Playing it safe with compressed air

Paul Boughton

Ben Rickards examines some key products and devices designed to help manufacturers manage compressed air system safety in accordance with the latest legislation.

Maintaining a safe working environment is vital for all manufacturers and is an area subject to increasingly stringent legislation such as the Machinery Directive, the Pressure Systems legislation and the Provision and Use of Work Equipment Regulations (PUWER).

All of these to some degree govern compressed air equipment and, in particular, focus on the need to address issues such as overpressure, sudden movement of parts, and treatment of exhaust air. A thorough risk analysis will identify all of the hazards in a pneumatic system.

Overpressure protection

Components in pneumatic systems often have a pressure rating which is lower than that generated at the compressor. To ensure that systems run at safe and efficient levels, pressure regulators are normally used, which reduce high generation pressures to lower levels. Components in pneumatic systems exposed to excess pressures often result in malfunction, or even extreme failure of the pressure-containing envelope should the upstream regulator fail or malfunction.

Of a number of solutions to combat overpressure conditions, the relief valve is the most popular option. In selecting the right valve manufacturers should consider the criteria specific to individual systems. All components and equipment within a pneumatic system should have a Safe Working Pressure (SWP) and an overpressure limit of 10 per cent. Regulators can be designed into a system to run it at pressures up to or below SWP. The additional 10 per cent safety factor is the maximum overpressure allowed for the system with the relief valve in operation.

A relief valve is a control valve with its outlet connected to a pressure system to enable the system pressure to be held at a constant level. This level should be above the required system operating pressure, which may be above or below the stated SWP. Avoid setting the valve too close to the system operating pressure, as the valve's operating characteristics can cause it to open and vent air during normal system operation - a costly waste of air.

There are several ways of ensuring a relief valve capable of maintaining pressure at the correct level is chosen. The relief device should have a flow capacity in excess of the compressor's free air delivery capacity in systems where no receiver exists. Alternatively, the device should have a capacity in excess of the flow through the smallest flow passageway of the equipment being protected when exposed to the highest possible pressure (usually the generated pressure at the compressor). Check existing tables of orifice flow. The smallest bore acts as a restriction to the flow downstream. This is important because, even if the distribution system is a mains system consisting of very large volume with large bore pipes and high capacity compressors, the device being protected might be fed by 1/8-in nominal bore tubing and it is this that will limit the flow capacity. As long as the relief valve can match or exceed these flows, then a constant, acceptable overpressure level can be achieved.

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 tee-piece. 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 within the area of moving parts of the system is paramount, both in terms of protecting the parts themselves against excessive wear, due to loading on start-up, or danger to personnel from sudden movement of parts.

This is where the use of 'soft start' ('slow start') valves is desirable. During normal operation, air is allowed to pass to a pneumatic system or device in a gradual manner. The rate of pressure build-up into the system can be adjusted through a restricted passageway in the valve, which is generally an internal poppet spring-operated device.

The poppet operates when the gradual pressure build-up produces a force in excess of that holding it closed. The poppet then moves to the fully open position at a condition known as the snap point, allowing full flow to proceed through the unrestricted passageways of the valve and rapidly fill the system to full line pressure. For most devices this snap point will be within the range of 40 to 70 per cent of full line pressure. It is important to locate these devices close to the piece of equipment they are used to protect. The devices deal with pressure build-up as a result of the system volume. Fitting a larger valve to a complete distribution system can often result in the system taking many minutes to fully pressurise.

It is extremely common to couple the slow start with a dump or exhaust function valve within one body for compactness. The 'dump' valve is designed to quickly exhaust the 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, thus providing a monitoring function which, in conjunction with other valves and relays - plus suitable system redundancy - can help comply 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 2ppm under average usage conditions.

Protection device selection

Firstly, decide which parts of the system cannot withstand the maximum pressure which can be developed in the distribution system (or compressor). Then, determine which type of relief valve is required to control this air pressure most effectively, taking into account failure flow through that part of the system. Consider using a restrictor (orifice) without producing excessive pressure losses in the normal operation of that part of the system to enable a suitable size of relief valve to be used.

For very large flows consider a pilot operated regulator as a dump valve, with a diaphragm relief valve pilot.
For machines consider an in-line device to build-up one complete integral modular preparation assembly for ease of piping, location and servicing.

Decide which parts of the system can suffer from problems on initial start up, or resetting where excessive initial speeds can lead to wear problems or entrapment, or where an emergency stop/dump function is required.

Employ one soft start/dump valve for each section of the system operated in this way. The larger the system the longer the dump or emergency stop function will take to fully empty the system. If there is a high safety risk in this part of the machine consider monitored soft start/dump versions along with appropriate other circuitry to comply with the machinery directive. Locate soft start/dump valves in the FRL assembly closest to the downstream end to prevent high back flows through the lubricator.

Where large volumes of air are to be exhausted consider fitting a silencer if the air cannot be piped away to a convenient position. Where rapid cycling of exhaust is present fit a heavy duty silencer. Where the exhaust air can be heavily laden with lubricant, usually from equipment requiring high levels of lubrication, fit a coalescing exhaust silencer.

Other products

In addition to the products and devices described above, there are other air line 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.

Ben Rickards is Product Technical Manager with Norgren, Lichfield, Staffordshire, UK.

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