Boilers and the Steam Plant: the Cycle, the Trips, and the Feed System

That burner's fuel oil valve closes on loss of burner flame under the burner safety trip control system. The master fuel oil valve and the other two burner valves stay open, because the boiler safety trip acts on loss of flame at all burners, not at one. If the answer choice says "the master fuel oil valve closes," it is wrong for this scenario.</details>

The feed system

Steam vessels, and motor vessels fitted with steam driven electrical generators, must have at least two separate means of supplying feed water for the boilers, and all feed pumps must be fitted with the necessary connections for this purpose 46 CFR §56.50-30. Feed pumps for water tube boilers must have freshwater connections only.

Valve placement is specific and it gets tested. Stop and stop-check valves go in the main feed line, attached as closely as possible to drum inlets or to the economizer inlet; auxiliary feed lines get stop and stop-check valves; and a boiler fitted with an economizer must have a check valve in the economizer discharge as close as possible to the drum feed inlet nozzle.

For the piping itself, feed piping from the boiler to and including the required stop and stop-check valves must have a design pressure exceeding the maximum allowable working pressure of the boiler by either 25 percent or 225 psig, whichever is less. Two numbers, take the smaller, add it to MAWP. Also, where feed water regulators or feed water heaters are installed, an alternate means of operation with those devices bypassed must be provided.

Boiler water

Scale is a hard mineral deposit that forms on the water side of the tubes when dissolved hardness salts come out of solution as the water boils; even a thin layer insulates the tube from the water, so the metal overheats, loses strength, and can rupture. Corrosion is chemical attack on the metal, driven chiefly by dissolved oxygen and by acidic or improperly conditioned water NAVEDTRA 14104 §4-5. Surface blows skim off concentrated surface water and floating impurities; bottom, or drum, blows remove settled sludge from the water drum. A sudden rise in chlorides warns of seawater contamination — a condenser tube leak, for instance — which must be found and corrected.

On auxiliary boilers the tested parameters are alkalinity, phosphate and chloride, and alkalinity is measured rather than pH because the higher alkalinity level is more easily measured by the alkalinity test than by the pH meter; the range is equivalent to a pH of 11.0 to 11.3 NAVEDTRA 14075 Ch. 7 — Water Treatment ¶1. High concentrations of dissolved solids lead to carryover with the steam, and excessively high pH causes caustic embrittlement and subsequent erosion of boiler metal parts.

Fittings and definitions that carry a point

• Fusible plugs — bronze casing with a tin filling that melts at 445 to 450 °F, intended to melt in the event of low water level 46 CFR §52.01-3
• Dry pipe — a perforated or slotted pipe placed in the highest part of the steam space to prevent priming, that is, water carryover
• Water column and test cocks — a fitting or tube equipped with a water glass for indicating water level, and small valves for indicating water level or sampling
• Blowoff valve — connected directly to the boiler for blowing out water, scum or sediment
• Superheater and economizer — the superheater increases steam temperature above saturation temperature; the economizer is a feed-water heater usually located in the uptake or casing to absorb heat from the waste gases
• Safety valve — a pressure relief valve actuated by inlet static pressure and characterised by rapid opening, or pop action, and it recloses; a rupture disk device is nonreclosing and functions by bursting

Which part of the CFR governs

A main power boiler generates steam for main propulsion; an auxiliary or donkey boiler serves general purposes other than main propulsion. Each main boiler meets the automatic safety controls of §62.35-20(a)(1); an automatically controlled auxiliary boiler under 12,500,000 Btu/hr meets part 63, and one at 12,500,000 Btu/hr or above meets part 62 46 CFR §52.01-10. Automatic boilers at 12,500,000 Btu/hr and above must also meet part 52 for the boiler itself 46 CFR §63.01-3.

Telling it apart

Boiler types — the criterion is what is inside the tubes

• Watertube — the tubes contain water and steam, and heat is applied to the outside surface of the tubes 46 CFR §52.01-3. Marine boilers are almost always this type NAVEDTRA 14104 §4-1.
• Internally fired firetube (scotch) — furnaces, combustion chambers and tubes or flues surrounded by water, with the products of combustion passing through them from the furnace to the uptake.
• Externally fired firetube or flue (horizontal return tubular) — part of the outer shell is itself exposed to fire or to the products of combustion. That exposed shell is the giveaway.
• Unfired steam boiler — steam generated by means other than fuel combustion.

