CWEA Mechanical Technologist Grade 3
Practice Exam with Practice Questions A
1. A centrifugal pump is operating at 1750 rpm, delivering 500 gpm against a total
head of 120 ft. The pump efficiency is 75%. If the impeller is trimmed to reduce the
diameter by 10%, and the affinity laws apply, what is the approximate new flow rate
and head?
A. 450 gpm, 97 ft
B. 450 gpm, 108 ft
C. 500 gpm, 120 ft
D. 550 gpm, 132 ft
Answer: A. 450 gpm, 97 ft
Rationale: Affinity laws: Flow varies linearly with impeller diameter (Q2 = Q1 × D2/D1 =
500 × 0.9 = 450 gpm). Head varies with square of diameter (H2 = H1 × (D2/D1)^2 = 120 ×
0.81 = 97.2 ft). Thus, 450 gpm and ~97 ft. Other options incorrectly apply linear scaling to
head or ignore the trim effect.
2. In a positive displacement rotary lobe pump used for sludge transfer, the discharge
pressure suddenly spikes and the relief valve lifts frequently. The pump is driven by a
VFD. Which condition is the most likely root cause?
A. Cavitation due to low suction pressure
B. Closed discharge valve or downstream blockage
C. Worn lobes causing slip
D. VFD set to constant torque mode
Answer: B. Closed discharge valve or downstream blockage
Rationale: A sudden discharge pressure spike with frequent relief valve lifting indicates a
blocked or closed discharge line. Cavitation would cause noise and flow reduction but not
necessarily pressure spikes. Worn lobes reduce pressure. VFD mode affects torque but not
directly cause pressure spikes.
Page 1 of 74
,3. A mechanical technologist is troubleshooting a motor that trips on overload after 10
minutes of operation. The motor nameplate indicates 50 HP, 460V, 3-phase, 60 Hz,
service factor 1.15. Measured current is 62A per phase. What is the most appropriate
next step?
A. Replace the motor with a higher horsepower unit.
B. Check for mechanical binding or misalignment in the driven equipment.
C. Increase the overload heater size to accommodate the current.
D. Reduce the voltage to lower the current draw.
Answer: B. Check for mechanical binding or misalignment in the driven equipment.
Rationale: Full load current (FLC) for a 50 HP, 460V motor is approximately 65A (NEC
Table 430.250). Measured 62A is within normal range, so overload trip suggests
mechanical load issues. Increasing heater size would be unsafe. Replacing motor or
reducing voltage does not address root cause.
4. A multistage centrifugal blower in an aeration basin is experiencing high vibration
at 2× running speed and elevated bearing temperatures. The blower has a gear-driven
speed increaser. Which analysis technique is most appropriate to identify the source?
A. Shaft alignment check with dial indicators
B. Gear mesh frequency analysis using a vibration analyzer
C. Ultrasonic leak detection on the discharge piping
D. Motor current signature analysis
Answer: B. Gear mesh frequency analysis using a vibration analyzer
Rationale: High 2× running speed vibration in a gear-driven blower often indicates gear
mesh issues, such as worn or misaligned gears. Gear mesh frequency analysis can pinpoint
this. Alignment check is less specific; ultrasonic leak detection is for leaks; motor current
signature analysis is for electrical faults.
Page 2 of 74
,5. A wastewater treatment plant uses a 480V, 3-phase, 100 HP induction motor for a
centrifugal pump. The motor is controlled by a soft starter. During startup, the motor
draws 600% of full load current for 10 seconds. What is the primary advantage of
using a soft starter over a direct-on-line starter in this application?
A. It increases starting torque to overcome high inertia.
B. It reduces mechanical stress on the pump and piping by limiting inrush current.
C. It allows the motor to run at variable speeds.
D. It provides better power factor correction during operation.
Answer: B. It reduces mechanical stress on the pump and piping by limiting inrush
current.
Rationale: Soft starters reduce inrush current and starting torque, minimizing mechanical
shock to the pump, coupling, and piping. They do not increase torque, provide variable
speed (that's a VFD), or correct power factor during run.
