Power Production Systems — October/November 2023

Question 1(a)

Define the following terms in relation to steam: (i) wet; (ii) superheated; (iii) dryness fraction.

(3 marks)

Answer

1. Wet steam: a mixture of saturated water vapour and suspended liquid water at the saturation condition.
2. Superheated steam: vapour at a temperature above the saturation temperature corresponding to its pressure.
3. Dryness fraction: the mass fraction of vapour in a wet mixture, x = m_vapour/(m_vapour+m_liquid). It lies between 0 and 1 and is not a quality variable for superheated steam.

Question 1(b)

With the aid of a sketch, explain the construction and operation of a throttling calorimeter.

(7 marks)

Answer

1. Construction: a representative steam-sampling connection leads to an insulated chamber and a small throttling orifice/valve. Pressure is measured upstream; downstream pressure and temperature are measured before safe discharge/condensation.
2. A small steam sample passes through the restriction from p₁ to a lower p₂. With negligible heat transfer, shaft work and kinetic/potential-energy changes, throttling is approximately isenthalpic: h₁ = h₂.
3. Choose the pressure reduction so that the downstream sample is superheated. Determine h₂ from its measured p₂ and T₂ using steam tables.
4. Upstream wet-steam enthalpy is h₁ = h_f(p₁)+x h_fg(p₁). Hence x = [h₂−h_f(p₁)]/h_fg(p₁).
5. If the downstream state remains wet, p₂ and T₂ do not determine enthalpy uniquely. A throttling-only calorimeter then cannot determine the original dryness; a separating-and-throttling arrangement or another method is required.

Question 1(c)

With the aid of sketches, explain the constructional difference between induced draught and forced draught cooling towers.

(5 marks)

Answer

The sketches use counterflow for a direct comparison. “Induced/forced draught” identifies the fan arrangement; it is distinct from “counterflow/crossflow”, which describes the relative air and water paths.

Question 1(d)

List five maintenance checks for each of the following: (i) surface condensor; (ii) cooling tower.

(5 marks)

Answer

These are inspection categories; maintenance requiring access follows isolation and the equipment procedure.

Question 2(a)

(i) With the aid of sketches, explain the cycle of operation of a four stroke spark ignition engine. (ii) Outline the procedure of determining indicated power by Morse test on a four cylinder petrol engine coupled to a dynamometer.

(16 marks)

Answer

One complete cycle requires two crankshaft revolutions, 720°. Real valve timing includes lead, lag and overlap.

1. Warm the engine and stabilise it at the specified test speed with all four cylinders firing. Measure total brake power B with the dynamometer.
2. Following the approved test method, disable combustion in cylinder 1. Reduce the dynamometer load to restore the same speed, keeping the other operating conditions as nearly unchanged as possible. Record brake power B₁ with that cylinder cut out.
3. Restore cylinder 1 and allow conditions to stabilise. Repeat independently for cylinders 2, 3 and 4, recording B₂, B₃ and B₄ at the same speed.
4. Assume total friction/pumping power is unchanged by each cut-out. The indicated power contributed by cylinder i is I_i = B−B_i.
5. Add the four cylinder contributions: I_total = 4B−(B₁+B₂+B₃+B₄). If required, friction power F = I_total−B and mechanical efficiency η_m = B/I_total.

The Morse result is approximate because friction and pumping losses can change when a cylinder stops firing. No numerical power is asserted because the paper supplies no dynamometer readings.

Question 2(b)

Outline four advantages of nuclear power plants.

(4 marks)

Answer

1. High energy output from a small mass of nuclear fuel.
2. Low direct greenhouse-gas and combustion-air-pollutant emissions during generation.
3. Ability to provide large, sustained power output when the plant is available.
4. Generation is largely independent of short-term wind and sunlight conditions, with relatively infrequent fuel deliveries.

Question 3(a)

(i) With the aid of a diagram, explain the operation of a spreader stoker used in a boiler. (ii) State two advantages and two disadvantages of the stoker in (a)(i).

(8 marks)

Answer

A feeder meters coal onto a rotating mechanical spreader or a suitable pneumatic distributor. Fuel is thrown across the furnace above the grate. Fine particles burn partly in suspension; larger particles land on the grate and finish burning in a relatively thin bed. Under-grate air supports bed combustion, and over-fire air can complete combustion above the bed. Ash is discharged from the grate arrangement.

Question 3(b)

Describe the following problems in boilers: (i) Scale formation. (ii) Corrosion; (iii) Priming.

(6 marks)

Answer

1. Scale formation: hard adherent deposits build up on heat-transfer surfaces when dissolved salts precipitate. The insulating layer reduces heat transfer and can cause local overheating.
2. Corrosion: chemical/electrochemical attack wastes boiler metal, for example from dissolved oxygen or unsuitable water chemistry, causing pitting, thinning or leakage.
3. Priming: liquid boiler water is carried into the steam outlet as droplets, often during sudden demand changes, excessive water level or foaming. The resulting wet/contaminated steam can foul downstream equipment.

Question 3(c)

With the aid of diagrams, explain the operation of a bag filter plant in pollution control.

(6 marks)

Answer

1. Dust-laden gas enters the dirty chamber and is distributed around the fabric filter bags.
2. Gas passes through the fabric and its accumulated dust cake; particles are retained on the dirty side.
3. Filtered gas flows through the bag interiors into a clean-gas plenum and then to the outlet fan/stack.
4. Periodic reverse-air, shaking or pulse-jet cleaning removes the dust cake. The illustrated design uses short compressed-air pulses.
5. Dislodged dust falls into the hopper and leaves through a sealed discharge device. Monitor differential pressure and bag integrity to detect excessive loading or leakage.

