Process Equipment Update
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
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Fig. 2. A soft start or dump valve will deliver a controlled increase of downstream pressure on start-up.
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
Fig. 3. A pressure relief valve helps enable system pressure to be held at a constant level.
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