Failure Modes of Positioners in Safety Applications

A few weeks ago, I highlighted a thread from the ISA Safety email list on partial stroke testing using control valve positioners. A few weeks later, this thread is still going strong. Emerson’s Riyaz Ali sent me some of the recent exchanges. A concern was expressed about the failure modes in an I/P [current-to-pneumatic] converter due to fouling (build-up) or plugging of the nozzle. Riyaz responded [with my light edits]:

Flapper nozzle technology has had more than four decades in the field and has been reliably used in conventional P/P [pneumatic-to-pneumatic] or E/P [electro-pneumatic] positioners. I would consider flapper nozzle technology mature enough to remove any of the initial stage design issues or any flaws due to components design or operating media concerns.

Positioners have been widely used in process manufacturing, immaterial of BPCS (Basic Process Control System) or SIS (Safety Instrumented System). I/P technology used may vary from manufacturer to manufactures. Flapper Nozzle, Piezo, Bang-Bang, etc., all have to go through the same orifice restrictions for controlled output. These are operated by the same air media or other compressible fluids (such as sweet gas, nitrogen etc.) Each technology has its plusses and minuses. Air or operating media, external factors, meeting ISA S7.01 guidelines, etc. need not pose any operational concerns/challenges for positioners after a long successful run of more than four decades.

Digital Valve Controllers when used for SIS applications, specific proprietary diagnostics (during normal operation and during partial stroke tests [PST]) are incorporated to overcome any technical challenges evaluated during failure modes, effects, and diagnostic analysis (FMEDA). These diagnostics run 24/7 checks on pneumatic systems of microprocessor-based devices. These Digital Valve Controllers already have more than a decade of proven-in-use operation for SIS applications providing confidence to process manufacturers for its usage.

Another safety list reader asked about a case where pneumatic positioners get dirt or water in them. This person noted that they’d seen water slugs due to some form of failure and suggested a solenoid or other suitable device be used between the positioner and valve. Riyaz responded [again with my light edits]:

Generally, P/P, E/P or microprocessor-based positioners’ I/P are designed for 40-micron particle sizes. Problems compound when moisture, water condensates or oil from compressors get into the air lines causing dust particles to begin to stick and grow in size. Though the process industries try to comply with the ISA S7.01 specifications for air quality, the general practice I’ve seen to avoid such cases is to have air filter regulators before the positioners associated with each valve.

If air particles due to the presence of moisture or oil starts binding or condensates begin to freeze up, it will restrict air passage. This causes the positioner to drive a valve to its minimum pneumatic output or fail-safe state for spring return actuators, which leads to a spurious trip.

Though I have yet to see this, it is a theoretical possibility for the I/P to drive a high pneumatic output, leading to a dangerous condition. Based on field experience, chances are minimal, but still should be considered during the FMEA [failure mode and effects analysis] stage and should be factored into the failure rates of the field equipment.

With the advent of microprocessor-based technology, applying basic physics laws to make a correlation between I/P current and air passage opening, intelligent field devices can diagnose and monitor build-up 24×7, on line, and will alert accordingly to the host system. Though rare and assuming worst-case conditions that this build-up remains unnoticed, it will be detected during the partial stroke test.

We’ll check back if necessary with Riyaz as the conversation continues on this thread.

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