Restart of a Laid-Up Firetube Boiler: What Actually Bars Firing Tonight?
A 350 hp natural-gas firetube boiler, MAWP 150 psig, has been in wet layup for 14 months while the plant finished a district-heat tie-in. It must be back on line before the winter peak. The insurance inspector's pre-fire walkdown found: (a) the single bottom-blowoff gate valve passes to the drain, the blowoff pipe is reduced from 2 in. to 1 in., and it ends 10 in. above a walkway floor; (b) the safety valve is stamped 150 psig set, 11,900 lb/h saturated-steam capacity, last bench-tested three years ago, seal intact; (c) both gauge-glass try cocks are plugged solid; (d) the low-water fuel cutoff was rebuilt, but the handle on its test/blowdown valve is missing so it cannot be functionally tested; (e) the operating pressure control cuts out at 145 psig and the manual-reset high limit is set at 148 psig. Feedwater is 105°F. The plant manager wants to fire tonight and "catch the rest at the spring outage."
1. Rank the five findings by whether they prohibit firing tonight, and justify each ranking against the function the item protects.
Read the worked answer
Answer: no firing tonight. Three items bar it, one is a documentation issue, one is acceptable.
The test is not "is it broken?" but "which independent protection does this defeat?"
- (a) Bottom blowoff - BAR. At 150 psig the blowoff is a pressure-boundary and water-level-control device. One passing valve with no second valve means there is no way to isolate and service it while the boiler is pressurized; the 2 in. → 1 in. reduction is a prohibited reduction on a blowoff line (it raises velocity and erosion); and a discharge ending 10 in. above a walkway will scald the first person who walks past during a blowdown. Correct fix: two valves in series, the downstream one quick-opening, full-size pipe carried to a safe point of discharge.
- (c) Try cocks - BAR. The gauge glass is the only continuous level indication, and the try cocks are the only way to prove the glass is telling the truth. A glass whose connections are partially plugged can show a level that does not exist. Restore the try cocks and prove a true level before firing.
- (d) LWCO test valve - BAR. A low-water fuel cutoff you cannot blow down is an untested safety device. Rebuild the test valve and prove the burner trips on a simulated low-water condition. This is the last line of defense if the glass is lost.
- (e) High limit - ACCEPTABLE. 148 psig is below the 150 psig safety valve setpoint, and the valve must limit accumulation to 10% (165 psig). Confirm the high limit is manual-reset and independent of the operating control.
- (b) Safety valve - PASS with a scheduling condition. Set pressure equals MAWP, which is correct, and capacity is adequate (see next task). Three years since a bench test is a test-interval issue, not a firing bar, provided the seal is intact and the insurer's interval is documented. Bench test at the first window.
2. Verify whether the existing safety valve can protect the boiler. Show the calculation and state what would change your answer.
Read the worked answer
Boiler horsepower is defined as 33,475 Btu/h, i.e. 34.5 lb/h of evaporation from 212°F feed. So the boiler's rated evaporation is 350 × 34.5 = 12,075 lb/h at 212°F.
At 150 psig the steam enthalpy is about 1,196 Btu/lb. With 105°F feedwater (h ≈ 73 Btu/lb), each pound of steam carries 1,196 − 73 = 1,123 Btu. The same heat input therefore produces:
12,075,000 ÷ 1,123 = 10,755 lb/h of actual steam, below the 12,075 lb/h nominal figure, because each pound carries more heat than the 212°F rating basis.
Compare with the valve nameplate: 11,900 lb/h at 150 psig, which is 111% of the required 10,755 lb/h. The valve is adequate.
What would change the answer:
- A firing-rate increase. Heat input is what drives relieving capacity. 12,075,000 Btu/h at 80% efficiency implies about 15.1 MMBtu/h of gas, roughly 14,800 ft³/h. A burner upgrade, a larger orifice, or a gas pressure increase above the data plate would require re-sizing the valve even though the boiler shell never changed.
- Valve capacity stamped at a different pressure. A 150 psig valve tested at 150 psig is fine; a valve de-rated to 125 psig is not.
- Cold feedwater. The colder the feed, the lower the lb/h and the easier the duty - colder feed never makes capacity worse, but hotter feed (a new deaerator) does.
3. The manager proposes blinding the blowoff drain line and running until spring because "the valve only weeps." Write your response, including what you would put in writing.
Read the worked answer
A blind on the downstream flange does not fix the valve; it hides it. The pressure boundary is still the leaking seat, and now the space between the seat and the blind is a trapped, undrainable volume that will corrode and will give no indication when the seat finally fails. A sudden failure then releases 150 psig saturated water at the flange face, next to a walkway, with no operable valve to shut it off.
Three more reasons it fails as an operating decision:
- You lose blowdown. Bottom blowdown is how you control boiler water dissolved solids and suspended sludge and how you dump water in an emergency. Blind it for four months and the boiler runs at rising conductivity, with carryover, foaming and a false level in the glass - which is exactly the condition the repaired LWCO is there to catch.
- It is an uncontrolled alteration of a pressure boundary. A blind installed by maintenance with no design basis, no drawing, and no management-of-change record is the classic origin of an incident that an insurer will not cover.
- A weeping valve degrades non-linearly. What weeps at 2 gpm in November can wash out the seat and fail wide open in January.
In writing, to the manager and copied to the insurer: the boiler will not be fired until the blowoff is replaced with two valves in series (downstream quick-opening), the line is restored to full size and rerouted to a safe discharge, and the repair is pressure tested and documented. The try cocks and LWCO test valve are restored and the LWCO trip is proven. The safety valve bench test is scheduled for the first available window with the insurer's concurrence. Include photographs of the current discharge location.