Whether it is to provide pressure as driving force for membrane separation, to pump out a flooded cellar or to circulate hot water in a central heating system: pumps are needed in many places. In fact, pumps account for as much as 10% of the world’s electricity consumption. How do pumps function, and why are various types of metal, advanced ceramics, polymers and rubber important for their performance?
A pump is a device that needs electricity, diesel or other forms of energy as input with the aim to make a medium (liquid or gas) flow faster, to boost it to a higher pressure or to lift it to a greater height. Pumps are used to pump sewage water to the wastewater treatment plant if it does not flow by natural downward slope. Pumps are also used to increase the water pressure on higher floors, so that water can be drawn from the tap if the pressure from the municipal water system is insufficient. In practice, the energy supplied is also needed to overcome flow losses in pipes after the pump.
Compare pumping with cycling from the Achterhoek area in the eastern part of the Netherlands to the more centrally located Hoge Veluwe area and back. The way out costs more energy because you have to cycle against the wind, as in the Netherlands it is usually wind from the west. Moreover, you have to cycle uphill, because the Hoge Veluwe is higher than the Achterhoek. The way back is faster, because you cycle with the wind and downhill.
Gases or vapours differ from liquids in that the former can be compressed, and in that they have a lower density so that they are less affected by gravity.
Flow rate and head
The output that a pump delivers is expressed in terms of the flow rate of the medium flowing, or in the pressure that you can achieve with the pump. If you convert this pressure into the height to which water or another liquid can be pumped, you talk about pump head. In a capacity curve of a pump, the head is graphically plotted against the flow rate. Both depend on each other: a high flow rate is associated with a low head, and a low flow rate with a high head. After all, the energy that a pump provides to the medium to flow faster cannot be used to increase the pressure.
If you have two identical pumps, you can double the total flow rate at the same pressure by connecting the pumps in parallel. And in a similar way, by connecting the pumps in series, the total pressure increases at the same flow rate. By analogy with electrical power, which is the product of voltage and current, pump power is flow rate times pressure.
Centrifugal and positive displacement pumps
In practice there are two types of pumps for pumping gases or liquids: centrifugal pumps and positive displacement pumps. In the case of the centrifugal principle, a rotating impeller with blades pushes the medium outwards, with a motor driving this impeller. The medium enters the pump at the centre of the impeller and leaves the pump at the outside, accelerated. The mechanical energy of the impeller is converted into kinetic (speed increase) and potential energy (pressure increase) of the medium. The centrifugal pump is a widely used pump type: for pumping chemicals in the chemical industry, to transport water in water supply and wastewater treatment, as well as for water pressure boosting and central heating circulation in buildings – just to name a few applications.
In the positive displacement principle, a piston or a gear transports the medium, with a motor causing the piston or gear to move. This works just like a piston in a combustion engine of a car. In a first stroke, the piston sucks the medium in through the suction inlet and in a second stroke, the piston forces the medium out through the outlet. Several successive strokes of these periodic, discrete steps produce a fairly continuous flow of liquid or gas. Gears are used in the same way, where the space between the teeth of two interlocking rotating gears determines the stroke volume. Actually, a vacuum pump is a ‘reverse’ pump, where the pressure side is at the inlet and the suction side is at the outlet.
Materials in pumps
A pump is intended to pump a certain gas, vapour or liquid – so that medium determines the choice of pump materials. This, of course, applies to the parts which come into direct contact with the medium. Those materials must be sufficiently wear-resistant to withstand abrasive particles in a liquid, such as when pumping water containing sand or small pebble stones. And when organic solvents or corrosive gases are pumped, the pump must be chemically resistant. If the pumped medium has a high temperature, the pump materials must be able to withstand this as well.
Pump parts that come into direct contact with the medium are the pump housing that contains the piston or impeller, as well as the suction pipe (at the inlet) and the discharge pipe (at the outlet). Typical materials for the pump housing are metals like cast iron or stainless steel, and (fibre-reinforced) plastics like polypropylene. Impellers and pistons are also based on (stainless) steel, sometimes with wear-resistant ceramic coatings.
Mechanical shaft seal
The drive motor and the moving parts in the pump housing are interconnected via a metal or ceramic pump shaft. A mechanical shaft seal is used to ensure that the pumped medium does not leak out via this pump shaft. One rotary seal face – a ring shape on the rotating pump shaft – is pushed by means of a spring against another stationary seal face which is connected to the pump housing, with a thin, lubricating layer of liquid between the two seal faces – which is sometimes the pumped medium itself. Silicon carbide is a commonly used hard and wear-resistant material for both seal faces. Due to its high thermal conductivity, silicon carbide can easily dissipate frictional heat and its low thermal expansion coefficient keeps the material stable during (sudden) temperature changes. As a secondary seal in the mechanical seal, to connect one seal face fluid-tight to the pump shaft and the other seal face fluid-tight to the pump housing, rubber O-rings are used – due to the elasticity of the material.
Mechanical shaft seal of a pump
When using an electric motor to drive the pump, copper wire around an iron cylinder for electromagnets are applicable materials, or ferrite or neodymium-iron-boron for permanent magnets.
The hypocaust from Roman times is a floor heating system avant la lettre. A central furnace in the crawl space heated the air, which was distributed as hot air under the entire floor. Nowadays, centrifugal pumps take care of the circulation of hot water in underfloor or central heating systems.
