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Bench Set, Positioners and Seating Force
Should I care about the bench set of an actuator if using a valve positioner?
By GustavoArg on 1 July, 2018 - 2:02 am

I'm very confused with what I've read so far about the "bench set" of a valve actuator.

The case I'm trying to analyze is an "air to close" globe valve with the main flow helping to open the valve. The bench set specified in the tag is 5-19psi and a stroke of 60mm, but with 19psi it only moves to 45mm. If I loosen the tension of the spring I can make it reach 60mm with 19psi applied but the valve starts to close with less than 5psi.

The question is, should I care about the bench set if I'll use a smart positioner with the valve? Does the bench set need to be corrected?
Who determines the seating force exerted on the shutter of the valve when a valve positioner is used?

In the tag of a valve I find
Range 7-28 PSI. Is this range the spring range?
Stroke 60mm.

I'm sorry if I'm all messed up, I'm new to the field.

Thanks in advance.

My simplistic understanding of bench set is the offset in spring pressure to balance out the pressure of the process trying to force the valve open or closed. There was a time when valves were sold without positioners so bench set was very important in order for the valve to stroke from 0 - 100% signal. The 0 -100% signal converts to 3 - 15 or 6 - 30 PSI

Now days most valves are sold with valve positioners which will increase or decrease the pressure as required to obtain the full valve movement. The 0 -100% signal converts to 0 - 100% Stroke (independent of PSI)

By GustavoArg on 14 July, 2018 - 4:45 pm

Thanks for your reply Roy,

I've found practically that for a 100% signal in an Air To Close valve, the pressure output of the positioner equals the input pressure to it. This gives the seating force I was concerned about.

My concern was that if the positioner only exerted the minimum pressure to make the valve position go to 100% a burst in the process pressure would tend to open the valve, regardless that subsecuently the positioner would force it close again.

For an Air To Open valve, I consider that the bench set is still important because for a 0% signal the only pressure keeping the plug of the valve on the seat is the spring pressure.

Please correct me if I'm wrong.

Best regards.

I believe you are correct. Unless you have enough spring pressure (bench set), a normally closed valve could be forced open by too much line pressure. In the case of a normally open valve, the air pressure needs to be great enough to force the valve closed.

You have obviously given this a lot of thought. Might I suggest you contact your valve vendors to see if they have a course available. There are a lot more tricks to valves than I can remember.