Playing it safe with compressed air

Overpressure protection

Relief valves are used to combat overpressure. This control valve has its outlet connected to a pressure system to enable system pressure to be held at a constant level. When selecting such a valve, all components should have a Safe Working Pressure (SWP) and a 10% overpressure limit, the maximum allowed with the relief valve in operation.

In systems without a receiver, relief valves should have a flow capacity exceeding the compressor’s free air delivery capacity. Alternatively, the device should have a capacity above the flow through the smallest flow passageway of the equipment being protected when exposed to the highest possible pressure.

The most common relief valve is the pop type, followed by the more accurate and repeatable diaphragm type. However, for optimum performance, pilot-operated valves are preferred.

An ‘in-line’ type is particularly popular, where all control equipment/protection devices are in one discrete position, usually in an assembly. This device has a relief port at 90 degrees to the flow direction. Flow passes through it to the downstream system unaffected, it only operating when the set relief pressure is exceeded. The pop or diaphragm types are connected via a teepiece, with flow only occurring when venting air to atmosphere.

Soft start/dump valves

The use of ‘soft start’ (‘slow start’) valves is desirable when working with moving parts, both to protect against sudden part movement and to minimise wear. During normal operation, air passes gradually to a pneumatic system or device. The rate of pressure build-up can be adjusted through a restricted passageway in the valve - generally an internal poppet spring-operated device. The poppet operates when the gradual pressure build-up produces a force exceeding that holding it closed. The poppet then moves to the fully open position at a condition known as the snap point, typically within 40-70% of full line pressure.

For compactness, the slow start valve can be coupled with a dump or exhaust function valve within one bod. The ‘dump’ valve quickly exhausts pressure from the downstream system. It can have solenoid or air pilot operators and often an override or emergency dump function.

Furthermore, a limit switch can be introduced to the device’s valve spool. This can give a spool open or closed signal, providing a monitoring function which, in conjunction with other valves and relays and suitable system redundancy, can aid compliance with legislation to make machines safer.

Exhaust air

To reduce noise and oil mist and minimise danger to personnel, exhaust air must be treated correctly. Where a dump valve is employed, large volumes of air can be released at high speed, creating high noise levels. Simple silencers may suffice but heavier duty silencers may be required. The most cost-effective choice is based on flow rate and required duty.

Oil is a major pollutant, particularly in incorrectly lubricated systems or those requiring high lubrication levels. In such instances, a coalescing exhaust silencer should be considered. This device results in a mist of small airborne particles merging into larger droplets and falling into a container for removal. Since these devices are on the exhaust side of the pneumatic system, they are exposed to sudden shock loading, meaning their oil removal capabilities are less than those using coalescing filters. A good exhaust silencer, however, should remove oil to around typically 2ppm under average usage conditions.

Note: 2006/42/EC replaces BS EN 983 Machinery Directive