As machine builders become more adept at the science of applying newer safety standards—such as ISO 13849—to their designs, they’ve started to take a deeper look at safety within their systems. Here are five of the most common safety functions within pneumatic systems.
1. Protection against unintended startup
Potentially the most common of all pneumatic safety, and often used in conjunction without pneumatic safety functions, protection from unintended startup is a vitally important safety function on many machines. This function is most often achieved by a purpose-built pneumatic safety valve that has been certified to achieve the required performance level. Simple, solenoid activated valves with a single valve element that are certified to performance level (PL) c are available from several manufactures. Applications requiring PL d or e often require a more sophisticated valve with redundant valve elements to achieve this function. When specifying this component, check to see if an external safe relay or safety PLC is required to comply with the certifying requirements. Some products achieve these performance levels with built-in electronics.
The requirements for this function are often very similar to the requirements for protection against unexpected startup, and this function can be realized within the same component that provides your protection against unexpected startup. In this case, because the exhausting device is also the pressure supply valve, care must be taken so that the failure of the device that shuts off incoming pressure doesn’t interfere with the exhausting behavior of the system. In the case of the devices that are certified to PL e, this is often accomplished by allowing much greater flow in the exhaust direction than the supply direction. In this case, the downstream pressure can still quickly decay to a safe level even if incoming flow fails to shut off.
Important note: The exhausting function can be easily defeated by a muffler element that is clogged with foreign material. Ensure that you get a safe muffler with your valve. Specially designed safe mufflers are available that prevent clogging by having a large exhaust path cross-drilled, thus ensuring exhaust capability.
3. Reversing the movement
In simple applications that require a low performance level, this function can be accomplished by applying a simple spring-return solenoid valve. In applications requiring a higher PL, such as “hands in press” applications, a more robust solution is required. This is accomplished using redundant valve elements, plumbed in such a way that they will only extend the cylinder when both valve elements have shifted. By sensing the position of each valve element and feeding these signals back to the safe PLC, Category 4 control architecture can be realized. While it is possible to achieve this performance with discreet components, certified components are available that achieve this function in an integrated package.
4. Stopping, holding, blocking
This safety function is applied in a wide variety of applications with varying performance level requirements. Simple applications can sometimes be satisfied by using a pilot-operated check valve installed in the cylinder port, while more complex applications with higher performance level requirements often require a mechanical rod brake or clamp that uses spring pressure to hold the piston rod in position when applied in combination with an appropriate valve.
5. Reducing speed
Reducing speed is probably the simplest of the common pneumatic safety functions. Simple flow controls can be made tamper resistant after initial validation to ensure that movement parameters are not altered at a later date.