4th Class Power Engineering boiler plant operations practice questions (4B).

Five free practice questions on boiler plant operations: unit B4 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 general preparation for a steam boiler startup

   A plant's procedure fills a cold, drained steam boiler with treated water only to a mark below its normal operating level before the burner is lit. The reason is that the

   1. smaller water mass shortens the warm-up and saves fuel
   2. low-water cut-off can then be proven to trip during the warm-up
   3. air in the drum needs a larger space to clear through the vent
   4. water expands as it heats and the level rises in the warm-up
   Show the answer

   D. Correct. Water at operating temperature takes up noticeably more room than cold water (about an eighth more at 180 °C than at 20 °C), and steam bubbles forming add to the rise. Filled to the normal mark cold, the boiler would rise toward the top of the glass and need blowing down; filling low leaves room for that growth.

   Why not the others

   A. The warm-up time is set by the allowed rate of temperature rise, not by how much water is in the drum, and the level comes back up anyway. Filling low leaves room for expansion.

   B. The cut-off is tested by its own procedure, and the starting level is set above its trip point, so the low fill does not test it. The level is set low because the water will expand as it is heated.

   C. The drum vent clears the air as steam forms whatever the starting level; a little more steam space does not change that. The low fill leaves room for the water's expansion as it heats.

2. Procedural · Describe how to start a single boiler steam plant

   During warm-up of a refinery boiler with a superheater, the main steam stop valve is still closed and the superheater outlet drain is open. At 30 % of operating pressure, a trainee wants to close that drain to save steam. What would closing it do?

   1. Stop the steam flow that keeps the superheater tubes cool
   2. Raise the drum pressure fast enough to lift the safety valves
   3. Let air back into the boiler through the superheater outlet
   4. Starve the economizer of the feedwater that cools its tubes
   Show the answer

   A. Correct. Superheater tubes hold no water; only steam flowing through them carries away the furnace heat they absorb. With the stop valve shut and the drain closed, the steam in them stands still and the tubes can overheat. The drain stays open until steam flow through the superheater is secured.

   Why not the others

   B. The small flow through the drain hardly affects the pressure rise, and 30 % of operating pressure is far below the safety valve setting. The harm is to the superheater, which would lose its cooling steam flow.

   C. At 30 % of operating pressure the boiler is well above atmospheric, so air cannot enter. Closing the drain would stop the steam flow that cools the superheater tubes.

   D. The economizer is protected on the feedwater side (by feeding or recirculating water through it); the superheater drain does not affect it. Closing the drain stops the steam flow cooling the superheater.

3. Recall · Describe routine steam boiler operating duties

   A packaged watertube boiler at a gas plant, designed for 1 000 kPa(g), is run at 300 kPa(g) while carrying its full rated steam output. The main risk is

   1. the tubes overheating at the lower saturation temperature
   2. the safety valves simmering at the reduced pressure
   3. scale forming faster on the cooler tube surfaces
   4. water carried over by the higher steam velocity
   Show the answer

   D. Correct. At a lower pressure each kilogram of steam takes up far more room (about two and a half times as much here), so the same output leaves the water surface, the drum separators and the outlet nozzle much faster and sweeps water droplets with it. The steam is wet and carries boiler-water solids; makers may derate a boiler run well below its design pressure.

   Why not the others

   A. A lower pressure lowers the water's boiling temperature, so the water and the tube metal run cooler, not hotter; tubes overheat when they lose water or are scaled. The risk at full output and low pressure is water carried over by the higher steam velocity.

   B. A safety valve's set pressure does not change with the operating pressure, and 300 kPa(g) is far below the setting of valves protecting a 1 000 kPa(g) boiler. The risk is wet steam from the higher steam velocity.

   C. Scale comes from hardness and other solids in the boiler water, and calcium carbonate is less soluble in hotter water, not cooler, so a lower pressure does not speed scaling. The risk at full output and low pressure is water carried over with the steam.

