A compressed-air system needs three devices that cannot be skipped: a relief valve that can pass the compressor’s whole output at its set pressure, a gauge that reads true, and a drain at the lowest point. A relief valve for a 235.6 L/min compressor at 9 bar absolute needs at least 2.2 mm² of open area in the ideal case, and a 10 mm seat at 8 bar gauge is held shut by a spring force of 62.8 N.
Part of Pneumatic Systems
The relief valve, the gauge, the drain and a few simpler devices stop a compressed-air system from exceeding the pressure its weakest part can hold.
A Relief Valve Is a Spring Holding a Seat Shut
A spring-loaded poppet or ball sits on a seat. Air pressure on the seat area tries to lift it, and the spring holds it down until the pressure reaches the set value.
Spring force = set pressure x seat area. For a 10 mm seat at 8 bar gauge:
- Seat area = π / 4 x 0.01² = 7.854 x 10⁻⁵ m² (78.5 mm²)
- Force = 800,000 Pa x 7.854 x 10⁻⁵ m² = 62.8 N
A smaller seat needs a lighter spring for the same pressure. Adjusting the spring’s compression changes the set pressure; lock the adjustment so nobody turns it.
It Must Pass the Compressor’s Whole Output
The relief valve is the last defence when the compressor keeps running and the load stops. It has to let out as much air as the compressor puts in, at a pressure close to the set value. The best-case flow through an orifice at the choked condition (formula recalled from compressible-flow texts) is a mass flux of about 2,153 kg/(m² s) at 9 bar absolute and 20 °C.
Worked example: the 235.6 L/min compressor.
- Mass flow = 0.003927 m³/s x 1.204 kg/m³ = 0.00473 kg/s
- Open area needed = 0.00473 / 2,153 = 2.2 x 10⁻⁶ m² = 2.2 mm², a hole 1.7 mm across
- Real valves pass less than the ideal because of friction and incomplete lift, and they need a little overpressure to open fully. A 10 mm seat of 78.5 mm² is 36 times larger than the ideal area, which leaves plenty of margin
Size relief valves to the largest compressor that could ever feed the vessel, and re-check if you fit a bigger one later.
Where It Goes and How to Set It
- On the vessel or on the pipe nearest to it, with no valve of any kind between it and what it protects.
- Pointed away from people, since the discharge is a loud, fast jet.
- Between the compressor and any valve that could close if it also protects the compressor.
- Set at or below the lowest rating of any part it protects, where the rating comes from each part’s own marking or a real reference. The carrier suggests 110% of working pressure on a vessel tested to 1.5 times; see Receiver Tank Construction.
Setting a valve safely. Do not set a relief valve by raising air pressure in the real tank until it lifts. Fit it to a small test vessel filled with water, pump with a hand pump against a gauge of known accuracy, and adjust until it opens at the chosen pressure; water at test pressure holds almost no energy. Then fit it to the tank. Re-check it with a gentle lift of its test lever at working pressure.
Testing. Lift the valve by hand at least monthly, at working pressure, with your face and body clear. A valve that does not lift freely, or that does not reseat, has corroded or stuck and must be replaced.
The Other Devices
- Gauge: read it from where you stand and check it against a reference. A gauge with a stuck needle is more dangerous than none. The corpus gives no gauge construction.
- Drain valve: at the lowest point of the tank and every low point of the pipework; see Moisture Removal.
- Check valve: on the tank inlet, so air does not run back through a stopped compressor.
- Pressure switch or unloader: stops or unloads the compressor at a set pressure. It is a convenience, never a replacement for the relief valve, because it can fail shut.
- Guards and a reachable stop on the belt, the flywheel and the compressor, as in Piston Compressor.
- Hose restraints on any hose that could whip, and an air line that does not end in an open hose.
Common Mistakes
- A plugged or tied-down relief valve. Boilers killed thousands of people this way, as the boiler article records.
- A relief valve set above the vessel’s tested pressure.
- A valve that has never been lifted. Corrosion can seize it shut without any outward sign.
- A gauge in the wrong place or with the wrong range. A gauge that tops out at the relief setting leaves no room to see a rising fault.
Safety
The device that saves you must be able to act
A relief valve that is blocked, corroded, shut off, undersized or set above the weakest part’s rating does nothing, and the tank then fails by bursting. Never put a valve in front of it, never plug it, never raise its setting to stop it lifting, and never test it on the real tank by raising the pressure with air. Air from a discharging relief valve can injure skin and eyes; stay out of its line.
What’s Next
- Learn next: Pneumatic Tools and Devices — what the protected air system runs
- Combine with: Piping and Fittings — keeping the pipework as strong as the relief setting