Boilers and the Steam Cycle — Classification, Steam Quality and the Feed Side

What the rule requires

A main power boiler is a steam boiler used for generating steam for main propulsion. An auxiliary or donkey boiler is a steam boiler used for general purposes other than main propulsion 46 CFR §52.01-3. Auxiliary boilers supply steam and hot water for galley, heating and other hotel services, and other auxiliary requirements in port NAVEDTRA 14104 Ch. 4 — Intended Service.

That is one axis. The second axis is the relative location of the fire and water spaces, which gives firetube and watertube. All propulsion boilers used in naval ships are of the watertube type; auxiliary boilers may be either firetube or watertube NAVEDTRA 14104 Ch. 4 — And Water Spaces, and firetube and watertube auxiliary boilers are natural circulation boilers NAVEDTRA 14075 Ch. 7 — Water Treatment ¶1. A question can come at you from either axis without saying which, so read for the discriminator: service, or which side of the tube wall the water is on.

An unfired steam boiler is a pressure vessel in which steam is generated by means other than fuel combustion .

The cycle in the order it is asked

The four areas of operation in a main steam system are generation, expansion, condensation and feed NAVEDTRA 14104 Ch. 3 — Main Steam System. Track the energy at each one and the sequence becomes hard to scramble:

Generation — combustion in the boiler furnace transforms the chemical energy stored in the fuel oil into thermal energy, which flows to the water and generates steam.
Expansion — steam enters the turbines and expands, and the thermal energy converts to mechanical energy that turns the shaft.
Condensation — steam exhausting from the LP turbine enters the condensate system, where it is condensed back to water NAVEDTRA 14104 Ch. 3 — Condensation.
Feed — the condensate is prepared and returned to the boiler. Throughout, the water and steam flow through the entire system without ever being exposed to the atmosphere NAVEDTRA 14104 Ch. 3 — Basic Steam Cycle.

The condensate system is the main condenser, the main condensate pump, the main air ejector condenser and the top half of the deaerating feed tank. The main condensate pump takes suction from the main condenser hot well; the air ejector removes air and noncondensable gases from the condenser. The DFT is the dividing line between condensate and feedwater, and it does three things: removes dissolved oxygen and noncondensable gases, preheats the water, and acts as a reservoir against fluctuations in feedwater demand or condensate supply. The main feed booster pump takes suction from the DFT and maintains a constant discharge pressure to the main feed pump, which discharges into the main feed piping system at 100 to 150 psig above boiler operating pressure on 600-psi plants and 200 to 300 psig above on 1200-psi plants. Water leaves the economizer about 100°F hotter than it entered.

Which pump takes suction from the main condenser hot well, and which takes suction from the DFT?

The main condensate pump takes suction from the main condenser hot well and delivers through the main air ejector condenser. The main feed booster pump takes suction from the DFT and holds a constant discharge pressure to the main feed pump. Answers that put the condensate pump on the DFT have the two ends of the condensate system reversed.

Steam quality, and the one calculation

Saturated steam is steam saturated with all the heat it can hold at the boiling temperature of water. Superheated steam has been heated above its saturation temperature at a given pressure, and the vessel in which saturated steam is superheated is the superheater. Wet steam is steam at the boiling temperature that still contains water particles. Desuperheated steam has been cooled by passing through a pipe extending through the steam drum, losing all but 20°F to 30°F of its superheat NAVEDTRA 14104 Ch. 2 — Energy Transformations ¶2.

Degree of superheat is the amount by which the temperature of superheated steam exceeds the temperature of saturated steam at the same pressure. Saturated steam at 620 psia has a saturation temperature of 490°F; superheated to 790°F, the degree of superheat is 300°F NAVEDTRA 14104 Ch. 3 — Generation. The subtraction is trivial. Entering the steam table at the right pressure is where candidates go wrong.

Superheating buys a greater temperature differential between boiler and condenser, so more heat is converted to work at the turbines, and superheated steam is dry and causes relatively little corrosion or erosion of machinery and piping. It is not used everywhere. Most auxiliary machinery operates on saturated steam, and reciprocating machinery in particular requires saturated steam to lubricate the internal moving parts of the steam end, which is why naval boilers produce both.

Combustion

Air is a mechanical mixture containing by weight 21 percent oxygen, 78 percent nitrogen and 1 percent other gases; only the oxygen is used, and nitrogen is inert. Complete combustion requires sufficient oxygen, thorough mixing of oxygen and fuel particles, temperature high enough to maintain combustion, and enough time to permit the process to finish. Perfect combustion has not been achieved in a boiler or in the cylinders of an internal combustion engine; complete combustion is obtained by supplying more oxygen than perfect combustion would need, so excess oxygen appears in the combustion gases.

