4th Class Power Engineering Practice Exam: 4A and 4B.

Part A (4A)

1. Procedural · Elementary Mechanics and Dynamics

   A 2.5 m steel tie rod in a boiler support frame stretches 1.2 mm under load. What is the strain in the rod?

   1. 0.480
   2. 0.00120
   3. 2 080
   4. 0.000480
   Show the answer

   D. Correct. Strain = change in length ÷ original length, both in the same units: 1.2 mm ÷ 2 500 mm = 0.00048. Strain has no units; it is often written 4.8 × 10⁻⁴.

   A. This divides millimetres by metres (1.2 ÷ 2.5). Put both in one unit: 1.2 mm ÷ 2 500 mm = 0.00048.

   B. This converts the 1.2 mm to 0.0012 m but never divides by the length. Strain = 0.0012 m ÷ 2.5 m = 0.00048.

   C. This divides the length by the stretch (2 500 ÷ 1.2 = 2 083), the ratio upside down. Strain = ΔL ÷ L = 1.2 ÷ 2 500 = 0.00048.

   Source: SOPEEC 4th Class syllabus

2. Recall · Electricity

   An autotransformer differs from a two-winding transformer in that

   1. it works on direct current as well as alternating
   2. its primary and secondary share part of one winding
   3. it isolates the load from the supply more completely
   4. it can step voltage up but never step it down
   Show the answer

   B. Correct. An autotransformer has a single winding tapped part-way along, and the common section carries current for both circuits. It is smaller and cheaper for a modest ratio (reduced-voltage motor starters, boosting a low supply), but primary and secondary are not electrically isolated.

   A. Like any transformer it relies on a changing flux and works only on AC. On steady DC it gives no transformation, and the winding would carry a heavy current limited only by its resistance.

   C. It is the reverse: with one shared winding, the secondary is connected directly to the primary, so there is no electrical isolation. A two-winding transformer isolates the circuits.

   D. An autotransformer can step up or step down, depending on which tap is the supply and which the load. Its defining feature is the single shared winding.

   Source: SOPEEC 4th Class syllabus

3. Recall · Boiler Systems

   Intermittent bottom blowoff from the shell of a firetube boiler is mainly done to remove

   1. oil and scum floating on the water surface
   2. air and other gases released from the feedwater
   3. hard scale already bonded to the tubes and shell plates
   4. sludge and sediment that settle out of the water
   Show the answer

   D. Correct. Suspended solids and sludge settle where circulation is slowest, at the bottom of the shell (or the mud drum of a watertube boiler). A short, wide-open blow every so often flushes them out before they build up, restrict circulation and insulate heating surfaces.

   A. Floating oil and scum are skimmed off through a skimming (scum) connection at the water line; the bottom blowoff is at the lowest point, where heavy solids collect.

   B. Gases are removed from the feedwater before it reaches the boiler, by the deaerator, and any released in the boiler leave with the steam; nothing gaseous gathers at the bottom of the shell.

   C. Hard scale is bonded to the metal and is not moved by water rushing past it during a blow; it has to be removed mechanically or chemically during an outage. A bottom blow carries out loose material that has settled to the bottom of the shell.

   Source: SOPEEC 4th Class syllabus

Part B (4B)

1. Recall · B3 Boiler Safety Devices

   How does the temperature element of a T&P relief valve open the valve when the stored water gets too hot?

   1. A bimetal switch in the probe energizes a solenoid that lifts the disc
   2. The water's rising vapour pressure lifts the disc against the spring
   3. A sealed probe in the water expands and pushes the disc off its seat
   4. A fusible alloy plug in the probe melts and lets the hot water escape
   Show the answer

   C. Correct. The probe reaches into the hottest water near the top of the tank and holds a material that expands strongly as it warms. At the set temperature the expansion pushes the disc off its seat; hot water discharges and cold water enters. When the water cools, the element contracts and the valve reseats.

   A. A T&P valve is self-contained and needs no electrical supply, so it still works in a power failure. Its probe's expanding material pushes the disc open directly.

   B. That describes the valve's pressure function, which opens on tank pressure. The temperature function works with the pressure below the setting: an expanding element in the probe pushes the disc off its seat.

   D. A fusible plug melts once and cannot reseat; it is found in the crown of some firetube boilers as a low-water warning, not in a T&P valve. The T&P probe holds a material that expands with heat and pushes the disc open, and the valve reseats when the water cools.

   Source: SOPEEC 4th Class syllabus

2. Recall · B6 Water Treatment

   In a medium-pressure boiler at a food plant, phosphate is fed to the boiler water mainly to

   1. turn residual calcium into a soft sludge that blowdown removes
   2. dissolve existing hard scale back off the heating surfaces
   3. remove dissolved oxygen before it can pit the drum and tubes
   4. neutralize the carbon dioxide that would acidify the condensate
   Show the answer

   A. Correct. Phosphate reacts with the small amount of calcium hardness that gets past external treatment, forming a soft, non-sticky precipitate instead of hard scale on the tubes. A sludge conditioner keeps it dispersed, and bottom blowdown removes it.

   B. Phosphate is a precipitating treatment: it stops new hard scale forming but does not remove scale already baked on, which takes mechanical or chemical cleaning.

   C. Removing dissolved oxygen is the job of the deaerator and an oxygen scavenger such as sulphite. Phosphate deals with hardness, precipitating it as sludge.

   D. Carbon dioxide leaves with the steam, so it is dealt with by amines in the condensate system. Phosphate stays in the boiler water and precipitates hardness.

   Source: SOPEEC 4th Class syllabus

3. Procedural · B7 Prime Movers and Engines

   A four-stroke engine's camshaft is gear-driven from the crankshaft. At what speed does it turn?

   1. twice crankshaft speed
   2. crankshaft speed
   3. half crankshaft speed
   4. a quarter of crankshaft speed
   Show the answer

   C. Correct. Each intake and exhaust valve opens once per cycle, and a four-stroke cycle takes two crankshaft revolutions. The camshaft, with one lobe per valve, turns once for every two turns of the crankshaft: a 2:1 drive ratio.

   A. Twice crankshaft speed would open each valve four times in a four-stroke cycle. A valve opens once per cycle of two revolutions, so the camshaft turns at half crankshaft speed.

   B. At crankshaft speed each valve would open every revolution, which is two-stroke timing. A four-stroke valve opens once per cycle of two revolutions, so the camshaft turns at half crankshaft speed.

   D. A quarter comes from dividing by the four strokes, but the four strokes take two revolutions, not four. Each valve opening once every two revolutions means half crankshaft speed.

   Source: SOPEEC 4th Class syllabus

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