About pipes.

Most people live in houses in which pipes are used for the plumbing, the central heating, the sewage system and for the gas supply. These domestic pipe systems are typical of the pipe systems used in general engineering and they can usefully be examined by anyone not familiar with pipe work. The pipes are, almost without exception, circular in cross-section and they are made of metal, (usually copper, steel or less frequently now, lead,) of plastic or of earthenware. The sewage pipes operate intermittently and, when they do, they function like a channel in that the waste runs along the bottom of the pipe. Normally the plumbing pipes and central heating pipes are full of water and the gas pipes are full of gas. This chapter is about pipes that, in operation, are full of fluid. Pipes that do not run full will be dealt with in the chapter on flow in channels. When studying the pipe systems in dwellings it should be realised that they will have been installed by plumbers and not by engineers. The sizes of the pipes will have been chosen from a limited range of available sizes by arbitrary rules that generally mean that the pipes are larger than is strictly necessary. The routing of the pipes may be dictated by the force of circumstance or be chosen either for the minimum cost or to be unobtrusive.

 

It will also be evident that the pipe work is made up of straight lengths of pipe that are joined together with manufactured fittings. It is the cost of these fittings that leads us to use a limited range of pipe sizes. The cost of making such fittings and holding them in stock ready for use, would be quite prohibitive if there was no standardisation of pipe sizes and if the commonly used pipes were not limited to a relatively few standard sizes. Typically, in Great Britain, these diameters are, for domestic copper pipes, 6, 10, 15, 22, 28, and 35 mm. These have bores of approximately 1/4², 3/8², 1/2², 3/4², 1², and 1 25² inch and were adopted at the time of metrication. Pipe sizes go on up to very large diameters but normally to about 6 feet.

 

Of course, for general engineering, a wide range of pipes is available, but as in domestic plumbing, fittings are held for only a relatively few preferred sizes. It should be remembered that, in equipment like heat exchangers and boilers, pipes are not joined by fittings and the size of pipe and the thickness of the wall may be important to the successful operation of the equipment. Then it may be convenient to use non-preferred sizes. This all means that, where the diameter of a pipe has to be chosen for some purpose in engineering, the preferred choice is normally made from those pipe sizes for which fittings can be purchased. This will be seen to be most important to us.

 

We know from our work on friction in fluids that we must expect a friction loss when a liquid flows in a pipe. It is known that the friction loss in pipes is partly dependent on the character of the internal finish of the pipe and that in turn depends on the way in which the pipe is made. Copper pipes are made by drawing, most steel pipes are made by rolling from sheet and electric welding, and plastic pipes are made by extrusion through dies. Larger pipes are often made of cast iron and concrete and these are made by spinning during setting, in horizontal moulds and very large pipes, such as underground aqueducts, are usually made by lining tunnels with steel rings or shaped blocks of concrete and spraying the inside surface with bitumen. This means that the number of surfaces of new pipes is relatively few but, of course, pipes do not always retain their manufactured finish especially when conveying water.