The short answer. A pressure switch starts the compressor at its cut-in pressure and stops or unloads it at its cut-out pressure. The gap between the two is the differential. Cut-out must never exceed the machine’s rated maximum pressure, and the safety valve must sit above cut-out but at or below the receiver’s design pressure. A safety valve that keeps lifting is never the fault itself: it is telling you that pressure went higher than the control should have allowed. Find out why before anyone touches the valve.
Who this page is for: plant engineers, maintenance supervisors, workshop and garage owners, and service technicians who set pressure switches on piston and small screw compressors, or who are dealing with a safety valve that lifts, hisses or chatters.
Key facts
- Cut-out is the pressure at which the compressor stops (start-stop control) or unloads (load-unload control). It must not exceed the maximum working pressure on the compressor nameplate.
- Cut-in is the pressure at which it restarts or reloads. Differential = cut-out minus cut-in.
- Typical differentials: about 1.5 to 2 bar on start-stop piston units, about 0.7 to 1 bar on load-unload screw machines. Narrower is not “tighter control”; it means more starts and more wear.
- The safety valve set pressure must be at or below the design pressure of the vessel it protects, and should normally be at least about 10 % above cut-out, and never less than about 0.5 bar above it, so that it does not simmer in normal running.
- Gauges drift. Before adjusting anything, check the working gauge against a reference gauge. An error of 0.3 to 0.5 bar on a ten-year-old dial gauge is common.
- Exact conversions used on this page: 1 bar = 14.5038 psi, so 7 bar(g) = 101.5 psig, 8 bar(g) = 116.0 psig, 10 bar(g) = 145.0 psig, 12 bar(g) = 174.0 psig.
How the switch, the controller and the safety valve work together
There are three layers of pressure protection on a typical package, and they must be set as a stack, never one at a time.
- The control layer sets normal running. On a piston compressor this is usually a mechanical pressure switch that stops the motor at cut-out and, through an unloader or a pressure-release valve, vents the delivery line so the motor can restart without load. On a screw compressor it is an electronic controller reading a pressure transducer, closing the inlet valve to unload and opening it again to reload.
- The high-pressure trip on screw machines, a controller alarm set a little above unload pressure. It shuts the machine down if pressure keeps rising after unload.
- The safety valve, a spring-loaded valve on the receiver, the separator vessel or between stages, which opens when everything above has failed.
The order must always read, from bottom to top: cut-in → cut-out → high-pressure trip → safety valve set pressure → vessel design pressure. If any two of these are too close together, the higher one will start doing the lower one’s job. That is exactly what a lifting safety valve usually means.
Typical settings by machine type
The table gives starting points for common Indian installations. Always confirm against the compressor nameplate, the receiver nameplate and the controller manual. Where they disagree with this table, they win.
| Machine | Control | Cut-in / load | Cut-out / unload | Differential | Safety valve, typical |
|---|---|---|---|---|---|
| Single-stage piston, workshop, 8 bar rated | Start-stop switch | 6 bar(g) (87.0 psig) | 8 bar(g) (116.0 psig) | 2 bar (29.0 psi) | About 8.8 to 9 bar(g) (127.6 to 130.5 psig), not above receiver design pressure |
| Two-stage piston, 12 bar rated | Start-stop switch | 10 bar(g) (145.0 psig) | 12 bar(g) (174.0 psig) | 2 bar (29.0 psi) | About 13.2 to 13.5 bar(g) (191.5 to 195.8 psig), receiver rated at or above this |
| Two-stage piston, garage or tyre shop, 10.5 bar rated | Start-stop switch | 8.5 bar(g) (123.3 psig) | 10.5 bar(g) (152.3 psig) | 2 bar (29.0 psi) | About 11.5 to 12 bar(g) (166.8 to 174.0 psig), not above receiver design pressure |
| Screw, 7.5 bar rated | Load-unload controller | 6.5 bar(g) (94.3 psig) | 7.5 bar(g) (108.8 psig) | 1 bar (14.5 psi) | Separator vessel valve per maker, commonly 1 to 1.5 bar above maximum working pressure |
| Screw, 10 bar rated | Load-unload controller | 9 bar(g) (130.5 psig) | 10 bar(g) (145.0 psig) | 1 bar (14.5 psi) | Separator vessel valve per maker |
| Variable speed screw | Set-point with speed control | Set-point, e.g. 7 bar(g) (101.5 psig) | Stop above set-point, typically +0.5 to +1 bar | Small band around set-point | Per maker |
Two rules sit behind every row. First, the cut-out cannot exceed the machine’s rated pressure, because the motor is sized for the power needed at that pressure, and the airend or valves are designed for it. Second, the plant rarely needs as much pressure as people set. Every 1 bar (14.5 psi) of unnecessary pressure costs roughly 6 to 7 % more energy on a typical machine, so set cut-out for the lowest pressure the tools at the far end of the line actually need, plus the measured pressure losses in between. Our pressure budget guide shows how to work that out.
