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Fisher Control Valve Sizing and Fp Coefficient 1

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gwalkerb

Petroleum
Jul 4, 2012
74
CA
I've been attempting to fully understand the valve sizing procedures that Fisher provides for their valves. Where I'm running into some issues is determining the Fp coefficient for scenarios other than where either the valve matches the linesize, or the valve is installed between identical reducers, in a larger pipe size. Historically, we've usually ignored Fp when sizing valves as we don't have full piping details that early in the process.

In the particular scenario I'm looking at, I'm coming from a larger header (4" XS), into a control valve I've originally estimated at 2", and then into some 1-1/2" (0.120" w.t.) tubing. The application is screw compressor oil injection. Because my piping design will have the valve connected directly to a reducing tee from the main line, I've treated this as a standard reducer at 4"x2". Then on the downstream side, I've considered the piping to tubing transition to be equivalent to a 2"x1-1/2" reducer. I was running into issues finding a 2" valve with a large enough Cv, so I was looking at larger valves, but the way the Fp formula works, I've actually getting reduced flow at larger sizes, which doesn't make any sense.

Are there limits on Fp? I'm getting values less than 0.5, which seems quite low, I wouldn't expect that the piping geometry could cut the total flow in half. I'm also wondering how far from the valve I should be considering fittings for affecting the valve sizing. Can I eliminate or reduce the effect of Fp by having some straight pipe at the valve size immediately upstream and downstream of the valve? If so, how much is needed? Or have I made incorrect assumptions by assuming that a reducing flange and tube fittings immediately downstream of the control valve would have a similar effect to a reducer fitting? In that case, how do I determine the effect on valve sizing?
 
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Why not ask your local Fisher (Emerson) office/representative?

Per ISO-4126, only the term Safety Valve is used regardless of application or design.
 
Unfortunately, I have not had much luck getting detailed answers from them. I'll see if they have any control valve sizing experts other than my primary contact, but based on my history with them, I don't have a huge amount of confidence.

Also, I find that the people on this forum have a large breadth and depth of knowledge that is unmatched elsewhere - Emerson should be able to explain the formulas, but someone with direct experience of sizing a valve and measuring the actual results vs the theoretical results in a real facility can't be matched.
 
Shame that. The bigger the company the less support you get. Hope someone can pick up your issue.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.
 
The Fisher Control Valve Handbook contains a brief section on determining Fp, as well as other necessary coefficients for control valve sizing. I'm looking at the Fifth Ed. (most current as of today), section 5.8.1 for liquids and section 5.9.1 for compressible media. I also checked my copy of the 4th Ed. and found the same information in Chapter 5, pg 110 Valve Sizing. I assume these are the same methods used by their valve sizing software. Your local rep should be able to get you a copy of the book, but sometimes even those are hard to come by (due to popularity).
 
Good one g_wiz type in fisher control valve handbook in a search engine and you should get a link to download the 300pp+ 5th Edition of the book from an Emerson Fisher website.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.
 
I do have the handbook, that's what I've been using to do my calculations to date. but it doesn't give a lot of information about determining Fp for piping configurations other than the simple reducers on either side scenario, or where the limits of fittings close to a control valve end. For example, how far downstream does a reducer need to be before it doesn't affect flow through the control valve? The book only references reducers directly attached to a valve, but I can't imagine that putting a couple inch pup between the reducer and the control valve would completely eliminate the effect of the reducer on the flow.
 
Dear gwalkerb,

I'm a new Process Engineer working for a plant for past 6 months. Can you please share the handbook with me as I'm trying to understand the Control Valve Sizing calculations as a beginner?
 
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