What is the difference between positive displacement and dynamic pumps?

Short Answer:

The main difference between positive displacement and dynamic pumps is how they move the liquid. A positive displacement pump moves a fixed amount of fluid by trapping it in a chamber and then forcing it out. In contrast, a dynamic pump increases the fluid’s velocity and converts that speed into pressure to move the fluid.

Positive displacement pumps are used for high-pressure and low-flow applications, while dynamic pumps are better for low-pressure and high-flow needs. The selection depends on the type of fluid, required pressure, and flow rate in civil engineering projects.

Detailed Explanation:

Difference between positive displacement and dynamic pumps

In civil engineering and fluid handling systems, pumps are chosen based on how they deliver energy to the fluid. The two main categories are positive displacement pumps and dynamic pumps, and they work in very different ways.

Positive Displacement Pumps

These pumps work by physically trapping a specific volume of fluid in a cavity and then pushing it out through the pump outlet. This creates a consistent, pulse-like flow, even at high pressure. The flow rate remains nearly constant regardless of pressure, making these pumps ideal for applications that require precision and steady delivery.

Types of positive displacement pumps include:

  • Reciprocating pumps (like piston or plunger pumps)
  • Rotary pumps (like gear, screw, or vane pumps)

Features of positive displacement pumps:

  • Delivers a fixed amount of fluid per cycle
  • Works well for high-pressure applications
  • Suitable for viscous (thick) fluids like oil or sludge
  • Self-priming (can start pumping without needing to be filled)
  • High efficiency at low flow rates

Dynamic Pumps

Dynamic pumps use a rotating impeller or similar mechanism to add velocity to the fluid. This velocity is then converted into pressure energy by the pump casing. These pumps are most effective when high flow rates are required at relatively lower pressures.

The most common example is the centrifugal pump, which is widely used in civil projects for water supply, drainage, and irrigation.

Types of dynamic pumps include:

  • Centrifugal pumps
  • Axial flow pumps
  • Mixed flow pumps

Features of dynamic pumps:

  • Fluid velocity is increased to create pressure
  • Suitable for high-flow, low-pressure applications
  • Works well with clean and low-viscosity fluids like water
  • Not self-priming, usually needs priming before use
  • Flow rate depends on system pressure

Key Differences:

  • Working principle: Positive displacement traps and pushes fluid; dynamic adds velocity and converts it to pressure.
  • Flow rate: Constant in positive displacement, variable in dynamic.
  • Pressure handling: Better in positive displacement for high-pressure use.
  • Fluid types: Positive displacement is good for thick or dirty fluids; dynamic is better for clean, thin fluids.
  • Self-priming: Most positive displacement pumps are self-priming; dynamic pumps are not.

Choosing between the two depends on what the civil engineering task requires. If a consistent flow is needed under high pressure, positive displacement is preferred. For high volumes of clean water at moderate pressures, dynamic pumps are more practical.

Conclusion:

The difference between positive displacement and dynamic pumps lies in how they move fluid. Positive displacement pumps trap and push a fixed volume, making them ideal for high-pressure, low-flow, and viscous fluids. Dynamic pumps increase fluid speed to build pressure, suitable for large volumes at lower pressures. Understanding these differences helps in selecting the right pump for various civil engineering applications.