Why the differential matters more than people think
The differential decides how often the motor starts. On a start-stop piston machine, the time between starts comes from the air stored between cut-in and cut-out in the receiver and the gap between compressor delivery and plant demand. Halving the differential roughly doubles the number of starts per hour.
Small induction motors are usually limited by their makers to somewhere around 6 to 15 starts an hour, depending on size and design. Exceed that and the starting current heats the windings faster than they cool, contactor tips burn and belts take repeated shock loads. If a machine is starting more often than its motor allows, widen the differential, add receiver volume, or move to load-unload control. Do not simply narrow the switch band to “keep pressure steady”.
On load-unload screw machines the differential sets how often the inlet valve cycles and how long the machine spends unloaded. A very narrow band of 0.3 to 0.5 bar makes the machine hunt between load and unload, which wears the inlet valve and the blowdown valve. See our page on controls and sequencing for multi-machine plants.
Worked example (illustrative): differential and starts per hour
A two-stage piston compressor delivers 0.6 m³/min (21.2 cfm) of free air into a 500-litre receiver. Cut-in is 10 bar(g) (145.0 psig), cut-out 12 bar(g) (174.0 psig), so the differential is 2 bar (29.0 psi). Plant demand averages 0.3 m³/min (10.6 cfm).
- Free air stored between cut-in and cut-out = receiver volume × differential ÷ atmospheric pressure = 0.5 m³ × 2 bar ÷ 1.013 bar = 0.987 m³ of free air.
- Pump-up time (compressor running, plant still drawing air) = 0.987 ÷ (0.6 − 0.3) = 3.29 minutes.
- Draw-down time (compressor stopped) = 0.987 ÷ 0.3 = 3.29 minutes.
- One full cycle = 6.58 minutes, so starts per hour = 60 ÷ 6.58 = 9.1 starts per hour.
Now someone narrows the switch to 11 to 12 bar(g) (159.5 to 174.0 psig) “for steadier pressure”. The stored air halves to 0.494 m³, each half-cycle drops to 1.65 minutes, and the motor now starts about 18.2 times an hour, likely beyond what a motor of this size is meant to do. The plant gains nothing it can measure, and the motor, contactor and belts pay for it. The correct fix, if steadier pressure is genuinely needed, is a regulator at the point of use or a larger receiver; the receiver storage calculator will size it.
Setting a pressure switch safely
Pressure switches are adjusted live, with the system under pressure and often with the cover off near mains terminals. That combination is how people get hurt. Follow a fixed method.
- Isolate and lock off the electrical supply before removing the switch cover. Read the switch type from its label: range spring, differential spring and whether the differential is adjustable at all. Some small switches have a fixed differential.
- Fit a reference gauge at the receiver, next to the working gauge. Note any difference; adjust against the reference, not against the old dial.
- Know which screw does what. On most mechanical switches the main range screw moves cut-in and cut-out together, and the differential screw moves only one of them: cut-out on some designs, cut-in on others. The switch’s instruction sheet or label says which. Set the range first, then the differential, in small steps of a quarter turn at a time, and replace the cover before each test.
- Restore power and run the machine up under observation. Record the actual cut-out on the reference gauge. Bleed air down slowly through a drain or a tool and record the actual cut-in.
- Repeat three cycles. A good switch repeats within about 0.1 to 0.2 bar. A switch that wanders by more than that is worn and should be replaced, not re-set.
