What is a Soft Start Dump Valve?

What is a Soft Start Dump Valve?

Soft start dump valves provide a soft start function that allows for a controlled increase of downstream pressure on initial startup and a dump function to ensure the system pressure is exhausted quickly when required. Fitting a soft start dump valve can assist designers in complying with the European Machineries Directive and, for existing machinery, to comply with the Provision and Use of Work Equipment Regulations.


How does a soft start dump valve work?

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 80 per cent of full line pressure. 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.

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.


How do I select a soft start dump valve?

Firstly, 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.

What flow is required; this will determine port size and, possibly, what model to choose. What is the maximum operating pressure; air pilot versions are capable of higher maximum pressures.

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. Our recommendation is to fit a heavy duty silencer (eg Quietaire) as standard; less robust types will not take the shock load from a rapidly exhausting dump valve. Where the exhaust air can be heavily laden with lubricant, usually from equipment requiring high levels of lubrication, fit a heavy duty coalescing exhaust silencer.


Types of soft start dump valves

Soft start dump valves are available for both Olympian Plus and Excelon ranges. For Olympian Plus, port sizes range from G1/4 to G1 ½; maximum flow ranges from 57dm3/s to 147dm3/s. The snap pressure is when downstream pressure reaches 50-80% of inlet pressure. The units can have solenoid, pilot or manual operation and maximum operating pressure is 10bar for solenoid and 17bar for pilot operators. A monitored dump valve version is also available, using a positive driven micro-switch for G1/4 to G3/4 options.

For Excelon, units are available G1/4 to G3/8, with maximum flow from 21dm3/s to 57dm3/s. Snap pressure and maximum operating pressure values are the same as above. A monitored dump valve is available for G1/2.


Typical applications of soft start dump valves

It is important to locate these devices close to the piece of equipment they are used to protect. Any machine design looking to conform to the European Machineries Directive should employ a soft start dump valve as part of a safety-based system.


Do I need anything else to make a soft start dump valve work?

As discussed above, a correctly chosen silencer could be an important additional component. A shut-off valve may also be a useful addition to the circuit. Wall mounting brackets, fittings and tubing are also available.

More information

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