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).
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.
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?
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  That standard was withdrawn.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.

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