6. A mechanical seal on a centrifugal pump is leaking. The seal is a single-spring,
pusher-type, with a carbon rotating face and silicon carbide stationary face. The
pumped fluid is abrasive sludge. Which seal design modification is most appropriate
to extend seal life?
A. Replace carbon face with silicon carbide and use a dual seal with flush.
B. Increase spring tension to improve face contact.
C. Switch to a packing gland with lantern ring.
D. Use a single seal with a flush plan 11.
Answer: A. Replace carbon face with silicon carbide and use a dual seal with flush.
Rationale: Abrasive fluids require hard faces (silicon carbide vs. silicon carbide) and a
dual seal with a clean flush to keep abrasives away from faces. Increasing spring tension
increases wear. Packing is not a seal upgrade. A single seal with flush may still allow
abrasives.
Page 3 of 74
, 7. A 12-inch diameter pipe carries water at 8 ft/s. A 6-inch branch is added. Assuming
incompressible flow and no losses, what is the velocity in the 6-inch branch if the flow
splits equally?
A. 4 ft/s
B. 8 ft/s
C. 16 ft/s
D. 32 ft/s
Answer: C. 16 ft/s
Rationale: Using continuity, Q_main = A_main × V_main = (0.5)^2 × 8 = 6.28 ft³/s. Half
flow to branch = 3.14 ft³/s. Branch area = (0.25)^2 = 0.196 ft². V_branch = 3..196
= 16 ft/s. Thus, C is correct. Other options misapply area ratios.
8. In a preventive maintenance program, a technologist uses ultrasonic testing to
detect early bearing degradation. Which ultrasonic parameter is most indicative of
incipient bearing failure?
A. Overall decibel level increase
B. High-frequency acoustic emission with periodic bursts
C. Low-frequency vibration amplitude
D. Temperature rise
Answer: B. High-frequency acoustic emission with periodic bursts
Rationale: Ultrasonic testing detects high-frequency acoustic emissions from friction and
impacts in early bearing degradation. Periodic bursts correspond to rolling element
defects. Overall dB increase can indicate lubrication issues, but bursts are more specific.
Low-frequency vibration and temperature rise occur later.
Page 4 of 74
Practice Exam with Practice Questions A
1. A centrifugal pump is operating at 1750 rpm, delivering 500 gpm against a total
head of 120 ft. The pump efficiency is 75%. If the impeller is trimmed to reduce the
diameter by 10%, and the affinity laws apply, what is the approximate new flow rate
and head?
A. 450 gpm, 97 ft
B. 450 gpm, 108 ft
C. 500 gpm, 120 ft
D. 550 gpm, 132 ft
Answer: A. 450 gpm, 97 ft
Rationale: Affinity laws: Flow varies linearly with impeller diameter (Q2 = Q1 × D2/D1 =
500 × 0.9 = 450 gpm). Head varies with square of diameter (H2 = H1 × (D2/D1)^2 = 120 ×
0.81 = 97.2 ft). Thus, 450 gpm and ~97 ft. Other options incorrectly apply linear scaling to
head or ignore the trim effect.
2. In a positive displacement rotary lobe pump used for sludge transfer, the discharge
pressure suddenly spikes and the relief valve lifts frequently. The pump is driven by a
VFD. Which condition is the most likely root cause?
A. Cavitation due to low suction pressure
B. Closed discharge valve or downstream blockage
C. Worn lobes causing slip
D. VFD set to constant torque mode
Answer: B. Closed discharge valve or downstream blockage
Rationale: A sudden discharge pressure spike with frequent relief valve lifting indicates a
blocked or closed discharge line. Cavitation would cause noise and flow reduction but not
necessarily pressure spikes. Worn lobes reduce pressure. VFD mode affects torque but not
directly cause pressure spikes.
Page 1 of 74
,3. A mechanical technologist is troubleshooting a motor that trips on overload after 10
minutes of operation. The motor nameplate indicates 50 HP, 460V, 3-phase, 60 Hz,
service factor 1.15. Measured current is 62A per phase. What is the most appropriate
next step?
A. Replace the motor with a higher horsepower unit.
B. Check for mechanical binding or misalignment in the driven equipment.
C. Increase the overload heater size to accommodate the current.
D. Reduce the voltage to lower the current draw.
Answer: B. Check for mechanical binding or misalignment in the driven equipment.
