Low-Water Cut-Out Bypassed During Night Shift
A 12 000 kg/h fire-tube boiler supplies process steam at 8 bar. At 02:40 the low-water cut-out (LWCO) trips and the burner locks out. The operator on shift finds that the gauge glass shows water at the bottom of the visible range, but he cannot see the water level move when he blows the glass down. He also notices the LWCO float chamber drain is blocked with sludge. To restore steam before the morning production run, he proposes to jumper the LWCO contacts, run the feed pump continuously, and watch the gauge glass. The boiler has been in service for 14 months since the last internal inspection, and feed water is softened but not demineralised.
1. Evaluate the operator's proposal. What should he do first, and why is jumpering the LWCO unacceptable here?
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The proposal must be rejected. A tripped LWCO with a stationary gauge glass is a classic indication of an unreliable level indication, not proof that the boiler is safely filled. Possible causes include a blocked water column or gauge-glass tappings, a stuck float, or a genuinely low level. Because the glass does not respond to blowdown, the operator cannot trust it, so he cannot verify the true level by eye.
The correct first action is to stop firing, keep the feed pump off until the level is proven, and isolate and blow down the water column and gauge glass to clear the blockage. If the level cannot be re-established and confirmed, the boiler must be cooled and the LWCO chamber and float examined. Jumpering the LWCO removes the last automatic protection against a low-water condition; in a fire-tube boiler, uncovering the crown sheet leads to overheating, collapse and possible explosion. Continuous feed does not substitute for a working cut-out because a blocked sensing line can leave the cut-out blind to the real level.
2. The water column is cleared and the LWCO now trips and resets correctly, but sludge is found in the chamber. What follow-up actions and records are required?
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The immediate fault is cleared, but the root cause (sludge carry-over from softened but not demineralised feed water) points to a water-treatment and blowdown problem. The boiler should be taken off line at the first safe opportunity for an internal inspection of the water column, LWCO chamber, and crown sheet area, and the feed-water hardness and suspended solids should be checked. Bottom blowdown frequency should be reviewed and increased if necessary.
Records should capture the trip time, the fault found, the corrective action, the person who carried it out, and the fact that the boiler was fired again only after the level was proven. Any bypass or defeat of a safety device, even briefly, should be reported to the responsible engineer. The incident should also trigger a review of the shift handover and the standing instruction that safety devices are never jumpered.
3. If the crown sheet had been uncovered for several minutes before the trip, what would you look for at the internal inspection, and why can re-filling a hot boiler be dangerous?
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Look for discolouration, sagging, bulging, or cracking of the crown sheet and staybolts, loosened or weeping staybolt heads, scale blistering, and any distortion of the furnace tube. Dye-penetrant or magnetic-particle inspection may be needed on suspect areas, and the staybolt holes should be checked for leakage.
Re-filling a boiler whose crown sheet has been overheated is dangerous because the metal may be at a temperature well above the water's saturation temperature. Sudden quenching by cold feed water can cause thermal shock, cracking, or a violent steam flash. The boiler should be allowed to cool in a controlled way, and the crown sheet inspected before any return to service.