Keeping water out of the steam

Cyclone steam separators remove moisture by spinning the steam or changing its direction, the water draining back into the drum while the steam continues up through a screen and scrubber; the dry pipe then directs the steam to the drum outlet nozzle NAVEDTRA 14104 Ch. 4 — Internal Fittings. Regulation defines the dry pipe by its purpose: a perforated or slotted pipe placed in the highest part of the steam space to prevent priming, that is, water carryover . Carryover is not a housekeeping matter. If excessive moisture is carried over into the superheater, serious damage results in the superheater tubes, piping and turbines, and high concentrations of dissolved solids in the boiler water are one route to it .

Feed arrangements and automatic controls

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, arranged as either a group feed system or a unit feed system 46 CFR §56.50-30.

In a group feed system, a vessel with a feed pump attached to the main propulsion unit must also have at least one independently driven feed pump capable of supplying the operating boilers at normal capacity, plus a second independently driven pump capable of 75 percent of normal capacity. Where two independently driven pumps are fitted, each capable of normal required operating capacity and neither used for other purposes, the third or emergency feed pump is not required. River or harbor steam vessels need two means, one of which must be an independently driven pump; the other may be an attached pump, another independently driven pump, or an injector.

In a unit feed system, used on vessels with two or more boilers, each boiler has its own independently driven main feed pump at 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 pump discharge and feed supply are automatically controlled by the water level in that boiler, with manual control provided in addition.

Two details that get asked on their own: stop and stop-check valves go in the main feed line, attached as closely as possible to drum inlets or to the economizer inlet, and feed pumps for watertube boilers must have freshwater connections only.

On the control side, safety trip controls and alarms must be provided for all main boilers regardless of mode of operation 46 CFR §62.35-50. On vessels propelled by steam turbines, the navigating bridge primary control system must include safety alarms for high and low boiler water levels and low steam pressure 46 CFR §62.35-5. For automatically controlled auxiliary boilers, the dividing line is a heat input rating of 12,500,000 Btu/hr (3.66 megawatts): below it, part 63; at or above it, the automatic safety controls of part 62 46 CFR §52.01-10, and the boiler itself must meet part 52 46 CFR §63.01-3.

An automatically controlled auxiliary boiler is rated at 14,000,000 Btu/hr heat input. Which part governs its automatic safety controls?

Part 62. The threshold is 12,500,000 Btu/hr (3.66 MW), and "at or above" sends the control system to part 62 and the boiler to part 52. Part 63 covers automatic auxiliary boilers below that rating.

Telling it apart

Which side of the tube wall the water is on

Watertube boiler — the tubes contain water and steam, and the heat is applied to the outside surface of the tubes. Misfiled when a candidate reasons from the furnace being full of fire and concludes the fire must be in the tubes.
Internally fired firetube (scotch) boiler — contains furnaces, one or more combustion chambers and tubes or flues surrounded by water, through which the products of combustion pass from the furnace to the uptake.
Externally fired firetube or flue boiler (horizontal return tubular) — part of the outer shell is exposed to fire or to the products of combustion, and it contains flues through which those products pass from the furnace to the uptake. The distinguishing feature is the exposed shell, not the flue count.

Tubes and flues are separately defined: flues are cylindrical shells of seamless or welded tubing that provide additional heating surface and form a path for the products of combustion, while tubes are cylindrical shells of comparatively small diameter that constitute the main part of the heating surface of a boiler or superheater 46 CFR §52.01-3.

Three heat-transfer surfaces sitting in the gas path

Superheater — an appliance, normally consisting of tube rows, for increasing the temperature of steam above the saturation temperature. It works on steam only, never on feedwater.
Economizer — a feed-water heater usually located in the uptake or casing of a boiler to absorb heat from the waste gases. All feedwater flows through the economizer tubes before entering the steam drum NAVEDTRA 14104 Ch. 4 — Internal Fittings. It is a preheater for water, not for combustion air.
Desuperheater — takes steam from the superheater outlet and is submerged in water in either the steam drum or the water drum, cooling that steam for use in the auxiliary steam systems.

Reclosing or not, and what actuates it

Safety valve — a pressure relief valve actuated by inlet static pressure and characterised by rapid opening or pop action; it recloses and prevents further flow once normal conditions are restored. The spring-loaded type holds the disk closed against the seat and opens and closes at predetermined pressures.
Rupture disk device — nonreclosing, actuated by inlet static pressure, functioning by the bursting of a pressure-retaining disk.
Fusible plug device — nonreclosing, functioning by the yielding or melting of a plug. Fusible plugs are made with a bronze casing and a tin filling that melts at 445 to 450 °F, and they are intended to melt in the event of low water level. This is the low-water device, and answering it to an overpressure question is the most common single error in this group.
Vacuum relief valve — admits fluid or gas to prevent an excessive internal vacuum, the opposite duty to the others.

