Dear All,

We are using 2 control valves in paralel for controlling the cooling water flow from the heat exchanger. The inlet pressure to the valve is steady at 5.3 barg. The first control valve (Valve-A) has 252155 kg/hr at 80% opening, and the second valve (Valve-B) is designed to deliver 2521550 kg/hr at 60% opening. We have a split range valve setup.

We control flow from 0 - 250000 kg/hr with the Valve-A, then when the flow above 250000 kg/hr, Valve-B start opening (Valve-A still opened at 100%). The small valve response very good when the cooling water is around 200000
kg/hr. But when the cooling water at about 300000 to 400000 kg/hr the cooling water flows erratically.

Can we modify the split range setup with a larger valve as a primary control?

Thanks

Lucky

 

Your bassic control stratagie is OK, but there has to be a little "over-lap" between the two
valves. Make sure that the bigger valve is sized
wright and has the correct CV (Linear or equal%)
I have seen big problems with the application of
butterfly (quick opening) valves in flow applications, since they are highly unlinear.

Good luck

Ps. You will always get better control over the entire 4-20mA range when using split-range, providing you are using the correct valves.

 

In working with parallel valves for air control, I have found that using the smaller valve as fine control and the larger valve as coarse control works best. In a typical application, I will always drive the larger valve to a feedforward position and then regulate the parameter (pressure or temperature) using the small valve.

If you are using hydraulically actuated valves, be certain to apply dither to the control valve to reduce stiction (90 to 150 Hz works well for many valve assemblies).