Pumps and Compressors Flashcards

What is the difference between a centrifugal pump and a positive displacement pump?

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.

What is NPSH and why is it critical for centrifugal pump operation?

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.

What are the common causes of excessive vibration in a centrifugal pump?

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.

What is the function of wear rings in a centrifugal pump?

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.

What is the purpose of a mechanical seal in a centrifugal pump?

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.

What is a pump curve and what information does it provide?

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.