Inherently Safe Design: A Way Out for Engineers Squeezed by New Rules on Quantitative Analysis

Related Vendors

Italvacuum, Srl

Donadon SDD Srl

ProMinent GmbH

Quantitative Risk Analysis

The two new standards specify precisely how to carry out a quantitative risk assessment.

Every risk situation is considered within a “risk tree”. First, the severity of any possible damage is assessed: from slight personal injury or environmental damage right up to a catastrophe with many fatalities.

The second branch of the tree considers duration. An engineer who is only exposed to danger during a weekly inspection round, for instance, is classified as being less at risk than the permanent operator of a machine.

Thirdly, the study considers whether the risk can be recognized so that the individuals affected can take defensive action. A hazard signaled by a noise or smell, for example, may allow people to escape or to trigger an emergency shutdown, while a sudden explosion gives no such warning.

EN IEC 61508 also includes the probability of occurrence as a fourth branch of the risk tree.

The result of the risk tree is a performance indicator known as the Performance Level (PL_r) (EN ISO 13849) or Safety Integrity Level (SIL) (EN IEC 61508). The PL_r or SIL defines the performance required to ensure that the machine provides adequate safeguards against this particular hazard.

The analysis requires that electronic and electrical components are considered in their entirety, including all sensors and actuators. Precise probabilities of failure play a key role in assessing the suitability of components for the construction of safety-related control circuits. This is a problem, because many component suppliers are only now starting to address the issue. For most components, the information is simply not yet available to calculate their PL_r or SIL classification.

But even if the relevant figures were available, many machine and plant constructors would face huge problems. The costs of developing safety controls is very high, and many sensible designs that have proved themselves over decades would have to be abandoned.

From Safety Controls to Inherent Safety

In many cases the best solution lies in the concept of inherent safety. Right from the original conceptual design stage of a new product or process, we should try to stop hazardous situations from developing in the first place. Only as a backup should we use controls to alleviate hazards.

(ID:35344730)