SI54P3OS, STRICKLAND & Co, Ltd., Dartmouth & Teddington. 



CHAPTER III. 



BOILER PRESSURES. 



regard to the best working pressure to use for launch machinery, 

 there is and has been a very great diversity of opinion. The general 

 opinion of the best engineering talent in the country appears, however, to 

 be that about 200 Ibs. is the best pressure for ordinary marine work : and 

 this is found to be satisfactory for launch work as well. For a long time 

 our standard working pressure for quadruples has been 175 Ibs., and we 

 have found this very satisfactory. With water tube boilers it is, however, 

 profitable to use slightly more than this, and we are accordingly making 

 all our standard water tube boilers for 200 Ibs., and the new d< 

 for the quadruple and triple engines are suitable for this. For comj 

 engines we do not see any advantage, except for very special purposes, 

 in exceeding 150 Ibs. It is perfectly true that by going to a higher pressure 

 more work can be got out of a given sized engine, but it is difficult to put 

 in sufficient bearing surfaces to enable it to be done without great wear and 

 tear, while there is certainly no gain in economy. 



There is an erroneous idea that high pressures are more dangerous 

 than low pressures. This as a fact is not so, as a boiler if properly 

 designed, has a bursting strain of from four to six or more times its 

 working pressure, and there is just the same margin in a boiler designed for 

 200 Ibs. as one for 100 Ibs. On the other hand we think it a mistake to go 

 too far in this direction, not because of risk, but because the use of 

 pressures higher than are usual is liable to lead to small troubles with 

 joints, stuffing boxes, &c., each of which may not be much consequence in 

 itself, but all of which put together make the difference between satisfactory 

 working and otherwise. This can be seen in the past, as when a few 



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