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INSTRUMENTATION • ELECTRONICS


DANGEROUS FAILURES A dangerous failure is one that prevents the solenoid valve from performing its safety function when required, on demand. This type of failure often occurs because friction and stiction have increased over time. The load holding the valve in the open position increases to the  close the valve. A dangerous failure occurs when spring force < friction + retentivity. To improve performance against dangerous


failures it is necessary to improve the force friction ratio (FFR). This is the ratio of the spring force to the friction and retentivity within the valve. Two steps can improve the FFR. Firstly, reduce the friction in the valve by minimising the number of dynamic seals. Dynamic seals add friction to the design, therefore using a poppet type valve rather than spool valve reduces the inherent friction. Secondly, increase the   power available is typically limited by the PLC output card.


SAFE FAILURES In safety-related systems it is important to periodically   function is working to the required performance. This means that it is essential to test the solenoid valve in addition to the actuator and process valve. Safe failures are the biggest concern when solenoid valves are tested. There are two types of safe failure: the spurious trip (that occurs during normal operation); and the failure to operate on demand, which causes a spurious trip during partial stroke testing (PST).  


  and thus the process valve closes. The safe failure rate will be improved if burn-out can be prevented, in which case the following may apply: reduce the coil running   integrated core, armature and coil construction may also help. The second type of safe failure is when valves fail to open on demand, which may be termed a pull-in failure. Over a period of time, cable damage or termination corrosion can cause an increase in resistance that subsequently causes a reduced voltage at the solenoid valve coil. Under such circumstances, the voltage may fall to below the required pull-in voltage of the solenoid. During a PST, the solenoid coil is de-energised and then re-energised at the end of the test. However, whilst the coil voltage  


50 www.engineerlive.com


its closed position. As a result, the process valve completely closes, causing an unintended trip. This can be avoided by checking the coil voltage prior to initiating a partial stroke test. One way to improve


safe failure rates is to  this allows higher margin   Fortunately, there are several ways of promoting optimal  valves use a wet armature arrangement where the internal magnetic components  media. Therefore a corrosion- resistant material must be used. Unfortunately, such hard magnetic materials   retentivity. In contrast, a dry armature, where the magnetic components are isolated from the instrument media, allows for soft magnetic materials to be used that  retentivity.  design, and in particular the magnetic path integrity, should also be considered. A core tube design leaks   


APPLICATIONS To summarise, optimising solenoid valve design, and in particular ensuring reliable actuation, is absolutely crucial to ensure good outcomes both in PST (where  spurious trips) and in daily applications. In fact, it is in day-to-day use that reliable actuation becomes most  and productivity. 


Richard Harvey is with IMI Precision Engineering. www.imi-precision.com


The ICO4-PST is a smart solenoid valve for integrated partial stroke testing


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