- Check the unloader on piston machines: when the motor stops, a short hiss from the unloader or pressure-release valve should vent the delivery pipe. If it does not, the next start will be against pressure.
- Record the final settings on the machine’s record sheet with the date. Our compressor log book format has space for it.
On screw machines with electronic controllers, load and unload pressures are parameters, often protected by a service password. Changing them beyond the machine’s rated range is blocked by good controllers, but not by all. Never raise unload pressure on a screw machine above the nameplate value, and never change the high-pressure trip to stop it alarming.
When the safety valve keeps lifting: a diagnostic sequence
A lifting safety valve is a symptom, as our page on safety valve identification and service boundaries explains. Work through the causes in this order, because it moves from the quickest checks to the most invasive, and from the most common causes to the rarest.
| Step | What you see | Most likely cause | Check |
|---|---|---|---|
| 1 | Valve lifts at the top of every cycle, gauge reads above the cut-out you expected | Pressure switch drifted high, or wrong setting | Reference gauge at the receiver; watch the actual stop or unload pressure over three cycles |
| 2 | Valve lifts while the gauge shows normal pressure | Working gauge reading low, or the valve set too close to cut-out, or a weak spring | Reference gauge; compare valve set pressure on its tag with the cut-out; check the margin is about 10 % of cut-out and not less than 0.5 bar |
| 3 | Screw machine: pressure keeps rising after it “unloads” | Inlet valve not closing fully, sticking piston or worn seal, faulty unload solenoid | Watch the pressure trend after unload; listen at the inlet; check the solenoid coil and the inlet valve service history |
| 4 | Piston machine: motor stops but pressure creeps up for a few seconds, or the valve lifts on restart | Unloader or pressure-release valve not working, leaking non-return valve | Listen for the vent hiss at stop; check the non-return valve between the compressor and the receiver |
| 5 | Separator-vessel valve on a screw machine lifts, plant pressure normal | Blocked separator element, minimum pressure valve sticking, or closed outlet valve | Separator differential pressure reading; check the outlet valve and the minimum pressure valve |
| 6 | Valve lifts when a downstream dryer or filter loads up, or after a valve is shut | Blocked filter element, iced or blocked dryer, closed isolation valve between the compressor and its receiver | Differential pressure across each filter and the dryer; walk the line for closed valves |
| 7 | Interstage valve lifts on a two-stage piston machine | Leaking high-pressure-stage valves, blocked intercooler | Interstage pressure against the maker’s value; head temperatures |
| 8 | Valve lifts but nothing above explains it | Wrong valve fitted, set pressure below cut-out, or a valve that has been tampered with | Read the tag and seal; compare set pressure and capacity with the vessel and compressor |
Two points need stressing. First, a valve that has lifted many times may no longer reseat cleanly; once the real cause is fixed, have it bench-tested or replaced, because a leaking safety valve wastes air continuously. Second, the replacement must match set pressure and relieving capacity: a valve with the right thread and the right pressure but too little capacity cannot protect the vessel if the inlet valve fails fully open.
Exceptions and cautions
- Never gag, plug, wire down or screw down a safety valve, and never raise its setting to stop it lifting. Doing that removes the last protection against a vessel failure.
- Receivers carry a design pressure on their nameplate. If the plate is missing or illegible, treat the vessel as unknown and have it assessed before raising any pressure. See receiver inspection and safety.
- Older receivers that have lost wall thickness to internal corrosion may need a lower working pressure than the original plate shows. Inspection findings override the plate.
- When two compressors feed one receiver, the safety valve must be sized for both machines delivering at once.
- Some screw machines use a pressure-relief valve set by the maker inside the package. Its setting is part of the package design; do not change it to match the plant.
