CPM Plant Equipment Maintenance & Troubleshooting 2 — Questions and Answers
Question 1: A centrifugal pump makes a crackling, gravel-like noise and its discharge pressure fluctuates. What is the most likely cause?
- Mechanical seal faces properly lapped
- Impeller rotating in the correct direction
- Cavitation from insufficient NPSH available (Correct answer)
- Discharge valve fully open at design flow
Correct answer: Cavitation from insufficient NPSH available
Cavitation occurs when vapor bubbles form and collapse because NPSH available falls below NPSH required, producing gravel-like noise and unstable pressure.
Question 2: Which action would most directly increase NPSH available to a centrifugal pump?
- Raising the liquid level in the suction tank (Correct answer)
- Increasing the fluid temperature
- Adding more elbows to the suction line
- Throttling the suction valve
Correct answer: Raising the liquid level in the suction tank
Raising the suction liquid level increases static head on the suction side, which increases NPSH available.
Question 3: A new mechanical seal leaks steadily right after installation. Which is the most likely cause?
- The pump was primed before startup
- Seal faces were damaged or contaminated during installation (Correct answer)
- The gland was tightened evenly
- The shaft sleeve was cleaned before assembly
Correct answer: Seal faces were damaged or contaminated during installation
Immediate leakage on a new mechanical seal usually means chipped, scratched, or dirty faces, or a damaged O-ring from installation.
Question 4: Why should packing on a centrifugal pump be allowed to leak slightly during operation?
- To reduce motor amperage to zero load
- To vent air from the volute
- To relieve discharge pressure
- To lubricate and cool the packing and shaft sleeve (Correct answer)
Correct answer: To lubricate and cool the packing and shaft sleeve
A small controlled leakage lubricates and cools the packing, preventing it from burning and scoring the shaft sleeve.
Question 5: A centrifugal pump runs but delivers no flow after a rebuild. Which should be checked first?
- Coupling guard paint condition
- Bearing housing oil color
- Whether the pump is primed and rotating in the correct direction (Correct answer)
- Baseplate anchor bolt torque
Correct answer: Whether the pump is primed and rotating in the correct direction
Loss of prime and reversed rotation are the most common reasons a rebuilt centrifugal pump produces no flow.
Question 6: Operating a centrifugal pump for extended periods against a closed discharge valve will most likely cause what?
- Increased flow to the system
- Lower bearing loads
- Reduced motor heating
- Overheating of the liquid in the casing and possible seal damage (Correct answer)
Correct answer: Overheating of the liquid in the casing and possible seal damage
At shutoff, nearly all input energy becomes heat in the trapped liquid, which can flash, damage seals, and overload bearings radially.
Question 7: On a positive displacement pump, what is the primary purpose of a relief valve on the discharge side?
- To protect the pump and piping from overpressure if discharge is blocked (Correct answer)
- To prime the pump automatically
- To regulate suction temperature
- To increase NPSH
Correct answer: To protect the pump and piping from overpressure if discharge is blocked
Positive displacement pumps keep delivering flow against a closed outlet, so a relief valve prevents dangerous pressure buildup.
