Proof test/‘trip testing

Class 1 safety instrumentation loops include alarms and trips on storage tanks containing flammable or toxic liquids, devices to control high temperature and high pressure on exothermic-reaction vessels, and control mechanisms for low-flow, high-temperature fluids on fired heaters. Other Class 1 instruments include alarms that warn of flame failure on fired heaters, and vapor detectors for emergency valve isolation and sprinkler-system activation. All of these alarms, shutdown valves, and other critical instruments are regularly proof-tested to a well-defined schedule. 

The sensor subsystem contributes an insignificant amount to the PFD of ESD2. That probably represents an opportunity to reduce the quantity of equipment and also reduce the lifecycle cost of manual proof testing. Comparing the failure rates of the switches that indicate closure of the valve to the failure rate of the radio system, it is clear that one of the two switches could easily be eliminated. This will reduce the false trip rate as well as lower capital and lifecycle cost. One could go further, however, by looking at the safety contribution of each of the sensor types. The PFD contribution of the ZS sensor subsystem (Gate Gil) and the PT sensor subsystem (Gate G12) is shown in Table 13-4. 

Operations and Maintenance records proof-test records trip reports near-miss evaluations diagnostic alarm performance records... 

The details of any demands on the system, and system performance on demand, should be recorded including data on any spurious trips, any revealed failures of the system or its components and, in particular, any failures identified during proof testing. 

Final element (Isolation valves, pump) should be tripped for a full proof test. 

Maintenance/proof testing for high level trip and alarm systems. 

On-line trip testing not really a diagnostic but a proof test. However if the valve can be arranged to trip fully shut (or open) without shutting down the process, perhaps by using parallel paths or bypasses then this test can be performed at regular intervals to improve the PFD of the valve. 

Clause 7.9.1 requires the design to allow for testing of the overall system, which means being able to demonstrate the final element response to sensor operation corresponding to a trip condition. On-line testing facilities are required when the intervals between scheduled process downtime is greater than the proof test interval. 

Reliability analysis results as shown in Figure 13.7 indicate that the SIL 2 target can be met with the assistance of proof testing at 2 times per year. The nuisance trip rate is predicted at 1.25 times per year based on the sum of the spurious trip rates found for the 3 elements of the loop. 

The first task is to calculate the spurious trip rate and the fail to danger rate (PFDavg) of a single channel version on the basis of proof testing every 6 months. 

Assume the 2003 relay stage uses 1 relay for the logic by using multiple contacts from the input stages. For the PFD and Spurious trip calcs assume the common cause factor is 10% and the proof test interval remains at 6 months. 

A 100% functional system test. In practice, this is only possible when the SIS is disconnected from the process. Hence on-line proof testing may leave a small fraction of the SIS untested. Also termed trip testing . 

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