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Playing It Safe With Compressed Air
The role of valves in industrial automation
Maintaining a safe working environment is vital for all manufacturers and is an area subject to increasingly stringent legislation - not least in the area of compressed air equipment where the focus is on the need to address issues such as overpressure, sudden movement of parts, and treatment of exhaust air.
Overpressure protection
To combat overpressure conditions, relief valves are widely used. This is a control valve with its outlet connected to a pressure system to enable system pressure to be held at a constant level. When selecting a valve, manufacturers should consider specific system criteria. All components should have a Safe Working Pressure (SWP) and a 10% overpressure limit - the maximum overpressure allowed with the relief valve in operation.
Relief valves should have a flow capacity exceeding the compressor's free air delivery capacity in systems without a receiver. 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 should be used.
An 'in-line' type is particularly popular, where all control equipment/ protection devices are in one discreet position, usually in an assembly. This device has a relief port at 90° to the flow direction. Under normal operating conditions, flow passes through it to the downstream system unaffected, only operating when the set relief pressure is exceeded. The 'in-line' device differs from the pop or diaphragm types which are connected via a teepiece, with flow only occurring when venting air to atmosphere.
Types of relief valves
To achieve different levels of performance of flow capacity and overpressure limitations, a number of types of relief valves are available. The one most commonly used is the pop type, followed by the diaphragm type, which is more accurate and repeatable. It is recommended that, for optimum performance, pilot operated valves are used. Of these, the integral pilot operated type is the most compact and cost effective.
An 'in-line' type is particularly popular with machine builders, where all the control equipment/protection devices are in one discreet position, usually in an assembly - aiding both installation and scheduled servicing. This device has a relief port at 90° to the direction of flow and, under normal operating conditions, flow passes through the body of the device to the downstream system unaffected. Only when the set relief pressure is exceeded will the relief port operate. The 'in-line' device differs from the pop or diaphragm types which are connected into the system on a teepiece. Flow through these pop or diaphragm relief valves only occurs when venting air to atmosphere. In-line and diaphragm valve types can have the exhaust flow piped away to an area where the noise and flow will not cause disruption or harm to the environment or the operators.
Soft start/dump valves
Protection of moving parts is paramount, both in terms of negating excessive wear due to loading on start-up, and reducing danger to personnel from sudden part movement.
This is where the use of 'soft start' ('slow start') valves is desirable. During normal operation, air passes to a pneumatic system or device in a gradual manner. 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.
The slow start can be coupled with a dump or exhaust function valve within one body for compactness. 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 - plus suitable system redundancy - can aid compliance with legislation to make machines safer.
Exhaust air
In order to reduce noise and oil mist, and to minimise danger to personnel, exhaust air has to 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 can often solve the problem but more heavy duty silencers might be required in more demanding applications.
Silencers are normally rated on their noise reduction and associated back pressure, the most cost effective choice of silencer would be based on the flow rate and duty required of the device.
Oil is the next major pollutant, particularly in incorrectly lubricated systems or those which require high levels of lubrication. In such instances, use of a coalescing exhaust silencer should be considered. Similar to those used for oil removal filters, this device results in small airborne particles in the form of mists merging into larger droplets and falling into a container for removal as liquid oil. 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 not as good as those using coalescing filters. A good exhaust silencer, however, should remove oil to around typically 2 ppm under average usage conditions.
Other products
In addition to the products and devices described above, there are a number of other air preparation products that can be used to help increase the level of safety in pneumatic systems. These include preset pressure regulators, lockable shut-off valves, and tamper resistant kits.
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