In larger facilities of the process and energy industries there are few plants that do not have one or more control rooms which are staffed by operators responsible for overseeing the smooth operation of the manufacturing or generating systems. It seems that the higher the level of investment in process and control systems, advanced control and in automated intelligence, the more likely there are to be humans keeping watch for potentially harmful out of specification situations so that appropriate action can be taken sufficiently early to prevent a disaster.
Introduction
When things start to go wrong, audible and visual alarms are the cues to which these control room operators react. The model is clearly reactive in nature: An alarm action causes an operator reaction. Training of operators in terms of what they need to do in any such event is important. But way back in the engineering phase, how was it decided that some particular parameter should be alarmed?
Poor alarm management is commonplace
In many incident reports by highly respected investigating bodies, poor alarm management (generally alarm flooding) is cited as a significant contributing factor to the incident. The UK HSE report on the Texaco incident at Milford Haven, Wales, in 1994, refers to alarm flooding and too many alarms defined as high priority (87% of 2040 alarms) despite being informative only. In the ESSO Longford incident up to 8500 alarms were reported. Refer to Table: 'Incidents in which alarm management cited' for more information from official incident reports.
What standards and guidelines are available?
As in most engineering disciplines there are bodies of standards, recommended practices and regulations which help to manage the process of alarm engineering.
EEMUA 191
To date, the de facto standard for alarm management practices has been the EEMUA document 191 'Alarm Systems: A Guide to Design, Management and Procurement'. The second edition of this was launched in June of this year.
ANSI/ISA 18.1 and 18.02
The existing standard: 'ANSI/ISA-18.1-1979(R2004), Annunciator Sequences and Specifications', that falls under this committee dates back to 1979, and was revised in 2004. This is primarily for use with electrical annunciators that call attention to abnormal process conditions by the use of individual illuminated visual displays and audible devices.
The ISA's SP-18 'Instrument signals and alarms' committee is busy working on a new standard tentatively titled 'ISA-18.02 Management of Alarm Systems for the Process Industries' and this is expected to result in a new ANSI standard being adopted in 2008. The next draft of this is expected to be available on 1 September 2007, and a further draft (final?) is planned for 1 January 2008.
IEC 62241, 61508 & 61511
IEC 62241 specification covers 'Nuclear power plants - Main control room - Alarm functions and presentation'. The IEC seems to be more interested in alarm systems for premises than in process alarm systems. IEC 61508 covers functional safety-related matters for electrical, electronic, and programmable electronic safety-related systems while IEC 61511 covers functional safety for instrumented systems in the process industry sector.
IEC 61508 mandates a set of key criteria to be met by a plant to demonstrate that appropriate functionality is built into trip and alarm equipment, and that it is maintained effectively. The specification requires that the design basis for trip and alarm systems must be documented and traceable and that the plant has onus of proof that it is testing the system for functionality and reliability by appropriate methods.
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