ISA/IEC sizing question
ISA/IEC sizing question
(OP)
I am looking for the equation for delta P across a control valve. I am looking for non-choked, compressible, turbulent flow with or without fittings attached. I have the equation to solve for C, but I was hoping someone has this equation re-worked for DP.
Thanks
Josh
Thanks
Josh





RE: ISA/IEC sizing question
For compressible flow you first have to determine the delta P to P1 ratio. Then Weight, Mass or Vapor basis and what kind of valve globe, ball or butterfly.
RE: ISA/IEC sizing question
Y=1/(3FkxT), x=DP/P1
RE: ISA/IEC sizing question
If
Cv= _Q_________
1360*P1*Y*Sqrt(X/GTZ)
And X=DP/P1
Substitute for X
Cv= _Q_________
1360*P1*Y*Sqrt(DP/P1GTZ)
Sqrt(DP/P1GTZ)=Q/CV*1360*P1*Y
(DP/P1GTZ)= (Q/CV*1360*P1*Y)^2
DP=Q^2*GTZ/P1^3*Cv^2*1360^2*Y^2
Q is in SCFH
P1 is inlet pressure in PSIA
Y is the expansion factor,(1-(X/3FkXt))
X Pressure drop ratio defined above
G specifig gravity (MW/29)
T=Temp in degrees R
Fk is specific heat ratio (ideal gas: 1.4)
Xt is terminal pressure drop ratio-from Mfr's tables)
Z is compressibility (ideal gas=1)
RE: ISA/IEC sizing question
But you still need to break Y out (because Y=1-X/(3FkxT)).
When I do that I get a cubic equation for DP.
If you use your equation or the ISA one I listed same result.
I was hoping someone had the equation solved for me.
Thanks
RE: ISA/IEC sizing question
If you assume the valve to be choked, start by using a Y of 0.667 and see where that gets you.
If the valve is not choked, use a Y value between 0.667 and 0.93.
If you want Y broken out, assume Fk=1 for air, Xt~0.5 to 0.75 for a globe valve, and the x/Xt factor to be the Xt value or some pressure drop ratio lower than the Xt.
What type of control valve are you using?
Regards,
Fred
RE: ISA/IEC sizing question
As for the type of control valves: I have used Fisher, Masoneilan, Samson, CCI, ValvTechnologies Xactrols, etc.
I know what Fk and xT values I will be using. I was just hoping some would have the equation written to solve for DP.
RE: ISA/IEC sizing question
If you have an inlet pressure and the xT value, you can calculate your downstream pressure and then the pressure drop.
RE: ISA/IEC sizing question
Help me out. Knowing the formula is C=W/(N6*Y*[x*p1*rho1]^.5)
Y=1-(x/(3FkxT)), x=DP/P1, N6=63.3
Known:
W, N6, Fk, xT, P1. rho1, C
This leaves DP the only variable I don't have. But I don't remember my algebra that well to solve for DP.
When I solve for DP I get:
DP^3/(3*Fk*xT*P1)^2 - 2DP^2/(3*Fk*xT*P1) + DP = (W/C*N6)^2/rho1
RE: ISA/IEC sizing question
As far as the equation goes, I wouldn't break up Y at first, use Y as 0.667 to begin with, or leave it as Y. When I went through the equation, I ended up with pressure drop on one side and W^2/(rho*C^2*N6^2*Y^2) on the other side, with the P1 term dropping off because it appears in the numerator and the denominator.