Validate, start-up, operate and maintain the strategy

Validation demonstrates that the installed SIS operates according to the design basis and that appropriate documentation is in place to support its long-term management. An input-to-output test is used to prove that the SIS functions properly and that the SIS equipment interacts as intended with other systems, such as the BPCS and operator interface. The Site Acceptance Test (SAT) also provides an opportunity for a first-pass validation of the operating and maintenance procedures. Validation must be completed prior to the initiation of any operating mode where a hazardous event could occur that would require the operation of a new or modified SIS. Some users require that validation be repeated after any major process outage or shutdown. 

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Installed safety equipment is subject to the same operational stresses as control equipment. However, when control equipment fails, the failure can be detected because the process behaves abnormally. In contrast, safety equipment typically operates on demand only, i.e., when an abnormal condition occurs, so failure may not be detected until it is required to act. Equipment often demonstrates a failure rate over time that follows a so-called bathtub curve. 

Early failures are often caused by manufacturing, assembly, test, installation and commissioning errors. Many early failures are the result of rough handling, improper pre-installation storage, poor installation practices, or sloppy construction practices. Rigorous inspection, commissioning and validation activities are necessary to identify and correct these failures. 

The wear-out period is characterized by an increasing failure rate over time. Poor mechanical integrity practices have been cited as a primary cause of equipment failure (HSE, 2005). Preventive maintenance can extend equipment useful life and improve its reliability. Mechanical integrity records provide data that equipment is being maintained in the as good as new condition and Justify its continued use. Consequently, maintenance personnel must be trained on the activities necessary to ensure equipment integrity. 

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Validate, start-up, operate and maintain the strategy implement the SIS following the design basis and detailed design documentation and define what is required of operation and maintenance personnel to sustain the SIL. 

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