Plug Valve K Factors
Plug Valve K Factors
(OP)
How do you deal with calculating the K factor (from Crane TP 410) of a plug valve where the area of the port is more than the area of the pipe size going to the valve?
Example is the Durco G432 plug valve (for natural gas). According to the literature, for the 1/4" to 1/2" valves, the %port size is greater than 100.
Would I do a pair of sudden/gradual enlargement/contraction?
I have never run into this and would appreciate any guidance.
Example is the Durco G432 plug valve (for natural gas). According to the literature, for the 1/4" to 1/2" valves, the %port size is greater than 100.
Would I do a pair of sudden/gradual enlargement/contraction?
I have never run into this and would appreciate any guidance.





RE: Plug Valve K Factors
You might want to take a glance at the page at the link below from the website of respectful member katmar:
http://www.katmarsoftware.com/articles/pipe-fittin...
RE: Plug Valve K Factors
And I got the Cv from the manufacturer. My issue is Cv is traditionally defined in regards to flow of liquids of low viscosity (flow in gpm which produces 1 psig drop) - and Crane TP 410 indicates the relationship between Cv (liquid) and K factor, but not Cv(gas) with K.
The issue I have is how to translate a Cv (gas), which I have or can calculate to K factor.
Per Crane, q'h = 24700*Y*d^2/Sg*sqrt (delta P*density/K)
The equation I have used to relate q'h to Cv is
q'h = 1391*Cv*sqrt(P2*delta P/ (Sg*T)).
This comes from http://www.fluidprocess.com/Atkomatic/pdf/Sizing.p... and I am assuming my pressure drop is less than the 55% for methane (natural gas)
So I put the two formulas on either side of an equal sign and solve until I get K.
Seems convoluted, but appears to be correct.
But I digress.
I use the information provided, but I like to know how it is derived.
I was wondering how to calculate the K factor for a plug valve when the port gets bigger.
RE: Plug Valve K Factors
Scientists are the ones who start the process, and their work is sometimes theoretical in manner and extremely precise, but engineers doesn't really need to go that far to serve their purpose.
RE: Plug Valve K Factors
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Plug Valve K Factors
RE: Plug Valve K Factors
There was a similar discussion a few years ago where I used the Fisher (Emerson) control valve handbook formulas to describe the conversion. See thread798-233225: Calculating specific gravity The Fisher handbook gives a table of the constants to use for various conditions and systems of units. Last time I looked this handbook was a free download from the Emerson site. I see that Fisher's constants are slightly different from those in your reference. This could be because of different definitions of standard conditions for gases.
One thing to be aware of is that in the realms of control valve sizing for gases, the specific gravity of a gas is defined as the ratio of the molecular weight of the gas to the MW of air (28.96).
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Plug Valve K Factors
The definition of the flow coefficient Cv is univocal and it’s specifically referred to water at 60 °F as fluid flowing through the valve, so my answer to your question is no.
What changes when working with compressible fluids (natural gas in your specific case) is the equation that relates the Cv of the valve to the pressure drop and to volumetric flow rate passing through the valve.
The Cv value you have to enter inside the equation – for instance the one you’ve quoted in one of your previous posts q'h = 1391*Cv*sqrt(P2*delta P/ (Sg*T)) - is always the one provided by the supplier/mfg and that is referred to water at 60 F. It’s the equation that has been re-arranged in order to take into account the nature of the fluid you are dealing with.
RE: Plug Valve K Factors
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Plug Valve K Factors
A published Cv relates to water at 60F - so if I want to find the Cv of the valve for natural gas (or any compressible fluid), I have to convert from the gpm indicated in the Cv value to scfh, taking into account differences in density at standard conditions.
RE: Plug Valve K Factors
Just to contradict myself, there is (was?) a form of Cv used by British companies that was based on the Imperial Gallon, and the Europeans have Kv, which is based on m3/h and bar. But if you are in the US and using US made valves these 2 situations can be ignored.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Plug Valve K Factors
I thought Cv was defined as the amount of flow in gpm that produces 1 psig drop in water at 60 F - so a valve with a Cv of 1 will produce 1 psig drop when water at 60 flows through it at 1 gpm.
So it would have units of gpm/psig - if I have 2 psig drop, I should only have 0.5 gpm flow.
So if I use Cv=1 in a compressible fluid equation, and put in 1 psig for the delta P, should I get the equivalent of 1 gpm of flow, once I change the units and take into account density?
RE: Plug Valve K Factors
Please consider the worked out example at page 25 (for a gas application) of the attached paper, it could be informative.
RE: Plug Valve K Factors
F = Cv x √(ΔP/S)
This gives Cv the units gpm/√psi
Being a bit pedantic - the g in psig is wrong. The pressure drop is a differential pressure and is sometimes written as psid, but should not be written as psig or psia.
Applying this formula to your example of the valve with Cv = 1, when you increase the differential pressure to 2 psi the flow will increase by the square root of 2 and will be 1.41 gpm.
Your last statement is correct. The value 1,391 in the Gas Flow equations on your sheet converts the gpm to SCFH, converts the SG relative to water to be relative to air, and includes the base temperature and pressure for the standard conditions (usually 520°R and 14.7 psia).
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Plug Valve K Factors
Thank you for the resource from Parker. It is definitely good.
Getting back to my original question, to convert Cv to K for friction loss calculations, I can use the Cv from the manufacturer in the equation K = 891*d^4/Cv^2, regardless of whether the process fluid is water or natural gas?
RE: Plug Valve K Factors
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Plug Valve K Factors
In short:
(Cv)^2 = const1 * (kinetic energy/pressure differential)
K = const2 * (pressure differential/kinetic energy)
Rearranging the two expressions above for pressure differential and equalling them, it is possible to find the correlation between the flow coefficient (Cv) and the resistance coefficient (K).
It turns out to be K = 891*D^4/(Cv)^2
Where Cv is indeed the published flow coefficient.
While what above is true for incompressible flow, for compressible non-critical flow it turns to be true in the measure you can assume the density of the fluid remains constant as it flows through the valve, that is under the same limitations over which the Darcy-Weisbach equation can be applied to compressible flow.