!!!)

Best regards,
Eduardo Camargo
• ###### Flow coefficient, Cv
Hello Eduardo,

have you set up your valve, in the same fashion as the ball valve tutorial?

take a look at:

C:\Program Files\SolidWorks Corp\SolidWorks Flow Simulation\ lang\english\Docs\Tutorial.pdf

Cheers, Rich.
• ###### Flow coefficient, Cv
Hello there Richie,

Is everything ok?

I've done the setup as the ball valve tutorial and tried many others situations. Even increasing the mesh quality I haven't been able to obtain by simulation the same result I have from the lab tests.

What could be influencing my resukts?

BEst regards,
Eduardo Camargo
• ###### Re: Flow coefficient, Cv
In my Ball Valve  example where I've added how to calculate Cv, known as the flow coefficient,  which is a very important measure of performance in the valve industry.  There  is a PDF file in the folder which shows how to calculate Cv:

Flow expressed by volume
Cv = q (SG / dp)^(1/2) ...(eqn. 1)
where
q = water flow (US gallons per minute)
SG = specific gravity (1 for water)
dp = pressure drop  (psi)

or alternatively in  SI units:
Cv = 11.7 q (SG / dp)^(1/2) ...(eqn. 1b)
where
q = water flow  (m^3/h)
SG = specific gravity (1 for water)
dp = pressure drop  (kPa)

In my example  (below), I use SI units.  The equation goal that I set up looks like this, where  11.7 is a coefficient from the eqn, 3600 is to convert from m^3/s to m^3/h, and  1000 is to go from Pa to kPa.

(11.7*3600*{Inlet Avg Vol Flow Rate})/({Avg Static  P Drop}/1000)^0.5

• ###### Re: Flow coefficient, Cv

I've been able to follow a bench test for this issue.

Modelling customer bench and valve correctly, using a Reynolds Number greater than 50000. We've been able to get results in order of 2,5% error margin for this valve.

The problem is that we're using GAL[UK]/min units. If we use GAL[US] it gives wrong results. As my customer hadn't specified which GAL thei pump uses, for the proximity of the results I guess it were used GAL[UK].

Best regards you all!

Thanks for the help!!

(I'm building a parametric model of a bench test, as soon I finish it I'll post here, ok?)

• ###### Re: Flow coefficient, Cv

Eduardo, check to see whether it is not because the flow Cv is based on US GPM at 60°F water?

• ###### Re: Flow coefficient, Cv

Hi Joe,

From where you got the constant  (11.7)  derived in below formula.

Cv = 11.7 q (SG / dp)^(1/2)

• ###### Re: Flow coefficient, Cv

It's the conversion value going from the standard units of how Cv is defined (GPM, psi) to SI units (m^3/h, kPa).