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Centrifugal Pumps VS Positive Displacement Pumps

Views: 29     Author: Site Editor     Publish Time: 2021-11-12      Origin: Site

Nearly all pumps fall within these two categories, with centrifugal being the most common. Both of which have their uses and best areas of application. It is important however to be able to identify when each pump type should be selected, which ultimately comes down to their working principle and the application at hand.


What is a centrifugal pump?


Centrifugal pumps are used to transport fluids by converting rotational energy driven by some sort of motor or engine into energy moving the fluid. The fluid enters the impeller of the pump by the rotating axis, where it is accelerated by the impellor towards a diffuser or scroll. The fluids gain both pressure and velocity while being pushed passed the impeller.


The most common type among centrifugal pump is the radial flow pump. These centrifugal pumps use a rotating impeller to create a vacuum in order to move fluid. The pump’s impeller rotates within the housing and reduces pressure at the inlet. This motion then drives fluid to the outside of the pump’s housing, which increases the pressure enough to send it out the discharge. There is also the axial flow centrifugal pump which uses a curved propeller-shaped impeller, whereas the impeller on a radial flow centrifugal pump looks more like a fan. Axial flow pumps move fluid by drawing fluid into their axis and using the impeller to send fluid out on the other side of the pump.


What is a positive displacement pump?


A positive displacement pump transports fluids by trapping a fixed amount of the liquid and forcing it into the discharge pipe. The movement is started by two or three spindles that move in opposite directions of each other; pumping and thus displacing the liquid.


Positive displacement pumps draw fluid into a compartment at the inlet and move it to an outlet for discharge, most typically using a rotary, reciprocating, or diaphragm method to move fluid. The main difference between these types of pumps and centrifugal is that positive displacement pumps will move fluid at the same speed regardless of the pressure on the inlet end and centrifugal pumps will not.


What are the differences?


1. Mechanics.              

The obvious difference between the two is in the way they operate. As displayed above, centrifugal pumps impart velocity to the liquid, resulting in pressure at the outlet. Positive displacement pumps capture confined amounts of liquid and transfers it from the suction to the discharge port. In short, with centrifugal pumps pressure is created and flow results, whereas with positive displacement pumps flow is created and pressure results.


2. Performance.

Due to the fact that the flow is the result of pressure, with centrifugal pumps the flow varies with changing pressure. Since positive displacement pumps work the other way round, their flow is consistent with changing pressure.


3. Viscosity.

For centrifugal pumps the efficiency decreases when viscosity increases. This is due to frictional losses inside the pump. In contrast, the efficiency of positive displacement pumps increases with increasing viscosity.


4. Efficiency.

Centrifugal pumps peak at BEP (best efficiency point). At higher or lower pressure levels, the efficiency decreases. Within a window of 80-110% of its BEP, this pump is adequate. For the positive displacement pumps, it applies that efficiency increases with increasing pressure.


5. Inlet Conditions.

With centrifugal pumps it is indispensable for liquids to be in the pump to create a pressure differential. With positive displacement pumps, a negative pressure is created at the inlet port (suction).


What applications are centrifugal pumps used for?


Centrifugal pumps are the most common pump type for the transfer of low viscosity fluids in high flow rate, low pressure installations, which makes them ideal for applications that require the pump to deal with large volumes. The centrifugal pump design is often associated with the transfer of water, but is also a popular solution for handling thin fuels and chemicals:


General water supply

Sea water transfer

Water circulation

Air conditioning

Boiler feed

Light fuel transfer

Petrochemical

Washing and firefighting

Irrigation


When to use a positive displacement pump?


There are two classifications of positive displacement pump; rotary and reciprocating. Whilst governed by the same working principle, all pumps types that fall within these classifications have their own design characteristics and benefits.


Rotary positive displacement pumps – operate via rotation of the pumping element


Progressing cavity pumps

Vane pumps

Screw pumps

Peristaltic pumps

Gear pumps

Lobe pumps


Reciprocating positive displacement pumps – operate via a constant back and forth motion.


Diaphragm pumps

Piston pumps


Positive displacement pumps are able to handle variations in pressure, flow and viscosity and remain efficient, unlike centrifugal pumps which do not operate well off the center of their curve. As their flow rate remains constant (proportional to the speed of operation), smooth and low pulsating despite changes in the pressure, positive displacement pumps such as peristaltic, piston and diaphragm pumps are ideal solutions for dosing applications as it allows accurate metering to be carried out.


So, which pump to use?


This question is not as simple to answer as it seems. Positive displacement and centrifugal pumps are essential tools for every fluid system.


AWS HYDRO focus on axial piston high pressure pumps, which belong to the category of reciprocating pumps in positive displacement pumps. It is a fixed displacement pump whose flow is proportional to the speed of the input shaft and the displacement of the pump. Axial piston high pressure pump is a new type of axial piston pump manufactured based on the principle of axial piston pump combined with new materials, which realizes the use of flowing liquid as the lubricating medium and eliminates the lubricating oil tank of the traditional plunger pump. It avoids the leakage and pollution of the overflow liquid, and is suitable for conveying low-viscosity liquids.



Contact us for expert assistance finding the right pumps for your facility, or request a quote to place your order today.