Valve Characteristics relationship with pipe length 2

I have found x2 comments made on the use of Equal% & Linear control valves on LinkedIn

I cannot understand the relationship between length of pipe & valve characteristics? Secondly per my operational experience the 2nd point is not true as even at very long Cl2 lines I can see Equal% characteristics very dominant using DCS trends of flow & Valve OP%

 

[RVAmeche]

In my experience I think this problem is mostly resolved these days since I think most people model the whole system and review system dPs, valve positions, etc. I think this was a bigger problem when this wasn't done as much and people didn't compensate for shifting dPs across the valve due to other piping losses.

To try to illustrate this I ran two simple flow models:
1) Simple 2 ft of 2" piping with a constant 10 psid across the system and the 2" control valve (linear characteristic).
1) More complicated system with 250+ ft of 2" piping, elbows, misc open manual valves, and a constant 50 psid across the system, and the same 2" control valve (linear characteristic).

You can see that in the "real" system the valve behaves very differently from the first, which is more representative of the standard bench tests for control valves. If you're including the valve Cv data in your model, I think you're fine.

 

[1503-44]

This YouTube video by professor at Uni Colorado on the difference between valve characteristics out of the box and how the installation can affect the valve selection options.topic.

https://youtu.be/f5lyKfhABFI?si=MHa1xiWt3KKt-l4b

Choosing a valve without this knowledge is like buying a pump without drawing the system curve.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."