---
title: "Centrifugal pump: parts and working"
subject: "Fluid Machines"
chapter: 4
key: "semester_5/fluid_machines:centrifugal-pump-working-components"
source: https://www.bctengineer.com/q/semester_5/fluid_machines/centrifugal-pump-working-components
site: BCT Engineering
---

# Centrifugal pump: parts and working

Fluid Machines · Chapter 4: Pump

- Asked in: year not given, 6 marks
- Video: [Centrifugal Pump | Construction | Principle | Working](https://www.youtube.com/watch?v=DmJCDOTIDRY) — ENGINEER'S ACADEMY
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/fluid_machines/centrifugal-pump-working-components
- Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-4

## Question

Explain the construction and working of a centrifugal pump with a neat sketch. Why is priming necessary, and how are the casing types classified?

## Short answer

A centrifugal pump uses a rotating impeller with backward-curved vanes to impart kinetic energy to liquid; the volute casing converts it to pressure. Priming is necessary to remove air, as air binding prevents suction. Casings are classified as volute, vortex, and diffuser types.

## Answer

A **centrifugal pump** raises the pressure of a liquid by the centrifugal action of a rotating impeller: the impeller adds kinetic energy and pressure energy to the liquid, and the casing converts a part of the kinetic energy into pressure.

```text
   Delivery pipe
        ^
        |      Casing (volute)
   +----+-------+
   |   .---.    |
   |  / ( ) \ <-+-- impeller with vanes
   | |  eye  |  |
   +-+---+---+--+
       |   |
 Suction pipe  <- foot valve with strainer
       |
   ~~~~~~~~~~~~~ sump
```

#### Main parts

1. **Impeller:** a wheel with backward-curved vanes (shrouded, semi-open or open) fixed on the shaft; it imparts energy to the liquid.
2. **Casing:** an airtight passage enclosing the impeller. Types: *volute* (spiral, with increasing area), *vortex* (a circular chamber between impeller and volute) and *diffuser* (guide vanes around the impeller, as in turbine pumps).
3. **Suction pipe with foot valve and strainer:** the foot valve is a non-return valve which keeps the pump primed; the strainer prevents debris entry.
4. **Delivery pipe with delivery valve:** the valve controls discharge, and is closed at starting to reduce the starting power.
5. **Shaft, bearings, stuffing box/gland** to seal the shaft.

#### Working

When the impeller rotates in a casing full of liquid, the liquid in the vanes is thrown outward by centrifugal force. A low pressure forms at the eye, so liquid from the sump (acted on by atmospheric pressure) flows continuously into the eye. As the liquid leaves the impeller at high velocity, the volute casing expands the flow area and the kinetic energy turns into pressure energy before the liquid enters the delivery pipe.

#### Priming

If air is inside the casing, the pump produces only a small pressure rise because the density of air is very small (**air binding**), so it cannot lift liquid. **Priming** is filling the suction pipe and casing with the liquid before starting, so the pump can create enough suction. It is done by the foot valve, a filling funnel, a vacuum pump or by an ejector.

The delivery valve is kept closed at starting (for the radial pump the power is minimum at zero discharge) and opened slowly after the pump has reached full speed.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the role of the volute casing?
   A. To add kinetic energy to the liquid
   B. To expand the flow area so that kinetic energy turns into pressure energy
   C. To create the vacuum at the eye
   D. To prevent cavitation by cooling the liquid
   Answer: B. The impeller gives kinetic energy; the volute's increasing area converts part of it to pressure. (Key point: Energy conversion in casing)

2. Why is priming necessary before starting a centrifugal pump?
   A. Air in the casing, of very low density, gives only a small pressure rise so liquid cannot be lifted (air binding)
   B. To reduce the weight on the shaft
   C. To increase the speed of the motor
   D. To cool the stuffing box only
   Answer: A. A pump full of air cannot create enough suction; filling it with liquid avoids air binding. (Key point: Priming)

3. Why is the delivery valve kept closed while starting a centrifugal pump?
   A. To fill the delivery pipe
   B. Because power is minimum at zero discharge for a radial pump
   C. To increase the head
   D. To avoid priming
   Answer: B. Starting at zero discharge needs the least power; the valve is opened slowly after full speed is reached. (Key point: Delivery valve at start)

## Related in Fluid Machines

- [Centrifugal pump head, discharge and power](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Acentrifugal-pump-head-power-numerical) — Fluid Machines, chapter 4
- [Reciprocating pump working, slip and air vessel](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Areciprocating-pump-slip-air-vessel) — Fluid Machines, chapter 4
- [Francis turbine: construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Afrancis-turbine-working-components) — Fluid Machines, chapter 3
- [Kaplan turbine construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Akaplan-turbine-working) — Fluid Machines, chapter 3
- [Pump specific speed and affinity laws](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Apump-specific-speed-affinity-numerical) — Fluid Machines, chapter 4
- [Pump characteristics, losses and selection](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Apump-characteristics-losses-selection) — Fluid Machines, chapter 4

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