Before the holidays, Dave Harrold wrote a post, A Wee Bit More About Safety Instrumented Systems, in his Dave @ AFAB Group blog. He describes his work with Dr. Angela Summers, founder/president of SIS-Tech Solutions on a guidelines book for the global IEC 61511 safety standards. Dave also referenced an SIS-related Q&A article Angela wrote for Flow Control magazine.
I forwarded the post and Flow Control article link to Riyaz Ali, whom you may recall from an earlier post. Riyaz wanted to add to the conversation and make three specific points in reference to the Flow Control article.
On the question regarding the use of digital valve positioners to perform partial testing and its relationship to the proof test interval, Riyaz agrees that the proof test is far more than a partial stroke test. The proof test can be performed on a final control element either on-line when a bypass valve exists or offline when the process is shutdown, such as during a plant turnaround. Many process manufacturers do not have large bypass valves and seek to extend the interval between plant turnarounds as long as possible. The on-line partial stroke testing provided by digital valve positioners can help extend the time between proof tests. They do not replace these tests. Riyaz points to a Control Engineering magazine article authored by Dr. Summers, Partial Stroke Testing of Safety Block Valves, in which she points out:
Also affecting the SIL is diagnostic coverage and testing intervals of partial-stroke testing to supplement full-stroke testing to reduce a block valve’s PFD.
Being a mechanical item, testing of SIS “Final Control Element” offers challenges but at the same time represents a significant failure contributor to SIF loop. Partial stroke test by digital valve positioners not only allows “audit documentation” but also allows diagnostics health of valve, a key feature to improve reliability of SIF loop.
Riyaz did take exception to a statement in the article about throttling valves:
Positioner failures are the leading cause of control failure, so the positioner should not be used to actuate the valve in an SIS application when preventing events associated with a loss of control. Instead, a solenoid-operated valve should be used to independently close the control valve.
He notes that control valves are better geometrically designed with proper actuator and valve plug connection to reduce hysteresis, dead motion, sticktion, backlash etc., compare to shut down valves those are typically keyed shaft and mainly used for On and Off function. The main concern for shut down valves is stuck condition. If initial inertia force is broken during normal exercise of valve either through partial stroke test or by modulating through DCS signal, it is very likely that valve will be available during a safety demand, when required to bring the process to safe state.
His final point is on the question regarding smart positioners for partial stroke testing of smart valves. Positioners operated by air have been used in process control industries for years to improve performance of control loop. It is becoming rarer to come across a process loop not without positioners, especially where the application improved process variability. Based on its usage and benefits in process control, process manufacturers have started using them for Safety Instrumented Systems also. Riyaz agrees with Dr. Summers comment that positioners have smaller orifice but any thing larger than 8″-12″ size valve, even otherwise a Quick Exhaust Valve or similar mechanical device will be used, if fast stroking speed is desired. Len Laskowski adds that the driving factor is process safety time. Many times larger valves do not need to close in one or two seconds, and in fact require a more controlled closure to avoid negative effects on process and utility equipment. It all hinges on the process safety time for each application.
Positioners by design are to bleed very small air to keep the air flowing as well keep pressure higher than atmospheric so as avoid any external atmospheric corrosive gas getting inside the housing. Also during partial stroke test positioners exhaust and fill the air, which makes its mechanical parts moving and avoid any build up.
Digital valve positioners allows partial stroke testing, while process is running and provides date and time stamp of test with capability to store and compare test results. Also, being a microprocessor based, these positioners allow remote testing and retrieval of data remotely. The main advantage is predictive maintenance by providing valve degradation analysis, which is important to critical valves in safety related systems. If by any chance valve is stuck, digital valve positioners are capable of providing alerts to operators to fix the problem.
