Reliable Pneumatics Ensure Safe Machines

The AV Series quick-release exhaust valve modules speed the exhausting from cylinders, clutches, and pneumatic actuators. An internal diaphragm opens in response to a differential pressure of only a few psi and opens quietly without chattering, while large internal passageways exhaust pressurized air quickly from the actuator.

Machinery Directive Provides Guidelines

In 2014, nearly 3.2 million non-fatal occupational accidents causing downtimes of at least four calendar days and 3,739 fatal accidents were recorded in the European Union. These figures reveal how important it is for machines not to present hazards to people when operated properly. In Europe, the Machinery Directive (2006/42/EC) sets specifications for the safety technology of all types of machines and systems. At the same time, it sets an example across the globe: The regulation serves as the foundation for other regional standards, such as Brazil’s NR 12. ISO 13849 plays an important role in the implementation of the machinery directive for the assessment of technical safety measures.

Engineers constantly have to ask themselves what happens when a power failure occurs or an operator presses the emergency OFF switch on a machine. In these cases, we need an external mechanism to safely cut off power to pneumatic cylinders, for example, so that they do not present a hazard.

In the example involving the packaging-machine operator, a quick-exhaust valve could have provided assistance, such as Emerson's AVENTICS’ AV rapid-exhaust valve module, which can be integrated into an AV valve manifold or stand alone. It exhausts the cylinder chambers without requiring additional energy. This means a cylinder can be moved to reposition workpieces because the otherwise pressurized cylinder chambers are open to atmosphere. If a body part is trapped, and a power failure prevents moving the cylinder, the exhaust module lets the cylinder’s piston rod be moved manually. The module offers full functional integration with simple connection to the cylinder. It significantly reduces the installation space required by the cylinder and can lower costs by up to 20%.

These AV modular valves have two independent signal paths for safety-related and other tasks. They are safeguarded against manipulation by not having a physical switch for configuration and can feature any valve to safety-zone ratio.

Assessing Performance Level

Engineers have to define a required performance level (PL), a value used to define the ability of safety-related parts of control systems to perform a safety function under foreseeable conditions. On the other hand, required performance level (PLr) is used in assessing the required risk reduction for each safety function. Therefore, the PL of safety-related parts of a control system must be equal to or higher than the PLr. For each safety function to be carried out by a safety-related part of a control system, the PLr must be assessed according to severity of injury, frequency of exposure to a hazard, and the possibility of avoiding a hazard or limiting harm.

Safe pneumatic switching processes and the reliability of the relevant safety components contribute to these efforts. The engineer calculates and documents the PL with reliability values, such as the B10 value. In valves, ISO 19973 defines this value as the number of switching cycles it takes for 10% of components to likely exceed defined limits, such as switching times, leaks, or switching pressure under specific conditions. Emerson sets benchmarks for SV07 and SV09 exhaust valves at 10 million cycles, and 75 million for AV valves.

Electronics plays a major role in the process. With galvanic isolation between the UL and UA, the valve electronics for the new AV valve generations meet proven safety principles. They have two independent signal paths for safety-related and other tasks. They are safeguarded against manipulation by not having a physical switch for configuration and can feature any valve to safety-zone ratio.

There are no easy answers to defining a security concept. Each case is different, and tailored solutions need to be found for each machine’s unique requirements. Creating safe pneumatic circuits is best achieved using proven and evaluated techniques, well-established components that have been tested for life cycle data, and properly sized components. Although the main objective is to make the machine safer, additional benefits can also make machines more reliable and efficient.

Andreas Blume is global account manager and Dusko Markovic is technical support—applications at Emerson Automation Solutions. For more information, visit www.aventics.com.