4th Class Power Engineering boiler and cooling water treatment practice questions (4B).

Five free practice questions on boiler and cooling water treatment: unit B6 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 water filtration and the removal of suspended solids

   A dual-media pressure filter at a gas plant holds anthracite and sand. After each backwash the bed settles with

   1. the finer sand on top, so the smallest grains meet the water first
   2. the coarser, lighter anthracite on top of the finer, heavier sand
   3. the two media mixed evenly to give one uniform filtering layer
   4. the anthracite at the bottom, where its weight packs the sand above
   Show the answer

   B. Correct. Anthracite grains are larger but less dense than sand, so after the bed is expanded in backwash they settle back on top. Water meets the coarse layer first, which holds much of the dirt in depth, and the fine sand below polishes it, giving long runs between backwashes.

   Why not the others

   A. With fine sand on top, the dirt would be caught in a thin surface layer that blinds quickly and needs frequent backwashing. Coarse over fine is the arrangement, and the media densities keep it that way.

   C. The media are chosen with different densities precisely so they separate back into layers after each backwash; a mixed bed loses the coarse-to-fine filtering in depth.

   D. Anthracite is the lighter medium, so it settles above the sand after a backwash, not beneath it; the heavier, finer sand ends up below.

2. Procedural · Describe internal boiler feedwater chemical feed systems

   A pulp mill's power boiler has an economizer between the feed pumps and the steam drum. Its phosphate is pumped through a chemical feed line straight into the drum rather than into the feedwater line. The reason is that phosphate fed into the feedwater would

   1. be too dilute in the feedwater to raise the drum's phosphate reading
   2. flash off with the steam in the economizer and never reach the drum
   3. react with the sulphite in the feedwater and use up the scavenger
   4. react with hardness and deposit in the feed line and economizer
   Show the answer

   D. Correct. Phosphate precipitates calcium wherever the two meet. Fed upstream, it would react with any hardness in the feedwater and lay calcium phosphate deposits in the feed line and economizer tubes; fed into the drum, it precipitates the hardness where blowdown can remove the sludge.

   Why not the others

   A. Every kilogram fed reaches the drum either way, so dilution would not lower the drum reading. Drum feed keeps calcium phosphate from depositing upstream in the feed line and economizer.

   B. Phosphate is not volatile, so it stays in the water. The concern with feeding it upstream is calcium phosphate depositing in the feed line and economizer.

   C. Phosphate and sulphite do not consume each other; both are often fed to the same boiler. Phosphate goes to the drum so it does not precipitate hardness in the feed line and economizer.

3. Recall · Describe condensate treatment and the effects of non-treatment

   A neutralizing amine fed to the feedwater at a chemical plant protects condensate lines hundreds of metres away because it

   1. reacts with the oxygen in the feedwater, so none reaches the return
   2. is volatile, leaving with the steam and dissolving in the condensate
   3. raises the boiler water's alkalinity, which then carries over with the steam
   4. lays a film on the inner surface of every line it passes through
   Show the answer

   B. Correct. Neutralizing amines are volatile, so they leave the boiler with the steam and condense with it wherever it condenses. There they neutralize the carbonic acid and hold the condensate pH up across the whole return system.

   Why not the others

   A. Reacting with oxygen is an oxygen scavenger's job. A neutralizing amine neutralizes carbonic acid, and it reaches the far lines because it is volatile and goes with the steam.

   C. Carried-over boiler water means wet steam and deposits, which the plant works to prevent. The amine reaches the lines as a vapour in the steam, not in water droplets.

   D. Laying a film on the metal is how a filming amine works. A neutralizing amine raises the condensate's pH after travelling with the steam as a vapour.

4. Procedural · Describe the effects of water on condensers and cooling tower materials

   Eddy-current testing of a turbine condenser's copper-alloy tubes at a pulp mill finds wall loss only in a short length at the inlet end of each tube. The cooling water carries some fine sand. What is the most likely cause?

   1. Steam impingement from the turbine exhaust on the tubes
   2. Dezincification of the tube alloy by the cooling water
   3. Pitting under slime deposits spread along the tube length
   4. Erosion-corrosion from turbulent flow entering the tubes
   Show the answer

   D. Correct. Water entering a tube is highly turbulent for a short distance before the flow settles, and with sand in it the turbulence strips the protective oxide film from the copper alloy, so the metal wears away there. Inlet inserts, screening and holding the water velocity within limits help.

   Why not the others

   A. Steam impingement attacks the outsides of the tubes in the rows facing the turbine exhaust. Damage at the water inlet end of every tube comes from the cooling-water side.

   B. Dezincification leaches zinc from brass and leaves porous copper wherever the alloy is exposed, not just a short length at each inlet. Loss confined to the inlet ends points to turbulent erosion-corrosion.

   C. Deposit pitting would be scattered along the tube length wherever slime settles, not confined to the inlet ends of every tube.

5. Procedural · Describe recirculating water system corrosion and deposition

   A strainer on a data centre's closed chilled-water loop, opened for cleaning, holds black slimy deposits that smell of rotten eggs, and the steel under them is pitted. What is most likely at work?

   1. Oxygen pitting from air drawn in at a leaking pump seal
   2. Sulphate-reducing bacteria growing under the deposits
   3. Galvanic attack from copper dissolved out of the coils
   4. Mill scale from the new piping flaking off and settling
   Show the answer

   B. Correct. Sulphate-reducing bacteria thrive without oxygen under deposits and in low-flow spots. They turn sulphate into sulphide, which gives the rotten-egg odour and black iron sulphide, and their attack pits the steel beneath. Cleaning, biocide treatment and keeping water moving through dead legs control them.

   Why not the others

   A. Air in-leakage gives oxygen pitting with reddish-brown rust, not black slime smelling of rotten eggs; that odour is hydrogen sulphide from bacteria.

   C. Copper plating out on steel can cause local pits, but it would not produce black slime with a sulphide odour; those point to sulphate-reducing bacteria.

   D. Mill scale flakes are hard and black but have no odour and do not grow as slime; the rotten-egg odour points to sulphide from bacteria under the deposits.

02Answer key

03What unit B6 covers

Water treatment across the plant: the impurities in raw water and how it is filtered, softened, demineralized and deaerated, internal boiler-water treatment, chemical feed and boiler-water tests, condensate treatment, cooling tower and condenser water, closed chilled-water circuits, and hot-water heating and glycol systems.

Part B of the SOPEEC 4th Class syllabus names this unit “B6 Water Treatment” and lists six objectives under it (objectives 22 to 27 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

Softening removes hardness, not dissolved solids

A sodium-zeolite softener swaps calcium and magnesium for sodium, so hardness falls to near zero while dissolved solids stay about the same and blowdown is still needed. Its run length is resin capacity divided by the hardness load of the make-up alone, since returned condensate does not pass through it.

Blowdown holds the boiler water's solids

The blowdown ratio is the feedwater's dissolved solids over the boiler water's limit, and it applies to the feed, which is steam plus blowdown. Boiler-water samples are drawn through a sample cooler, because a hot sample flashes and reads high.

Oxygen goes before the feed pumps

The deaerator removes most of the dissolved oxygen and sulphite fed into its storage section scavenges the rest before the feed pumps. To test the deaerator itself, the sulphite feed is stopped and cleared first, or the result flatters it.

Condensate and cooling water have their own chemistry

Carbon dioxide from the feedwater's alkalinity makes condensate acidic; a neutralizing amine leaves with the steam and protects the return lines. In a cooling tower, cycles of concentration fix the bleed-off: the water leaving other than by evaporation equals evaporation divided by cycles minus one.

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.