4th Class Power Engineering pumps and compressors practice questions (4B).

Five free practice questions on pumps and compressors: unit B2 of paper B (4B) of the 4th Class Power Engineer certificate exam, written to the SOPEEC syllabus. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Recall · Describe the common pumps found in plants

   In a single-stage volute pump, the impeller throws the water outward at high velocity. The job of the volute casing around it is to

   1. slow the water gradually, converting its velocity into pressure
   2. speed the water up before it enters the eye of the impeller
   3. limit leakage from the discharge back to the impeller eye
   4. balance the axial thrust that pushes the impeller toward suction
   Show the answer

   A. Correct. The volute's passage widens steadily toward the discharge nozzle, so the fast water leaving the impeller slows down and its velocity head becomes pressure head. A diffuser pump does the same job with fixed guide vanes around the impeller.

   Why not the others

   B. The water reaches the eye from the suction pipe at modest velocity, and the eye is the lowest-pressure point in the pump; the casing does not speed it up there. The volute works on the discharge side, slowing the water and raising its pressure.

   C. That is the job of the wear rings: a small, renewable clearance between the impeller and the casing that limits recirculation from discharge back to suction. The volute is the spiral passage that turns velocity into pressure.

   D. Axial thrust is handled by balance holes, back vanes, a double-suction impeller or a thrust bearing. The volute is the spiral passage around the impeller that converts the water's velocity into pressure.

2. Recall · Describe the types of shaft seals

   Packed centrifugal pumps usually have a renewable sleeve fitted over the shaft where it passes through the stuffing box. The sleeve is there so that

   1. the lantern ring is held in line with the flushing connection
   2. the packing can be tightened enough to stop all leakage
   3. packing wear falls on a cheap spare part and not on the shaft
   4. the shaft is insulated from the heat of the pumped liquid
   Show the answer

   C. Correct. Packing rubs where it grips, and over time it grooves the surface under it. A hard, corrosion-resistant sleeve takes that wear and is renewed at overhaul, saving the shaft; it is sealed to the shaft so liquid cannot leak along underneath it.

   Why not the others

   A. The lantern ring's position is set by the number of packing rings fitted on each side of it, so that it lines up with the flushing connection. The sleeve is a wear part under the packing.

   B. Packing needs a small, steady leak to lubricate and cool it, sleeve or no sleeve; tightened until the leak stops, it overheats and scores whatever it runs on. The sleeve's purpose is to take the wear in place of the shaft.

   D. A thin metal sleeve does little to insulate the shaft, and protecting it from heat is not its job. It is a renewable wear surface where the packing rubs.

3. Recall · Describe gaseous compression systems

   In a plant compressed-air system supplied by a reciprocating compressor, the air receiver between the compressor and the distribution header mainly

   1. raises the air to a higher pressure than the compressor delivers
   2. cools the air to the dew point needed by the plant's instruments
   3. stores air and damps the pulses so the compressor cycles less
   4. strips oil vapour from the air before it reaches the header
   Show the answer

   C. Correct. The receiver's volume absorbs the pulsing delivery of a reciprocating machine and holds a reserve for peaks in demand, so the compressor can load and unload, or start and stop, less often. As a large vessel it also lets some condensed water settle out, which is why it has a drain.

   Why not the others

   A. A receiver is a storage vessel; it cannot raise the air above the pressure the compressor delivers to it. A higher pressure would need a booster compressor.

   B. Lowering the dew point is the dryer's job, using a refrigerated or desiccant dryer downstream. A receiver drops out some water that has already condensed, but it does not dry the air to a set dew point.

   D. Oil aerosols are removed by coalescing filters and oil vapour by activated-carbon filters. A receiver lets some liquid oil and water settle, but it does not clean the air to instrument quality.

4. Recall · Describe the construction and operation of seals for compressors

   A food plant's reciprocating air compressor is a non-lubricated (oil-free) type, with no oil fed to its cylinders. Its piston rings and rider rings are usually made of

   1. a filled PTFE or carbon material that can run dry
   2. cast iron, lubricated by oil mist carried in the air
   3. bronze, kept wet by water injected with the inlet air
   4. hardened steel, which wears too slowly to need oil
   Show the answer

   A. Correct. With no oil in the cylinder the rings must slide on the liner dry, so they are made of self-lubricating materials such as PTFE filled with glass, carbon or bronze, or carbon-graphite. Rider rings carry the piston's weight so the piston itself does not touch the liner. These rings wear and are renewed on a schedule.

