433A Pumps and Compressors 2 — Questions and Answers
Question 1: What is the purpose of priming a centrifugal pump?
- To lubricate the pump bearings
- To fill the pump casing and suction line with liquid to displace air, since centrifugal pumps cannot pump air (Correct answer)
- To test the pump motor
- To adjust the impeller clearance
Correct answer: To fill the pump casing and suction line with liquid to displace air, since centrifugal pumps cannot pump air
Centrifugal pumps must be primed (filled with liquid) because they cannot create sufficient vacuum to lift liquid when filled with air. The impeller needs liquid present to function — it imparts energy to the liquid, not to air.
Question 2: What are the signs of cavitation in a centrifugal pump?
- Smooth, quiet operation
- Noise like pumping gravel, reduced flow and pressure, and erosion damage to the impeller (Correct answer)
- Increased flow rate above design
- Lower bearing temperatures
Correct answer: Noise like pumping gravel, reduced flow and pressure, and erosion damage to the impeller
Cavitation produces a distinctive rattling or gravel-like noise as vapor bubbles implode. It causes reduced flow and head, pitting and erosion damage to the impeller vanes (especially the low-pressure sides), and increased vibration.
Question 3: What is a gear pump and what type of fluid transfer is it used for?
- A pump driven by a gear motor
- A positive displacement pump using meshing gears to move fluid, used for viscous liquids and hydraulic systems (Correct answer)
- A centrifugal pump with gear-shaped impeller
- A pump used only for gear oil
Correct answer: A positive displacement pump using meshing gears to move fluid, used for viscous liquids and hydraulic systems
A gear pump is a positive displacement pump that uses two meshing gears to trap fluid between the gear teeth and the casing, carrying it from the suction to the discharge side. It is well-suited for pumping viscous fluids, oils, and hydraulic systems.
Question 4: What is the function of a packing gland on a pump?
- To pack the pump for shipping
- To compress packing rings around the shaft to control leakage while allowing slight dripping for cooling and lubrication (Correct answer)
- To increase pump pressure
- To protect the pump from overheating
Correct answer: To compress packing rings around the shaft to control leakage while allowing slight dripping for cooling and lubrication
A packing gland uses the gland follower to compress soft packing rings around the rotating shaft to control leakage. A slight controlled drip is intentional — it cools and lubricates the packing. Over-tightening burns the packing and scores the shaft sleeve.
Question 5: What is a diaphragm pump and what is its main advantage?
- A pump that uses sound waves
- A positive displacement pump using a flexible diaphragm, capable of handling abrasive, viscous, and chemically aggressive fluids without contamination (Correct answer)
- A high-speed centrifugal pump
- A pump used only for water treatment
Correct answer: A positive displacement pump using a flexible diaphragm, capable of handling abrasive, viscous, and chemically aggressive fluids without contamination
A diaphragm pump uses a flexible membrane that reciprocates to displace fluid. Since the pumped fluid contacts only the diaphragm and check valves (no rotating seals or shaft penetrations), it excels at handling corrosive, abrasive, and shear-sensitive fluids with zero leakage.
Question 6: What is the best efficiency point (BEP) of a centrifugal pump?
- The maximum pressure the pump can generate
- The flow rate and head at which the pump operates at its highest efficiency, with minimum vibration and longest life (Correct answer)
- The point at which the pump shuts off
- The minimum speed the pump can operate at
Correct answer: The flow rate and head at which the pump operates at its highest efficiency, with minimum vibration and longest life
The BEP is the operating point where the pump converts the most input energy into useful hydraulic output with the least waste. Operating at or near BEP minimizes vibration, radial bearing loads, cavitation risk, and component wear, maximizing pump life.
433A Pumps and Compressors 2
What is the purpose of priming a centrifugal pump?