When it comes to safeguarding machinery, many designers of safety systems do not look past electrical control, leaving pneumatic and hydraulic driven loads with only electrical safety integrity.
With today's safety standards requirements, electrically monitored pneumatic and hydraulic valves have become a key component in machinery safety solutions, providing the interface between the fluid power operation and electrical safety circuits.
Failure of the safety-related component of a fluid power control system could potentially lead to exposure of a person to risk of serious injury or death. Therefore, careful selection of a suitable safety product is of extreme importance. Fitting a sensor or switch to a spool or cartridge valve may provide what can be termed for automation purposes as a 'monitored valve', but does it have the integrity to provide a safety function?
Risk categories are applied to hazards of a plant so that the safety component of the control system can be suitably designed. The risk assessment process takes into account severity of injury, frequency of exposure and possibility of avoidance. There are five risk categories that can be determined when applying these factors to Appendix F of AS4024.1. The categories include B, 1, 2, 3 and 4 with category 4 being the highest level of risk and safety.
Fluidsentry's monitored pneumatic and hydraulic safety valves have recently become key components in the safety upgrade of an ageing JC Smale brick palletising line. The risk assessment of this operation indicated the majority of hazards to be no lower than category 3. A number of hazards were found to be category 4 risks.
A purpose-built valve bench fitted with a combination of the monitored safety valve systems was integrated with the brick palletising line, providing a fully redundant interface between the fluid power operation and the electrical safety circuits monitoring the emergency stops, guarding and associated interlocking devices. The safety valves prevent single faults within the fluid power safety system from leading to the loss of the safety function. Correct monitoring ensures single fault detection and prevents further operation of the secondary or redundant valve until the fault is corrected.
A safety system that isolates only the electrical solenoid of a non-monitored valve may typically at best only meet category 1. According to the AS4024.1 table 10.3 - Categories of safety-related parts of control systems - category 1 allows the loss of the safety function when a fault occurs without a requirement for detection. For example, if a valve spool fails to return to its home position, even by just 1 mm, it can allow slow pressurisations of a fluid power system. Without high integrity monitoring, the fault may not be detected and a serious accident could be the unfortunate result.
Companies invest many thousands of dollars protecting their employees from dangerous machinery. Many systems are designed with the goal of meeting single fault and detection requirements of categories 2, 3 and 4. Designers of safety systems should not only assess the dangers of fluid power loads, they also need to carefully consider all potential failure modes of their fluid power circuits, to ensure risk category compliance. Without consideration of the failure modes and installation of compliant fluid power control methods, the resulting safety system could comprise high integrity interlocking and monitoring systems that provide very little to no fluid power safety.
All the Fluidsentry products undergo a complete F.M.E.A (failure modes and effects analysis) in conjunction with third party conformity as part of its product Ce certification. With the inclusion of the Fluidsentry monitored valves, the brick manufacturer was able to produce a safety system where both the electrical and fluid power systems met their risk category requirements. What they achieved is a practicable and complete solution for the safety-related parts of its machine control system. They are now protecting operators from not only the electrically driven loads, but also from the hydraulic and pneumatic hazards within the dangerous area.
For further information contact Fluidsentry Pty Ltd
PO Box 8135, Carrum Downs 3201