MAY-JUN 2019

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36 INTECH MAY/JUNE 2019 WWW.ISA.ORG SPECIAL SECTION with the same calibration range and configuring discrepancy alarms for them ● having permanently or temporarily installed pressure gauges to compare to the nearby pressure transmitters' readings ● local level gauges (sight glass, magnet- ic level indicator) or infrared cameras to cross check the level instruments' (displacer, differential pressure, radar, etc.) readings ● checking the online temperature sensor reading with a temperature gauge, a test temperature element, a portable temperature detector, or with an infrared detector/camera. SIS preventive maintenance Diagnostics provided by the logic solver, input devices (like transmitters), and output devices (like emergency isolation valves) should support the overall pre- ventive maintenance program for the (SIL) study/verification is conducted. The SRS specifies the required testing intervals for the SIS equipment (logic solver, input and output devices). To define and further optimize the safety instrumented system (SIS) by re- ducing the required physical preventive maintenance. The safety requirement specification (SRS) document should be developed after the safety integrity level Figure 2. Probability of failure on demand calculation and safety integrity level Instrument description PM task performer Online/ turnaround PM interval Task description SIS input devices Maintenance Online Per SIL study • Conduct visual check (leaks, grounding, insulation, cables, terminations and electrical fittings, heat tracing, corrosion, enclosure/cabinet doors). • Zero check and functional check (use IAMS, if available for GWR, noncontact radar, and similar devices) SIS valve Operation Online Per SIL study • Conduct visual check and conduct partial/full stroke testing. Maintenance Turnaround • Conduct full stroke test. • Conduct leak test without removing the valve if possible. • Replace the valve soft parts, per manufacturer recommendations. Interposing relay Maintenance Turnaround • Conduct functional check for the relay. • Confirm relay mounting and wiring are well tightened. • Inspect relay for any sign of corrosion, color change. Skid (instrument air/nitrogen/ hydrogen) Maintenance Online Per manufacturer • Secure a complete set of identical valves for one skid. During PM, remove the com- plete valve set, replace with the spare set, and return the skid back to operation. • For the removed valves, inspect and replace the valve's soft parts. Control valve Operation Online Six months • Visual inspection: Check for leak, vibration, heat tracing and insulation, corro- sion, loose wiring, air regulator and filter, pressure gauges, auxiliary devices, hunting. Compare DCS signal versus valve opening, noise, and enclosure tightness. • Stroking if valve at fixed position for long time. Maintenance Online Two years • Check diagnostics via IAMS or a laptop. • Apply lubrication per manufacturer recommendations. • Stroke check and leak test prior to T&I. Maintenance Turnaround (based on findings from above task) • Overhaul and check. • Replace soft parts as recommended by manufacturer. • Conduct functional test. • Perform leak test. Recommended preventive maintenance for process instrumentation Part 1 Safety integrity levels and performance requirements (for the entire system) ISA S84 safety integrity levels (SIL) Safety availability Probability of failure on demand (pfd) (1 – safety availability) Risk reduction factor (RRF) or (1/pfd) 3 99.9% – 99.99% 0.001 – 0.0001 1,000 – 10,000 2 99% – 99.9% 0.01 – 0.001 100 – 1,000 1 90% – 99% 0.1 – 0.01 10 – 100 0 Process control – not applicable DU PFD avg × TI 2 [ ] PFD avg, SIF = PFD Sensors + PFD Logic Solver + PFD FE + PFD Power Supplies

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