170 STEAM BOILERS 



reseated and reground before the hydrostatic test is made. 

 After a boiler passes the hydrostatic test, the clamp locking the 

 safety valve is removed, and by running the pressure up once 

 more, the point at which the safety valve opens can be noted by 

 watching the steam gauge, which is supposed to have been tested 

 and corrected. If the safety valve does not open at the work- 

 ing pressure allowed or opens too soon, it is readjusted. If the 

 safety valve is locked by a seal, as is often required by official 

 regulations, the seal is applied after adjustment of the valve. 



Testing of Steam Gauge. The steam gauge should be tested 

 before the hydrostatic test, and at each inspection, with a so- 

 called boiler inspector's testing outfit. If the gauge under test 

 is more than 5% incorrect, most inspectors will condemn it, 

 although some will condemn gauges showing a much smaller 

 error. In most cases the gauge can be repaired at small expense 

 by the makers. 



Inspection of Water Gauge and Blow-Off. The connections 

 of water columns and water-gauge glasses require examina- 

 tion in order to see that they are clear throughout their whole 

 length. The blow-off pipe also requires examination in order 

 to see that it is clear. 



BOILER EXPLOSIONS 



As a boiler inspector is usually called on to investigate the 

 circumstances of a boiler explosion, and to render an opinion 

 concerning it, he must be familiar with the causes of such explo- 

 sions. Boiler explosions are really due to overpressure of steam. 

 This may occur because the boiler is not strong enough to 

 carry safely the working pressure used, or because the pressure, 

 through some cause, such as sticking or overloading of the safety 

 valve, has been allowed to rise above the ultimate strength 

 of the boiler. A boiler may be unfit to bear its working 

 pressure, for any of the following reasons : defective design ; 

 defects in workmanship or material; corrosion, and wear and 

 tear in general; and mismanagement in operation. 



The common faults in design that have led to boiler explo- 

 sions are: insufficient staying, the stays being too small or too 

 few in number; the cutting away of the shell for the dome, 

 manhole, and other mountings, without reinforcing the edge 





