Calculate the flow coefficient and pressure drop through valve

[daftdave11]

Hi
I am wondering would ye be able to help me out? I am instructed with the task of calculating the pressure drop and flow coefficient for a valve. Please see basic diagram attached for layout of valve. I need to find the pressure drop and flow coefficient when the ram is closed at different percentages. would ye be able to tell me the necessary equations that i need to do this. note internal diameter is 100mm at inlet and 50mm at outlet.The valve shown would be connected to the bottom a reactor etc. the pressure entering is set at 100 bar. Thanks for yer help in advance

Regards Dave

 

[pmover]

the best course of action is to forward your request to the valve mfg - they should be able provide you with the requested information.

if this is a custom made valve, then you will need additional means of measuring pressure and flow. no doubt, a time-consuming task.

good luck!
-pmover

 

[BigInch]

Valve coefficients are determined from flow and pressure drop data using cold water. You match the data points to a suitable equation. You have no chance to calculate it accurately from valve geometry, unless that geometry can be represented by a simplified orifice coefficient.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[davefitz]

I suggest you borrow a copy of teh "ISA handbook of control valves" from the library and use their procedure for determining the vale Cv and Xt.

The Cv is measured using simple tests with air or water. The Xt is important only if the application will involve a flow of gas that involves a pressure drop greater than about 50% of the inlet absolute pressure. So, if the Reactor is below 50 bar when the supply pressue is 100 bar, then you will need to also calcualte the Xt value.

 

[daftdave11]

i cannot contact the manufacturer as the are closed for the following week. i know that the fluid been tested is to be natural gas and also water, which i have the properties of both. is there any formula i could use to get a ruff estimate of valves for the flow coefficient and for the pressure drop.. badly stuck for this info? thanks in advance

 

[CRG]

Find valves of similar construction and contact the manufacturers to get the flow coefficients. Look at the spread in the data to determine if you are comfortable with taking the average.

If you cannot find manufacturers with similarly constructed valves, then ask your professor that gave you the problem for some help.

 

[jmw]

You could post in the valve forum but in any case there is a nice program downloadable from Fisher that may help.
It will at least provide a near equivalent which can help you see if your calculations are on the right lines.
The link is here FISHER:

"Designed for engineers, Fisher® Specification Manager software offers a powerful set of tools for producing an ISA specification sheet faster, improving noise prediction calculations and exporting dimensional data for Fisher and Baumann™control valves. You'll find it easy to learn and use. Fisher Specification Manager software helps take the worry out of control valve specification."

JMW

http://www.ViscoAnalyser.com

 

[BigInch]

But the problem is that his valve doesn't look anything like a control valve, or any normal type valve. I don't see any hydraulic possibilities to scale it to a control valve. Most valves flow through some kind of circular orifice, closed by a plug or plate. This would be blanking of an eliptical orifice by flow past the upper edge of a truncated cylinder. Highly 3D. I wouldn't believe any calculation of Cv, without some special CFD work.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[novis76]

If you simply want a rough estimate you could, at least for small opening percentage, assume that the flow is a ideal nozzle. Assuming no pressure recovery (P_throat=P2) the pressure drop can be calculated according to

dP = 0.5*rho*Q^2*(1/Ac^2 - 1/A1^2)

(Ac=throat area, assume physical area-circular segment). Using following conversion-factors an expression for the flow coefficient Cv (US gpm @ 1 psi pressure drop)

1 psi = 6894.7 Pa
1 US gpm = 6.309E-5 m^3/s

Cv = sqrt(6894.7*(1/Ac^2-1/A1^2)^(-1) /(rho * 0.5) )/6.309E-5

At large/fully open the geometry kind of resembles a junction. Perhaps the loss coeff for this geometry can be used as a rough estimate

Good luck

 

[racookpe1978]

Well, this is the "next week" (Jan 3) ... So, call the manufacturer of the valve.