CPE Mechanical Systems & Equipment 2 — Questions and Answers
Question 1: A centrifugal pump is operating in cavitation. Which symptom is MOST characteristic?
- Steady increase in discharge pressure
- Rattling or crackling noise with erratic flow (Correct answer)
- Smooth flow with reduced power consumption
- Gradual rise in suction pressure
Correct answer: Rattling or crackling noise with erratic flow
Cavitation causes vapor bubbles to implode violently, producing a rattling noise and unstable flow.
Question 2: What is the primary purpose of a mechanical seal in a rotating pump?
- To balance axial thrust on the impeller
- To prevent fluid leakage along the shaft (Correct answer)
- To reduce bearing friction losses
- To align the pump with the driver
Correct answer: To prevent fluid leakage along the shaft
Mechanical seals use lapped mating faces to prevent process fluid from leaking along the pump shaft.
Question 3: Which gear type is BEST suited for transmitting high torque between parallel shafts with low noise?
- Spur gears
- Helical gears (Correct answer)
- Bevel gears
- Worm gears
Correct answer: Helical gears
Helical gears have angled teeth that engage gradually, reducing noise and enabling higher torque transmission between parallel shafts.
Question 4: The Net Positive Suction Head Required (NPSHr) for a pump is determined by:
- The plant piping layout and fluid temperature
- The pump manufacturer based on impeller design (Correct answer)
- The fluid viscosity and specific gravity only
- The discharge pressure and pipe diameter
Correct answer: The pump manufacturer based on impeller design
NPSHr is a pump characteristic established by the manufacturer through testing and is fixed for a given pump and flow rate.
Question 5: A V-belt drive shows excessive slippage. The FIRST corrective action should be:
- Replace all belts with a larger cross-section
- Increase center distance to add belt tension
- Check and adjust belt tension to specification (Correct answer)
- Lubricate the sheave grooves
Correct answer: Check and adjust belt tension to specification
Insufficient tension is the most common cause of belt slippage, and adjusting tension to the manufacturer's specification corrects this before replacement is considered.
Question 6: What does the "specific speed" of a centrifugal pump describe?
- The maximum allowable shaft speed in RPM
- The pump's shape and performance characteristics (Correct answer)
- The ratio of impeller diameter to shaft diameter
- The flow velocity at the pump inlet
Correct answer: The pump's shape and performance characteristics
Specific speed is a dimensionless index that classifies pump impeller geometry and predicts performance behavior at the best efficiency point.
Question 7: In a hydraulic system, a pressure relief valve is set above the normal working pressure to:
- Increase system efficiency during peak loads
- Protect system components from overpressure (Correct answer)
- Maintain constant flow rate at all conditions
- Reduce heat generation in the hydraulic fluid
Correct answer: Protect system components from overpressure
The relief valve bypasses fluid back to the reservoir when system pressure exceeds the set point, protecting hoses, cylinders, and valves from damage.
CPE Mechanical Systems & Equipment 2
A centrifugal pump is operating in cavitation.
Which symptom is MOST characteristic?