The safety instrumented function (SIF) is used to monitor a selected process parameter and takes an executive action to make the process safe, if certain limits are exceeded.
An illustration of safety instrumented function in a process plant is shown below:
The figure above illustrates a gas pipeline which supplies a feed to a power station. The gas passes from the left to right, through a shutdown valve before it reaches the pressure control valve (PCV). The PVC is controlled by a pressure controller (PC) which maintains the pressure of the gas to below 47.9 bar, the safe rating of the export pipeline. The failure of this pressure control function could lead to over-pressurization of the downstream pipeline, possible bursting of the pipeline, ignition and fatality and thus a safety function has been designed to safeguard against such a scenario. The safety function is made up of a separate pressure transmitter (PT), emergency shutdown (ESD) logic and a shutdown valve (SDV) which is actuated by a hydraulic solenoid operated valve (SOV) to shut off the gas supply in the event of the downstream pressure going beyond a pre-set trip level.
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