Steam conditions — the criterion is temperature against saturation at the same pressure

• Saturated steam — steam saturated with all the heat it can hold at the boiling temperature of water NAVEDTRA 14104 Ch. 2 — Energy Transformations ¶2
• Superheated steam — hotter than saturation at that pressure. Degree of superheat is the amount by which superheated steam exceeds saturation temperature at the same pressure: saturated steam at 620 psia with a saturation temperature of 490 °F, superheated to 790 °F, has 300 °F of superheat NAVEDTRA 14104 Ch. 3 — Generation
• Wet steam — at the boiling temperature but still carrying water particles
• Desuperheated steam — cooled by a pipe passing through the steam drum, losing all but 20 °F to 30 °F of its superheat

Group feed against unit feed — the criterion is whether each boiler owns its main feed pump

• Group feed — a vessel with a feed pump attached to the main propulsion unit must have at least one independently driven feed pump used exclusively for feed and capable of supplying the operating boilers at normal capacity, plus a second independently driven pump capable of 75 percent of normal capacity. Two independent pumps each at full normal capacity, neither used for other purposes, remove the requirement for the third or emergency pump.
• Unit feed — used on vessels with two or more boilers; each boiler has its own independently driven main feed pump at its normal operating capacity, plus an auxiliary independent feed pump of the same capacity, and with three or more boilers no one auxiliary pump may serve more than two boilers. Each boiler gets a separate feed line, and feed supply is automatically controlled by that boiler's water level with manual control in addition.

Working a question

You have the order to light off No. 2 main boiler, oil-fired, fully automatic, and the question asks which condition must be satisfied before fuel reaches the first burner. Work the interlock sequence in order and the distractors fall away.

1. Registers and dampers open, air flow proven. At least 25 percent of the full load volumetric air flow must be proven before the purge period commences. Air before purge, purge before fuel.
2. Prepurge. A continuous purge of the combustion chamber and convecting spaces sufficient for a minimum of 5 changes of air, not less than 15 seconds in duration, immediately prior to trial for ignition of the initial burner. It must be complete before that trial begins.
3. Igniter proven. The igniter must be in position and proven energized before fuel is admitted, and must stay energized until the flame is established and stable or the trial period ends. If a light oil pilot is fitted, the main burner trial for ignition must not proceed until the pilot flame is proven, and the pilot trial must not exceed 15 seconds.
4. One burner, 15 seconds. Only one burner per boiler is in trial for ignition at any time, and the trial period must not exceed 15 seconds.
5. Failure to light. Failure of the burner to ignite during a trial automatically actuates the burner safety trip controls — valve shut inside 4 seconds, and you do not get a second attempt without the programmed sequence starting over.
6. Coming up on the line. A low fire interlock prevents high firing rates and superheater damage during boiler warm up.

The distractor most often planted in this family of questions is automatic post-purge after a trip. An automatic purge equal to the volume and duration of the prepurge follows normal shutdown; following boiler safety trip control operation the air flow must not automatically increase, and post purge is under manual control. A trip and a normal shutdown do not end the same way.

Where candidates lose the point

• Reading "watertube" as gas in the tubes. The name describes the tube contents, not the fire path: water and steam inside, heat on the outside. An option saying the products of combustion pass through the tubes describes a firetube boiler.
• Answering that the boiler trips on low water. The alarm is on high and low water level; the trip is on low-low water level. A question that offers both is separating exactly these two.
• Securing the boiler for one burner's flame failure. Loss of flame at one burner closes that burner's valve. The master valve closes on loss of flame at all burners.
• Cross-connecting feed to salt water in a pinch. Feed pumps for water tube boilers have freshwater connections only. Any answer that lets seawater into a water-tube boiler's feed system is wrong on its face.
• Swapping economizer and superheater. The economizer heats feedwater with waste gas before the steam drum; the superheater raises steam temperature above saturation after it leaves the drum. Economizer surface heats feed water before it enters the steam drum; superheater surface heats steam after it leaves the drum NAVEDTRA 14104 Ch. 4 — Boiler Terminology.
• Treating rising chlorides as a chemistry problem. More treatment chemical does not answer a chloride climb; find the seawater in-leakage, and the condenser tubes are the first place to look.
• Taking the larger figure on feed piping design pressure. The rule says 25 percent or 225 psig, whichever is less. On a low-MAWP boiler the percentage governs; the 225 psig figure is a cap, not a default.

Check yourself