Specifying gas valves for wide duty range 1

[Jobst]

Hello Everybody,

I have a case where a valve is needed for regulating the pressure inside a process. The issue is that the pressure setpoint(and flow required to maintain this pressure) varies quite a lot. The valve(s) is installed downstream of the process, and the process itself has inlet flow controlled.

The minimum flow is on the order of 10% of design flow, max flow 120% of design. Pressure setpoint can vary anywhere between 3 barg to 8 barg (but not likely high pressure at low flows).

So far my approach has been to identify the range of design points as "max flow at min pressure" and "min flow at high pressure". I'm then following the Emerson/Fisher control valve handbook (4th edition) to calculate a Cv and choose a valve for each case. Then to get an idea of the useful range of each valve I've looked at the Cv for the valve chosen at 20%, 50% and 80% open.

The two valves I've ended up with seem to be able to do the job, the smaller 20%-80% Cvs being 0.15-0.83, the larger 0.85-7.07. Both are equal percentage characteristic. I'm a bit worried that there might not be enough overlap here.

Being a bit of a rookie at this I'm mainly looking for feedback on the method used, and any issues to hold in mind when specifying a dual valve system like this. Some of the "unknown unknowns" that a newbie like me might run into.

Thanks for any advice.

 

[steamdog]

You can do interesting things with the controls that you may have with these two valves, like what we do here in NYC steam stations.
The basic idea is first to consider that most globe style valves have the best control between about 15% and 80% open. So we size the small valve so that your minimum flow would mean the valve would be no less than 10% open (how often is there really a calculated minimum??). Then size the other valve for the full 100% maximum flow at 80% open.
Now for the controls, when the small valve is 80% open, shut the small valve and open the big valve. Then, when the big valve is 20% open, close the big valve and open the small valve.
This works great for us, making sure neither valve is controlling close to its seat, which wears the valve, or above its optimum control range.

 

[gerhardl]

Following on valve selection:

a) The best test for all valve selection is to give a complete perofomance requirement description, including pressures before and after valve, fluid and flow descriptions, temperatures, operation, certificates and tests, actuation and operation details, closing and opening times etc. etc. In addition give a sketch of required deliverytimes, expected lifetime and number off.

b) Second step ask two or three quality valve suppliers for a preliminary offer including alternatives and suggestions.

c) Base your official inquiry on what you have gathered of information on commercial available solutions on this.

d) On the small sizes you mention a standard valve with variated trim or special types may cover your total requirement.

Following on regulating purpose:

aa) In processes regulating requirements are often set 'too fine' complicating the process control. Be sure that the valves actually are necessary. (are inlet control alone enough, combined with some overpressure limitiation device, for instance?).

bb) Same as above: check other cheaper solutions for control

cc) I have seen alarmpoints in processes set too fine, or to react too soon, and process extra controls then required not to release the (not needed) fine alarmpoint. Be sure this is not the case.

dd) I have on steam seen unnecessary double controls, for instance outlet and inlet, working against each other, keeping heat exchange surfaces half drowned in condensate in stead of free against steam, reducing heat transfer to much less then half capacity. Any similarity?

ee) Possible swinging curve in regulation if inlet and pressure control not properly damped or tuned too fine?

 

[Jobst]

Thank you both for your comments.

2x equal percentage valves cover the range without going outside the 15-80% opening window so I'm happy about that.

Doing a bit more analysis (part a of gerhardl's suggestions) shows the duty is not as bad as I first thought, as the downstream operation is pressure controlled as well, leading to a fairly stable dP across the control valves..

A very good list gerhard, unfortunately one of the problems I often have (not just with valves) is getting a good spec from whoever has made the request!

 

[steamdog]

Very funny Jobst!!