---
title: "Pump characteristics, losses and selection"
subject: "Fluid Machines"
chapter: 4
key: "semester_5/fluid_machines:pump-characteristics-losses-selection"
source: https://www.bctengineer.com/q/semester_5/fluid_machines/pump-characteristics-losses-selection
site: BCT Engineering
---

# Pump characteristics, losses and selection

Fluid Machines · Chapter 4: Pump

- Asked in: year not given, 6 marks
- Video: [Pump Chart Basics Explained - Pump curve HVACR](https://www.youtube.com/watch?v=U8iWNaDuUek) — The Engineering Mindset
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/fluid_machines/pump-characteristics-losses-selection
- Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-4

## Question

Write short notes on (a) the efficiencies and energy losses in a centrifugal pump, (b) the main characteristic curves of a centrifugal pump, and (c) the factors considered in the selection of a pump.

## Short answer

Centrifugal pump losses: hydraulic (shock, friction), volumetric (leakage), mechanical (bearing, disc friction). Overall efficiency = manometric × volumetric × mechanical. Characteristic curves: H-Q falls, P-Q rises with flow; efficiency peaks at BEP. Selection considers duty point, liquid type, NPSH, specific speed, and cost.

## Answer

#### (a) Losses and efficiencies

Energy losses in a centrifugal pump:

1. **Hydraulic losses:** shock at impeller entry (off-design flow), friction in impeller passages, casing and volute, and eddy losses due to sudden changes of area.
2. **Leakage (volumetric) losses:** liquid leaking back from the delivery side to the suction side through the clearance between the impeller and the casing.
3. **Mechanical losses:** friction in bearings and stuffing box, disc friction on the outer faces of the impeller.

| Efficiency | Definition |
|---|---|
| Manometric | $\eta_{man}=\dfrac{H_m}{H_e}=\dfrac{gH_m}{V_{w2}u_2}$ |
| Volumetric | $\eta_v=\dfrac{Q}{Q+q}$, where $q$ is the leakage |
| Mechanical | $\eta_m=\dfrac{\text{power given to the impeller}}{\text{shaft power}}$ |
| Overall | $\eta_o=\dfrac{\rho gQH_m}{P_{shaft}}=\eta_{man}\,\eta_v\,\eta_m$ |

#### (b) Characteristic curves

Curves are drawn for constant speed, with discharge on the x-axis.

```text
 H, P,       H-Q (falls with Q)
 eta   \  .-------eta
        \/   ..  
        /\       P rises with Q
       /   \
      0---------------------- Q
 (shut-off head at Q = 0)
```

- **Head-discharge (H-Q):** head is highest at shut-off ($Q=0$) and falls as discharge increases (for backward curved vanes).
- **Power-discharge (P-Q):** power rises almost linearly with discharge; power at zero discharge is small, hence the pump is started with the delivery valve closed.
- **Efficiency-discharge ($\eta$-Q):** zero at $Q=0$, rises to a maximum at the **best efficiency point (BEP)** and falls again.
- **Main (constant-speed) and operating curves** show the performance at different speeds; **constant efficiency (Muschel) curves** show the lines of equal efficiency in the H-Q plane. The system curve cuts the H-Q curve at the operating point.

#### (c) Selection of a pump

- Required discharge and head (duty point), and the nature of the system curve; the duty point should be near the BEP.
- Type of liquid: clean water, sewage, slurry, corrosive or viscous liquid (viscosity reduces head and efficiency and raises the power).
- Suction conditions: NPSH available, to avoid cavitation.
- Specific speed to choose between radial, mixed and axial types; choose multistage if the head is high.
- Efficiency and power cost; the motor and speed; variation in demand (parallel units or variable speed).
- First cost, maintenance, spare parts availability, space, and the reliability required.
- For small discharge at very high head, or for viscous liquids and metering duties, a reciprocating or other positive displacement pump is selected.

## Check yourself

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

1. How is the overall efficiency of a centrifugal pump related to the other efficiencies?
   A. Sum of manometric, volumetric and mechanical
   B. Product of manometric, volumetric and mechanical efficiencies
   C. Manometric divided by volumetric
   D. Equal to the mechanical efficiency
   Answer: B. eta_o = eta_man x eta_v x eta_m. (Key point: Overall efficiency)

2. Which loss is the leakage (volumetric) loss?
   A. Friction in bearings
   B. Disc friction on the impeller
   C. Liquid leaking from delivery side back to suction through the clearance
   D. Shock at impeller entry
   Answer: C. Volumetric efficiency is Q/(Q+q), where q is the leakage through the impeller-casing clearance. (Key point: Leakage loss)

3. Which describes the standard constant-speed curves of a centrifugal pump with backward vanes?
   A. Head is zero at shut-off
   B. Head is highest at shut-off and falls with Q; efficiency peaks at the BEP
   C. Efficiency is highest at Q = 0
   D. Power falls linearly with Q
   Answer: B. H-Q falls from shut-off head, power rises with Q, and efficiency is zero at Q = 0, peaks at BEP and then falls. (Key point: Characteristic curves)

## Related in Fluid Machines

- [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
- [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

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