TechArchive

Original article date: November 1999

What do you need to think about for safety-critical pneumatic systems? We asked STEVE PARTINGTON at Norgren

Whoever puts a machine on the market is responsible for its safety, and legislation means that, in most cases, machine builders must also self-certify their machines as being safe. There are no fail-safe rules for avoiding mistakes, but there are procedures, such as risk assessment, to help you to minimise hazards.

According to Steve Partington of Norgren, his company’s sales engineers complete a control system specification form with their customers to help identify the potential hazards on each item of equipment in the pneumatic circuit. He says: “Every actuator or valve is considered in isolation to check whether malfunction or mis-function could lead to a hazard. This approach gives designers the chance to design-out anything in the machine that has the potential to cause a hazard. If the potential hazard cannot be designed-out, it is the designer’s responsibility to build adequate safeguards into the machine to reduce the level of risk to an insignificant or negligible level.

“Risk assessment is a subjective process and designers have to reach their own conclusions, but a quantitative risk assessment, which allocates a ‘hazard rating number’ (HRN) to each hazard, helps to identify the best method.”

In some cases, hazards can be eliminated by using specially-designed safety equipment to replace the potentially hazardous equipment. For instance, one of the most commonly occurring hazards in a pneumatic system is the uncontrolled movement of vertically mounted cylinders when loaded. This usually occurs when the air supply to the cylinder fails. But cylinders fitted with a passive piston rod lock avoid this hazard by triggering a locking action when the air supply is removed.

Steve Partington also says: “If the hazard cannot be designed-out, different safeguarding methods can be compared with each other and with the original level of risk because each method has been allocated an HRN. This gives designers a clear indication of how successful their methods will prove.

“Typical safeguards include fixed guards, interlocked guards and protection devices such as two-hand start units. Regardless of whether or not any of these safeguards are used, designers must issue warnings to operators, and manufacturers must provide instructions and training to ensure operators are aware of the risks.”

Help is also at hand from BS EN983, the standard which specifically deals with pneumatic systems and components on machinery, giving guidance on the hazards a designer should be considering. Also, Norgren’s Guide to Products for Safe Pneumatic Systems helps designers meet the legislative requirements by including references to the relevant legislation and standards alongside product information.

Typical products include monitored dump valves that have a built-in monitoring switch linked to the control system. If the valve fails, the control system shuts down the machine and this cannot be restarted until the valve is replaced. These solenoid/spring 3/2 valves can be used to dump air from a system in response to an emergency stop signal and can be used in redundancy. The monitoring capability adds an additional safety element. 

•  Norgren

November 1999