Pumps are devices that move fluids by applying mechanical force. They are widely used in various applications such as water supply, irrigation, oil and gas production, chemical processing, and power generation. Pumps can be classified into two main types:ย positive displacement pumpsย andย non positive displacement pumps. In this article, we will explain the basic differences between these two types of pumps and their advantages and disadvantages.
Positive Displacement Pumps
Positive displacement pumps work by trapping a fixed amount of fluid in a closed space and forcing it out through a discharge valve. The fluid is moved by the action of a piston, a diaphragm, a gear, a screw, a lobe, or a vane. The flow rate of a positive displacement pump depends on the speed and size of the pump, and is independent of the pressure. Some examples of positive displacement pumps are:
- Reciprocating pumps: These pumps use a piston or a plunger that moves back and forth in a cylinder to create suction and discharge strokes. They can handle high pressures and viscous fluids, but they have low efficiency and high maintenance costs. They are used for applications such as metering, dosing, and injection.
- Rotary pumps: These pumps use rotating elements such as gears, screws, lobes, or vanes to move the fluid. They can handle moderate pressures and viscous fluids, but they have low efficiency and high wear and tear. They are used for applications such as lubrication, fuel transfer, and food processing.
- Diaphragm pumps: These pumps use a flexible membrane that is moved by a mechanical or pneumatic actuator to create suction and discharge strokes. They can handle low to high pressures and corrosive, abrasive, or sensitive fluids, but they have low efficiency and pulsating flow. They are used for applications such as chemical transfer, wastewater treatment, and paint spraying.
Non Positive Displacement Pumps
Non positive displacement pumps work by imparting kinetic energy to the fluid and converting it into pressure energy. The fluid is moved by the action of an impeller, a propeller, a jet, or a fan. The flow rate of a non positive displacement pump depends on the pressure and the characteristics of the fluid and the system. Some examples of non positive displacement pumps are:
- Centrifugal pumps: These pumps use a rotating impeller that creates a centrifugal force to move the fluid. They can handle low to high pressures and clean, non-viscous fluids, but they have low efficiency and high cavitation risk. They are used for applications such as water supply, cooling, and irrigation.
- Axial flow pumps: These pumps use a propeller that creates a thrust to move the fluid. They can handle low pressures and large volumes of fluid, but they have low efficiency and high axial thrust. They are used for applications such as drainage, flood control, and marine propulsion.
- Jet pumps: These pumps use a high-velocity jet of fluid or gas to entrain and move another fluid. They can handle low to high pressures and mixed or multiphase fluids, but they have low efficiency and high noise. They are used for applications such as well pumping, steam injection, and ejector refrigeration.
Comparison Table
The following table summarizes the main differences between positive displacement pumps and non positive displacement pumps:
| Feature | Positive Displacement Pumps | Non Positive Displacement Pumps |
|---|---|---|
| Flow rate | Constant and independent of pressure | Variable and dependent on pressure |
| Pressure | High | Low to high |
| Viscosity | High | Low |
| Efficiency | Low | High |
| Cavitation | Low | High |
| Pulsation | High | Low |
| Wear and tear | High | Low |
| Maintenance | High | Low |
| Noise | High | Low |
| Applications | Metering, dosing, injection, lubrication, fuel transfer, food processing, chemical transfer, wastewater treatment, paint spraying | Water supply, cooling, irrigation, drainage, flood control, marine propulsion, well pumping, steam injection, ejector refrigeration |
