Repeated Auxiliary Boiler Low-Water Trip with Cargo Heating Demand
At 04:20 the auxiliary boiler, rated 1.5 t/h at 0.7 MPa, trips on low water level for the third time in 24 hours. Steam demand is high because of cargo tank heating. The feedwater pump is a reciprocating unit with discharge pressure fluctuating between 0.45 and 0.85 MPa. The gauge glass shows a level about 40 mm above the bottom of the visible range, but the control panel indicates 62% and the low-water alarm has just reset. The duty engineer suspects a faulty level electrode and proposes to bypass the low-water trip and run the boiler in manual to keep cargo heating. There are no spare electrodes on board; the next port is 36 hours away. The boiler is a composite oil-fired unit with an economizer; the burner flame is stable.
1. Evaluate the proposal to bypass the low-water trip and run in manual. What immediate actions would you take?
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Reject the proposal to bypass the low-water trip. The trip protects the boiler against overheating, tube failure, steam-space collapse and a serious explosion risk. Bypassing a safety device to maintain cargo heating is not a justified operational choice.
Immediate actions: reduce steam demand, stop or reduce cargo tank heating where possible, and inform the bridge and chief engineer. Put the boiler in manual only with continuous local gauge-glass monitoring by a second person and clear authorization, but keep the low-water trip in service. If the true level cannot be maintained, shut the boiler down.
The fact that there is no spare electrode 36 hours from port is a spares and planning issue, not a reason to defeat the protection. Alternative heating arrangements or reduced cargo temperature should be discussed with the cargo control room.
2. How would you establish the true boiler water level and identify the cause of the trip?
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Blow down the gauge glass and check that the water column and connections are clear. Compare the gauge glass with the test cocks and with any independent level indication. A gauge glass reading of 40 mm while the panel shows 62% suggests an instrument or connection fault, but the glass must be proven first.
Check the feedwater pump discharge pressure, suction strainer, recirculation line and feedwater control valve. The fluctuating pump pressure may be causing level swings or affecting the differential pressure type level transmitter.
Inspect the level electrode and its reference pot, check for fouling, moisture or a blocked sensing line, and test the alarm and trip circuit. Check for foaming by testing boiler water conductivity, carrying out a controlled blowdown and reviewing chemical treatment.
Do not restart until the actual level is known and the protective trip has been proven to operate correctly.
3. What must be done before the boiler is returned to service, and what records and notifications are required?
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Repair or renew the level electrode or sensing line, then prove the gauge glass, low-water alarm and low-water trip by an actual test. Check the feedwater control valve, pump and recirculation, and verify water quality before lighting off.
Carry out the normal pre-start checks: furnace purge, burner checks, all drains and vents, and warm-through. Bring the boiler up slowly and monitor level, pressure and flame stability. Keep steam demand reduced until the level control is stable.
Record the trip, the investigation, the defective part, the test results and the return to service in the engine log and maintenance system. Raise a defect report and requisition the spare electrode. Notify the company and, if the safety trip was found defective, consider class or flag reporting under the SMS. If the boiler is not available, record the alternative cargo heating arrangements.