Control valve range

[Drexl]

Hi,

Does anyone know any respectable source for range (of travel or Kv/Cv) in which different types of control valves can perform well.

One rule of thumb i've heard is 10%-90% of travel, but i'm looking for something more comprehensive on the topic.

best regards
Drex

 

[stanier]

I would get hold of the Fisher design guide.

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf.

It is just shy of 300 pages.

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[stanier]

You certainly would use that wide criteria for all types of valves used in control. e.g. butterfly valves.

You need to engineer the system. A 10% open valve could well cavitate in one system and not another. Also do you have a range of conditions and the odd one is for a very short period and thus you can operate outside the norm for a short while.

If your fluid is solids bearing then the operating range could be totally different.

“In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be."

Lord Kelvin [PLA, 1883-05-03]


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[Drexl]

Thanks for the answer. Unfortunately the Fisher handbook does not comment on recommended control range (at least what I have seen).

I would say problems like cavitation and capacity is a different question that has to be investigated case by case. I'm more interested in general range in which the control valve can still control with some reasonable accuracy.

best regards
Drex

 

[gerhardl]

Hello Drexl and stanier!

I see that the Fisher handbook actually defines a control valve with what we in Europe normally would call a regulating valve.

If then your question is valid for all types of control/regulating valve constructions, all sizes, all pressure classes, all fluids etc, I would go straight for staniers answer:

You will always have to select a control valve from the flow (lower, average and max), fluid type, pressure (and delta p) and necessary response time.

I could also add that for most 'common' applications you would probably find special valves adapted to regulate from 90-95% opening down to 10-5% opening, if it is suited for the flow conditions you have.

Some specialized control valves will regulate from 2-5% opening (needed to be free from the seat) and up to (almost) full opening.

Quite another thing is if you can utilize the full control range of the valve! For butterfly valves, in a typical application, often only a smaller part of the range (say 20% of the opening range) will actually be used.

My advice is like staniers: put your efforts into finding the requirements and actual flow, and regulating parameters. Quite often this is neglected or overseen or not given enough engineering thought, ending up with wrongly selected control valves, often both type and size.

With correct information, any good supplier of control valves would be able to help you.

If anything goes wrong with a control application, uneven regulation, breakdown or eccessive wear or noise, you can be 100% sure that engineering/selecting and/or installation is not done properly.

Any selection of a control valve should include full analysis of flow, delta p and cavitation curve under all actual flow conditions. (And if your supplier cannot give you this, drop the supplier and find one better!)

PS: regulating speed is very often set too fast in a regulating circuit, giving undamped swingings and killing valves and actuators.

 

[stanier]

If all fails get your minimum, normal and maximum operating conditions defined and go along to your local control valve purveyor and get them to size it for you. After all that is what most highly paid consultants do before they charge their customers a fortune.

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[Drexl]

Thanks for the valuable answer. In this case the valve is 20 years old and the manufacturer don't exist anymore. The change in process values is evaluated as precise as possible and if needed we can modify the valve seat and plug. The valve is a linear globe type valve.

 

[gerhardl]

Advice: ask several (2-4?) reliable suppliers, giving all data and also available info and materials on existing valve and ask for advice/offer for new valve, type and materials, and expected lifetime and necessary maintenance. Also ask for alternatives regarding quality and technical comments.

 

[zdas04]

It used to be common for valve suppliers to publish a %opening vs. %of flow graphs, but I haven't seen them in a while. You might just be able to find one for a 20 year old valve.

David.

 

[stanier]

Drexl,

What you are searching for doesn't exist and that is why reference isn't made in respected texts. There is NO rule of thumb for % travel.

What would you do with this information anyway? It is meaningless, irrelevant and frivolous.

There are instances where a control valve could be closed or open for legitimate process reasons.

Remember the maxim " All blackbirds are black birds but not all black birds are blackbirds"!

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[katmar]

I am absolutely with stanier on this one. The valve cannot be considered separately from the overall system. LIke stanier, I have seen control valves legitimately operate anywhere between 0% and 100%.

With a 20 year old valve you may have wear and backlash that make it unstable at any degree of opening, so any generic answer would be questionable in your situation.

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[gerhardl]

..... and I am also ending ending with still agreeing with stanier!