What is Positive Displacement Pumps | Types and Advantages

Positive Displacement Pumps
Positive displacement pumps, in contrast, have very little slips, are self-priming and pump against very high pressures, but their volumetric capacity is low. Positive displacement pumps have a very close clearance between rotating and stationary parts and hence are selfpriming. Positive displacement pumps eject a fixed amount of fluid into the hydraulic system per revolution of the pump shaft. Such pumps are capable of overcoming the pressure resulting from mechanical loads on the system as well as the resistance of flow due to friction. This equipment must always be protected by relief valves to prevent damage to the pump or system. By far, a majority of fluid power pumps fall in this category, including gear, vane and piston pumps.Performance curves for positive and non-positive displacement pumps are shown in Fig

Performance curve for positive and Non Postitive displacement Pumps
Performance curve for positive and Non Positive displacement Pumps

Also Read : What is Non-Positive Displacement Pumps | Advantages and Disadvantages

Positive displacement pumps are classified based on the following characteristics:

1. Type of motion of pumping element: Based on the type of motion of pumping element, positive displacement pumps are classified as follows:

  • Rotary pumps, for example, gear pumps and vane pumps.
  • Reciprocating pumps, for example, piston pumps.

2. Displacement characteristics: Based on displacement characteristics, positive displacement pumps are classified as follows:

  • Fixed displacement pumps.
  • Variable displacement pumps.

3. Type of pumping element.

Advantages Of Positive Displacement Pumps :

The advantages of positive displacement pumps over non-positive displacement pumps are as follows:
1. They can operate at very high pressures of up to 800 bar (used for lifting oils from very deep oil wells).
2. They can achieve a high volumetric efficiency of up to 98%.
3. They are highly efficient and almost constant throughout the designed pressure range.
4. They are a compact unit, having a high power-to-weight ratio.
5. They can obtain a smooth and precisely controlled motion.
6. By proper application and control, they produce only the amount of flow required to move the load at the desired velocity.
7. They have a great flexibility of performance. They can be made to operate over a wide range of pressures and speeds.

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