Oversized Control Valve Issue

[chemE72]

Recently a new engineer oversized a control valve for a water inlet to a funnel pot where no tuning allows for effective control of flow. Usual operating flows would like to be from 15 gpm to 50 gpm, but the valve simply cannot control anywhere near the lower end of the flow requirements. Without replacing the new control valves, can anyone think of a possible solution to this? I am not looking for exact numbers, just suggestions.

I was thinking adding an orifice plate beforehand to reduce the flow to ~60 gpm to increase control. Will this cause any issues?

Thanks

 

[moltenmetal]

Replace the trim (plug and seat) with a smaller Cv trim. Also make sure that you select the correct trim taper- for flow control you want an equal percentage taper rather than a linear taper, as the equal percentage will give you more sensitivity at the low end of the flow range.

A fixed resistance in series upstream such as an orifice plate will increase the ability of the existing valve to control at low flows, but if you size that orifice plate wrong, you won't be able to achieve your maximum flow even with the control valve fully open.

 

[chemE72]

That is what I feared as well. Thank you for the expertise.

 

[TugboatEng]

What is the motive force for the valve? If it's pneumatic, a stiffer spring will give a smaller movement for a given pressure input which may help give better flow control.