Low-Water Trip With Water Visible in the Gauge Glass
At 2:00 a.m. a 250 hp, 150 psig gas-fired fire-tube boiler trips on low-water cutoff. The operator finds the burner off, drum pressure 110 psig, stack temperature 380°F, feed pump running, and the gauge glass showing water at the bottom of the visible range. The low-water cutoff float chamber was last blown down a month ago. The operator wants to open the feedwater valve wide, reset the cutoff, and restart because “there is still water in the glass.” The plant needs steam by 5:00 a.m. The boiler has a manual reset low-water cutoff and a separate auxiliary low-water cutoff.
1. State immediate actions and whether water may be added.
Read the worked answer
Keep the burner off and do not reset the cutoff. Verify the level by blowing down the gauge glass and water column, then watch whether water returns to the bottom of the visible range. If water returns and remains visible, the boiler is low but not proven dry; water may be added slowly with the burner off while watching the glass and listening for abnormal noise. If the glass empties and does not return, treat it as a low-water condition: do not add water, shut the feedwater and fuel, and let the boiler cool for inspection. Because the cutoff is manual reset, the cause must be found before reset. The need for steam by 5:00 a.m. does not justify bypassing the low-water protection.
2. What does the gauge-glass reading prove and not prove? Include a false-level scenario.
Read the worked answer
Water at the bottom of the glass proves only that some water is communicating with the lower gauge-glass connection at that moment. It does not prove that the cutoff is false, that the water column is clear, or that the drum level is adequate. A clogged lower connection, closed glass valve, or blocked float chamber can give a false reading. For example, the glass may show water because the upper connection is plugged and the glass is filled from the lower tap, while the true drum level is lower. Blowing down the glass and water column is the check. The auxiliary cutoff and the manual-reset cutoff should both be treated as independent protections, not as nuisances.
3. Define restart criteria and what to inspect before returning to service.
Read the worked answer
Restart only after the cause of the trip is identified, the water column and float chamber are blown down and proven clear, the gauge glass reads correctly at normal level, and both low-water cutoffs are tested and reset according to the manufacturer's instructions. Inspect for evidence of overheating: discolored or sagging furnace parts, distorted tube sheets, leaking tubes, or a melted fusible plug if fitted. Check feedwater control and pump operation. If there is any evidence of low water or overheating, perform an inspection and any required hydrostatic test before firing. Then follow a normal warm-up procedure; do not rush the restart.