Answer
Setting an EGB safety valve requires specific precautions because you cannot isolate the economizer from the main engine's hot exhaust gas stream while the ship is underway.
Under statutory maritime regulations and Classification Society rules, setting boiler safety valves must be carried out under the supervision of a Class Surveyor. However, in emergency scenarios or post-overhaul adjustments at sea, a Chief Engineer holding a Class 1 Certificate of Competency (CoC) may perform the adjustment, subject to official written notification sent to the Class Society prior to arrival at the next port.
Preparation & Pre-Checks
Calibrated Gauges: Mount two freshly calibrated master pressure gauges on the drum/economizer local monitoring point to ensure redundant, high-precision readings.
Mechanical Integrity: Ensure the safety valve has been overhauled, the valve stem/spindle is verified straight, valve seats are properly lapped, and the drain line from the valve chest is clear and free of obstruction.
Free Movement: Confirm that spring compression rings are removed and the valve stem/spindle can move freely up to a lift of $D/4$.
Safety & Easing Gear: Disconnect the manual easing gear before beginning the adjustment process.
Preparation & Pressure Accumulation: Isolate the EGB steam lines from the auxiliary boiler. Accumulate steam pressure within the EGB. This is typically done during a tightly controlled sea trial where the main engine is run at a steady load to generate sufficient exhaust gas thermal energy.
Fitting the Mechanical Gag: EGBs are fitted with twin safety valves. Fit a specialized manufacturer-supplied mechanical gag onto the spindle of Valve 'A'. This locks its disc securely against the seat, preventing it from lifting while you test the opposite valve.
Adjustment of Valve 'B': Slowly raise the steam pressure until Valve 'B' lifts. Adjust the spring compression nut until the valve pops at exactly its designated overpressure limit—which is typically no more than 10% above the maximum allowable working pressure (MAWP) of the plant. Ensure it exhibits a clean reseating blowdown.
Reversing the Setup: Once Valve 'B' is set, drop the steam pressure, transfer the mechanical gag to Valve 'B', and repeat the identical adjustment process for Valve 'A'.
Final Operational Check: Remove all gags. Raise system pressure once more to verify that both valves lift at their respective set pressures and reseat cleanly without chatter or leakage.
Surveyor Certification: The attending Classification Surveyor must physically witness the lifting and accumulation trials. They will place a lead security seal and stamp on the valve caps to prevent unauthorized adjustments, subsequently issuing the statutory certificate credit.
Set Pressure Limits & Statutory Criteria
Maximum Pressure: Safety valves must be set to lift at a pressure not exceeding 103% of the approved maximum working pressure (MWP) / design pressure (or as stipulated by Class guidelines).
Differential Setting (Staggered Opening): For dual-valve setups:
Valve 1 (Primary): Set to blow off at 100% of Design Pressure.
Valve 2 (Secondary): Set to blow off at 102%–103% of Design Pressure to prevent unnecessary simultaneous blowdown during minor overpressure events.
For Forced-Circulation EGB Units (Separate Drum Type): The EGB heating coil safety valve is typically set higher than the auxiliary boiler steam drum safety valve to prevent steam flashing in the economizer tubes and to account for circulating pump head pressure.