The greater the risk, the higher the SIL

Steve Meadows of Asco Joucomatic explains the function of Safety Integrity Levels in the new draft IEC61511 standard for the process industry sector.

The continuing drive to improve upon the performance, reliability and standardisation of safety systems has led to the development and introduction of a number of international standards including IEC61508 - "Functional Safety of electrical/ electronic/programmable electronic safety related systems", and IEC61511 "Functional safety instrumented systems for the process industry sector".

IEC61511 is a draft international standard that has been developed as a process sector implementation of IEC61508, the latter a generic standard, which applies to any industry that uses electrical systems to perform safety functions.

IEC61508 can be looked upon as an umbrella standard from which more industry specific standards such as IEC61511 emanate.

As evidence, of this IEC61508 was the first IEC standard to define safety integrity levels, which are measures of the safety of a given process.

These same SIL are also one of the cornerstones of IEC61511.

Safety Integrity Levels (SIL) refer specifically, to the probability of failure on demand (PFD) - or to what extent the end user can expect his process to perform safely, and in the case of failure, fail in a safe manner.

SIL have been the subject of much confusion in the market, as they have been construed as a strict indicator of reliability.

In fact reliability is a byproduct of SIL, experience in the field revealing that many of the parameters central to Safety Integrity Levels, once optimised, provide added reliability and up time for the processes concerned.

What is important to note is that individual products do not carry SIL ratings they can only be certified for use within a given SIL environment.

In its draft form, IEC61511 defines a total of four Safety Integrity Levels, with SIL 4 being the highest achievable safety level.

Level 1 is defined as minor property and production protection.

Level 2 refers to major property and production protection (possible personnel injury).

Level 3 covers employee and community protection.

And Level 4 covers catastrophic community impact.

By selecting devices that are rated for use within a desired SIL environment, end users can optimise their processes for risk.

As an example of this, if the required SIL value for a process is SIL 3, then by using components rated for use within an SIL environment this objective may be achieved.

However, simply combining process components rated for use in a given SIL rated environment does not guarantee the process to be rated at the specified SIL.

To determine whether a safety process loop fulfils a certain SIL, a simplified calculation technique can be employed to determine the PFD (probability of failure on demand) factors for each component in the loop.

The PFD for the loop (PFDtotal) can be determined by the sum of the PFDs of each subsystem.

It should meet the range of PFD, which relates to the required SIL value.

(Refer to Asco Joucomatic for a copy).

Although the main focus of SIL ratings is the statement of the inherent safety of a process, an important byproduct of the statistics used in calculating SIL ratings is the statement of a product's reliability.

To ascertain whether a product such as a process valve can be used in a given SIL environment, the product must be shown to "be available" and up and functioning when needed to perform its assigned task.

A number of considerations are taken into account when determining "availability", including mean time between failure (MTBF) and probability of failure on demand (PFD).

These considerations, together with variations based on the system architecture, determine the reliability of the product.

Subsequently, this reliability data, combined with statistical measurements of the likelihood of the product to fail in a safe manner, known as the Safe Failure Fraction (SFF) determine the maximum rated SIL environment in which the device or devices can be used.

The benefits to users of using products certified within given SIL environments are many.

First, there is the ability to purchase a "black box" in respect of SIL requirements.

Secondly, with such a package reliability calculations are already performed and available to the end user.

Thirdly, the user has the security of knowing that a third party has reviewed the reliability statistics for the product.

Finally, and most important, is the attached certification report that accompanies each product.

This contains important information ranging from restrictions of use to diagnostics within the device and also reliability statistics.

Several of Asco Joucomatic's process valves, such as the 327 Series of high flow, balanced poppet valves have TUV certified functional safety data and can be used up to SIL-4/AK-7.

In addition, Asco also supplies factory assembled and tested safety enclosure systems which provide added levels of operating safety in emergency shutdown systems (ESD) and other critical applications in water treatment, pharmaceutical and petrochemical industries.

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