Rationale: Full load current (FLC) for a 50 HP, 460V motor is approximately 65A (NEC
Table 430.250). Measured 62A is within normal range, so overload trip suggests
mechanical load issues. Increasing heater size would be unsafe. Replacing motor or
reducing voltage does not address root cause.
4. A multistage centrifugal blower in an aeration basin is experiencing high vibration
at 2× running speed and elevated bearing temperatures. The blower has a gear-driven
speed increaser. Which analysis technique is most appropriate to identify the source?
A. Shaft alignment check with dial indicators
B. Gear mesh frequency analysis using a vibration analyzer
C. Ultrasonic leak detection on the discharge piping
D. Motor current signature analysis
Answer: B. Gear mesh frequency analysis using a vibration analyzer
Rationale: High 2× running speed vibration in a gear-driven blower often indicates gear
mesh issues, such as worn or misaligned gears. Gear mesh frequency analysis can pinpoint
this. Alignment check is less specific; ultrasonic leak detection is for leaks; motor current
signature analysis is for electrical faults.
Page 2 of 74
,5. A wastewater treatment plant uses a 480V, 3-phase, 100 HP induction motor for a
centrifugal pump. The motor is controlled by a soft starter. During startup, the motor
draws 600% of full load current for 10 seconds. What is the primary advantage of
using a soft starter over a direct-on-line starter in this application?
A. It increases starting torque to overcome high inertia.
B. It reduces mechanical stress on the pump and piping by limiting inrush current.
C. It allows the motor to run at variable speeds.
D. It provides better power factor correction during operation.
Answer: B. It reduces mechanical stress on the pump and piping by limiting inrush
current.
Rationale: Soft starters reduce inrush current and starting torque, minimizing mechanical
shock to the pump, coupling, and piping. They do not increase torque, provide variable
speed (that's a VFD), or correct power factor during run.
6. A mechanical seal on a centrifugal pump is leaking. The seal is a single-spring,
pusher-type, with a carbon rotating face and silicon carbide stationary face. The
pumped fluid is abrasive sludge. Which seal design modification is most appropriate
to extend seal life?
A. Replace carbon face with silicon carbide and use a dual seal with flush.
B. Increase spring tension to improve face contact.
C. Switch to a packing gland with lantern ring.
D. Use a single seal with a flush plan 11.
Answer: A. Replace carbon face with silicon carbide and use a dual seal with flush.
Rationale: Abrasive fluids require hard faces (silicon carbide vs. silicon carbide) and a
dual seal with a clean flush to keep abrasives away from faces. Increasing spring tension
increases wear. Packing is not a seal upgrade. A single seal with flush may still allow
abrasives.
Page 3 of 74
, 7. A 12-inch diameter pipe carries water at 8 ft/s. A 6-inch branch is added. Assuming
incompressible flow and no losses, what is the velocity in the 6-inch branch if the flow
splits equally?
A. 4 ft/s
B. 8 ft/s
C. 16 ft/s
D. 32 ft/s
Answer: C. 16 ft/s
Rationale: Using continuity, Q_main = A_main × V_main = (0.5)^2 × 8 = 6.28 ft³/s. Half
flow to branch = 3.14 ft³/s. Branch area = (0.25)^2 = 0.196 ft². V_branch = 3..196
= 16 ft/s. Thus, C is correct. Other options misapply area ratios.
8. In a preventive maintenance program, a technologist uses ultrasonic testing to
detect early bearing degradation. Which ultrasonic parameter is most indicative of
incipient bearing failure?
A. Overall decibel level increase
B. High-frequency acoustic emission with periodic bursts
C. Low-frequency vibration amplitude
D. Temperature rise
Answer: B. High-frequency acoustic emission with periodic bursts
Rationale: Ultrasonic testing detects high-frequency acoustic emissions from friction and
impacts in early bearing degradation. Periodic bursts correspond to rolling element
defects. Overall dB increase can indicate lubrication issues, but bursts are more specific.
Low-frequency vibration and temperature rise occur later.
Page 4 of 74
