4th Class Power Engineering piping, valves and steam traps practice questions (4A).

Five free practice questions on piping, valves and steam traps: unit G of paper A (4A) 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 methods and devices used to allow for pipe expansion and support

   During erection, a hot line is deliberately made shorter than the gap between its anchors by half its calculated expansion and pulled into place cold. This cold spring

   1. lowers the forces on anchors and equipment when the line is hot
   2. cancels the expansion, so the line grows no longer when hot
   3. lets the line be built without any loops, bends or expansion joints
   4. proves the welds by loading them before the hydrostatic test
   Show the answer

   A. Correct. Built short and stretched into place, the line starts with stress in the opposite sense. As it heats it first relaxes and is then stressed by the rest of the expansion, so the hot reactions on anchors, nozzles and equipment are lower. The total expansion is unchanged.

   Why not the others

   B. Cold spring does not change how much the steel expands; the line still grows by its full calculated amount. It shares the stress between the cold and hot states, lowering the hot loads on anchors and equipment.

   C. The line still needs enough flexibility, from loops, bends or joints, to absorb its expansion. Cold spring only shifts part of the stress into the cold condition so the hot loads on anchors and equipment are lower.

   D. Welds are proved by inspection and the pressure test, not by cold spring. Cold spring is set into the line to lower the forces it puts on anchors and equipment when hot.

2. Procedural · Describe a typical steam pressure reducing station, and the design and operation of steam system pressure-reducing valves

   Why is the bypass around a steam pressure-reducing valve usually fitted with a globe valve?

   1. It opens fully with little pressure drop, so the low side gets full flow.
   2. It can be throttled by hand to hold the low side while the PRV is out.
   3. It closes by itself if the low-side pressure rises too high.
   4. It stops steam flowing back from the low side to the high side.
   Show the answer

   B. Correct. When the PRV is isolated for repair, an operator supplies the low side through the bypass, opening it only enough to hold the required pressure while watching the low-side gauge. A globe valve throttles well and resists wire-drawing, which a gate valve does not.

   Why not the others

   A. Wide open, the bypass would pass high-side pressure straight to the low side and could overpressure it. The bypass valve is chosen to throttle, which is why it is a globe valve.

   C. Closing automatically on high pressure is not a hand valve's job; the low side is protected by its own safety valve. The bypass is a globe valve so it can be throttled by hand.

   D. Preventing reverse flow is the job of a check valve, and the high side is at the higher pressure anyway. The bypass is a globe valve so it can throttle steam to hold the low-side pressure.

3. Recall · Explain the methods used to promote good drainage of steam pipes, including the installation and maintenance of steam traps, to reduce the effects of water hammer

   An inverted bucket trap is classed as a mechanical trap because it tells steam from condensate by the difference in their

   1. temperature, which expands or contracts an element
   2. density, which floats or sinks the bucket
   3. velocity, which snaps a disc onto its seat
   4. conductivity, which switches a probe circuit
   Show the answer

   B. Correct. Steam entering the open bottom of the inverted bucket collects under it and makes it buoyant, so it rises and closes the valve. Condensate displaces the steam, the bucket loses buoyancy and sinks, and the valve opens. Float traps work on the same density principle.

   Why not the others

   A. Responding to temperature through an expanding bellows, capsule or bimetal element is how thermostatic traps work. The inverted bucket responds to density: steam floats the bucket, condensate lets it sink.

   C. A disc snapped shut by fast-moving flash steam is the thermodynamic (disc) trap. The inverted bucket is mechanical: it works on the density difference between steam and condensate.

   D. Conductivity probes are used in some electronic trap monitors and level controls, not in an inverted bucket. The bucket works on density: steam floats it and condensate lets it sink.

4. Procedural · Discuss valve details, including materials of construction and identification markings

   A valve is being chosen for a superheated steam line at 4 100 kPa (gauge) and 400 °C. Which body material suits this service?

   1. carbon or alloy steel
   2. grey (flake-graphite) cast iron
   3. cast tin bronze
   4. forged yellow brass
   Show the answer

   A. Correct. Carbon or alloy steel bodies, cast or forged, keep their strength and toughness at high pressure and temperature and stand up to shock and thermal stress. The grade is matched to the temperature, with alloy steel for the hotter services.

   Why not the others

   B. Grey cast iron is brittle, weak in tension and can crack under water hammer or sudden heating, so it is limited to lower pressures and temperatures. A 4 100 kPa (gauge), 400 °C steam valve needs a steel body.

   C. Bronze is used for small valves at low and moderate pressures and temperatures, and it loses strength well below 400 °C. Superheated steam at this pressure and temperature calls for steel.

   D. Brass is a valve material for low pressures and temperatures, and it weakens as temperature rises. Superheated steam at 4 100 kPa (gauge) and 400 °C needs a steel body.

5. Recall · Describe methods of connection for screwed, flanged and welded pipe; identify fittings and their markings

   In a run of small screwed pipe, which fitting allows a valve or a section of pipe to be taken out without unscrewing the whole line?

   1. a close nipple
   2. a coupling
   3. a reducing bushing
   4. a union
   Show the answer

   D. Correct. A union has two threaded ends joined by a separate nut. Undoing the nut parts the joint without turning the pipes, so a valve or piece of equipment can be removed. Unions are placed next to items that will need servicing.

   Why not the others

   A. A close nipple is a very short piece of pipe threaded along its whole length to join two fittings closely. It cannot be undone without turning the pipe; a union can.

   B. A coupling joins two pipe ends with right-hand threads at both ends, so it can only come apart by unscrewing a whole length. A union breaks the line without turning the pipe.

   C. A reducing bushing screws into a fitting to change the pipe size; it gives no way to part the line. A union is the fitting that breaks a screwed line without turning the pipe.

02Answer key

03What unit G covers

Plant piping and the valves on it: pipe sizes and schedules, pipe materials for high temperatures, welded, flanged and screwed joints, expansion joints and cold spring, steam traps, drains and water hammer, insulation, and the design and use of gate, globe, needle, check and non-return valves, OS&Y valves, pressure-reducing valves and their bypasses, backseats and valve exercising.

The SOPEEC 4th Class syllabus names this unit “Piping and Valves” (ABSA numbers it A7) and lists two objectives under it. 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

Schedule changes bore, not OD

Pipes of one nominal size share the same outside diameter, so a heavier schedule means a thicker wall and a smaller bore. The inside diameter is the outside diameter less twice the wall thickness.

Water hammer is a moving slug

Condensate lying in a low point can be picked up at steam velocity. Water is dense and nearly incompressible, so a slug stopped at a bend or valve gives a damaging pressure spike. Mains are pitched to trapped drip legs to prevent it.

Globe valves throttle, gates pass straight

A globe valve turns the flow up through its seat, so it causes a large pressure drop even when open but throttles well, which is why it serves as a pressure-reducing valve bypass. A fully open gate valve leaves an almost straight bore.

Expansion follows ΔL = αLΔT

Steel grows by its expansion coefficient times length times temperature rise, and an expansion joint between two anchors takes the growth of the whole run. Cold spring lowers the hot forces on anchors and equipment but leaves the total expansion unchanged.

05The other units

Paper 4A has twelve units, A to L. Each has its own page of free questions:

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