For all ordinary and high pressures used in connection with land boilers, steel pipe is almost invariably adopted, the longitudinal joints being lap-welded. Cast steel is largely employed for bends and elbows, although copper is used in high-class work. Many old plants with pressures up to 100 lb. per square inch are working with cast iron pipes. On board ship, pipes are usually made of copper.
Pipes of small diameter are generally solid drawn, but many steam pipes on board ship are made with brazed joints. In their construction, makers usually allow a factor of safety varying from 10 to 15 tons per square inch, assuming the copper to possess an ultimate tenacity of about 15 tons per square inch.
Steam pipes expand and contract about one inch in fifty feet, through variation of temperature. It is best to allow for this movement, when possible, by arranging springing lengths, so that the whole arrangement may be elastic. When there are long lengths between fixed supports, expansion sockets are sometimes adopted. These, however, should always be fitted with guard bolts, to prevent the pipe from being accidentally drawn apart.
Steam pipes should always be kept free from water, and drain taps should, consequently, be fitted wherever necessary. Should an accumulation of water accidentally occur in a long horizontal length of pipe, its drainage under steam pressure is very liable to cause fracture. Therefore drainage should not be attempted without first isolating the boilers so as to minimize the danger.