THE STEAM ENGINE 337 



External corrosion does not as a rule give much trouble, 

 but under certain conditions the combined action of heat and 

 moisture on asbestos pipe-covering will set up pitting. This, 

 however, can be prevented by painting the pipes with any 

 good graphite paint before the covering is applied. 



378. Piping Material. For all ordinary and high pres- 

 sures 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 Ib. 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 



