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control valve size

control valve size

control valve size

(OP)
why control valve size is normally 1 or 2 size smaller than pipe size?

Thanks you for the answers!

RE: control valve size

Because a control valve makes its living creating pressure drop and the object is usually to make plenty of it anyway, so why not use a smaller pipe diameter to help it do its job. Besides the fact that a smaller valve is cheaper.

you must get smarter than the software you're using.

RE: control valve size

If this is for new pipeline design, it should be understood reduced port valves may limit some future "pigging", if desired for various purposes including pipeline condition/damage assessment etc (e.g. http://en.wikipedia.org/wiki/Pigging).

RE: control valve size

To some extent this practice is a carry over from the past when energy was cheap. As a generalization (dangerous, I know) the higher the pressure drop across the valve the more responsive is its control action. The rule of thumb was to allocate around 30% of the system pressure drop to the control valve, and this was best achieved by making the valve a size or two smaller than the line.

By avoiding rules of thumb and applying more mental energy than pumping energy it is often possible to get away with less than 30% across the valve. And in many applications in the industries where I work the control valves are being eliminated completely and are being replaced by variable speed drives on the motors. In the end everything is driven by economics, and the economics have changed over the last 40 years - and continue to change.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: control valve size

Because we live in a commercial capitalist world in the main. If I can give you a valve which does your duty and is cheaper because it's two pipe sizes smaller then I'll get the order. A lot of time you're trying to minimise pressure drop / velocity in your lines so they are bigger than they might need to be, but a control valve is the opposite. It all depends what your minimum pressure drop is at highest flow - this is what drives the sizing of the body.

Rconner is correct in terms of reduced port ball valves and gate valves / plug valves, but nearly all control valves are not piggable regardless of the size.

My motto: Learn something new every day

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

RE: control valve size

A lot of valves in vapor or gas service are deliberately downsized to avoid an excessive release rate of gas into lower pressure downstream systems in a control valve failure case, which otherwise would have the effect of needlessly driving up relief valve, tank vent and flare system sizes. The answer to most control valve sizing usually is found in the drawings and in the system or connected systems that are often quite a ways away from the immediate vicinity of the valve.

RE: control valve size

Given that so many control valves are of the globe body style, it should come as no surprise that they are not piggable!

Piping Design Central

RE: control valve size

Only once!

you must get smarter than the software you're using.

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