Fabric selection and gas temperature must suit the process; this equipment removes particulate matter and does not by itself remove all gaseous pollutants.

Question 4(a)

(i) Explain the need for energy audit in industries. (ii) Describe the following types of energy audit: (I) preliminary; (II) comprehensive; (III) system specific.

(8 marks)

Answer

An energy audit identifies where and how energy is used, quantifies significant losses and develops justified improvement opportunities. It supports cost reduction, prioritisation of investment and verification of energy performance relative to production.

Question 4(b)

Outline four elements of energy monitoring and targeting system.

(6 marks)

Answer

1. Measure and record energy use and relevant drivers such as production, operating hours and weather.
2. Analyse the data to establish a baseline and meaningful energy-performance indicators, accounting for changes in the drivers.
3. Set realistic targets/budgets and compare actual performance with expected consumption.
4. Report deviations, investigate causes, implement corrective actions and verify the resulting savings.

Question 4(c)

Explain the following steam distribution system problems and state two causes of each: (i) steam leakage; (ii) pipeline erosion; (iii) water hammer.

(6 marks)

Answer

Question 5(a)

(i) State four desired properties of a moderator in a nuclear plant. (ii) Outline five factors considered when selecting site for locating a nuclear power.

(9.5 marks)

Answer

1. Good slowing-down ability: a large reduction in neutron energy per scattering collision, generally favoured by light nuclei.
2. A high probability of neutron scattering.
3. Low neutron absorption, so useful neutrons are not unnecessarily lost.
4. Adequate thermal, radiation and chemical stability under reactor operating conditions.

Question 5(b)

Outline three types of fuels used to generate nuclear power.

(4.5 marks)

Answer

If fuel families are named by element, distinguish uranium-based fuel, plutonium-containing fuel and thorium-based fuel cycles. Thorium-232 itself is fertile: it supports a cycle that produces fissile uranium-233 and is not interchangeable with a directly fissile fuel. Likewise uranium-238 is fertile rather than a stand-alone thermal-reactor fissile fuel.

Question 5(c)

(i) State four maintenance tasks carried on sight glasses in a steam distribution system. (ii) State two causes and their remedies for each of the following problems in a steam trap: (I) trap blows steam; (II) condensate won’t drain.

(6 marks)

Answer

1. Clean viewing surfaces to restore visibility using the approved isolated/cool maintenance procedure.
2. Inspect for cracks, erosion, clouding or other damage; replace damaged glass with the specified type.
3. Check gaskets, seals and connections for leakage and condition.
4. Inspect protective guards and associated isolation/drain fittings for correct condition and operation.

Visible discharge vapour can include flash steam, which is not automatically a leaking live-steam valve. Confirm the diagnosis using the appropriate trap-testing method.

Question 6(a)

With the aid of diagrams, explain the difference between impluse and reaction water turbines.

(7 marks)

Answer

Question 6(b)

With the aid of a diagram, explain the operation of a francis turbine used in a hydropower plant.

(5 marks)

Answer

1. The penstock supplies pressurised water to the spiral casing around the runner.
2. Stay/guide passages distribute flow; adjustable wicket gates regulate the quantity and direction of water entering the runner.
3. Water moves inward through the runner and then turns toward the axial outlet. Changes in pressure and momentum produce runner torque.
4. The shaft transmits power to the generator.
5. The expanding draft tube carries discharge to the tailrace and recovers part of the remaining velocity head.

Question 6(c)

Explain the following methods used to store idle boilers (i) wet; (ii) dry.

(5 marks)

Answer

Choice and detailed preservation conditions depend on boiler design, outage duration, ambient conditions and the approved maintenance program.

Question 6(d)

Outline the procedure of hydrostatic test of boilers.

(3 marks)

Answer

1. Prepare the boiler under the authorised test plan: isolate it, make it safe for examination, fit the specified test connections/gauges, and fill with suitable water while venting trapped air.
2. Use a controlled hydraulic source to raise pressure gradually to the test pressure specified by the applicable boiler procedure and authorised examiner; hold and inspect as required for leakage, deformation and pressure behaviour.
3. Depressurise safely, drain/restore the required fittings, record findings and carry out the required checks before returning the boiler to service.

This is an examination outline. The paper gives no boiler rating or test standard, so no universal test pressure, hold time or temperature is invented. Compressed gas is not substituted for the hydrostatic test medium.

Question 7(a)

With the aid of sketches, explain the difference between vertical and horizontal wind turbines.

(6 marks)

Answer

The names describe the shaft orientation; a vertical-axis turbine does not require the wind itself to blow vertically.

Question 7(b)

With the aid of a sketches, explain the following methods of boiler feed water treatment: (i) internal; (ii) external.

(8 marks)

Answer

Softening is not the same as complete demineralisation. Internal treatment supplements the required external treatment; it does not make arbitrarily poor feedwater acceptable. Treatment selection and dosing require the actual pressure, boiler type and water analysis.

Question 7(c)

(i) Outline three maintenance checks carried out on steam turbines. (ii) Outline two problems caused by wet steam in steam turbines.

(6 marks)

Answer

1. Check lubrication condition, oil pressure/temperature and bearing condition.
2. Trend vibration, alignment, seals and evidence of abnormal rubbing or leakage.
3. Inspect/check the governing and protective systems, including scheduled overspeed-trip verification under the approved procedure.
4. Wet-steam droplets can erode blades and associated flow surfaces, particularly at high relative velocity.
5. Moisture increases flow losses and reduces stage efficiency; significant water carryover can also cause mechanical damage.