Pipeline size VS control valve size 1

[valves4life]

Hey everyone! I have been in a debate with one of our reps, regarding control valve sizing.


If I have a 12" line, and a 6" or 8" triple offset valve can do the required flow control (ie it has the CV and it isn't at too far opening angle), should I quote that? Is there a rule of thumb?

I was always told ISA best practices was not to drop more than 2 pipeline sizes below the line for a control valve. So in my example, the smallest valve should be a 12"er. However, I've tried to check this and the only thing I could find was in a book "control valve primer" by Hans Baumann, saying never go below HALF the pipeline size.

By the way, the media is water at ambient temp.


anyone have an idea what the right answer or rule is here?

 

[DRWeig]

There is no rule, other than choosing your control valve by Cv, not pipeline size. If the step is huge enough that it will create a turbulent mess at the valve inlet and outlet, do a reduction for 10 or 20 pipe diameters upstream and downstream. To minimize, look into a valve with reduced port or trim features.

Often, pipes are sized for future flows in my experience. I can count dozens of pipes in which the valve size was half or less to achieve proper authority and good control. We generally do the lengthened reducers when the size will be 2/3 or less on larger pipes, but not at all on 1-1/2" and smaller. The few feet of higher-velocity water makes a nice place to stick a flow meter too.

Try this book: Fisher Control Valve Handbook

Rather than a rule-of-thumb or a limit on valve vs pipe size, the methods and equations are in there to determine the effect of reducers and such on performance.



Best to you,

Goober Dave

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[LittleInch]

I agree with all of Daves very good response. Choosing a control valve, like many tings is a compromise between competing aspects and all depends on your aprticualr pressure and flow requirments. The more there are in terms of set points the harder it is to choose a correct valve which does all the things required well.

If a pipeline has been designed and operater in the most econoimc way for velocity etc, then, generally the control valve will be at least one pipe size and oftern two smaller, but it just depnds what you're trying to do. A triple offset valve will work well betwene certain limits but is still a fairly crude control valve and will create a lot of noise and turbulence at certain opening points, but fully open will have a very low resistance to flow.

The hard part for a CV design is trying to spec all the reasonable cases that the valve will operate for ilet / outlet pressure and flow rate. Then just accept what the vendors give you to do the duty.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[valves4life]

Thannk you Gentlemen.


I understand the sizing limitations on a Triple Offset, and its benefits. This Q was more of a pipleine vs valve size rule. I'm glad to hear there is none. I guess I was told that 2 pipe diameters rule very early on, and for some reason it stuck.


Thank you for your replies,


John