To ensure optimum machine safety, design engineers need a solid understanding of the Machinery Directive 2006/42/EC and how to comply with required safety levels.
A safety exhaust valve when incorporated into an air preparation system, lets the user safely and reliably shut off the pneumatic energy, stopping compressed air-flow to the machine and allowing downstream pressure to exhaust.
Installing a pneumatic safety exhaust valve is a simple and cost-effective way to achieve machine safety and to comply with the directive.
Here are four things to consider for your application:
1. Select a series-parallel design
To eliminate the danger of residual energy making its way into the machine, select a solution which offers a series-parallel flow design, such as Parker’s P33 valve. This permits higher exhaust flow capability and ensures low residual pressure during a fault.
Essentially, the two valve elements are arranged such that air from inlet to outlet must pass through both valves in series, but the flow path from outlet to exhaust is in parallel.
Cross-flow technology ensures that both valve elements (redundant design) must shift to supply air downstream and, if either valve element is out of position with the other, downstream air will dump to exhaust in parallel.
2. Utilise external monitoring
Monitoring detects faults or failures in control systems, and checks for short-circuit faults. The monitoring portion of a safety system must check if both sides of the safety valve shift together every time by monitoring the condition of pressure-operated sensors. These sensors are hardwired into the controls and monitored by the external control system.
This is generally done with most safety relays and safety PLCs. The use of sophisticated controls and monitoring ensures sensors are not bypassed and faults are detected so the valve functions as intended.
3. Optimise component life
Statistical component life (B10d) is key to any safety component. When designing a safety system according to ISO 13849-1, each component in the system needs a B10d or a mean time to dangerous failure (MTTFd).
Engineers use a B10d value, along with the number of operations (nop) to determine the MTTFd of the component for the application:
MTTFd = B10d/nop.
Valves that use electromechanical components for monitoring are usually limited by the life of the circuit boards or mechanical wear parts.
Using solid-state electronic pressure sensors for monitoring greatly improves the B10d numbers as there are no mechanical wear components.
4. Evaluate Performance Level
The required Performance Level (PLr) should be determined by a risk assessment. Once a PLr is determined, application statistical component life (MTTFd), controls architecture (Category), diagnostic coverage (DC), and consideration of common-cause failures (CCF) can be used to determine the system PL. The system PL must equal or exceed the Performance Level.
For applications where the severity of injury and level of exposure to the hazard are high, the percentage of diagnostic coverage of the monitoring system must be high as well. Depending on the safety relays or safety PLCs used, the system can achieve a High-Performance Level, up to PL e and Safe Integrity Level to SIL 3.
If a risk assessment demands a safety rating of PLc or higher, a redundant safety exhaust valve is a simple-to-implement and cost-effective way to attain the required safety level.
Our white paper Selecting & Integrating Pneumatic Safety Exhaust Valves provides more in-depth support with the correct specification and integration of safety exhaust valves.
Article contributed by Linda Caron, global product manager for Factory Automation, Pneumatic Division.