Basic Engineering Practice and Safety
Core concepts
Candidates must read P&IDs and ISA symbology, distinguish a PFD from a P&ID, and follow loop numbering and documentation. Know the safety lifecycle: hazard analysis, HAZOP, LOPA, SIL determination, SIS design, proof testing, and management of change. Understand fail-safe design, de-energize-to-trip philosophy, redundancy, diagnostics, and bypass controls. Electrical area classification, intrinsic safety, purging, grounding, and shielding are common construction and safety topics. Commissioning includes loop checkout, calibration records, trip setpoint verification, alarm testing, and as-built documentation.
Traps
SIL is assigned to a safety function or loop, not to a single device. A SIS must be independent of the basic process control system; sharing a transmitter can defeat that independence. Fail-safe usually means de-energize to trip, so loss of power or air must be evaluated. An intrinsically safe barrier is not simply a fuse. Do not treat a PFD as a P&ID; the P&ID carries instruments, interlocks, and control loops. Bypasses and overrides need authorization and tracking.
Worked example
For a SIL 2 safety function, the average probability of failure on demand, PFDavg, is between 1×10^-3 and 1×10^-2, corresponding to a risk reduction factor of 100 to 1000. Verify the loop PFD by combining sensor, logic solver, and final-element contributions, then confirm proof-test intervals and diagnostics support the claimed SIL.