Q & A > Question Details
- I have seen most of the cases Control valve( CV) with 1or 2 sizes less than the pipe size and with Reducer and expander u/s and d/s of CV.
- Is there any reason to select lower size CV?
- What are advantages of Reducer and Expander of piping u/s and d/s of CV
- If i need to select CV size is same with the pipe based on Valve coefficient,what is the the impact Of "no reducer and expander"on CV performance.
08/10/2013 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
As the other answer indicated, the different criteria used for sizing piping and control valves, commonly results in a smaller CV size than the pipe diam. The control valve is designed to pass the desired amount of flow over the range of expected operating conditions and to operate in a range where changes in valve position result in changes in flow. If you arbitrarily select your CV size to match the line size, you will typically get an oversized control valve with very poor turndown characteristics as well as paying more than necessary for the valve. The valve will operate almost fully closed even when processing the maximum required flow.
If you arbitrarily pick the line size to match the CV size, then you will typically end up with an undersized line that has excessive pressure drop and limited capacity.
03/10/2013 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
For a new design, criteria for sizing pipe diameter between items of equipment are based on economics, except where minimum or maximum velocities are involved for special situations. Similarly, criteria for sizing control valves are used. As you have pointed out, this often results in a CV and the “line size” being different. Using reducers become necessary, and a decision to match the block and bypass valve sizes with the smaller size is a matter of economics and has no effect on performance.
For revamp situations, you do what you can to make it work O.K., sometimes leading to unusual appearing designs.