Best practice is to have 10 clear straight diameters of pipework of the same nominal diameter before (upstream) and 5 clear straight diameters after the CV.
How many diameters are required before and after Commissioning Valves (CVs) for accurate flow measurement ?
The diameters before the CV are important for flow measurement accuracy and the diameters after the CV maximise system pressure recovery.
When installed with 10 and 5 diameters, other pipework components, i.e. bends/ control valves etc., can be ignored and have no effect on accuracy or system pressure recovery.
Where it is not possible to install with 10 and 5 diameters, they can be decreased to 5 and 3 diameters with only slight reduction in accuracy.
However, where the upstream diameters are decreased to 5 clear straight diameters, it becomes important to consider components that are installed within 5 diameters upstream of the CV.
For example, control valves (with convoluted water passage through them) would have a greater impact on accuracy than a concentric reducer or an elbow which would have a greater impact than a slow bend.
Where the downstream diameters are decreased to 3, almost all the system pressure has been recovered so the change from 5 to 3 has little impact.
Ref - cranefs