Instrumentation/control systems process safety practices

Listed below are some process safety practices which can help reduce accidents due to instrumentation and control systems. 

All equipment used in the SIS must be classified as a safety instrumented system. The design, installation, operation and maintenance process must follow all the rules of ANSl/lSA-84.00.01-2004 (lEC 61511 Mod), put there to prevent systematic faults. If this is not done, the standard clearly states that any safety instrumented function cannot have a risk reduction greater than 10. This is the bottom of SlLl range so, in effect, that design cannot meet SIL 1 requirements. The practical effect of this requirement is that a designer cannot combine control functions and safety functions in the same equipment imless the equipment is classified as a safety instrumented system and follows aU the design rules of the standard. 

For example HSE Programmable electronic systems in safety related applications Vols 1 and 2 (HMSO, 1987). BS 5750 Quality Systems, Parts 1-6. BS 6739 Codes of Practice for Instrumentation in Process Control Systems Installation, Design and Practice. 

Certification does not remove the need for the end user to deeide if he can tolerate a failure of the control network and the safety system at the same time. This can be acceptable in machinery applications but requires great care in process situations. This area of technology is just emerging and it will take some time before its impact on process safety instrumentation practices will be clear. The position regarding the use of Foundation Fieldbus is also unclear at the moment as the original position adopted several years ago for ISA S84 was that Fieldbus should not be used in safety applications. 

The normal practice in the process industries is for the operations staff to take over ownership responsibilities for all instrumentation and control systems at the start of process commissioning, i.e. before process materials are introduced to the plant. This allows all plant safety procedures to be enforced from the moment there is any potential... 

Increasingly, newer fired process heater installations are adding more fuel-air combustion controls and safety instrumentation systems. However, the decision on the extent of fired heater combustion controls, instrumentation, and safety systems to employ is fundamentally a loss prevention and risk tolerance issue, rather than a fire protection one. The following recommended practices, codes and standards apply to fired heater and dryer controls and instrumentation ... 

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