What Is a Pump?
A pump is a device that moves a fluid (gas or liquid) from one location to another, making them popular for a variety of uses. How a pump moves a fluid determines which category it falls within – displacement, direct lift and gravity pumps.
In this post, we will cover the three popular types of pump: gear pumps, screw pumps and centrifugal pumps and how they work.
How Does a Pump Work?
A pump works on the basis that they require energy input to output mechanical work to move the fluid. The energy input for pumps comes in a great variety such as manual (hand), electrical, wind power, engines. Pumps are widely used across many industries and applications such that there is a pump, big and small, for the majority of required applications.
The primary working principle of pumps is to create a pressure difference which ‘sucks’ in the fluid to the pump and allows it to be expelled in the desired location.
As a pump begins operating, the moving components of the pump push out any air in the pump and create a vacuum (low pressure). The resultant pressure difference between the pump and the fluid means the fluid is attracted to and forced into the pump.
In the presence of a pressure differential, a fluid always wants to travel from a high-pressure environment to a low-pressure environment. A great example of this is when you blow up a balloon. If you fill a balloon with air and release it, all the air is expelled because the fluid (air in this case) wants to move from the higher pressure environment (balloon) to the lower pressure environment (atmosphere).
Generally, a pump has two connections an input (known as suction) and an output (known as discharge).
Types of Pumps
As mentioned above, pumps are categorised depending on the method of transporting the fluid. The categories mentioned above are split into more detail: positive-displacement pumps, impulse pumps, velocity pumps, gravity pumps, steam pumps and valveless pumps.
Positive Displacement Pumps
The positive displacement pump group gets its name from trapping a fixed amount of fluid and displacing it by force into the discharge pipe.
Positive displacement pumps are further broken down into subcategories – rotary, reciprocating or linear.
Gear Pump
A gear pump is the most popular and well-known type of rotary positive displacement pump. A gear pump uses the rotation and meshing of gears to move the fluid. The direction of meshing for the gears is crucial as this generates the suction. As the gears rotate, they open up on the inlet side, creating a void that sucks in the fluid.
As the fluid is drawn in, the rotating gears transport the fluid around the casing and then produce a final displacement of the fluid during meshing to force the fluid into the discharge pipe.
The tight clearances within the gears and the speed of rotation help prevent any backflow in the pump.
Gear pumps under high-pressure environments are extremely efficient. Factors that affect their efficiency are gear backlash and clearances. A high gear backlash increases the probability of fluid backflow and leakage. However, on the other hand, gear backlash improves efficiency by reducing wasted energy from pressure trapping.
Screw Pump
A screw pump is another type of positive displacement rotary pump; however, it is more complicated than the gear pump. A screw pump generally consists of two screws (sometimes three) that rotate about their axis and have opposing threads resulting in them rotating in alternative directions (towards each other). For example, one screw will rotate clockwise, and the other will rotate anti-clockwise.
The screws are mounted on parallel shafts, with the ends in gear mesh meaning everything stays in place during rotation. As the screws rotate towards each other, the fluid is compressed and displaced up the screw axis towards the discharge pipe.
Like a gear pump, the clearances between the screws and the casing are marginal to help control the direction of the fluid flow. Additionally, on start-up, the air is displaced out of the pump creating a vacuum resulting in the suction of the fluid into the pump – like a gear pump.
One of the main advantages of a screw compressor is that it can be scaled straightforwardly to meet the customers’ requirements – either up or downscaling. Additionally, they are very robust, and due to the internal compression, they tend to be very efficient.
The traditional application of screw pumps is in irrigation systems and agricultural systems.
Radial Flow Pump
Centrifugal Pump
A centrifugal pump utilises rotational energy to move a fluid from one point to another. In centrifugal pumps, an impeller or impellers are rotated, which result in the fluid being sucked into the pump. The fluid hits the rotating impeller as it enters the pump – the fluid enters the pump by hitting the top of the impeller in the centre (known as the eye). As the impeller continues to rotate, the centrifugal forces result in the fluid forced to the end of the vanes – an impeller is made up of many curved vanes.
Once the fluid reaches the edge of the vanes it leaves the pump through the discharge pipe. Like most pumps, there are small clearances between the pump’s impeller and casing. A centrifugal pump can reach a rotational speed of up to 5000 rpm making them very efficient for use on low viscosity fluids.
There are two types of centrifugal pump casings, volute and diffuser – both designed to ensure the fluid leaves through the discharge pipe.
A volute casing is designed in a manner such that the impeller is offset. A curved funnel with an increasing cross-sectional area is created around the impeller towards the pump’s discharge.
The diffuser casing design works off the same principle. A diffuser design implements static vanes surrounding the impeller. As the fluid leaves the curved vanes of the impeller, the pressure of the fluid increases as it travels through the stationary vanes.
Diffuser designs can be tailored to certain applications making them more efficient.
Summary
We have covered three popular types of pumps and how they work. A pump works based on creating pressure differentials to manipulate the direction of fluid flow.
A pump has many applications and is used in a variety of different sectors across the world. Pumps are categorised into one of three categories: displacement, direct lift and gravity pumps. Displacement pumps are the type of pump most used and work in a range of environments.
Pumps can be a daunting prospect to try and understand but at their core, they work based on simple principles.
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