Low-Water Cutoff Trips While the Gauge Glass Shows Water

When two level devices disagree, you do not pick the one you like. You get a third, independent measurement.

Establish the true level with the try cocks, burner off:

• Open the lowest try cock. Steady water means the level is above that point; steam means it is below.
• Work upward until you find the water/steam transition. That is the real level, regardless of what the glass says.

What the fast refill means: after a proper column blowdown the glass should refill at a rate you can watch. A near-instantaneous snap back to mid-glass suggests the glass is not being fed from the drum at the bottom connection - it is being fed from the top only, or the column is holding water that never drained. Likely causes:

• Bottom connection of the glass or the water column plugged with sludge or scale.
• Water column drain or blowdown valve partly plugged, so the blowdown only emptied the glass, not the column.
• Float in the column stuck or flooded, so the float-type cutoff sees a level that never existed.

A glass fed only from the top reads high or full no matter how low the drum really is - which is exactly the failure mode that destroys boilers.

Most probable root cause: a restricted lower connection to the water column or gauge glass, aggravated - not created - by the unstable feedwater regulator. The regulator hunting pulled the actual drum level down during a high-fire week; scale and sludge carried over from that cycling collected in the small-bore lower connections until the glass and the float chamber stopped tracking the drum. The probe-type cutout, sensing the shell directly, caught the event and did its job.

The manual reset is the evidence. It exists so that a genuine low-water condition cannot be cleared with a button push once the water returns. It tripped; therefore the boiler really did go to a low-water condition, or that device is also lying. Both must be resolved before restart:

• Blow down and rod out the column, glass, and their connections; confirm the float actually moves.
• Test each cutout independently - drain the column and watch for a trip, refill and watch for reset (manual-reset device must stay locked out until deliberately reset).
• If the probe device is the one that tripped, inspect the probe for scale bridging, which can hold it in the 'wet' or 'dry' state.

A manual-reset trip is never a nuisance alarm.

1. Keep the burner off until the true level is confirmed by try cock and both cutouts pass a functional test. Unknown level on a hot boiler means no firing, and no feedwater added until the drum level is established.
2. Feedwater on manual, watching the glass and the try cocks. Do not return the regulator to automatic while it is hunting.
3. Reduce load if possible - tell the process the boiler is running limited until the level instrumentation is proven.
4. Clean the water column and glass connections, verify the float and probe, and check that the blowdown piping is clear.
5. Test both low-water cutouts on their own, then test them together, and log the results with times and who witnessed it.
6. Have the feedwater regulator serviced and trend drum level before returning to automatic.
7. Notify supervision and the inspector if the manual-reset device tripped in service - many jurisdictions require reporting, and the log entry protects the operator.

The one thing not to do is reset the annunciator to make the panel quiet and restart because the glass 'looks fine.'