Pressure settings record and safety valve check sheet
| Item | Nameplate or tag value | Measured on reference gauge | OK / action |
|---|---|---|---|
| Compressor maximum working pressure (bar g) | — | ||
| Receiver or vessel design pressure (bar g) | — | ||
| Cut-in or load pressure (bar g) | |||
| Cut-out or unload pressure (bar g) | |||
| Differential (bar) | |||
| High-pressure trip, screw machines (bar g) | |||
| Safety valve set pressure (bar g) and capacity | — | ||
| Margin: safety valve set minus cut-out (bar), target about 10 % of cut-out, never below 0.5 | — | ||
| Working gauge error against reference (bar) | — | ||
| Starts per hour at normal demand (piston) | Motor limit: | ||
| Unloader vents at stop (piston): yes / no | — | ||
| Pressure rise after unload (screw): none / creeping | — | ||
| Date, hour-meter reading, name |
Common mistakes
- Raising cut-out because the far end of the plant is short of air. The usual cause is pressure loss in pipes, filters and hoses, or a leak. Raising pressure at the compressor hides the problem and adds energy cost.
- Narrowing the differential to “steady” pressure, and doubling motor starts in the process.
- Adjusting against a worn gauge. The switch is then set right to a wrong number.
- Replacing a safety valve with one that only matches the thread. Set pressure and relieving capacity both matter.
- Treating a lifting valve as a valve fault and replacing it repeatedly, while the inlet valve or unloader that causes the over-pressure keeps failing.
- Working inside the switch housing with the supply on.
How Reddy Compressors helps
Setting a pressure stack in the right order is routine work for us: Reddy Compressors has sold, rented, serviced and supplied spare parts for compressors of all makes since 2002, as an independent multi-brand business. Our service engineers set and verify pressure switches and controllers against a reference gauge, trace the cause of lifting safety valves, and replace inlet valves, unloaders, non-return valves, switches and safety valves matched to the vessel and the machine. Where a machine has to come out of service, our own rental fleet, from 50 to 2,000 cfm (about 1.4 to 56.6 m³/min) and 2 to 30 bar, can keep air on while the repair is done. We work across Telangana, Andhra Pradesh and the districts within about 300 km of Hyderabad, and supply across India.
What to send us
- Photographs of the compressor nameplate, the receiver or vessel nameplate, and the safety valve tag.
- A photograph of the pressure switch label or the controller’s pressure settings screen.
- The pressures at which the machine actually stops or unloads and restarts or reloads, and the pressure at which the valve lifts, if you have seen it.
- When it started happening, and anything that changed: a new dryer, a filter change, a different machine on the line.
Send them through our compressor repair page or on WhatsApp to 99590 60718 or 92468 88812.
Frequently asked questions
What should the differential be on a piston compressor pressure switch?
Usually about 1.5 to 2 bar, for example cut-in at 6 bar(g) (87.0 psig) and cut-out at 8 bar(g) (116.0 psig) on an 8-bar workshop machine. A narrower band means more motor starts per hour; a wider one means bigger pressure swings. Check that the resulting starts per hour stay within what the motor maker allows.
How much higher than cut-out should the safety valve be set?
Normally at least about 10 % above cut-out, and never less than about 0.5 bar above it, while staying at or below the design pressure of the vessel it protects. Too small a margin makes the valve simmer or lift in normal running.
Why does my safety valve hiss even when the compressor is stopped?
Either the receiver is at or above the valve’s set pressure, which means the control stopped too late, or the valve no longer reseats because its seat is damaged or dirty. Check the pressure against a reference gauge first. If the pressure is normal, the valve needs testing or replacement.
Can I raise the cut-out pressure to get more air at the end of the line?
Only up to the machine’s rated maximum, and it is rarely the right fix. Low pressure at the far end usually comes from pressure losses or leaks. Raising pressure at the compressor costs roughly 6 to 7 % more energy for every bar.
Why does pressure keep rising after my screw compressor unloads?
The inlet valve is not closing fully, or the unload solenoid is not working. Air keeps entering, and pressure climbs until the high-pressure trip or the safety valve acts. Have the inlet valve and solenoid checked before running the machine again.
Is it safe to adjust a pressure switch myself?
Only if the supply is isolated before the cover comes off, adjustments are checked with the cover back on, and the setting stays within the machine’s and vessel’s ratings. If the switch does not repeat within about 0.2 bar over three cycles, replace it.
Related pages
- Safety valve identification and service boundaries
- Air receiver inspection and safety
- Compressor troubleshooting and diagnostics
- Working pressure and pressure budget
- Receiver storage calculator
- Inlet and unloader valves
- Piston compressor maintenance
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 5 November 2026.