CPM - Certified Plant Mechanic Power Plant Machinery Questions and Answers — Questions and Answers
Question 1: In a steam power plant, what is the primary function of a deaerator in the boiler feedwater system?
- To filter out suspended solids and sediment.
- To increase the pressure of the feedwater before it enters the boiler.
- To remove dissolved gases, primarily oxygen, from the feedwater. (Correct answer)
- To chemically treat the water to adjust its pH level.
Correct answer: To remove dissolved gases, primarily oxygen, from the feedwater.
The primary purpose of a deaerator is to remove dissolved gases, especially oxygen and carbon dioxide, from the boiler feedwater. Dissolved oxygen is a major cause of corrosion and pitting in boiler tubes and other components, which can lead to equipment failure and reduced efficiency. By removing these gases, the deaerator protects the entire steam system.
Question 2: A plant mechanic observes a centrifugal pump making a loud rattling noise, similar to gravel passing through it, along with fluctuating discharge pressure and flow. Which of the following is the MOST likely cause of these symptoms?
- Pump-motor misalignment
- Worn bearings
- Cavitation (Correct answer)
- A clogged discharge line
Correct answer: Cavitation
The symptoms described—loud rattling or crackling noise, vibrations, and fluctuating pressure/flow—are classic signs of pump cavitation. Cavitation occurs when vapor bubbles form in a low-pressure area at the pump's suction and then violently collapse as they move into a higher-pressure zone, causing noise, damage to the impeller, and erratic performance.
Question 3: Which type of steam turbine is designed to exhaust steam at a pressure below atmospheric pressure into a condenser, maximizing the energy extracted from the steam for power generation?
- Back-pressure turbine
- Extraction turbine
- Topping turbine
- Condensing turbine (Correct answer)
Correct answer: Condensing turbine
A condensing turbine is specifically designed to expand steam to a very low pressure (a vacuum), which is maintained by a condenser. This large pressure drop across the turbine allows for the maximum amount of energy to be extracted from the steam, making it highly efficient for dedicated power generation. Back-pressure and extraction turbines, by contrast, exhaust steam at higher pressures for use in industrial processes.
Question 4: What is the principal role of an economizer in the boiler system of a power plant?
- To superheat the steam after it leaves the boiler drum.
- To preheat the incoming boiler feedwater using waste heat from flue gases. (Correct answer)
- To control the air-to-fuel ratio for optimal combustion.
- To remove moisture from the steam before it enters the turbine.
Correct answer: To preheat the incoming boiler feedwater using waste heat from flue gases.
An economizer is a heat exchanger that captures residual heat from the hot flue gases exiting the boiler. This captured heat is used to preheat the boiler feedwater before it enters the boiler drum. This process improves the overall thermal efficiency of the boiler by reducing the amount of fuel needed to turn the water into steam.
Question 5: In large rotating machinery like turbines and generators, what is the specific function of a thrust bearing?
- To support the radial load or weight of the rotor.
- To absorb and manage axial forces along the length of the shaft. (Correct answer)
- To reduce the rotational speed between the turbine and generator.
- To dampen torsional vibrations within the shaft.
Correct answer: To absorb and manage axial forces along the length of the shaft.
Thrust bearings are designed specifically to handle axial loads, which are forces that act parallel to the shaft's axis of rotation. In a turbine, for example, the flow of steam or gas creates a significant axial thrust that must be controlled to prevent the rotor from moving horizontally and making contact with stationary parts. Radial bearings, in contrast, support the weight of the shaft.
Question 6: Which of the following inspections on a large industrial gas turbine is the most comprehensive, typically requiring the turbine casing to be removed for direct examination of hot gas path components like blades and nozzles?
- Borescope Inspection
- Combustion Inspection
- Major Inspection / Hot Gas Path Inspection (Correct answer)
- Vibrational Analysis
Correct answer: Major Inspection / Hot Gas Path Inspection
A Major Inspection (MI) or Hot Gas Path Inspection (HGPI) is the most extensive type of scheduled maintenance for a gas turbine. It involves disassembling the unit, including removing the turbine casing, to allow for direct visual and non-destructive testing of critical internal components like turbine blades, nozzles, and combustion liners. Borescope and combustion inspections are less intrusive and do not typically require full disassembly.
CPM - Certified Plant Mechanic Power Plant Machinery Questions and Answers
In a steam power plant, what is the primary function of a deaerator in the boiler feedwater system?