Positive Displacement Pump Explained

A positive displacement pump is also known as PD pump in industry terminology. Thermodynamically, positive displacement means the movement or displacement of something in a single direction (i.e., forward) only. This means that there is no backward flow or backward movement. Similarly, the working fluid in the PD pump moves in the forward direction only without any backflow during the compression stage under ideal conditions.

Working Principle of Positive Displacement Pump

Any positive displacement pump works on the fundamental principle “A positive displacement pump operates by capturing a consistent volume of liquid in its chamber and then pushing it out to the discharge side.” One of the common examples is a reciprocating piston pump.

How does a Piston Pump work?

A piston pump is a type of positive displacement pump that operates using a piston, which moves back and forth within a cylinder to draw in and expel fluids. The operation of a piston pump can be explained in four main stages:

Intake Stage:

• The piston starts at the far end of the cylinder, creating a large volume in the cylinder.
• The intake valve (also known as the suction valve) opens, allowing fluid to enter the cylinder.
• As the piston moves away from the intake valve, it creates a vacuum, drawing the fluid into the cylinder.

Compression Stage:

• The piston changes direction and moves towards the intake valve, reducing the volume of the cylinder.
• The intake valve closes to prevent backflow of fluid.
• The reduction in volume pressurizes the fluid within the cylinder.
• This compression forces the fluid to move towards the outlet valve (discharge valve).

Discharge Stage:

• The outlet valve opens as the piston continues to move towards it.
• The pressurized fluid is pushed out through the outlet valve and into the discharge pipe.
• This stage provides the pumping action, as the piston’s movement forces the fluid out of the cylinder.

Exhaust Stage:

• The piston reaches the end of its stroke in the direction of the outlet valve, creating a larger volume in the cylinder again.
• The outlet valve closes to prevent reverse flow.
• The cylinder is prepared to start the cycle again by moving the piston towards the intake valve.

The piston pump then repeats these stages in a continuous cycle to provide a consistent flow of fluid. The piston’s back-and-forth movement in the cylinder is what creates the necessary pressure and flow of the fluid.

Positive Displacement Pump Characteristics

Theoretically, the backflow of a working fluid in a PD pump/compressor is zero. However, in practice, there may be some leakage from the high-pressure side to low pressure. This is due to slight clearance between moving and stationary parts (the gap between moving & non-moving parts is mandatory to avoid wear due to metal-to-metal contact) during high-pressure air/gas generation as shown in the following figure.

The above leak is shown in the PD pump’s characteristic curve (Pump’s head vs Flow rate). In practice, the actual flow rate deviates from the ideal flow due to the leakage of working fluid between the piston and casing as shown in the above figure. This phenomenon is called slippage. Slippage causes a reduction in the flow delivered by the pump resulting in the shifting of the curve towards the left in the characteristic curve as shown in the following figure.

Positive Displacement vs. Centrifugal Pump

In the case of non-positive displacement pumps like centrifugal pumps, the working fluid can undergo backflow or recirculation under certain conditions. This occurs when the resistance to flow at the discharge side is high or the discharge valve is closed as shown in Figure 4.

When the discharge valve of a centrifugal pump is closed while the pump is running, the working fluid undergoes recirculation and backflow without raising the casing pressure. This is why these types of pumps are called non-positive displacement pumps.

In contrast, if the discharge valve of PD pump is closed, working fluid pressure will rise significantly since there is no possibility to undergo backflow or recirculation theoretically. Therefore, positive displacement pumps are equipped with pressure relief valves at their discharge side to prevent damage due to excessive pressure.

Due to the absence of backflow or recirculation, a positive displacement pump delivers a constant flow at a fixed speed irrespective of upstream pressure. Hence PD pumps are called constant flow devices. For example, a piston pump traps a fixed amount of fluid during the suction stroke and delivers the same during the discharge stroke since there is no backflow or recirculation.

Types of positive displacement pumps

• Progressive cavity pump (or) single screw pump
• Piston pump
• Plunger pump
• Rotary vane pump
• Diaphragm pump
• Gear pump
• Lobe pump
• Liquid Ring Vacuum Pump
• Peristaltic pump
• Reciprocating pump
• Reciprocating compressor
• Screw compressor

Applications of Positive Displacement Pumps

PD pumps are preferred in the following conditions.

• Precise control of the flow of working fluid is required in the pharma industry, process plants and diesel pumps in automobiles for injecting a metered quantity of fuel into the combustion chamber..etc. The flow is controlled either by controlling the speed in rotary pumps or stroke length and speed in reciprocating pumps.
• For transferring highly viscous fluids like oils, paints and resins which cannot be transferred using centrifugal pumps.
• For transferring shear-sensitive fluids which undergoes shear thinning or thickening in centrifugal pumps due to churning action.
• PD pumps are preferred where the discharge flow is constant irrespective of resistance in the upstream side.
• For generating high discharge pressures and low flow rate combination.

FAQ

Why Positive Displacement Pumps are called Self Priming Pumps?

The impeller in the centrifugal pump cannot pump out the air from the casing because the impeller cannot impart energy to the air efficiently. Centrifugal pumps require water to displace the air in the casing. But in the case of PD pump, the piston, vane, lobe or screw displaces the air positively out of the casing. Hence positive displacement pumps are called self-priming pumps.