---
title: "Working principle of centrifugal and reciprocating pumps"
subject: "Hydro Power"
chapter: 7
key: "semester_6/hydro_power:centrifugal-reciprocating-working"
source: https://www.bctengineer.com/q/semester_6/hydro_power/centrifugal-reciprocating-working
site: BCT Engineering
---

# Working principle of centrifugal and reciprocating pumps

Hydro Power · Chapter 7: Hydro-Electric Machines

- Asked in: 2077 Chaitra, 5 marks
- Video: [Centrifugal Pump Basics - How centrifugal pumps work working principle hvacr](https://www.youtube.com/watch?v=XpcCUtYzwy0) — The Engineering Mindset
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/centrifugal-reciprocating-working
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7

## Question

Discuss working principle of centrifugal and reciprocating pumps.

## Short answer

A centrifugal pump uses an impeller's centrifugal force to draw liquid in through the eye, converting kinetic energy to pressure in the volute casing; priming is essential. A reciprocating pump displaces liquid by a piston's reciprocating motion, with suction and delivery valves directing pulsating flow; it is self-priming.

## Answer

Pumps convert mechanical energy into pressure energy of a liquid. A **centrifugal pump** does this by a rotating impeller (rotodynamic action); a **reciprocating pump** does it by a piston moving to and fro in a cylinder (positive displacement).

#### Centrifugal pump

```text
        delivery pipe
             ^
             |
     +-------+-------+
    /   volute casing \
   |    +---------+    |
   |    | impeller|<---- shaft (motor)
   |    +----^----+    |
    \        |        /
     +-------+-------+
             |  eye
        suction pipe
             |
        foot valve + strainer
```

1. **Priming:** the casing and suction pipe are first filled with liquid so that no air remains (air cannot create enough suction).
2. The motor rotates the impeller. The liquid in the impeller is forced outward by **centrifugal force**, which creates a low pressure at the eye (centre).
3. Atmospheric pressure on the sump surface pushes liquid up the suction pipe into the eye.
4. The vanes give the liquid high velocity and pressure. In the **volute casing** the area increases gradually, so kinetic energy is changed into pressure energy.
5. The liquid leaves through the delivery pipe at high pressure. Flow is continuous and smooth.

The head developed is the forced-vortex rise $\frac{\omega^2 r_2^2 - \omega^2 r_1^2}{2g}$, so a centrifugal pump needs a certain minimum speed to start delivering (minimum starting speed).

#### Reciprocating pump

```text
   delivery valve      suction valve
        |                  |
   +----+------------------+----+
   |  cylinder  <==piston==>    |---- crank & connecting rod
   +----------------------------+
```

1. A crank driven by a motor moves the piston to and fro through a connecting rod.
2. **Suction stroke:** the piston moves outward, the cylinder volume rises and pressure falls. The suction valve opens and liquid enters; the delivery valve stays closed.
3. **Delivery stroke:** the piston moves inward and pushes the liquid out. Pressure rises, the suction valve closes and the delivery valve opens.
4. Discharge per revolution (single acting) is $Q = \frac{ALN}{60}$, where $A$ is piston area, $L$ stroke length and $N$ rpm. Flow is pulsating; air vessels are fitted to smooth it.

**Use:** centrifugal pumps suit large discharge at low to medium head (water supply, irrigation, dewatering of powerhouses); reciprocating pumps suit small discharge at high head (boiler feed, hydraulic testing).

## Check yourself

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

1. How does a centrifugal pump raise the liquid pressure?
   A. A piston pushes a fixed volume of liquid out each stroke
   B. Air is compressed in the casing to push the liquid up
   C. The rotating impeller flings liquid outward by centrifugal force; the volute casing then turns kinetic energy into pressure energy
   D. The foot valve pumps the liquid by opening and closing
   Answer: C. Impeller vanes give velocity and pressure, and the gradually widening volute converts velocity head to pressure. (Key point: Centrifugal action)

2. Why must a centrifugal pump be primed before starting?
   A. To lubricate the piston rings
   B. To raise the delivery valve pressure
   C. To increase the number of impeller vanes
   D. Air in the casing and suction pipe cannot create enough suction, so they must be filled with liquid first
   Answer: D. Without priming the impeller churns air and cannot lower the eye pressure enough to draw water up. (Key point: Priming)

3. What happens in the suction and delivery strokes of a reciprocating pump?
   A. On suction the piston moves out, pressure falls, suction valve opens; on delivery the piston moves in, delivery valve opens and suction valve closes
   B. On suction the delivery valve opens; on delivery both valves stay shut
   C. The suction valve opens during the inward stroke, the delivery valve during the outward stroke
   D. Both valves stay open all the time and the liquid is thrown outward
   Answer: A. Valve action follows the piston: volume rises and pressure falls on suction; liquid is pushed out on delivery. (Key point: Reciprocating strokes)

4. Which statement about flow and use is correct?
   A. Reciprocating flow is smooth and suits large discharge at low head
   B. Reciprocating flow is pulsating (air vessels smooth it) and suits small discharge at high head; centrifugal suits large discharge at low to medium head
   C. Centrifugal pumps suit small discharge at very high head such as boiler feed
   D. Both give pulsating flow and need air vessels
   Answer: B. Positive displacement gives pulsating flow at high head; the continuous centrifugal pump suits big flows at moderate head. (Key point: Flow and applications)

## Related in Hydro Power

- [Centrifugal versus reciprocating pumps](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Acentrifugal-vs-reciprocating) — Hydro Power, chapter 7
- [Working of a centrifugal pump](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Acentrifugal-pump-working) — Hydro Power, chapter 7, asked 3×

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