Process Equipment Update
Playing it safe with compressed air
Ben Rickards examines some key products and devices designed to help manufacturers manage compressed air system safety in accordance with the latest legislation.
Ben Rickards untersucht Schlüsselprodukte, die Hersteller bei der Gewährleistung der Sicherheit in Druckluftsystemen gemäß der neuesten Gesetzgebung unterstützen sollen.
Ben Rickards examine certains produits et dispositifs phares conçus pour aider les fabricants à gérer les systèmes à air comprimé conformément aux dernières réglementations en vigueur.
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aintaining 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
Fig. 1. A pressure regulator will reduce high generation pressures to lower levels. All components should have a Safe Working Pressure and overpressure limit of 10 per cent.
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,
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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
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