Don’t Be Alarmed, Be Informed

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Alarm systems in large process plants should guide and inform operators without overwhelming them. This requires a structured approach, says Jeremy Gadd, Head of Instrumentation, Control & Automation at GSE Systems.

In July 1994, an electrical storm at the Texaco Milford Haven refinery set in motion a chain of events that ultimately resulted in the release and explosion of 20 tonnes of flammable hydrocarbons. The blast injured 26 people and caused £48 million in damage. The subsequent fires burned for three days.

Like most significant industrial accidents, the Milford Haven explosion was caused by a number of technical and management failures. A key area of concern in the HSE’s investigation of the incident, however, was the role played by the plant’s alarm system. In the final 11 minutes before the explosion, two operators had to recognize, acknowledge and act on 275 separate alarms.

For any owner of a large process plant, especially if their site falls under the Control of Major Accident Hazards (COMAH) legislation, proper alarm management is a vital part of overall functional safety. Alarm system design and implementation has a direct impact on functional safety, since it will determine how a Safety Instrumented Function (SIF) operates as an independent layer of protection, and will also affect operator performance. ‘Alarm floods’, where operators are overwhelmed by multiple alarms, have been seen in many major incidents. Likewise, frequent spurious alarms can mean operators become desensitized to their significance, or may take unauthorized action to override the alarm.

There are a number of benchmark standards for the design and operation of alarm systems. HSE’s guidance for COMAH inspections uses EEMUA191 Edition 3, for example. Other standards include IEC 62682 and ISA 18.2. Whatever benchmark your organisation uses, effective alarm management requires three interlinked elements: a clearly defined ‘alarm philosophy’; comprehensive and accurate data; and a robust process for auditing, rationalizing and continually improving the performance of the alarm system. Read More

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