STEAM BOILERS 165 



the water should be blown out. This should be done every 

 morning, so that any impurities in mechanical suspension in 

 the water that settled during the night may be blown out. 

 Great care should be exercised while blowing down that too 

 much water is not blown out; from 3 to 4 in. as shown by the 

 gauge glass, is sufficient. Under no circumstances should the 

 attendant leave the blow-off while it is open. Disaster to the 

 boiler is liable to follow a disregard of this injunction. Next, 

 all the valves, except the stop-valve, which were shut the night 

 before should be opened and tried to see that they are free and 

 in good working order. 



BOILER INSPECTION 



NATURE OF INSPECTION 



The inspection of a boiler usually consists of an external 

 examination of the complete structure, and of the setting if 

 the boiler is externally fired, and an internal inspection. The 

 examination of the boiler consists of an ocular inspection for 

 visible defects, and a hammer test or sounding for hidden defects 

 of plates, stays, braces, and other boiler parts. The hammer 

 test is made by tapping the suspected parts with a light ham- 

 mer and judging the existence and extent of defects from the 

 sound produced by the hammer blow. If the examination 

 discloses marked wear and tear, a series of calculations is often 

 required to find the safe pressure that may be allowed on the 

 worn parts, using such formulas or rules as laws, ordinances, 

 and regulations may prescribe for the particular official inspec- 

 tors. In the absence of officially prescribed formulas and rules, 

 the inspector should use such rules as he deems best appli- 

 cable or in best accordance with good practice. The inspec- 

 tion is usually, but not always, completed by a so-called 

 hydrostatic test, which is generally prescribed by official 

 regulations. 



EXTERNAL INSPECTION 



Preparation. Before a boiler that has been in use can be 

 inspected, it must be blown out and must be allowed to cool off. 



