split range flow control valve size issue

[koba78]

Dear eng-tips members

The 14 tons of hot water supply enter to the reboiler and
return through two valves in parallel,and the pipe size is 8" the hot water return header.
And, the split range control is used here to control flow
in a broad range.

Our leader suggest that the low range valve is changed from 8" to 6"and high range valve is set to be 8".
He said that the low range valve should be smaller size than high range valve size.

But the flow meter FT located on the upstream 8" pipe is common to both valves. so is it necessary to change to low range valve size to be smaller 6"?

And, usually the smaller valve is sized to 1/3 of the system capacity, and large valve is twice as big so is that means I
have to size the smaller valve based on the 1/3 flow capacity?


Thank you very much.

 

[hsbcn]

What kind of valves are you considering?

My focus: Alloy Valves Super Duplex Valves Monel Valves Incoloy Valves [link titanium-valves.com]Titanium Valves[/url] Hastelloy Valves

 

[gerhardl]

Your leader is theoretically correct: usually you design a split range system with two different valve sizes because you have two different variable working situations (each changing and variating over time): one needing a lower range capacity, the other a larger capacity.

Each single regulating valve (also for parallell mounted valves in a split-range system) is always dimensioned to give optimum regulating conditions for the given flow.

Pipelines are also dimensioned for flow.

Eg.: each valve must be dimensioned for pressure before the valve, pressure after the valve (delta p for all working situations), flow range and fluid condition, and selected accordingly with suitable Cv and size.

The trim for a single dimension of regulating valve can vary (fabricated with different sized trim/inner parts).

The answer to your question is hence not that straightforward, and can not be answered before all dimensional and working condition details mentioned above is taken into consideration, and the cv of the valve is given.

If your system is working perfectly today with optimum effect (minimum energy consumption) it is not worth the effort to do anything.

If you have difficulties or any components are suspect (or total system is of some age) you ought of course to go through a technical/economical revision of the complete system including controlling size, type and dimension for all components suitabillity after todays working parameters.

PS. Checkvalves in addition?