Repeated Low-Water Trips on a Cold Monday Morning
At 6:40 a.m. Monday, a 100 hp low-pressure steam boiler in a school trips on low water twice in twelve minutes while warming a cold building. The gauge glass reads at the bottom of the visible range; the glass was blown down, the flow sputtered weakly then steadied, and the level read the same. Both the primary float-type low water fuel cutoff and the manual-reset secondary cutoff have tripped. The custodian resets and restarts, and it trips again. The feed pump runs continuously at 25 psig discharge, the condensate receiver is full, and the makeup water meter has not moved since Friday. Boiler pressure is 4 psig and falling. The feedwater regulator is a pneumatic level controller whose air supply was worked on Friday afternoon.
1. Is this a true or false low-water condition, and what do you do right now?
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Treat it as a true low water condition until it is proven otherwise.
Why: the gauge glass was blown down, the flow steadied, and the level came back to the same low reading. A false indication from a clogged lower connection usually changes or clears when the glass is blown down. Two independent devices - the primary and the manual-reset secondary - agree, and the boiler pressure is falling, which is consistent with a burner that is no longer firing because the water is genuinely gone from the crown of the furnace.
Immediate actions:
- Secure the fuel and confirm the burner is locked out. Do not reset again.
- Do not jumper, wedge, or bypass either cutoff, and do not remove the manual-reset feature.
- Do not feed water into a hot boiler whose heating surface may be uncovered. Adding water to an overheated crown sheet or tube sheet can cause sudden steaming, distortion, or cracking. Let the boiler cool before restoring level, then inspect.
- Notify the building administrator that the school is without heat, and log the times, readings, and actions.
Repeated resets without finding the cause is how low-water cutoffs end up bypassed and how boilers fail.
2. With a full receiver and a running feed pump, why is the level not recovering? List the likely causes and the check for each.
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A running pump with a full receiver and no rise in the boiler means the water is not getting from the pump discharge into the boiler, or the boiler is losing it as fast as it arrives.
- Feedwater regulator stuck closed. The pneumatic level controller's air supply was disturbed Friday. Check controller output air pressure and the valve stem position; a regulator that fails closed will do exactly this. Verify by carefully using the manual bypass around the regulator while watching the gauge glass.
- Discharge check valve stuck or installed backwards after Friday's work. Feel the pipe downstream of the check: if it stays cold while the pump runs, there is no flow.
- Strainer or feed line plugged. Compare pump discharge pressure to the boiler pressure; a large gap with no flow points to a restriction.
- Pump cavitating or air-bound. Listen for noise and check suction conditions; a pump can run, show pressure on a gauge, and move almost nothing.
- A bypass or drain left open, dumping feedwater to the floor or the receiver - check the floor and the receiver level trend.
- Real steam/water loss through a failed return, trap, or relief discharge - walk the system.
The makeup meter not moving is informative: either the system is not losing water and this is entirely a feed-side delivery problem, or the meter is not seeing the makeup you think it is.
3. What must be done before the boiler is returned to service, and what tests are documented?
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Return to service only after the cause is found and corrected, not after the boiler cools enough to accept water.
- Repair the feedwater delivery problem - regulator, air supply, check valve, or strainer - and prove flow into the boiler with the burner off.
- Refill slowly to the normal level, and confirm the gauge glass agrees with the try cocks.
- Blow down and test the primary low water fuel cutoff and confirm it cuts the burner at the proper level, then test the manual-reset secondary cutoff by its test procedure and confirm it requires a manual reset.
- Restart at low fire and warm up normally, watching the gauge glass and the feed regulator response through the first load swing.
- Re-test both cutoffs at operating pressure with the boiler stable, and record the water level at which each one actuated.
- Log everything: the failure, the cause, the repair, the technician, the test results, and the times. Note in the log that the manual-reset feature is required to remain in service.
If the cause cannot be identified with confidence, the boiler stays out of service pending a proper inspection of the fireside and waterside for overheating damage.