Gas Blending Control; Pipe Schedule and Control Valve Size

Any Thoughts on Where to Go for Reference Material and/or Subject Matter Experts for Help with Controls Engineering Project? Is It Mandatory That Pipe Line Schedule Be Mentioned for Control Valve Sizing?

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Kailash Mandal
Kailash_Mandal@fwuk.fwc.com

A: I would stress the fact that the valve rating should be the same as the pipe, independent from valve size. There could be a contractual constraint calling for minimum rating 300# for valves up to a certain size (3" or 4", according to the customer specifications).

Pipe schedule can play a role in determining the velocity in the outlet piping that has some constraints (sometimes contractual or good engineering practice). If the schedule is very high (special thickness for very high design pressure and temperature), the area of the pipe is much smaller than with a lower schedule, with consequent higher velocity. Sometimes, it is not possible to specify a body size equal to the trim size because of too high velocity inside the valve that could produce cavitation, flashing or erosion, thus reducing valve lifetime.

In case of valves with butt welding ends (typical of power industry) the piping and the valve internal and external diameters should match exactly. This is the case where the piping schedule is essential for valve manufacturer.

Alberto Rohr
alcamapa@tin.it

A: I understand that the question concerns the line schedule rather than the flange rating. In most cases the pipe schedule probably won't matter much as the impact on valve sizing will be minimal. However if there is cavitation and/or flashing, then the pipe schedule will impact the approach and exit velocities, and it can have a significant impact on the sizing calculations. It is always better to include the information if possible.

Hunter Vegas
hvegas@att.blackberry.net

A: A DN25 (1NS) control valve can have a valve coefficient of Cv=12 or even higher. Such valve can be installed in DN25 Sch 40 pipe, or even Sch 80. BUT many piping systems specify Sch160 or XXH for DN25 for mechanical strength or your DN25 valve may be in DN50 pipe, chosen for support strength.  In such a case, the bore of the connecting pipe can be limiting the Cv to a maximum value of Cv=6.  In such a situation, I would avoid using DN25 valves, and use DN40 because the price is not much higher, and the chance of embarrassment much lower.  The swage correction practice using the nominal pipe size just doesn't work at the bottom end of the valve size range.

I recently saw a small valve installed which was trying to pass Mach 1.5 through the swage. It failed to do so. The pipe schedule is a significant component in the noise calculation – Sch80 pipe can give 6dB reduction relative to Sch40. So the valve supplier needs to know the downstream pipe schedule (and material) as well as the size. In gas flow, the guideline is to keep the valve body exit velocity below 0.3mach (absolute maximum 0.5mach if using a close coupled noise-reduction restrictor.

Ian H. Gibson CPChem RPEQ FSEng
gibs0108@optusnet.com.au

A: The concern is around the stresses caused by thermal variation and imperfect support which a large pipe can exert on the valve body. One simplistic rule of thumb is a avoid valve bodies sizes that are half or less of the nominal pipe size. This is far too general.

I classify control valve designs as heavy duty, medium duty and light duty. For valves which dissipate high energy, the pipe wall thickness has a significant effect on aerodynamic noise radiation through the pipe. For severe cavitation or severe corrosion, a heavier pipe will stand up for a longer time, but the body material may also make a difference.

As I see it, the pipe schedule has very little effect on the flow capacity of a control valve except in extreme cases, and it is necessary to match the flange classification used.

Cullen Langford PE
CullenL@aol.com

A: The only time the downstream pipe schedule should be considered in sizing of the valve is in predicting the noise level. If the noise level is too high, valve design changes can be made to bring it down. This may only mean the selection of a different valve trim. If the primary noise source is the outlet jet from the valve, increasing the valve size will reduce the noise.

The in and outlet pipe schedule will also impact the weld prep shape for a valve that meets ASME weld prep standards. However, I would not list this as a sizing issue.

Herb Miller, PE
hlmiller37@cox.net

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