Steam Generators and Boilers Questions and Answers

Answer: A

The feed check valve is a boiler mounting. Mountings are the fittings fixed on the boiler for its safety and control, such as the safety valve, stop valve, water level gauge, blow-off cock and the feed check valve. An injector, feed pumps and a steam trap are accessories or devices that work with the boiler and are not mountings in this classification.

Answer: C

The advantage of an injector for a boiler is that it has no moving parts, hence there are no dynamic parts. An injector uses a steam jet to raise the pressure of the feed water, so nothing in it rotates or reciprocates, which makes it simple. Its pumping efficiency is low, it cannot force very hot water into the boiler, and it does not work smoothly in extreme variation of load and pressure, so the other options are wrong.

The steam used by the injector condenses in the feed water, so its heat returns to the boiler, which is why the injector is a compact and cheap feed device on locomotives and small boilers.

Answer: B

The statement that is correct for water tube boilers when compared with fire tube boilers is that the various parts are more accessible. In a water tube boiler the water is in tubes and the hot gases are outside, which makes the tubes easy to inspect, clean and replace. Water tube boilers are suited to large power plants, they occupy less floor area for a given power, and they require more skill to work efficiently, so the other statements are wrong.

Answer: C

The Yarrow boiler is an example of a water tube boiler. In a water tube boiler the water flows inside the tubes and the hot gases pass over them. The Lancashire boiler is a fire tube boiler of the Cornish type with two flues, the Cochran boiler is a vertical multitubular fire tube boiler and the locomotive boiler is also a fire tube type, so those are not water tube boilers.

Other familiar water tube boilers are the Babcock and Wilcox, Stirling and La Mont boilers.

Answer: D

A dead weight safety valve is mainly used for low pressures, low capacity, stationary boilers of the Cornish type. In it a weight placed on the valve holds it on its seat, and the weight is lifted when the steam pressure rises, which makes it suited to low pressure. A spring loaded valve is used for higher pressure and for locomotives, a lever safety valve is used for stationary boilers with a heavier weight on a lever, and the high steam and low water valve gives a warning for both conditions, so the dead weight valve is the key.

Answer: C

Energy is recovered by flashing the blowdown to generate low-pressure steam.

Blowdown is hot water drawn from the boiler at boiler pressure to limit dissolved solids. In a flash tank its pressure is reduced, part of it flashes to clean low-pressure steam that can feed the deaerator or process heating, and the remaining hot water can preheat the make-up water in a heat exchanger.

Blowdown cannot be avoided, because solids would build up and cause scaling and carryover. Recycling it to the drum or mixing it with feedwater would return the concentrated solids to the boiler, defeating its purpose.

Answer: A

Boiler rating is usually defined in terms of steam output in kg/hr.

A boiler's capacity is stated as the mass of steam it can generate per hour, often reduced to an 'equivalent evaporation' from and at 100°C so that boilers working at different pressures and feed temperatures can be compared.

The maximum steam temperature describes the quality of the steam, not the quantity a boiler can supply. A heat transfer rate in kJ/hr is not the conventional rating figure, and the heating surface area is a design dimension of the boiler rather than a measure of what it delivers.

Answer: D

An air preheater is installed between the economizer and chimney.

The flue gases leaving the furnace pass in turn through the superheater, the economiser and then the air preheater before going up the chimney. The air preheater uses the last useful heat in the gases to warm the combustion air, which improves boiler efficiency.

Placing it before the superheater or the economiser would rob those higher-temperature heat exchangers of the heat they need, so they always come first in the gas path.

Answer: A

The boiler mounting is the blow off cock.

Mountings are fittings mounted on the boiler for its safe operation: safety valves, water level indicator, pressure gauge, steam stop valve, feed check valve, fusible plug and blow off cock. The blow off cock drains water and sludge from the bottom of the boiler.

The feed pump, economizer and superheater are accessories, which improve efficiency but are not needed for safety.

Answer: D

Boiler rating is usually defined in terms of steam output in kg/hr.

A boiler's capacity is stated as the mass of steam it generates per hour, often as equivalent evaporation from and at 100 °C, which makes boilers working at different conditions comparable.

Steam temperature, heat transfer rate and heating area all affect performance, but none of them is the standard rating.