Safety Systems for Non-Engineers

What All the Hullabaloo Over Safety Instruments Systems Is About?

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Looked where at the what?

"We did a preliminary look at the PFD of a few of these SIF's and we think there is something we can do in the BPCS."

PFD is another acronym, and it means Probability to Fail on Demand. The probability to fail on demand (PFD) can be calculated using the dangerous failure rate (λD) and the testing interval (TI). The mathematical relationship, assuming that systematic failures are minimized through design practice, is as follows:  PFD = λD * TI/2.  The equation shows that the relationship between PFD and TI is linear, thus longer times between tests results in larger PFD values.

PFD means that when a demand (in this case an unsafe condition) occurs, there is a possibility that an undetected failure in some element of the safety loop (SIF) will prevent the SIS from performing the necessary shut-down action.

In terms of automobiles, PFD is the likelihood that the air-bag will not deploy in an accident.
Adding PFDavg to the SIL table.

BPCS is defined in safety standards as, "system which responds to input signals from the process, its associated equipment, other programmable systems and/or an operator and generates output signals causing the process and its associated equipment to operate in the desired manner, but which does not perform any safety instrumented functions with a claimed SIL ≥1.”

The short definition is that the BPCS is what for decades has been called the process control system. The BPCS can be a single-loop, panel-based instrument, a distributed control system (DCS) or a programmable logic controller (PLC) with some type of operator interface.

What Gomer is telling Mr. B. is that he and his buddies took a cursory look at the probability that at the same time a demand occurred (unsafe condition existed) that one of the devices (sensor, logic solver, etc.) that made up the few safety loops (SIF) would actually fail (on demand) and prevent the automated shut-down logic from being executed. Gomer was also suggesting that there was a possible solution to preemptively detect these sorts of failures using the basic process control system (BPCS) – though his explanation was way too simplified.

What Gomer had in mind was to access the HART diagnostic information that often goes unmonitored in safety system devices. Since the Springfield Snacks and Pesticides plant already was using HART multiplexers to access the diagnostic information inside most of the process system transmitters, Gomer's thinking was that by adding another HART multiplexer just for safety system devices, they would be able to detect device failures before a demand thereby improving the SIS's reliability.

While I applaud Gomer's forward thinking use of HART diagnostics to improve SIS reliability, that solution may not be appropriate for every company.

Reaping the benefits of HART diagnostics requires that the organization embrace a proactive maintenance culture accompanied by an investment in HART diagnostic utilization training.

When installed properly, HART multiplexers can extract diagnostic information from safety system devices without influencing the reliability of the SIS.

We'll talk a bit more about the relationship between the BPCS and the SIS later. In the meantime, keep in mind that 21st-century operator interface terminals provide a window to a variety of systems and applications, including the BPCS, HART multiplexers, safety system logic solvers, data historians, online modeling and optimization applications, etc. The days of "different tubes for different views" is long gone.

Unsafe production

"Also Mr. Barns, ever since corporate insisted on extending the time between scheduled shutdowns, it has been playing havoc with our full- and partial-stroke testing periods.”

Full-stroke testing is the term used to define when each SIF (safety loop) is fully tested, meaning each discrete sensor is forced to its action state; each analog transmitter is forced to its action value; the logic solver is permitted to execute its programmed logic; and final elements are permitted to change to whatever state they've been instructed (i.e., on/off valves fully open or close).

Because full-stroke testing is a complete test of the automated shut-down system, it is usually only conducted when the process is shut down for scheduled maintenance (i.e., scheduled turnaround).

Note: Remain aware that testing the SIS when the process is shut-down means final elements are not working against "true" process conditions (i.e., flow, pressure). The most serious risk is that a valve might have undetected leak through.

Partial-stroke testing is almost always a test of final elements (i.e., on/off valves). Partial-stroke testing is the term used to define how parts of an SIF (safety loop) are tested without actually permitting devices to go all the way open/close. When properly performed, partial-stroke testing should never interrupt production. Note: Partial stroke testing is conducted under actual process conditions.

Note: Clamping mechanical travel stops to the valves shaft permits manual partial-stroke testing, but often eliminates other final device elements from the test. You may only take credit for the SIS elements you actually test.

During the design of a safety instrumented system (SIS), several assumptions are used in calculations, including establishing the safety integrity level (SIL) value assigned to each safety loop (SIF). One of those assumptions is the amount of time that will elapse between when each safety loop (SIF) can be full-stroke tested. The longer the time between each full-stroke test, the greater the PFD thus the greater the SIL value that is required for that safety loop. Note: PFD (probability to fail on demand), MTTD (mean-time-to-detect, and MTTR (mean-time-to-repair) are some of the assumed values used in determining an SIS's design requirements.

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