Norgren Collateral

ICO4-PST Solenoid Valve 05 There are two main types of intelligent system suitable for Partial Stroke Testing. These are Positioners and Electronic Systems. Both offer different approaches but each of these involves a series of compromises in order to achieve the goal. The Problem with Current PST Solutions Positioners Electronic systems Positioners are currently the most common type of intelligent PST system in use and are derived from control valve positioner technologies. They were the quickest development path available as they already had the technology built in to reliably move a valve to a given position. In fact these made for relatively simple systems as the device could easily be configured to give any desired percentage of partial stroke. With the addition of an ESD function, these devices became part of the control system for the actuator and therefore could be used to effectively create a redundant ESD operation when used with a solenoid valve. There are a number of manufacturers of positioner systems and all use a similar fundamental approach to partial stroke testing. However, as time progressed, the compromises created by these systems became more apparent as other technologies were developed. » Requires reconfiguration of pneumatic control » Limited flow rates » Reduced Diagnostic Coverage » Does not test the valve at the designed operating speed » High safe failure rates » Not compatible with all valve types » Difficult to install, commission and service As the industry sought PST systems to alleviate the issues with Positioners, many manufacturers turned to electronic systems that were able to perform a partial stroke in a similar manner to that required in an emergency. Many of these systems use electronic control boxes that are fitted close to the actuator and connect the power supply to the solenoid valve. To perform the test, these systems de-energise the solenoid valve and monitor the valve position and/or instrument pressure to determine the success of the test. Electronic systems can deliver a high level of diagnostics while limiting many of the issues associated with the positioners. However, even these systems are not entirely free of problems. » Additional equipment required - The most common is an electronic control box » Spurious trip risk » Solenoid valve selection resulting in poor SIL performance » Difficult to service » Costly Each technology clearly has its strengths, but it's also acknowledged that neither option currently offers a wholly satisfactory or reliable solution.

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