Safety instrumentation systems action taken

An important issue to be considered at an early stage is whether there are any common oause failures between redundant parts within each layer (for example, between 2 pressure relief valves on the same vessel), between safety layers or between safety layers and the BPCS. An example of this could be where failure of a basic process control system measurement could oause a demand on the safety instrumented system and a device with the same characteristics is used within the safety instrumented system. In such cases it will be necessary to establish if there are oredible failure modes that could cause failure of both devices at the same time. Where a common cause of failure is identified then the following actions can be taken. 

Which measurements need to be taken to detect the onset of the hazardous conditions. A simple example could be that a pressure rise above a specified vaiue needs to be detected. The value of the parameter at which action shouid be taken wiii need to be outside the normal operating range and less than the value that will result in the hazardous condition. An allowance will need to be made for the response of the system and the accuracy of measurement. In setting the limit, there will therefore need to be a discussion with those responsible for the safety instrumentation system design and implementation. 

Safety instrumented systems are designed to respond to conditions of a plant that may be hazardous in themselves or if no action were taken could eventually give rise to a hazard. They must generate the correct outputs to prevent the hazard or mitigate the consequences ... 

An operator-initiated SIF is often associated with a never exoeed never deviate alarm, where the operator is expected to mitigate risk in much the same manner as an automated SIF. Operator-Initiated SIFs are generally used when it is not possible to completely automate the function. The manually initiated action is typically comprised of the sensor detecting the hazardous condition, the logic solver that determines that the safety condition exists, alarm presentation, human response, and the equipment used by the operator to bring the process to a safe state. When risk reduction is taken for an operator-initiated SIF, the PFDavg should be determined for the instrumented system. This is discussed further in B.6. 

Demand mode SIF Demand mode safety instrumented function (SIF) where a specified action is taken in response to process conditions or other demands. In the event of a dangerous failure of SIF, a potential hazard only occurs in the event of a failure in the process or the basic plant control system. 

The Data Processing System displays all operating parameters and the state of the plant. Alarms are displayed separately. Operator action is taken immediately to clear operational faults. Plant defects relating to safety are investigated and cleared if possible in 1 day. Lesser defects are cleared within 3 days. Reasons for all standing alarms are recorded for the information of all operational staff. Major faults may result in the immediate shutdown of the reactor or wait for a planned outage. Safety related plant is covered by alternative conventional instrumentation as backup to the computer-derived data. 

Inspection is a crucial part of any operating and maintenance system. It is vital that equipment and instrument items that could fail—particularly where that failure could lead to a safety or environmental violation—are monitored, and that corrective action is taken before problems arise. 

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