Water Level Control Failure During Peak Demand
A fire-tube boiler supplies a reciprocating engine through a pressure-reducing station. During a plant peak the engine demand rises sharply and the boiler drum level gauge glass shows the level dropping fast. The feedwater pump is at maximum stroke, the feedwater temperature is 95 C, and the feed check valve is warm but not hot. The low-water cutoff has not tripped. The operator wants to add feedwater as fast as possible and to bypass the low-water cutoff to avoid a nuisance trip. Steam pressure at the boiler is falling from 10 to 9 bar.
1. Assess the situation and explain the risks of the operator's proposed actions.
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The boiler is losing water faster than the feed can replace it, and the low-water cutoff has not yet acted. That is a dangerous combination: the cutoff is the last automatic protection against a low-water condition that can overheat the crown sheet and cause a catastrophic failure.
Bypassing the low-water cutoff is never acceptable. It removes the protection precisely when the boiler is closest to the condition the device exists to prevent.
Adding feedwater as fast as possible also carries risks. Cold feed at 95 C entering a hot boiler causes thermal stress, and in a fire-tube boiler a sudden large feed addition can cause level swell and misleading gauge readings. More importantly, if the level is already below the safe limit, adding water to a hot, possibly uncovered crown sheet can cause violent flashing and a pressure surge.
The falling pressure with rising demand suggests the boiler is at its capacity limit, not that the feed system alone is at fault. The warm feed check valve hints at leakage or a passing check, which would reduce effective feed flow.
2. Give the correct sequence of actions, and explain how you would determine whether the problem is feed capacity, feed temperature, or a passing check valve.
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Correct sequence:
- Reduce steam demand immediately - throttle the engine or shed load - to bring the boiler back within capacity.
- Do not bypass the low-water cutoff. If it trips, treat it as a real event: stop firing, do not add water until the boiler has cooled per procedure, and investigate.
- Maintain feed at the maximum safe rate, watching the gauge glass and the low-water cutoff.
- If level continues to fall despite maximum feed, shut down the boiler on the low-water trip and do not force it.
Diagnosis:
- Feed capacity: compare pump stroke and discharge pressure against boiler pressure. If the pump cannot overcome 10 bar, the check valve or pump is the limit.
- Feed temperature: 95 C feed is close to boiling at low pressure; flashing in the pump or line reduces delivered flow. Check for cavitation noise and pump discharge pressure fluctuation.
- Passing check valve: a warm check valve downstream of the pump when the pump is not running, or a feed line that stays hot, indicates leakage back to the feed tank. Isolate and inspect the check valve.
Confirm by isolating the feed line and observing whether the boiler level holds with firing steady; a falling level with the feed isolated confirms a leak or a demand problem, not just feed capacity.