In industrial environments, pneumatic lines can pose significant hazards to workers and associated equipment – both the lines themselves and the moving components they operate.
Systems have been developed to help minimize these hazards and to comply with two ISO safety standards (EN ISO 13849 and ISO 13118:2000), which mandate the dissipation of pneumatic energy to prevent unintended startup or movement in a machine.
Design engineers can utilize a pneumatic safety exhaust valve, such as Parker’s P33 Safety Exhaust Valve, into an air preparation system to comply with these standards and increase levels of plant safety.
Several important points should be considered when specifying a safety exhaust valve:
1. Measure the exhaust time to faulted condition
Take the time to understand the worst-case scenario as when a safety valve is in faulted condition, standard exhaust times (assuming normal stop) do not apply. In this situation there is a failure in the valve or control system and the exhaust flow may be restricted, thereby increasing exhaust time.
2. Switching time is important
The faster a machine can stop, the closer to the machine that the guards, light curtains or other presence-sensing devices can be installed. Your switching off impacts your calculations for safe stopping distance and your switching time on impacts the lag time to fill the machine.
3. Product design
Adopting a safety exhaust valve with a series-parallel flow design means benefiting from the best of both series and parallel arrangements, ensuring that both valve elements (redundant design) must shift to supply air downstream. If either valve element was to be out of position with the other, then the downstream air will be dumped to exhaust in parallel.
This arrangement allows for higher exhaust flow capability and ensures very low residual pressure during a fault, eliminating the danger of residual energy making its way into the machine. Consequently, if two exhaust valves are used in series, the air supply to the first valve flows through to the second and then downstream. When in exhaust mode, this design flows most of the exhaust through the second valve creating a lag in exhaust time.
4. Know the B10 value
The valve’s B10 value is its life expectancy in switching cycles and is based on B10 testing (the point at which 10% of a sample has failed). It’s an important consideration when determining the MTTFd. The higher the quality of the components is, the longer the B10 life of the machine into which they are built will be. This is an important consideration for Category 4 applications where a high diagnostic coverage is needed.
To achieve the highest level of diagnostic coverage, it’s critical to employ all the best aspects of safety circuit architecture - redundancy (dual channel circuits) and monitoring to detect faults or failures in control systems and check for short circuit faults. The monitoring portion of the safety system must check to see if both sides of the valve are shifting together every time. This is generally done with most versions of safety relays and safety PLCs that can also perform pulse test monitoring.
These type of safety relays and safety PLCs make for very reliable systems with high diagnostic coverage – especially short circuit faults in dual channel systems. The use of sophisticated controls and monitoring ensuring the valve is functional.
Safety allows no room for error. If the risk assessment requires a safety rating of PLC or higher for the pneumatic system, then a dual redundant safety exhaust valve offers a simple-to-implement and cost-effective way to attain the required safety level.
Our white paper Selecting and Integrating Pneumatic Safety Exhaust Valves outlines more considerations for specifying safety exhaust valves.