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

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Example From The Water Treatment Industry

An example from the water treatment industry shows how lateral thinking can eliminate hazards at source. In the on-site generation of chlorine dioxide for disinfection, sodium chlorite solution and hydrochloric acid are metered into a reactor together with dilution water. If the water pump fails, an explosive concentration of chlorine dioxide can form in the reactor.

The traditional EN 954-compliant solution is to add safety systems to ensure dilution. First, a software interlock only permits the metering of chemicals in conjunction with the metering of water. Second, flow sensors ensure that the metering strokes triggered by the controller actually produce a flow of water. Third, the flow sensor signals are evaluated separately by a second controller, and if the signals are not validated the system shuts down immediately.

Under the old rules, this arrangement provided one more layer of safety than was actually required. Since all three layers depend on control systems, however, under the new rules they would have to be quantitatively assessed in line with EN ISO 13849 — at an impractical cost.

Additional Information

Key Facts for Users

Under the new requirements, risks in machines and systems associated with controls must be evaluated quantitatively as well as qualitatively.

The risk tree delivers a performance indicator known as the Performance Level (PL_r) (EN ISO 13849) or Safety Integrity Level (SIL) (EN IEC 61508).

Controls must be constructed in such a way that they meet the relevant PL_r or SIL required to overcome the hazard.

In future, companies would be well advised to pay attention to simple, transparent and inherently safe processes even at the conceptual design stage.

This accords with the strategic background of the new standards, which is that hazards should be prevented directly where possible, and only mitigated by controls as a second choice.

Re-thinking the Chlorine Dioxide Process

Water treatment specialist ProMaqua has developed an alternative solution that includes equipment to pre-dilute the concentrated hydrochloric acid to around five percent. This eliminates the risk that both concentrated chemicals could become mixed without dilution water due to a control fault. Since the pre-diluter operates purely hydraulically and mechanically, it does not have to undergo a risk analysis according to EN ISO 13849.

The separate pre-dilution process also improves safety at a wider level, because it allows the two concentrated chemicals to be stored completely separately, and it increases flexibility for users by allowing varying concentrations of hydrochloric acid to be used.

This example shows that the tightening of directives and standards does not always have to lead to obstacles. Taking a wider view, the opportunity to question older processes can generate new and better solutions.

* The author is Technical Director with ProMaqua GmbH, Heidelberg/Germany.Contact: Phone +49 (0) 62 21 / 64 89 - 0

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