---
title: "Turbomachines versus positive displacement machines"
subject: "Fluid Machines"
chapter: 1
key: "semester_5/fluid_machines:turbomachine-vs-positive-displacement"
source: https://www.bctengineer.com/q/semester_5/fluid_machines/turbomachine-vs-positive-displacement
site: BCT Engineering
---

# Turbomachines versus positive displacement machines

Fluid Machines · Chapter 1: Introduction of Turbomachime and Dynamic Action of Fluid

- Asked in: year not given, 4 marks
- Video: [Comparison of Turbo machine with Positive Displacement Machines](https://www.youtube.com/watch?v=H1e145HyAAM) — Dr Vishal Gupta Sir
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/fluid_machines/turbomachine-vs-positive-displacement
- Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-1

## Question

Differentiate between turbomachines and positive displacement machines.

## Short answer

Turbomachines use dynamic blade action for continuous, steady flow; they are compact, high-speed, and need no valves. Positive displacement machines trap a fixed volume, causing intermittent flow; they are bulky, low-speed, require valves, and can generate very high pressure but need relief valves.

## Answer

Both transfer energy between a machine and a fluid, but the mechanism is different. In a turbomachine the energy exchange is due to the dynamic action of the fluid on rotating blades, whereas in a positive displacement machine a fixed volume of fluid is trapped, moved and pushed out by a moving boundary.

| Point | Turbomachine | Positive displacement machine |
|---|---|---|
| Principle | Change of angular momentum (Euler's equation) | Change of volume of a closed space |
| Flow | Continuous, steady | Intermittent, pulsating |
| Moving part | Rotor with blades, pure rotation | Piston, gear, vane, screw (often reciprocating) |
| Capacity | Large discharge, moderate head | Small discharge, very high head possible |
| Speed | High speed, direct drive from motor | Low speed, often gear or crank drive |
| Size and weight | Compact, less weight per unit power | Bulky, heavy |
| Balancing, vibration | Easily balanced, smooth | Unbalanced forces, more vibration |
| Valves | Not needed | Suction and delivery valves needed (reciprocating type) |
| Maintenance | Low | High (wear of valves, seals) |
| Examples | Centrifugal pump, turbines | Reciprocating pump, gear pump, piston engine |

In a turbomachine the delivered head depends on speed and discharge, so closing the delivery valve gives a limited head. A positive displacement pump will build up pressure until it fails, so it needs a relief valve.

## Check yourself

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

1. The basic working principle of a turbomachine differs from a positive displacement machine in that it uses:
   A. Reciprocating pistons
   B. Trapping a fixed volume of fluid
   C. Change of angular momentum (Euler's equation)
   D. Change of volume of a closed space
   Answer: C. A turbomachine works by dynamic action and angular momentum change; positive displacement machines change the volume of a closed space. (Key point: Principle)

2. Which pairing of capacity is correct?
   A. Both give small discharge and low head
   B. Turbomachine: large discharge, moderate head; PD: small discharge, very high head possible
   C. Both give large discharge and very high head
   D. Turbomachine: small discharge, very high head; PD: large discharge
   Answer: B. Turbomachines handle large discharge at moderate head, while PD machines give small discharge with very high head. (Key point: Capacity)

3. The delivery flow of a reciprocating (positive displacement) pump is:
   A. Intermittent and pulsating
   B. Continuous and steady
   C. Independent of speed
   D. Always zero at no head
   Answer: A. PD machines deliver intermittent, pulsating flow; turbomachines give continuous steady flow. (Key point: Flow nature)

4. What happens if the delivery valve is closed against a positive displacement pump?
   A. Flow reverses
   B. Nothing; it simply stops
   C. Pressure builds until failure, so a relief valve is needed
   D. Head stays limited, as in a centrifugal pump
   Answer: C. A PD pump keeps building pressure and can fail; a turbomachine develops only a limited head. (Key point: Delivery valve closed)

## Related in Fluid Machines

- [Turbomachines: definition and classification](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aturbomachine-definition-classification) — Fluid Machines, chapter 1

## Related in other subjects

- [Turbomachines versus positive displacement machines](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aturbomachine-vs-positive-displacement) — Turbo Machines, chapter 1