Water Level Lost on a 150-psig Firetube Boiler
At 06:40 a 300-hp, 150-psig natural-gas firetube boiler is carrying 120 psig at 80% firing rate. The operator notices the gauge-glass level drop from normal to the bottom nut in less than a minute. The burner remains firing. The low-water cutoff alarm has not sounded, although the float chamber drain was last tested 30 days ago. The feedwater pump is running with 158-psig discharge pressure, but the feedwater flow indicator reads zero. The shift lead says the gauge glass may be false and wants to open the feedwater bypass wide to restore the level before stopping the burner. A second operator reports steam hammering near the feedwater check valve. The plant has no spare feedwater pump and the laundry load is waiting.
1. What are your first actions, in order, and why?
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- Trip the burner immediately at the emergency switch and close the fuel valve. A disappearing gauge-glass level with the burner still firing is a low-water emergency until proven otherwise.
- Do not add water yet. If the boiler is overheated, sudden feedwater can cause violent flashing and tube or shell damage.
- Verify the level with the gauge-glass try cocks and check whether the glass is false. A false glass can be caused by clogged connections or a closed glass valve, but you cannot assume that under these symptoms.
- Check the low-water cutoff and feedwater system. The fact that the LWCO did not alarm is itself a serious defect; the float chamber may be plugged, the switch may be stuck, or the drain test was not effective.
- Notify the shift supervisor and boiler inspector/authority as required, and keep the boiler on natural draft if possible while it cools. The ordering matters: stopping fuel removes the heat source; confirming level with try cocks prevents a dangerous wrong assumption; and withholding feedwater protects a hot boiler from thermal shock.
2. The shift lead wants to open the feedwater bypass wide to restore the level before stopping the burner. Evaluate that instruction.
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The instruction is unsafe and should not be followed. The boiler may have lost water below the visible range, meaning the crown sheet or tube sheet could be overheated. Opening the bypass wide adds cold water to hot metal, which can cause sudden contraction, flashing, and a pressure surge. The steam hammering near the check valve suggests water hammer or check-valve problems, so forcing flow through that line adds risk. The correct sequence is to stop firing first, let the boiler cool, and then investigate. If the gauge glass is false and the try cocks show a safe level, feedwater may be restored only after the cause is found and the LWCO is proven operable. The lead's approach reverses that sequence. It also keeps the burner firing while the level is unknown, which defeats the low-water protection. If production pressure is the concern, the supervisor should arrange standby steam or an alternate boiler, not risk a pressure-part failure.
3. After the boiler is secured, what inspection and testing must be completed before return to service?
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The boiler must be cooled slowly, opened, and inspected internally for overheating, distortion, leaking tubes, and tube-sheet damage. The gauge glass and its connections should be removed, cleaned, and proved clear. The low-water cutoff float chamber and switch must be opened, cleaned, and tested by draining the chamber to simulate low water while the burner is off; the burner must trip and the alarm must sound. The feedwater pump, check valve, and bypass valve should be repaired and flow-tested. The operating controls, safety valve, and pressure gauge should be checked, and the boiler should be warmed slowly and watched for leaks and abnormal level behavior. A hydrostatic test may be required after pressure-part repairs. The authority having jurisdiction should be notified before return to service if the boiler was damaged or the low-water cutoff failed.