Installation of control valve

[PW2006]

Some control valves, like Samson model 3241, strongly recommend that the pipeline upstream and downstream of the control valve should be straight and free of obstructions for a length of at least 6 times the pipe diameter (DN).

My questions are:
(1) Is this a common rule that people do follow? We are not quite aware of it and wonder how worse it would be if we don't have such long straight pipes at both ends of the control valve.

(2) For example, if the control valve size is 2", the pipe size is 3" and we need to install 3x2 reducers at both sides of this control valve, in order to follow the above rule, do we (a) install the 3"x2" reducers right next to the control valve and make 3x6=18" long straight 3" pipe at both sides of the 2" control valve, or (b) install 2"x6 = 12" long straight 2" pipe at both sides of this 2" control valve, and install the 3"x2" reducers 12" away from the control valve?

(3) If we need to install a fully open gate valve within this 6xDiameter straight pipe length, is it OK or do we have to exclude it when we count the 6xDiameter straight pipe length?

Thanks in advance for your replies.

 

[JLSeagull]

ISA used to have a control valve manifold standard. That standard was withdrawn.

It is normally good advise to follow the manufacturer's instructions but the six diameter requirement seems wrong.

I have never heard or read of a requirement for 6 diameter straight run upstream or downstream of a control valve. This would be very uncommon in the refineries, chemical plants and offshore platforms where I have worked. Perhaps I would disregard the Samson criteria of six D up and down. However, piping designers lay out the piping and I am not involved much. I check isometric diagrams for most head type flow elements, accessibility, etc. and not control valve manifolds.

 

[PW2006]

Thank you for your quick answer. I kind of agree with you.

I have talked to Samson and they said that shorter straight pipe length should also be acceptable but not what they would recommend. The control accuracy may suffer due to this. But they were not be able to give any detail explanation.

 

[Mass44]

Hi there,

You should really just look at the process line to decide how many pipe diameters will be acceptable. Six on both sides is playing it completely safe but look for yourself and estimate what the flow would look like inside the pipe. For instance directly after a bend in the line or close to a pump or a reduction in pipe size or something like these will cause the flow to become turbulent so you need to get as far away as possible from these. I would say not closer that 3 to 4 pipe diameters.