Liquid pumps

A wide variety of liquids are now being used in chemical plants, and these usually have to be pumped through pipelines. In choosing the type of pump most suitable for any specific operation, a number of problems have to be taken into account. In the first place the quantity of liquid, and the pressure at which it is required to be pumped, must be considered. Neglecting all other considerations, the reciprocating pump is ideal for pumping small quantities of liquid at high pressure, the amount of fluid delivered depending on the volumetric displacement of the pistons. The delivery however is rather uneven a defect which can only be remedied by compounding a number of cylinders, thus making the machine rather large and expensive.

The viscosity of the liquid is another factor which must be taken into account, in that it largely determines the frictional losses which will occur. Rotary- type pumps are widely used in the handling of highly viscous liquids. They differ from reciprocating pumps in that they deliver an even flow of liquid, but they are unsuitable for pumping liquids of low viscosity, which tend to leak past the tips of the gear teeth. They are mainly used for the pumping of oils and similar liquids of high viscosity, which are less liable to leakage and which moreover provide the necessary lubrication for the moving parts of the pump, thus obviating the need for a separate lubricant. A further consideration involved in the choice of pump is- whether or not the liquid is corrosive or contains solid particles in suspension. In such cases, precautions have to be taken to avoid damage to the mechanism. With regard to suspensions, the clearances in the pump must be large enough to permit the particles to pass, and from this point of view, the rotary pump is not suitable, clearances necessarily being small to reduce leakage. The centrifugal type of pump is more commonly used when suspensions are present, since various types of impeller can be fitted, thus enabling the pump to handle a wide variety of liquids, including those with suspensions.

Механико-технологические специальности

Smart Materials

Smart – or shape memory – materials are an invention that has changed the world of engineering. There are two types: metal alloys and plastic polymers. The metal alloys were made first and they are usually an expensive mixture of titanium and nickel.

Shape memory materials are called ‘smart’ because they react to change in their environment, for example:

• plastics that return to their original shape when the temperature changes. One use is in surgery where plastic threads ‘remember’ the shape of a knot, react to the patient’s body temperature and make themselves into stitches;

• metal alloys that have a ‘memory’ and can return to their original shape. They are used in medical implants that are compressed so they can be put inside the patient’s body through a small cut. The implant then expands back to its shape. More everyday uses are for flexible spectacle frames and teeth braces;

• solids that darken in sunlight, like the lenses in some sunglasses;

• liquid crystals that change shape and colour. These have been used in climbing ropes that change colour if there is too much strain and weight on them.

The future of these materials and their possible uses is limited only by human imagination. One clever idea is that if cars were made of smart metal, a minor accident could be repaired by leaving the car in the sun!