"Installed Valve Characteristic" vs "inherent valve characteristic" is it compar

[ManTicora]

Is the a rational explanation why DP across a given valve at a given flow rate and opening will be different than when this same valve is installed in a system with Pump, Elbow and restriction?
I'm trying to comprehend the notion of "Installed Valve Characteristic" vs "inherent valve characteristic". To me those advocating for such a concept are comparing apple to peanuts....
Thanks for your contributions

 

[LittleInch]

For an installed valve the flow path is not the smooth homogeneous flow that you get in a lab / test facility. Unless you have long lengths upstream of straight full bore pipe (>20D) or a flow straightener in front of your valve, you will have additional swirl, eddies and non linear flow impacting on the valve body and internals creating a different pressure drop than the perfect flow.

Plus all valves are manufactured slightly differently than its neighbour so the installed valve may differ from the theoretical.

My motto: Learn something new every day

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

 

[gerhardl]

For common type valves at low flow the two definitions may not differ much in practical use. For more specialised valves, for instance regulating or throtteling valves, usually allowed used at larger ranges, the definition is more important, as small differences will multiply over the range.