4. Recall · Describe routine maintenance activities for boiler plant operation

   A district energy plant has two identical boiler feed pumps, one running and one on standby with its suction open. Each week the operators change over, so the standby pump runs and the other stands by. What is the main purpose?

   1. To prove the standby pump will run, and to share wear between them
   2. To let the duty pump's motor cool before its windings overheat
   3. To vent air that gathers in the standby pump's casing, so it stays primed
   4. To keep the standby pump's packing wet, so its gland does not dry out
   Show the answer

   A. Correct. A standby pump is only useful if it starts and carries the load when the duty pump trips. Running it each week proves it, shows up faults while there is time to fix them and spreads the running hours; bearings that stand still beside a running machine can also be marked by its vibration, which regular running helps prevent.

   Why not the others

   B. Feed-pump motors are rated for continuous duty and need no rest periods. A motor that overheats in normal service has a fault to find (overload, poor cooling, bearing trouble), not a schedule to change.

   C. A standby pump with its suction open stays flooded, and it is vented when it is placed on standby. Venting is not the reason for a weekly change-over.

   D. A flooded standby pump with its suction open keeps water at its packing anyway. The change-over is about proving the pump and sharing wear, not wetting the packing.

5. Recall · Describe steam boiler shutdown and lockout procedures

   As a steam boiler that has been shut down cools, the operator opens its drum vent while the pressure is still a little above atmospheric. Why?

   1. So a vacuum cannot form as the steam left inside condenses
   2. So the boiler cools faster as the remaining steam escapes
   3. So dissolved solids can leave the drum with the venting steam
   4. So the safety valves are not left holding pressure as they cool
   Show the answer

   A. Correct. Steam trapped in a closed drum condenses as it cools and pulls a vacuum. Boilers are built for pressure from inside: rolled tube joints and inside-fitted manhole and handhole covers can leak under vacuum, water can be drawn in through leaking valves from connected lines, and the boiler will not drain until air can enter.

   Why not the others

   B. Speed is not the aim, and the vent is opened only once the pressure is low. Cooling a boiler quickly by blowing off steam strains the drum and tube joints; the pressure is let fall naturally.

   C. Dissolved solids stay in the water when it turns to steam; they leave by blowdown, not with vented steam. The vent is opened to let air in, not to clean the water.

   D. Safety valves are shut at these pressures and come to no harm as they cool. The vent is opened to stop a vacuum forming as the steam condenses.

02Answer key

03What unit B4 covers

Running a boiler plant from start to lay-up: preparing auxiliaries and starting pumps and fans, warming a cold boiler, starting and cutting in hot-water and steam boilers, routine operation, emergencies and troubleshooting, the shift checks, standard operating procedures and logs, shutdown and lay-up, water hammer, and what to do when equipment fails.

Part B of the SOPEEC 4th Class syllabus names this unit “B4 Plant (Boiler) Operations” and lists six objectives under it (objectives 12 to 17 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

Know the water level first

The gauge glass is the first reading on every boiler at the start of a shift. If the glass shows no water and the burner is still firing, the fuel is shut off first; no feedwater goes into a boiler whose level is out of sight, because cold water on overheated metal can crack it.

Warm a cold boiler slowly

Thin tubes heat faster than the drum and headers, so a cold boiler is warmed at low fire and judged by water temperature, not by the gauge, which does not move until the water boils. The first hour, from cold to boiling, is where a 50 °C-per-hour limit can be broken while the gauge still reads zero.

Keep steam moving and condensate out

Superheater tubes hold no water, so the superheater outlet drain stays open during warm-up until steam flow is secured. When a boiler is cut in to a common header, the drain between the non-return valve and the header stop valve is opened first so condensate is not driven into the header.

Lay-up keeps oxygen and water off the metal

A wet lay-up fills the boiler right to the top with treated water so no air space is left; a dry lay-up drains and dries it, adds desiccant and seals it, or holds nitrogen slightly above atmospheric so any leak flows outward. The drum vent is opened while a cooling boiler still has a little pressure, so no vacuum forms.

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.