2nd Class Power Engineering 2A3 Practice Exam Part 1

2ND CLASS POWER

ENGINEERING 2A3

PRACTICE EXAM PART 1

Question 1: What is the primary purpose of a zeolite softener in a boiler water

treatment system?

Choices:

1) To remove dissolved oxygen 2) To reduce scale-forming hardness by exchanging calcium and magnesium ions for sodium ions 3) To �lter out colloidal silica 4) To neutralize acidic return condensate Correct Answer: To reduce scale-forming hardness by exchanging calcium and magnesium ions for sodium ions Explanation: A zeolite softener uses ion exchange to remove calcium and magnesium (hardness) and replace them with non-scale-forming sodium ions.Page 1

Question 2: How is top dryness de�ned in a boiler steam-water mixture?

Choices:

1) The percentage of water in the steam drum 2) The percentage of steam, by mass, in the steam-water mixture at the top of a riser tube 3) The moisture content of steam leaving the superheater 4) The ratio of steam to feedwater entering the economizer Correct Answer: The percentage of steam, by mass, in the steam-water mixture at the top of a riser tube Explanation: Top dryness is the percentage of steam by mass in the mixture at the top of a riser where heating ceases. It normally runs from 5% to 20%.Question 3: Which chemical is typically added to a boiler feed system to chemically scavenge dissolved oxygen?

Choices:

1) Sodium hydroxide 2) Sodium sul�te 3) Sodium bicarbonate 4) Trisodium phosphate

Correct Answer: Sodium sul�te

Explanation: Sodium sul�te (or hydrazine) is commonly used as an oxygen scavenger to prevent pitting corrosion.Page 2

Question 4: What is a major advantage of a once-through supercritical boiler?

Choices:

1) It requires no boiler feed pumps 2) It does not require steam drums or steam separating equipment 3) It can operate using untreated raw water 4) It produces steam at lower e�ciencies but higher mass �ow

Correct Answer: It does not require steam drums or steam separating equipment

Explanation: Once-through boilers operating at supercritical pressures do not have a distinct liquid-to-vapor phase change, eliminating the need for steam drums.

Question 5: Why is colloidal silica problematic in a boiler system?

Choices:

1) It forms a highly insulating scale and cannot be removed by standard ion exchange 2) It highly corrodes copper in the condenser 3) It causes immediate tube failures by blocking �ow 4) It lowers the pH of the boiler water excessively Correct Answer: It forms a highly insulating scale and cannot be removed by standard ion exchange Explanation: Colloidal silica is non-ionic, meaning standard zeolite or simple ion exchange won't remove it. In the boiler, it can bake onto tubes or volatilize and deposit on turbine blades.Page 3