Pumps

Centrifugal - Positive Displacement

Relevant Products

Applications:

Different applications and liquid mediums require specific pumps: their type & physical size, capacity/duty, and material construction play a major role, from simply transferring water through to highly viscous products as well as high dry matter, abrasive, and difficult slurries. Typically manufactured in stainless steel, cast iron, aluminium, bronze alloys as well as plastic and composite materials.

Two Pumping Principles:

There are two different pumping methods, centrifugal and positive displacement (PD), each having different characteristics that lend them to different applications.

PD Pumps work by pulling the liquid medium into a cavity and then mechanically forcing it out. There are many mechanical mechanisms: diaphragm; gear; lobe; piston; peristaltic; vane and rotor stator. Each of these mechanisms is designed for pumping different liquids for their physical characteristics: viscosity; temperature; ignition point; abrasiveness; thickness; solid particle size; delicateness & hygiene, and the desired delivery requirement: accuracy, flow rate, and pressure. This mechanical principle means that very high pressures can be achieved and will pump to the design pressure until something breaks, therefore an overpressure bypass or similar is recommended.  

Centrifugal Pumps are a simple principle that works by having a fast-spinning Impeller within a chamber. The Impeller pulls the liquid medium into the chamber’s inlet port and forces it out through the outlet port at an increased speed/velocity. A combination of RPM speed, impeller diameter, dept & design, as well as the inlet and outlet port sizes, determine the pump’s performance. This mechanical principle generally works well in situations when pumping to a dead-end or there’s a risk of blockages etc. as the pump can run without damage to the pump or pipework. 

Which Pump Type

Deciding on whether to use a centrifugal or positive displacement pump is not always clear, please see the chart below that will hopefully give some clarity:

Duty

Centrifugal

PD

Increase in line pressure/head

Flow decreases

Flow is generally not affected

Increase in medium viscosity

Flow decreases

Flow generally increases, as the liquid medium fills clearances in the pumping mechanism making it more efficient

Inlet lift/suction

Up to 4.5 metres for self-priming pumps                           

9 metres

Maximum pumping pressure

Up to 40 Bar

Up to 340 Bar

Maximum flow rate

High – up to 1,000 m3/hr

Medium – up to 400 m3/hr

Pumping consistency / metering control

Bad – Flow fluctuates with slight changes in liquid medium and head

Good – Flow is generally constant with any changes in the liquid medium or head, and can even be regulated with speed control

Seal life

Average – High RPM speeds

Good – Low RPM speeds

Pump wear

Average to good – Generally internal pumping components are well spaced that is good for wear, but high RPMs can negate this

Low to average – Internal pumping components are close fittings, causing wear but slow RPMs can negate this

Cost

Tend to be lower priced as generally a simpler design

Tend to be higher priced as generally a more complex design

Generally, a PD pump will satisfy almost all pumping requirements except for very high flow rates, but if a medium to high flow rate with low viscosity, abrasive liquid, and if a variable flow rate is not an issue, a centrifugal pump is a good option.  

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