Applying Hazardous Risk to Safety Integrity Levels
W hen people think of automation applied to industry, they think of increased productivity and profits. But when specialized systems are applied for the purpose of critical control, the goal is not solely one of protecting the bottom line. Instead, the goal is achieving the delicate balance between safety and productivity. How user representatives evaluate and rank hazards and risk can result in either an over-protected process that shuts down too 'easily,' or an under-protected process that shuts down too late, or perhaps never. Achieving the right balance is important to ensure a safe and a productive process.
Factors requiring consideration to arriving at the right balance for a Safety Instrumented System (SIS) include:
Analysis tools, such as Hazard and Operability (HAZOP) studies;
Industry accepted components and technologies;
Probability to fail on demand (PFD);
Mean time between failure (MTBF) data;
Industry accepted fatal accident rates;
Severity of service; and
When examining and evaluating each factor independently, user evaluators can quickly become confused and may design an over- or under-protected system. Treating the evaluation process like a picture puzzle with each individual piece adding clarity to the final results is more likely to deliver a SIS well suited for the industry and the process.
Falling in place
Understanding how the evaluation of individual factors can produce an inadequate SIS is revealed by municipal transit systems. If safety standards (e.g., ISA/S84 and IEC 661508) developed and used by the chemical processing industry are applied to municipal transit systems, the third-rail power de-energization and stuck door detection systems essentially have zero safety integrity. [Safety integrity levels (SIL) are defined as the probability of the SIS to fail on demand. A demand occurs whenever the process to be protected reaches the trip condition and causes the SIS to take action. For additional information, read ' Understanding Safety Integrity Levels .']
A key factor causing the municipal transportation industry to have such low safety integrity levels results from the use of 'energize to trip,' relay-based systems with a test frequency often exceeding five years, and untested, nonredundant field sensors and control devices. Although many of these systems are being upgraded to programmable logic controllers, common industry practice continues to implement nonredundant technologies with an 'energize to trip' methodology. Such human life protection systems would be assigned much higher SIL requirements in other industries.
How can municipal transportation agencies justify the low SIL requirement when assuming the risk for travelers, security officers, or maintenance workers contacting the 750-volt third rail? It's simple; they accept a higher fatality rate and apply a low demand rate to the statistically-based system performance calculation. The result is a very low SIL requirement level.
At the other end of the spectrum, industries such as petrochemical are expected to operate within industry-established guidelines and mandated 'best engineering practices.' For safety applications, the operating philosophy is typically 'de-energize to trip,' resulting in an automatic shut down of the process. Traditionally, these industries accept very low fatality rates, thus requiring safety integrity levels as high as three or four. Additionally, process industry engineering guidelines and associations bring to light issues such as MTBF data sources and severity of service issues, thus avoiding the pitfall of using a device's most favorable MTBF data and ignoring the severity of service environment impact resulting in an artificially low SIL requirement.
For example, a sensor or final control device tested at room temperature, under moderate conditions, will perform according to the published MTBF data as long as the device is applied in those same conditions. Most likely, applying that same unit in a corrosive environment subject to extreme hot and/or cold temperatures, will dramatically shorten the devices MTBF. Overcoming the reduced MTBF brought on by changes in the environmental surroundings is commonly achieved by applying redundancy and/or increasing the dynamic testing frequency to achieve the required level of safety integrity system performance.
Even though these two scenarios are based upon divergent industry philosophies, they serve to emphasize how many different ways the effectiveness of the SIS can be affected. In the final review, it is the integrity of the hazardous risk analysis, combined with the reliability of the applied data, which will strike the balance and define the applications true SIL value. Only then can an appropriately rated SIS be specified and correctly applied.
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