Initial Spring Setting - Control Valve 2

[ryldbl]

I have a Fisher control valve and on the front of the actuator is a table listing "initial spring settings (PSI)" for various valve configurations. What does this mean? Does this mean that the actuator spring has been tightened to the point where it takes that much pressure to start the actuator moving?

 

[yogibear1]

I would say, yes you are correct.
Your valve should have come with a manual, however, I have pasted the Control Valve Handbook address from Fisher Controls, if you would like to read more about it.

http://www.documentation.frco.com/groups/public/documents/book/cvh99.pdf

 

[JimCasey]

Other sources refer to this as the "Bench setting"
This is the preload that you have to put on the spring before the valve is installed so that when it is in service at the specified pressure, it starts to open at 3 psi.

You probably have an EZ or another unbalanced valve. The flow comes in under the plug and tries to force the plug to open. 100 psi differential against a 4" plug would make a force of 1256 pounds, and you need another 200 lbf per inch of seat:2513 pounds (total=3769) so in a typical actuator with 150 square inches effective diaphragm you would need a preload of about 25 psi to get class IV shutoff. It is necessary to have either a HUGE actuator or a substantial spring preload to force the valve closed against the process pressure.

Balanced valves (e.g.: ED) have substantially lower requirement for spring preload.

Rotary valves such as Ball valves have almost no process pressure effect on actuator force required.

Butterfly valves handle high differentials easily when closed, but make an enormous dynamic torque when open as a function of velocity....and velocity is a function of differential pressure.

 

[ryldbl]

OK, that makes sense, but how do you measure the preload that you put on the spring before its installed? If have the thing taken apart in pieces and I want to put it back together, how do I know when I have the spring preloaded to XX.X PSI?

 

[bigTomHanks]

ryldbl

I'm in the valve industry and to set our valves we apply a constant specified inlet pressure and adjust the spring to achieve a certain outlet pressure. That outlet pressure is probably the XX.X psi that you see on the valve.

bigTomHanks

 

[JimCasey]

Expanding on what Big Tom correctly says: Setting the preload is pretty simple. The spring perch is adjustable. If the actuator does not have an adjustment screw, you can adjust how far the valve stem screws into the actuator stem.
With the actuator assembled to the valve, apply the calculated spring preload pressure. if the valve opens, thread the adjuster down until the plug touches the seat. Don't rotate the plug against the seat. If the valve does not open with the specified preload, add enough pressure to topen the valve a bit, then back off on the adjuster a little, and check again.

 

[RR05]

The bench set is correctly set on a valve when the actuator is not attached to the valve body. I would suggest you get a maintenance manual from the Fisher web site to properly follow all the procedures and follow safety requirements. Benchset that is not set correctly can cause valve leakage (to little spring load lets the line pressure lift the plug in closed postion) or not get full travel from valve (spring goes solid stopping travel).

 

[ncstate86a]

Not meaning to beat a issue to death, but the bench set is the air pressure required to move the valve through its full stroke (ie, "open to close" or "closed to open", depending on whether the actuator is a reverse or direct acting actuator. The "bench set" values are based on the fluid system pressures that you provided the manufacturer (ie, Fisher) when you purchased the valve. By knowing your system conditions, the vendor then selects the correct actuator and spring to meet your system conditions and shut off requirements. Be careful not to confuse the bench set values with the spring range, which can also be stamped on the nameplate. The spring range is a nominal range of a spring (3-15, 5 - 12, etc) based on its spring rate and travel and is not a parameter to be utilized when setting up the valve. As one of the previous replys noted, the actautor is setup either on a bench or in the field, but always with the actuator stem disconnected from the valve stem. The the "lower bench setting" air pressure is applied to the actuator and until the actuator just begins to move (the spring tension adjuster is adjusted util travel begins at the lower bench set) Then the travel or stroke of the actuator is measured as pressure is increased util rated travel is attained (should almost exactly at the upper bench setting). After setting up the actuator, the assembly of the valve and actuator can be tested together in the field by using the Flowscanner, which measures things such as packing load, hysteresis, positioner signal etc. Best to call and talk with a Fisher Rep.