Pneumatic Dump Valve
Original article date: July 1996
Norgren has developed a monitored pneumatic dump valve. This will help pneumatic system designers comply with the Machinery Directive.
Whilst the majority of components are not designed to be safety components some are employed to perform safety-related functions. Although the Machinery Directive and its supporting European standards do not state that a monitored dump valve must be used the standards are quite clear about the levels of safety which must be achieved for different degrees of risk.
A typical case to consider would be the solenoid-operated spring return three-port two-position valve. This is a standard product and it is not designed for any particular application which can be used on many different ways. This type of valve is often found dumping compressed air from a cylinder or system in response to an emergency stop condition. If for any reason the valve should fail to operate correctly the cylinder or system could remain pressurised.
This is an action which must no comply with the Machinery Directive and provisional standard EN 954 Part 1: General principles of design which applies to safety-related parts of control systems irrespective of type – electrical hydraulic or pneumatic.
With this standard risk assessments must be carried out by machine builders in order to place components or systems into one of five categories ranging from B (the lowest) to 4 (the highest or worst case).
When the circuit designer carries out the risk assessment on the proposed system the hazard could be placed in risk category 3. Following the guidelines in PR EN 954-1 redundancy on the function of the dump valve should be employed.
Where this is incorporated in a safety dump system two valves are mounted in series. To ensure that each valve has operated its function needs to be positively monitored. This enables potentially hazardous situations to be identified.
PR EN 954-1 Annex C does in fact list possible faults for hydraulic and pneumatic components which whilst not being exhaustive do give examples for consideration. In this case where a valve is dumping air in response to an emergency the main applicable fault is the lack of switching or incomplete switching of the moving element due perhaps to a valve piston sticking.
Obviously if the valve piston fails to move the valve will not dump the compressed air. So how can this fault monitoring and detection be achieved? A pressure switch fitted downstream of the valve does not detect valve piston movement. Further its contacts could conceivably weld together so defeating the monitoring action.
The new Norgren monitored dump valves (MDV) allows this monitoring requirement to be met simply and cost-effectively. The MDV is a solenoid/spring 3/2 valve which incorporates a special normally-closed positively-driven switch directly operated by the valve poppet. If the switch contacts then weld together the design of the switch is such that it will force them apart. By adopting this method of fault detection this valve now satisfies the requirements of EN 954-1.
In practice monitored dump valves should be used in conjunction with an emergency stop safety relay such as for example that produced by Pilz. These relays test themselves for faults before energisation and allow an electrical signal to be fed to the machine. A feedback loop is employed which can monitor external switches and will prevent energisation if any faults are detected. This ensures that a fault in either valve will prevent re-start from taking place while still maintaining the dump facility.
When the start button is pressed the relay checks itself and the switches of the MDVs. If everything is satisfactory the relay will energise switching on its output. The MDV solenoids are powered via the relay output and will then operate allowing compressed air to the machine.
Upon operating the E-stop device the relay will de-energise and remove power from the MDVs which will exhaust the compressed air. If either of the MDVs fail to operate though a fault in the valve switch or cable the other will carry out the dump function. When the operator actuates the start button the system will be prevented from re-starting until the fault has been rectified. Used in this manner the MDV can prevent the hazardous situation occurring.
The MDV valve is designed for air filtered to 40aem at operating pressures from 2 to 10 bar. It has a flow of 4460 litres/minute between ports 1 to 2 and 4775 litres/minute between ports 2 to 3. Port sizes are G1/2 and a 24Vdc 8W solenoid coil is the standard option fitted although other standard solenoid operating voltages are available. The microswitch on the valve has a 2m long flying lead.
- IMI Norgren
- Contact: Alan Satchwell
- Tel: 01543 414333
- Fax: 01543 268052
- www.norgren.com
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July 1996