control valve pressure drop

[koshyeng]

I am sizing a pipeline in my facility.Exisitng 1" pipe sch 80 has a globe control valve with max Cv 5.4. Using hydraulic institute pipe friction manual I found out equivalent length for control valve is 136 feet.

K = (891 d^4) / Cv^2 d = 0.957"

f = friction factor. Use 0.015 for fully turbulent flow
L/D = K/f
L = (L/D) * D = 136 feet
I am startled with the equivalent pressure drop with this control valve(Used for acetone service).
being a new engineer, i want to know do we expect this kind of large pressure drop across control valves?

 

[Latexman]

That is not unusual. A wide open control valve usually has significant pressure drop compared to the total system frictional losses. An old rule of thumb I have says 40%. This is because you want the control valve to start controlling when you start closing it, say at 0 to 20% open. If it just started to control when it was halfway closed, 50% open, you'd lose half its controllability.

Good luck,
Latexman

 

[JoeWong88]

Latexman,
Please allow me to cut in...

koshyeng,
While designing a hydraulic system involve control valve, sufficient pressure drop across control valve is required for proper controllability from minimum to maximum flow. As rule of thumb, pressure drop (dP) across control valve may be at least 20% - 25% of maximum line loss with minimum 0.7 bar.

Another perspective you may need to look at is the operating scenario of the delivery system and destination system. Large dP across control valve may be derived from pressure difference of both delivery and destination systems.

Hope this help.

JoeWong

 

[koshyeng]

Thanks for clarifying my doubt- Latexman and joe

 

[sheiko]

K = (894 d^4) / Cv^2 (not 891 because 29.9^2 = 894.01)

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[psafety]

sheiko,
I believe it is 891; Crane TP-410 concurs.

 

[vzeos]

I get 890.26927613123 because 29.837380517251^2 = 890.26927613123. Do the math.

 

[JimCasey]

I plugged a 1" valve with a Cv of 5.4 into the Flowserve sizing equation.


I get 6.074 gpm through a Cv 5.4 valve (Valtek Mark1) with 1.0 psi drop.

For noncritical flows, Cv=Q SQRT(Gf/dp) the Delta-P increases as the SQUARE of the flowrate.

Why use K-Factor when you can directly calculate the lost pressure in the valve with the Cv equation.

And Latexman: I always heard "30%" of the system pressure loss in the open valve. Either way: usable numbers. Nine out of every four customers have no idea of the available pressure loss anyway.

 

[vzeos]

JimCasey,
You would use the K factor (or the equivalent length derived from the K factor) to include the valve in a pressure drop calculation of a pipe run that may include various piping components such as straight pipe, reducers, elbows as well as valves. The equation, K = (891 d^4) / Cv^2, is the Darcy equation (h_L=Kv² /2g) rewritten in terms of Cv and allows valves to be considered as pipes.

 

[JimCasey]

Thanks for that vzeos.
I should have embellished my point more eloquently.

If you go to a recognized text such as the Crane "Flow of Fluids" they just give you a range of K-Factors for a valve, and the K factor for YOUR valve probably falls within that range. Valve manufacturers do not publish K-factor. THey DO publish Cv.

When you evaluate the system you are actually looking for the pressure drop. Grind through the Cv equation for the particular valve and you will get the specific pressure drop for thet valve in that application. Use the K-factors for the balance of the system since "an elbow is an elbow" is closer to being correct than "A control valve is a control valve".