SECT, in.] CONDENSATION OF STEAM. 139 



SAFETY VALVES. 



259. The precautions for safety are of much importance : the boilers of steam 

 engines should never be constructed without them, and they should be done with 

 every care to render them effective in preventing accidents. l 



Safety valves are called external or internal, according to the nature of the 

 evil to be prevented. An internal safety valve is to prevent the pressure of the 

 atmosphere crushing in the sides of the boilers or pipes to which it is applied. 

 It is usually an inverted conical valve, retained in its seat by a rod connected to 

 a lever, having a weight at its opposite end, such that the force of the atmospheric 

 pressure will overcome it, when its pressure is three or four pounds on the circular 

 inch greater than the elastic force of the steam in the boiler. 



In Plate i. Fig. 1. this valve is shown as inserted in the man-hole plate ; a being 

 the valve kept in its seat by the weight on the lever at b. 



260. The external safety valve is to prevent the risk of explosion, should the 

 steam become stronger than that which the boiler is intended to confine ; therefore 

 it is of the greatest importance that it be properly constructed and not liable to 

 derangement. 



The application of a loaded valve to limit the force of steam appears to have 

 been first made by Papin to his digesters ; and it was applied by Savery to the 

 boilers of his steam engines. It consisted of a conical valve retained in its seat 

 by a weight on a lever ; and, from its resemblance to a steelyard, was called the 

 steelyard safety valve. It is still much used, but it has the obvious defect that 

 the weight may be increased at the will of the workman, or even may be done 

 through the ignorance of a stranger; hence valves of this form should not be 

 employed unless the lever and valve be wholly inclosed in a box kept locked by 

 the proprietor. Such a box should have a pipe leading into the chimney, to carry 

 off the steam, and a slight wire or chain to lift the valve by, lest it should stick 

 fast by corrosion. 



261. For low pressure steam, the form is rendered more convenient. The 

 conical valve has its load directly upon it, and it ought to be sufficiently large. 

 Its clear area in the narrowest part not being less than is calculated by the 

 annexed rule, and the power of the steam having been determined, a fixed and 

 unalterable weight agreeing with that power should be formed and attached to the 

 rod on the top of the valve ; and the whole should be inclosed in a metal box, 

 having a passage larger than the area of the valve, to convey the steam away to 

 the chimney or other place. 



1 See description of safety valve applied to Boulton and Watt's Marine Engine, Plate xxi. 



