Safety Systems for Non-Engineers

What All the Hullabaloo Over Safety Instruments Systems Is About?

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LoPA is short for Layer of Protection Analysis. It is a simplified form of risk assessment that is often used as an extension to or in conjunction with other process hazard analysis methodologies. LoPA considers the different and diverse hazard mitigation "layers" available. For example, two layers of protection typically provided for holding tanks is 1) an automated overfill protection system and 2) locating the tank inside an enclosed dyke or berm area.

SIF (Safety Instrumented Function) is defined in safety standards as a "safety function with a specified safety integrity level which is necessary to achieve functional safety and which can be either a safety instrumented protection function or a safety instrumented control function." That's really not much help.

To really understand and appreciate the nuances of the SIF will require some additional homework on your part, but for now accept that the SIF is the safety loop equivalent of a single process control loop―something akin to a flow, pressure, level or temperature control loop. But instead of controlling level to an operator determined setpoint, the SIF (safety loop) monitors for a pre-determined unsafe condition (i.e., high level) and automatically initiates the appropriate action necessary to mitigate the unsafe condition.

SIL (Safety Integrity Level – see table) is defined in safety standards as a, "discrete level (1-4) for specifying the safety integrity requirements of the safety instrumented functions to be allocated to the safety instrumented systems. Safety integrity level 4 has the highest level of safety integrity; safety integrity level 1 has the lowest."
SIL values are assigned to each SIF using a systematic methodology, such as LoPA.

Note: When the SIS logic solver is microprocessor-based (i.e., DeltaV SIS, Triconex, GE Fanuc, Allen-Bradley GuardLogix, Honeywell Fail Safe, etc.) it will very likely host multiple safety loops (SIFs) with different SIL value assignments.

Even among experienced safety system practitioners, SIL assignments are often misunderstood. Concern often arises when an identified risk is assigned a SIL 3 value. What some people have overlooked is a small footnote in the safety standard that permits dividing the mitigation of a risk across multiple SIL values. For example, rather than incur the life-cycle costs associated with a single SIF (safety loop) classified as SIL 3, you are permitted to split the SIF into "mini-functions" meaning you may, for example, implement both a SIL 1 and a SIL 2 solution for that particular risk and still meet the SIL 3 requirements.

What Gomer was explaining to Mr. Burns was that he and his buddies had systematically examined the hazards using two independent methodologies (HAZOP and LoPA) and had found that several of the safety loops (SIFs) were not assigned the correct risk reduction values (SIL), thus the plant's safety instrumented system (SIS) wasn't providing the level of protection expected or needed.

Good news & bad news

"We also found a few SIL 2 SIFs that could qualify as SIL 1s, but since we've always maintained diversity among our SIS logic solvers, we can't just reclassify these SIFs."

It sounds like Gomer is talking in circles, but he really isn't.

What Gomer is telling Mr. B. is that a few of the safety loops (SIFs) were originally designed and are now being operated, and maintained as SIL 2 (risk reduction 100 to 1000) when in fact the risk is not very severe, so a few SIFs could be designed, operated, and maintained as SIL 1 (risk reduction 10 to 100).

Gomer's good news is that the company is spending more money to operate and maintain these improperly assigned safety loops than is necessary. Gomer's bad news is that there is a safety instrumented system (SIS) logic solver for each SIL value (diversity) – "…we've always maintained diversity among our SIS logic solvers..." – meaning there is a logic solver that hosts all the SIL 1 safety loops, another logic solver that hosts all the SIL 2 safety loops, etc. In order for the Springfield Snacks and Pesticides Plant to maintain its logic solver diversity practice requires physically moving each reclassified SIF―wiring, program logic, documentation, etc.―from one logic solver to another.

Gomer's implication is that this requires some serious change control procedures and follow-up testing of both logic solvers to ensure each is working properly after all the changes have been completed.

Note: Safety standards do permit different SIL-valued safety loops to reside in the same logic solver. However, doing so means that the logic solver must always be designed, operated and maintained per the requirements of the highest-valued SIL safety loop residing in that logic solver.

A frequently overlooked footnote in the IEC 61511-1 safety standard is Note 2 of paragraph 3.2.74 which says, "It is possible to use several lower safety integrity level systems to satisfy the need for a higher level function (for example, using a SIL 2 and a SIL 1 system together to satisfy the need for a SIL 3 function)."

A common application of this concept often occurs in reactor designs where there is a need to prevent reactor run-away. You can design a single SIF that will likely be classified as SIL 3; OR you can design two SIFs, each with lower SIL values.

In choosing the second option, the first SIF is designed to close/open valves, stop/start pumps, etc., bringing the process to a safe state. The second SIF is designed to inject a "kill" solution (chain-stopper) into the feed stream. If one of these SIFs achieves a SIL-1 risk-reduction value, which is easy to achieve, and the other a SIL 2 risk-reduction value―not very difficult to achieve―then when combined, the two designs provide the same risk-reduction as a single SIL-3 design. However, and here's the best part, both SIFs can likely be cost effectively designed to achieve a SIL-2 risk-reduction value, thus producing an overall more reliable solution without incurring the life-cycle cost typically associated with a single SIL-3 design.

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