Steady Gauge Glass, Falling Pressure, Rising Stack Temperature
You are the only operator on shift with a 125 psig firetube boiler firing No. 2 oil. Over about 20 minutes, steam pressure falls from 100 to 78 psig while the burner stays at high fire. The feedwater regulator has gone wide open. Stack temperature has climbed from 480 F to 590 F. Through all of this the gauge glass reads a steady level at about one-third of the visible length - it has not moved. Four hours earlier you blew the glass down and it returned to a normal level. You now open the glass blowdown valve: water runs out for a few seconds, then only steam, and after you close the valve the glass stays empty.
1. Explain what the combination of symptoms tells you, and what the empty glass after blowdown proves.
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Three symptoms are telling the same story from different angles:
- Pressure falling while the burner is at high fire means the heat being released is not becoming steam.
- Stack temperature up 110 F means less heat is being absorbed by the waterside - the classic signature of tubes with water on one side and vapor or hot gas on the other, i.e. uncovering or a waterside problem.
- Feedwater valve wide open with no rise in level means water is leaving the boiler as fast as it goes in - a leak, or a level indicator that is not reporting the truth.
Against that, the glass has been frozen at one-third for 20 minutes. A real level never behaves that way during a 22 psig pressure swing.
The blowdown result is the deciding fact. After the glass emptied, it did not refill. Only two things produce that:
- The lower cross-connection or water column is plugged, so the glass has been showing stale water the whole time, or
- The boiler water level is genuinely below the lower gauge connection.
You cannot tell which from the glass, so you must assume the worst case - (2) - because that is the case that destroys the boiler. The steady reading was never trustworthy; you were reading water that had been sitting in the column since the morning blowdown.
2. State your immediate actions in order, and list what you must not do.
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In order, now:
- Shut off the fuel to the burner immediately - trip it, don't ramp it down. Do not wait to confirm the level. An uncovered pressure vessel being fired is the highest-consequence event in the plant.
- Confirm flame is out and keep the forced draft fan running for a post-purge to clear combustible gases from the furnace and breeching.
- Do not feed water and do not blow down. If the tubes have been overheated, cold feedwater hitting hot metal can cause violent flashing and can turn a repairable tube into a catastrophic failure; blowing down drops pressure and does the same thing.
- Isolate the boiler from the steam header if the piping arrangement allows it, so the rest of the plant is not exposed.
- Notify supervision and the chief engineer, log times and readings, and keep people out of the area.
- Check whether the low-water fuel cutoff should have tripped and why it did not.
What you must not do:
- Do not add water to "see if it comes back."
- Do not open the safety valve to drop pressure.
- Do not re-light on the strength of the one-third glass reading you have already disproven.
- Do not attempt a hydrostatic test or refill until the boiler has cooled and a qualified person has inspected the fireside and waterside.
The boiler returns to service only after the water column and glass connections are proven clear and the low-water cutoff passes a functional trip test.
3. Once the boiler is cool, how do you establish what actually happened, and what maintenance practice would have prevented it?
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Find out whether the glass lied, or the boiler was truly low:
- Drain the gauge glass, remove the glass and both try cocks, and rod out the upper and lower cross-connections - do not assume a blowdown cleared them. Sludge and iron oxide pack the lower connection routinely.
- Remove the water column for internal inspection. Look for a solid plug of deposit at the boiler-side tapping.
- Open the low-water cutoff chamber (float or probe type) and confirm it is free and correct.
- With the connections proven clear and a new glass installed, refill slowly and cross-check the glass against the try cocks at three points: bottom, middle, top. If they disagree, the glass is still lying.
- Have the fireside and waterside inspected for tube bulging, sagging, and black/blue oxide before any return to service.
- Finally, perform a functional trip test of the low-water cutoff - drain the chamber with the burner at low fire and confirm shutdown before the level reaches the bottom of the visible glass.
Prevention: the failure chain started with a blowdown procedure that was not verified. A glass should be blown down with the proper sequence (water connection, then steam connection, then both) and the operator should watch it return and settle, not just open and close a valve. Keep a written blowdown log with the level observed before and after. A column that is blown but never rodded out will plug eventually - put an annual column removal and internal cleaning on the maintenance schedule, and make sure the low-water cutoff is tested, not just tagged.