Pneumatic Safety Systems: Redundant Safety Exhaust Valve vs. Zoned Safety for OEM Applications
Written by Tom Voth, Product Marketing Manager, Emerson, and Jeff Welker, product manager, FS Eng (TUV Rheinland, #18333/ 19, Machinery), Emerson
OEMs who design equipment with pneumatic safety systems are repeatedly challenged with the dual task of protecting operator safety while also delivering maximum uptime. To accomplish safety goals, OEMs have traditionally integrated redundant safety exhaust valves (RSEV) and multiple manifolds into their pneumatic safety system designs. While the RSEV approach can satisfy operator safety requirements like ISO 13849-1, it can also increase design complexity and sacrifice equipment efficiency when applied to equipment with more complex safety needs — two consequences that run counter to manufacturers’ quest for greater efficiency and sustainability.
One more efficient alternative to the RSEV system is zoned safety. This approach provides OEMs the tools to design pneumatic safety systems and circuits for high-demand, high-frequency safety functions using fewer components. Zoned safety also improves equipment efficiency by taking a functional approach that only applies pneumatic safety circuits where they are needed and consolidating them on one zoned safety manifold. This allows release of energy, stop motion, or return home safety functions to occur at an individual operator station instead of throughout an entire process.
While RSEVs and zoned safety are both capable of satisfying pneumatic safety requirements, zoned safety is often the more efficient option for many applications, particularly those with complex safety demands. To determine which pneumatic safety solution will produce the greatest efficiency within a machine operation, a basic understanding of the difference between these safety architectures and their application parameters can help OEMs make the best choice.
Comparing Pneumatic Safety System Architectures
Picture a production line in which an operator must load a part for welding. When the operator reaches into the welding area, all equipment motion must stop to protect the operator’s safety. To meet ISO 13849-1, OEMs have historically applied redundant dump valves to this type of operation to shut off the pneumatic air supply, exhaust air from the entire pneumatic system and disable the operation. Each time the safety function is triggered, the entire operation is suspended while the operator loads a part. Once the part is loaded, the operation then requires additional time to return home and resume running.
From the perspective of functional safety, which strives to gain efficiencies by applying safety functions only where they are needed, using RSEVs across multiple manifolds for a welding operation like this would be considered overkill. The entire welding operation is shut down, when only one area within the operation requires shutdown for operator safety. Applying these same functional safety and efficiency principles, the RSEV system also adds an unnecessary level of complexity to the equipment, as each operator station would have its own manifold.
To address these inefficiencies and help OEMs design simpler, more efficient pneumatic safety systems for loading/unloading and similar safety-related tasks, pneumatics engineers developed the zoned safety approach. Zoned safety is the inclusion of multiple safety functions and pneumatic safety circuits on one manifold. Each discrete safety “zone” on the manifold includes its own redundant features and one manifold can accommodate up to three zones, each with the potential for their own mix of safety functions.
To further illustrate the difference between these two pneumatic safety systems, applying zoned safety to the welding example would mean an operator could load a part while the air and power were disabled only at that loading station. The rest of the welding stations would remain operational, adding a level of efficiency and productivity that could not be accomplished using a traditional RSEV configuration.
Guidelines for Choosing the Most Efficient Pneumatic Safety System
Because the release of pneumatic energy to prevent unintended movement or start-up of equipment is a well understood and easy-to-integrate safety function as laid out in ISO 13849-1, OEMs often integrate RSEVs to satisfy the standard. For some equipment and applications, RSEVs are the safest, most efficient solutions. For others, completely exhausting or dumping air from the entire machine is unnecessary to protect operator safety and can result in higher operating costs and lower productivity. It can also add an unnecessary level of complexity to the machine’s design and increase machine cost.
The traditional dump valve design represented in an RSEV system is most effective for equipment that has a low demand on the safety function, such as those with an emergency-stop function, and a low cycle count. The RSEV approach is also best-suited for higher exhaust flows in the range of 5,000 to 9,000 l/min, as the infrequency of demand and rare occurrence of the safety function make related costs nominal.
Comparatively, zoned safety is recommended for equipment with a requirement for multiple safety functions and higher cycle demands of up to 6,000 cycles per day. Lower exhaust flows of 1,000 to 2,200 l/min are also characteristic of zoned safety, as only one area is being exhausted at a time for a release of energy safety function. A zoned safety manifold allows OEMs to define and configure multiple independent safety functions within up to three discrete safety zones on one valve manifold. This solution can achieve up to Category 3, PLd, as defined by the ISO 13849-1 standard, making it a practical solution for most manual load/unload stations, as well as a wide variety of other industrial applications with higher daily cycle counts.
Reasons to Consider Zoned Safety for Pneumatic Safety Systems
One of the most important benefits of zoned safety for OEMs is the ability to create redundant pneumatic safety circuits while greatly simplifying design. Whereas multiple RSEV manifolds add components, complexity and cost to equipment, a zoned approach can reduce the number of safety system components by as much as 35%. The zoned approach also reduces the overall size of the pneumatic safety system, which frees up valuable space within the equipment that can be used for other purposes. OEMs have used RSEV-based designs for pneumatic safety systems for years, which has created a level of comfort and a default to the familiar that does not always produce optimal results. What all OEMs share in common is a desire to build equipment with simpler designs and fewer components that create efficiencies for both themselves and their customers. This goal should drive OEMs to continually explore new technologies like zoned safety to ensure they are taking advantage of the best solutions the industry has to offer.