433A Pumps and Compressors 1 — Questions and Answers
Question 1: What is the difference between a centrifugal pump and a positive displacement pump?
- Centrifugal pumps are always larger
- A centrifugal pump uses an impeller to impart velocity energy that converts to pressure, while a positive displacement pump traps and moves a fixed volume of fluid per cycle (Correct answer)
- Positive displacement pumps cannot handle high pressures
- Centrifugal pumps have no rotating parts
Correct answer: A centrifugal pump uses an impeller to impart velocity energy that converts to pressure, while a positive displacement pump traps and moves a fixed volume of fluid per cycle
Centrifugal pumps use a rotating impeller to add kinetic energy to the fluid, which is then converted to pressure energy in the volute casing. Positive displacement pumps trap a fixed volume of fluid and mechanically push it through the discharge.
Question 2: What is NPSH and why is it critical for centrifugal pump operation?
- Nominal Pump Shaft Horsepower — it determines motor size
- Net Positive Suction Head — the absolute pressure available at the pump suction above the fluid's vapor pressure, required to prevent cavitation (Correct answer)
- Normal Pump Speed and Head — rated operating conditions
- Net Pressure System Height — the total system elevation
Correct answer: Net Positive Suction Head — the absolute pressure available at the pump suction above the fluid's vapor pressure, required to prevent cavitation
NPSH is the absolute pressure at the pump suction inlet minus the vapor pressure of the liquid. Available NPSH must exceed the pump's required NPSH to prevent the fluid from vaporizing (cavitating) as it enters the impeller eye.
Question 3: What are the common causes of excessive vibration in a centrifugal pump?
- Operating at the best efficiency point
- Misalignment, imbalance, cavitation, bent shaft, or worn bearings (Correct answer)
- Using the correct impeller size
- Proper foundation mounting
Correct answer: Misalignment, imbalance, cavitation, bent shaft, or worn bearings
Common causes of pump vibration include shaft misalignment with the driver, impeller or rotor imbalance, cavitation, bent shaft, worn or damaged bearings, loose foundation bolts, and operating far from the best efficiency point.
Question 4: What is the function of wear rings in a centrifugal pump?
- To support the shaft
- To provide a renewable close-clearance seal between the impeller and casing, minimizing internal recirculation (Correct answer)
- To prevent cavitation
- To direct flow into the impeller eye
Correct answer: To provide a renewable close-clearance seal between the impeller and casing, minimizing internal recirculation
Wear rings are sacrificial close-clearance components between the impeller and pump casing that minimize internal leakage (recirculation) from the high-pressure discharge side back to the low-pressure suction side. They are designed to be replaceable when worn.
Question 5: What is the purpose of a mechanical seal in a centrifugal pump?
- To connect the pump to the motor
- To prevent fluid leakage along the shaft where it exits the pump casing (Correct answer)
- To increase the pump's pressure rating
- To cool the pump bearings
Correct answer: To prevent fluid leakage along the shaft where it exits the pump casing
A mechanical seal prevents pumped fluid from leaking along the rotating shaft at the point where it passes through the pump casing. It uses precision-lapped rotating and stationary faces in contact to create a near-zero leakage seal.
Question 6: What is a pump curve and what information does it provide?
- The physical shape of the pump casing
- A graph showing the relationship between flow rate, head (pressure), efficiency, power, and NPSH required at various operating points (Correct answer)
- The direction of pump rotation
- The maintenance schedule for the pump
Correct answer: A graph showing the relationship between flow rate, head (pressure), efficiency, power, and NPSH required at various operating points
A pump performance curve plots head (pressure), efficiency, brake horsepower, and NPSH required against flow rate. It enables the millwright and engineer to select the right pump and predict its performance at any operating point.
433A Pumps and Compressors 1
What is the difference between a centrifugal pump and a positive displacement pump?