--- title: "Working of a centrifugal pump" subject: "Hydro Power" chapter: 7 key: "semester_6/hydro_power:centrifugal-pump-working" source: https://www.bctengineer.com/q/semester_6/hydro_power/centrifugal-pump-working site: BCT Engineering --- # Working of a centrifugal pump Hydro Power · Chapter 7: Hydro-Electric Machines - Asked in: 2073 Bhadra, 4 marks; 2071 Bhadra, 3 marks; 2070 Bhadra, 3 marks (asked 3 times) - 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-pump-working - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7 ## Question How does a centrifugal pump work? ## Short answer After priming, a motor spins an impeller, forcing liquid outward by centrifugal action, creating low pressure at the eye. Atmospheric pressure pushes liquid into the suction pipe. The impeller imparts velocity and pressure; the volute casing converts kinetic energy to pressure energy for delivery. ## Answer A **centrifugal pump** is a rotodynamic pump that raises the pressure of a liquid by the **centrifugal action** of a rotating impeller: it converts mechanical energy into pressure energy through forced-vortex motion. #### Main parts Impeller (with backward-curved vanes), volute (spiral) casing, suction pipe with foot valve and strainer, delivery pipe with delivery valve, and the shaft driven by a motor. ```text delivery pipe || _______||______ / volute casing \ | _______ | | / \ | / \ | suction|===|-- (eye) --| | pipe | \ / | \ / | | impeller | \_________________/ | foot valve + strainer (in sump) ``` #### Working 1. **Priming:** the pump casing and suction pipe are first completely filled with liquid (priming) to remove air; an impeller rotating in air cannot create enough suction. 2. **Starting:** the motor rotates the impeller with the delivery valve closed (minimum power at zero flow). 3. **Forced vortex and suction:** as the impeller rotates, the liquid in it rotates as a forced vortex. Centrifugal force throws the liquid radially outward from the eye towards the tip, creating a **low pressure (partial vacuum) at the eye**. Atmospheric pressure on the sump surface pushes liquid up the suction pipe into the eye. 4. **Energy transfer:** the vanes give the liquid high velocity and pressure. The rise in pressure head due to the forced vortex is $\dfrac{u_2^2 - u_1^2}{2g}$, where $u$ is the peripheral speed. 5. **Conversion in casing:** the liquid leaves the impeller with high kinetic energy and enters the **volute casing**, whose area increases gradually; the velocity is reduced and kinetic energy is converted into **pressure energy**. 6. **Delivery:** the delivery valve is opened and the high-pressure liquid flows up the delivery pipe to the required height. 7. **Stopping:** the delivery valve is closed before stopping the motor to avoid backflow and water hammer. A centrifugal pump works only after priming and is used for large discharge at moderate heads (water supply, irrigation, dewatering in powerhouses). Reversed, a pump-turbine works as a Francis turbine in pumped-storage plants. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why must a centrifugal pump be primed before starting? A. To cool the motor B. To fill the casing and suction pipe with liquid and remove air, since an impeller in air cannot create enough suction C. To increase the delivery pressure directly D. To open the foot valve Answer: B. Air has too low density to produce the needed low pressure at the eye, so the pump must be full of liquid. (Key point: Priming) 2. How is liquid raised into the pump eye? A. Centrifugal force throws liquid outward, creating low pressure at the eye, and atmospheric pressure on the sump pushes liquid up B. The delivery pipe pulls liquid through the pump C. The volute casing sucks liquid directly D. Gravity feeds the liquid Answer: A. Outward flow leaves a partial vacuum at the eye and atmospheric pressure on the sump pushes liquid in. (Key point: Suction mechanism) 3. What is the role of the volute casing? A. Convert pressure to velocity B. Store liquid C. Increase area gradually so velocity falls and kinetic energy is converted to pressure energy D. Prevent priming Answer: C. The gradually widening volute slows the flow, converting kinetic energy into pressure energy. (Key point: Role of volute) 4. Why is the pump started with the delivery valve closed? A. Power required is minimum at zero flow B. Priming is not needed then C. The impeller then rotates in the opposite direction D. Maximum discharge is obtained Answer: A. Centrifugal pump power demand is lowest at zero discharge, so starting with the valve closed lowers the starting load. (Key point: Starting procedure) ## Related in Hydro Power - [Working principle of centrifugal and reciprocating pumps](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Acentrifugal-reciprocating-working) — Hydro Power, chapter 7 ## Related in other subjects - [Centrifugal pump: parts and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Acentrifugal-pump-working-components) — Fluid Machines, chapter 4 - [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 - [Shunt motor pump: resistance for 800 rpm](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashunt-motor-centrifugal-constant-torque) — Electrical Machine, chapter 4, asked 2×