   Why not the others

   B. Cast-iron rings are used in lubricated cylinders, where oil is fed to the walls. In an oil-free machine there is no oil mist to rely on, and dry cast iron would score the liner.

   C. Reciprocating air compressors are not normally lubricated by injecting water: liquid in a cylinder risks broken valves and corrosion. The rings themselves are made of a dry-running material.

   D. Hard steel rubbing dry on an iron or steel liner would gall and score it quickly; hardness does not replace lubrication. Dry-running rings use self-lubricating materials such as filled PTFE.

5. Recall · List the common pump applications

   At a hospital heating plant that keeps heavy fuel oil as its standby fuel, the oil is heated in a day tank and pumped to the boilers' burners. The pumps on this service are usually

   1. air-operated diaphragm pumps, fed from the plant air system
   2. single-stage centrifugal pumps, sized for the burners' full flow
   3. rotary positive-displacement pumps, such as gear or screw pumps
   4. vertical turbine pumps hung inside the day tank
   Show the answer

   C. Correct. Thick oil robs a centrifugal pump of head and efficiency, but a gear or screw pump moves a nearly fixed volume each turn, and thick oil only seals its clearances better. That gives the steady flow and pressure the burners need, with a relief or back-pressure valve returning the surplus to the tank.

   Why not the others

   A. An air-operated diaphragm pump delivers in pulses, and a standard (1:1) unit's outlet pressure is limited to about its air-supply pressure, so it does not give the steady pressure a burner's atomizing system needs. It suits transfer and drum-emptying duties.

   B. A centrifugal pump loses head and efficiency quickly as viscosity rises, and heavy oil's viscosity changes with its temperature, so the delivery would wander. Centrifugal pumps suit thin liquids such as water and light oils.

   D. A vertical turbine pump lifts large flows of cold water from wells, sumps and intakes. It is a centrifugal machine, so it shares the centrifugal pump's poor handling of thick oil.

02Answer key

03What unit B2 covers

The pumps and compressors a power engineer runs: pump applications and performance terms, centrifugal, positive-displacement and special pumps, packing, mechanical seals and alignment, priming and start-up, the classes of compressors, their valves, seals and auxiliaries, capacity control, and routine compressor maintenance.

Part B of the SOPEEC 4th Class syllabus names this unit “B2 Pumps and Compressors” and lists four objectives under it (objectives 3 to 6 of 52). SOPEEC does not publish how many exam questions come from each unit, and neither do the ABSA, Technical Safety BC, TSASK or TSSA pages we checked. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Head comes from the pressure rise

Total head is the pressure the pump adds divided by density times g. With both gauges at the same height and both reading gauge pressure, atmospheric pressure cancels; the suction pressure is subtracted, not added. Motor input is hydraulic power divided by the pump and motor efficiencies.

Centrifugal and positive-displacement pumps start differently

A radial-flow centrifugal pump draws least power at zero flow, so it is started against a closed discharge that is then opened slowly. A positive-displacement pump is the opposite: suction, discharge and delivery line are opened fully first, because a closed outlet drives the pressure up until something gives.

Watch the margin above vapour pressure

A pump taking saturated water depends on the height of water above it to stay ahead of the water's vapour pressure. When a deaerator's pressure falls faster than its stored water cools, that margin shrinks and the feed pump can cavitate or become vapour-bound.

Compressor ratios use absolute pressure

For a two-stage machine with equal stage ratios, convert both pressures to absolute, take the square root of the overall ratio and convert back for the gauge. Suction-valve unloaders cut output by holding the valves open; a centrifugal compressor blows off air at low demand to stay out of surge.

05The other units

Paper 4B has twelve units, B1 to B12. Each has its own page of free questions:

The 4th Class Power Engineering practice exam (4B) page describes the full question bank; paper A has its own 4A practice exam. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.