Bench set is a no-load measurement
The spring range stamped on an actuator — 0.2 to 1.0 bar, 3 to 15 psi — is measured on a bench with the actuator separated from the valve and nothing opposing the spring. It describes the spring, not the installation.
Fit that actuator to a valve carrying process pressure and the plug experiences a force trying to push it off its seat. The spring now has to overcome that as well, so the whole operating range shifts upward.
Worked example
0.2–1.0 bar bench set, 500 cm² diaphragm, unbalanced 50 mm plug, 10 bar shutoff differential:
- Unbalance force = 10 × 10⁵ × (π/4) × 0.05² = 1,963 N
- Shift = 1,963 ÷ (0.05 m² × 10⁵) = 0.393 bar
- Installed range becomes 0.59 – 1.39 bar
- With 1.4 bar supply, there is almost nothing left — the valve will just reach full travel and nothing more
That last line is the whole point of the calculation. A valve that strokes beautifully on the bench can fail to seat, or fail to reach full travel, once process pressure is applied — and the symptom looks like a positioner fault rather than a sizing one.
Why balanced plugs exist
A balanced plug has passages that let pressure act on both faces, so most of the unbalance force cancels — typically leaving around 10% as residual. In the example above that would reduce the shift from 0.39 bar to about 0.04 bar, and the valve would operate essentially at its bench set.
The trade-off is seat leakage. Those balancing passages need a seal, and that seal is what limits a balanced plug to lower leakage classes unless soft seats are used — see the seat leakage class calculator. Tight shutoff and low actuator thrust pull against each other.
Field notes
- Supply pressure must exceed the installed range, not the bench set. A regulator set at 1.4 bar is ample for a 0.2–1.0 bar bench set and marginal for the same valve in service at 0.59–1.39 bar. Check the supply holds up at flow — see instrument air sizing.
- Seat load is force per unit of seat circumference, and metal seats need considerably more of it than soft seats. The manufacturer states the figure; it is what determines whether the specified leakage class is actually achieved in service.
- Fail position depends on this working. A fail-closed valve whose spring cannot overcome process pressure does not fail closed — it fails somewhere in between, which is usually the worst place to be.
- Springs relax with age and heat. A valve that seated fine at commissioning and now passes slightly may have lost spring force rather than seat condition.
- The signal still arrives as 4-20 mA. The I/P converter maps it to 3-15 psi before the actuator ever sees it — see the I/P scaling calculator, and the characteristic calculator for what happens to the travel in between.
Frequently asked questions
What is bench set on a control valve?
The spring compression range, measured with the actuator off the valve on a bench and no process load applied. A 0.2 to 1.0 bar bench set means the stem starts moving at 0.2 bar and reaches full travel at 1.0 bar with nothing opposing it.
Why is the installed spring range different from the bench set?
Once the actuator is on the valve, process pressure acting on the plug and the seat load both oppose the spring. The operating range shifts upward by the equivalent pressure of those forces, which is why a 0.2–1.0 bar bench set can operate at 0.6–1.4 bar in service.
How do I calculate actuator thrust?
Thrust equals signal pressure times effective diaphragm area. A 500 cm² diaphragm at 1.0 bar produces 5,000 N. Subtract the spring force at that travel to get the net force available for seating.
What causes a valve not to close fully?
Usually insufficient seating force: the actuator cannot overcome process pressure pushing the plug off its seat. The cure is a larger actuator, a higher spring range, higher supply pressure, or a balanced plug that removes most of the unbalance force.
Simplified calculation for reference and education; ignores friction, packing load and dynamic effects. Use the manufacturer's actuator sizing data for selection. See our disclaimer.