What is Non-Positive Displacement Pumps | Advantages and Disadvantages

Non-Positive Displacement Pumps

Non-positive displacement pumps are primarily velocity-type units that have a great deal of clearance between rotating and stationary parts. Non-displacement pumps are characterized by a high slip that increases as the back pressure increases, so that the outlet may be completely closed without damage to the pump or system. Non-positive pumps do not develop a high pressure but move a large volume of fluid at low pressures. They have essentially no suction lift. Because of large clearance space, these pumps are not self-priming.

In other words, the pumping action has too much clearance space to seal against atmospheric pressure. The displacement between the inlet and the outlet is not positive. Therefore, the volume of fluid delivered by a pump depends on the speed at which the pump is operated and the resistance at the discharge side. As the resistance builds up at the discharge side, the fluid slips back into the clearance spaces, or in other words, follows the path of least resistance. When the resistance gets to a certain value, no fluid gets delivered to the system and the volumetric efficiency of the pump drops to zero for a given speed. These pumps are not used in fluid power industry as they are not capable of withstanding high pressure. Their maximum capacity is limited to 17–20 bar.These types of pumpsare primarily used for transporting fluids such as water, petroleum,etc.,from one location to another considerable apart location. Performance curves for positive and non positive displacement pumps .

The two most common types of hydrodynamic pumps are the centrifugal and the axial flow propeller pumps.

Advantages and disadvantages of non-positive displacement pumps

The advantages are as follows:
1.Non-displacement pumps have fewer moving parts.
2.Initial and maintenance cost is low.
3. They give smooth continuous flow.
4. They are suitable for handling almost all types of fluids including slurries and sledges.
5.Their operation is simple and reliable.

The disadvantages are as follows:
1.Non-displacement pumps are not self-priming and hence they must be positioned below the fluid level.
2. Discharge is a function of output resistance.
3.Low volumetric efficiency.