A negative displacement pump is a type of pump that uses centrifugal force to move fluids or gases through the system. Unlike positive displacement pumps, which trap a fixed volume of fluid and push it out, negative displacement pumps draw fluid into an intake and seal it before ejecting it. The flow rate of a negative displacement pump depends on the pressure difference between the inlet and the outlet, and it decreases as the pressure increases.
There are three main types of negative displacement pumps: centrifugal pumps, multi-stage pumps, and axial pumps. Each type has its own advantages and disadvantages, depending on the application and the characteristics of the fluid.
Centrifugal Pumps
A centrifugal pump is the most common type of negative displacement pump. It consists of a rotating impeller that creates a vacuum at the center of the pump, which sucks the fluid in. The fluid then follows the curved path of the impeller blades, which impart kinetic energy to the fluid. The fluid exits the pump at the outer edge of the impeller, where it is converted into pressure energy by a diffuser or a volute.
Centrifugal pumps are suitable for low to medium pressure applications, such as water supply, irrigation, drainage, and cooling systems. They can handle a wide range of flow rates and viscosities, as long as the fluid is not too thick or abrasive. Centrifugal pumps are also easy to operate and maintain, as they have few moving parts and no valves.
However, centrifugal pumps have some limitations. They are not very efficient at high pressures, as they require more power to overcome the friction losses. They are also prone to cavitation, which is the formation and collapse of vapor bubbles in the fluid, causing noise, vibration, and damage to the pump. Cavitation can be avoided by maintaining a sufficient net positive suction head (NPSH), which is the difference between the pressure at the pump inlet and the vapor pressure of the fluid.
Multi-Stage Pumps
A multi-stage pump is a type of centrifugal pump that has two or more impellers arranged in series or parallel. By adding more impellers, the pump can increase the pressure and the flow rate of the fluid. A multi-stage pump can have either a single or a double suction inlet, depending on whether the fluid enters the pump from one or both sides of the first impeller.
Multi-stage pumps are used for high pressure applications, such as boiler feed, reverse osmosis, desalination, and oil and gas production. They can also handle high temperatures and corrosive fluids, as they are made of durable materials and have special coatings and seals. Multi-stage pumps are more efficient than single-stage centrifugal pumps, as they reduce the friction losses and the risk of cavitation.
However, multi-stage pumps are more complex and expensive than single-stage centrifugal pumps. They require more space and more maintenance, as they have more components and connections. They are also more sensitive to alignment and balance issues, which can affect the performance and the lifespan of the pump.
Axial Pumps
An axial pump is a type of negative displacement pump that has a propeller-like impeller that rotates along the axis of the pump. The fluid enters the pump parallel to the axis and is accelerated by the impeller blades. The fluid exits the pump in the same direction as the inlet, with a higher velocity and a lower pressure.
Axial pumps are used for low pressure and high flow rate applications, such as flood control, irrigation, and water transport. They can handle large volumes of fluid and solids, as they have a large inlet and outlet and no diffuser or volute. Axial pumps are also simple and compact, as they have only one impeller and no casing.
However, axial pumps have some drawbacks. They are not very efficient at low flow rates, as they produce a lot of turbulence and backflow. They are also susceptible to erosion and wear, as they are exposed to the fluid and the solids. Axial pumps also require a high NPSH to prevent cavitation, which can limit their operating range.
Comparison of Negative Displacement Pump Types
The following table summarizes the main features and differences of the three types of negative displacement pumps: centrifugal, multi-stage, and axial.
| Feature | Centrifugal | Multi-stage | Axial |
|---|---|---|---|
| Pressure | Low to medium | High | Low |
| Flow rate | Wide range | Moderate to high | High |
| Viscosity | Low to medium | Low to medium | Low |
| Temperature | Low to high | Low to high | Low to high |
| Corrosion | Low to moderate | Low to high | Low to moderate |
| Solids | Low to moderate | Low | High |
| Efficiency | Moderate | High | Low |
| Cavitation | High risk | Low risk | High risk |
| Complexity | Low | High | Low |
| Cost | Low | High | Low |
| Maintenance | Low | High | Low |
As you can see, each type of pump has its own pros and cons, and the choice of the best pump depends on the specific application and the fluid characteristics.
