Pneumatic Amplifiers & Volume Boosters
A pneumatic amplifier (volume booster, relay) is a flow amplifier: it takes the positioner's low-capacity output signal as a pilot and passes supply air to the actuator at a flow rate the positioner alone could never deliver. The signal decides what pressure the actuator should see; the booster decides how fast it gets there.
Why big actuators need one
A positioner's internal pilot moves a few litres per minute; a large piston cylinder can demand hundreds just to stroke once in a few seconds. Without a booster, the positioner saturates trying to fill the volume and the valve crawls — the classic "positioner is fine on the bench, slow in the field" symptom.
Forward and reverse boosters
- Forward booster — on the fill path: multiplies the positioner's increasing output to fill the actuator quickly.
- Reverse booster — on the vent path: dumps actuator volume fast on a decreasing signal.
- Fast-trip installations often parallel a solenoid and quick-exhaust valve instead — the booster serves modulating speed, the solenoid serves the trip.
The tuning problem
A booster in parallel with the positioner can overshoot: the booster fills faster than the positioner's loop expects, the positioner sees overshot position and throttles back, and the pair oscillate against each other. Practical fixes:
- Bypass orifice / acceleration limit — most boosters have an adjustable bypass that bleeds the pilot; tune it so the booster tracks, not leads, the positioner.
- Dead-band type boosters — stay inactive within a small band around the positioner output and only cut in for large demand changes, keeping steady-state control entirely with the positioner.
- Stroking-time verification after tuning — the whole point of the booster is measurable speed, so test it.