When evaluating a production system’s adherence to safety standards, electrical considerations are usually the top priority. A thorough risk assessment, however, requires that the fluid power components also be reviewed, especially considering they are often the final control element.
One of the most important factors in any pneumatic safety system is the machine stopping time. In the case of a sticky directional control valve, for example, cylinder motion could continue until the supply pressure is exhausted. Because of this, the valve exhaust response time is a critical factor in maintaining a safe stopping time. The term “safe stopping time” (per the ISO-Safety Related Parts of the Control System standard) means that the machine must be able to stop in a time period such that no worker should be able to reach a hazard area before the machine has time to stop. The faster the safety exhaust valve can dump the pressure from the system, the faster the machine will stop. This depends on the individual machine, but a faster stopping time means that the guards could be located closer to the machine, thus, allowing faster machine access and freeing up valuable floor space.
Valve exhaust time is a function of the response time and the flow rate. A valve that is slower to respond will have a longer exhaust time, and, potentially, a longer machine stopping time than a valve with the exact same CV rating. Manufacturers usually provide flow charts or CV data on their safety valves representing normal operation as well as flow during a faulted condition. Always consult this data in order to choose an appropriately sized valve for your safety system’s required stopping time.
The faulted response times of exhaust valves are particularly important attributes when serious hazards exist. A faulted valve will always increase the exhaust time and potentially the machine stopping time. The faulted valve response time depends entirely upon how a valve is constructed. Therefore, choosing a product designed to minimize these times is vital.
The ROSS DM2 Series C size 4 valves (½” port size) will increase exhaust response time by less than 33% in a faulted state, whereas other competitive valve designs are known to increase the exhaust response time by 60 to 80% (based on published data). For example, a 100 milliseconds exhaust time in a system with 30 in.3 of downstream system volume would increase to less than 133 milliseconds during a fault condition with the ROSS valve, or 160 to 180 milliseconds with a different company’s product. Due to the potential for injury, damage to machinery, and downtime, these values must be seriously considered in a safety system design.
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