Working a question

Feed piping from a boiler with a maximum allowable working pressure of 600 psig is being renewed. What design pressure governs the section running from the boiler to and including the required stop and stop-check valves?

Establish which side of the stop-check valves the question puts you on. Discharge piping from the pump up to, but not including, those valves is designed for the feed pump relief valve setting, or for the pump shutoff head if no relief valve is fitted. From the boiler to and including those valves, the design pressure is driven off the boiler's MAWP 46 CFR §56.50-30. The two answers are different numbers, and the question tells you which side by naming the valves.
Take the first candidate margin: 25 percent of 600 psig is 150 psig.
Take the second: the flat figure of 225 psig.
Apply the operative word. The design pressure must exceed the MAWP by 25 percent or 225 psig, whichever is less — so 150 psig governs, and the design pressure is 750 psig.
Test the crossover so the method survives a different boiler. At 1200 psig MAWP, 25 percent is 300 psig, which is more than 225, so 225 governs and the answer is 1425 psig. The two criteria cross at 900 psig MAWP: below that the percentage governs, above it the flat 225 psig does.
Then select the allowable stress at a temperature not below that for saturated steam at the boiler's maximum allowable working pressure. Not ambient, and not superheater outlet temperature.

Where candidates lose the point

Answering superheater when the question asks what heats the feedwater. The superheater raises steam temperature above saturation; the economizer is the feed-water heater in the uptake or casing living off the waste gases. Both sit in the gas path, which is what makes the distractor work.
Answering that the fusible plug protects against excessive pressure. It is a nonreclosing device that functions by melting at 445 to 450 °F in the event of low water level. Overpressure belongs to the safety valve, actuated by inlet static pressure with pop action.
Taking the greater of 25 percent and 225 psig on feed piping design pressure, or adding the two. The regulation says whichever is less, and at a 600 psig MAWP that changes the answer by 75 psig.
Reading the 12,500,000 Btu/hr threshold as an open interval and sending a boiler rated exactly at it to part 63. Part 63 applies to auxiliary boilers of less than that rating; at or above, the controls go to part 62.
Putting the main condensate pump on the DFT. It takes suction from the main condenser hot well; the top half of the DFT is the last component of the condensate system, and the main feed booster pump is the one that draws from the DFT.
Assuming everything in the plant runs on superheated steam. Most auxiliary machinery operates on saturated steam, and reciprocating machinery requires it to lubricate the internal moving parts of the steam end — which is the whole reason the desuperheater exists.

Check yourself

You are assigned to an auxiliary boiler in which the products of combustion pass from the furnace, through tubes surrounded by water, to the uptake. What class is it, and could this same design serve as a naval propulsion boiler?

It is an internally fired firetube boiler, the scotch boiler, which contains furnaces, one or more combustion chambers and tubes or flues surrounded by water. It could not serve as a naval propulsion boiler: all propulsion boilers used in naval ships are watertube, while auxiliary boilers may be either firetube or watertube.

A boiler has a maximum allowable working pressure of 900 psig. What is the design pressure of the feed piping from the boiler to and including the required stop and stop-check valves?

1125 psig. Twenty-five percent of 900 is 225 psig, which equals the flat figure, so the two criteria give the same margin at this MAWP. This is the crossover point: below 900 psig the percentage is the lesser figure, above it the 225 psig is.

The boiler is operating at 630 psia and the superheater outlet thermometer reads 567°F. What is the degree of superheat, and what does the reading tell you about the steam?

Zero. The saturation temperature for water at 630 psia is 567°F, so steam at the outlet is at saturation temperature and carries no superheat. Degree of superheat is the amount by which superheated steam exceeds saturated steam temperature at the same pressure, and here there is no excess.

Water level in a firetube auxiliary boiler falls below the crown of the furnace. Which fitting is designed to respond, and at what temperature does it act?

The fusible plug, a nonreclosing pressure relief device that functions by the yielding or melting of its plug. The plug has a bronze casing and a tin filling that melts at 445 to 450 °F, and it is intended to melt in the event of low water level.

You are standing a watch on a vessel propelled by steam turbines. Which boiler-related safety alarms must the navigating bridge primary control system include?

High and low boiler water levels, and low steam pressure. Separately, safety trip controls and alarms are required for all main boilers regardless of mode of operation, so the presence of automatic control does not remove the requirement.

A steam vessel has one feed pump attached to the main propulsion unit. What else must be fitted, and under what condition is the emergency feed pump not required at all?

At least one independently driven feed pump capable of supplying the operating boilers at their normal capacity, plus a second independently driven pump capable of 75 percent of normal capacity. Where two independently driven pumps are provided, each capable of the normal required operating capacity and neither used for other purposes, the third or emergency feed pump is not required.