Control Valves

[KC12]

I need some practical insight here, gentlemen.

I have a flow of about 350 cubic meters per hour.

This works fine for 10 inch piping (falls well within the range of recommended velocities).

What would work better:

A) having a 10 inch control valve straight on the line with no reducers?

B) having the 10 inch reduce to 8 inches then have an 8 inch control valve, then expand back to 10 inches? Will this ensure more proper control than option A?

Or... is this all valve manufacter dependant aside from rules of thumb.

 

[BigInch]

Usually you can drop down a size, both to get a good %open ratio for control, the most economic design and the reducers even preconditon flow into and out of the valve.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[katmar]

There is simply not enough information given to be able to say categorically which will work better. If you can get away with the 8" valve then that will most probably be cheaper - cheaper valve, cheaper flanges, but reducers more expensive than same length of pipe. But unless we know how much pressure you have to work with and what your range of pressures and flows is we cannot say that you do not need the 10" valve.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[KC12]

Thanks Big Inch... That is the advice I have also received else where.

 

[Latexman]

My recollection is 80% or more control valves in a chemical plant are one line size smaller than the pipe.

Good luck,
Latexman

 

[katmar]

These rules of thumb will probably give the right answer in 90% of cases. The other 10% cause unnecessary problems.

Control valve sizing is a well established procedure. Why not just do the calculation properly and know that you will have the correct answer? It's not beyond the bounds of possibility that you actually need a 6" valve.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[CJKruger]

Gbur, calculate the control valve opening for both rated and turndown cases and select the size that is between 10% and 90% open for both cases.

Bear in mind that both the flow rate and pressure drop across the valve changes between the rated and turndown case.

Cilliers

http://www.korf.co.uk

 

[KC12]

"Gbur, calculate the control valve opening for both rated and turndown cases and select the size that is between 10% and 90% open for both cases.

Bear in mind that both the flow rate and pressure drop across the valve changes between the rated and turndown case."

Do you have a reference/calculation method I can follow for this?

I understand the Cv calculation method for control valve sizing, but I am having difficulties getting a Cv table from the vendors.

 

[BigInch]

You've got to get that table first.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[davefitz]

For long term reliability is may be wise to use a "reduced port" trim in a slightly oversized valve. For example, it may be that a 6" body with a 6" trim provides lowest cost and best control range using design predictions. But it can happen that design predictions are not exactly correct- if you later find you need a 8" trim, you're stuck . So in this hypothetical case , one may select an 8" body with a 6" reduced port trim.

 

[rneill]

in regards to Davefitz's post ... what happens if you go with the 6" trim in the 8" valve and discover that you really needed 4" trim. There is usually a limit to how small a trim set you can get in a given valve as well.

 

[BigInch]

Why not just figure out what all the conditons are and make the correct selection.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/