5. Overheating of the plates, caused by shortness of water. When water is poured on red-hot surfaces it does not touch the surface, but remains in the spheroidal state at a little distance from it, being apparently surrounded by an atmosphere of steam. It assumes this state above 340°; when the temperature falls to about 288° it touches the surface and commences boiling.
6. By accumulation of scale, mud, or other deposit, which prevents the water gaining access to the iron. This causes the seams to leak, the crown-sheet to bulge or come down.
One is unable to find any proof that boilers do generally explode at about starting time, nor is that statement, to the best of information, founded on any basis of fact, but was first affirmed by parties who had designed a boiler especially arranged to avoid that imaginary danger.
No one supposes that inspection will absolutely prevent all explosions; but rigid inspection will discover defects that might end in explosion.
Low water is dangerous from the fact that it leaves parts of the boiler to be overheated and the strength of iron rapidly decreases in such a case. In fact, an explosion caused by low water might be expected to be less disastrous than if the water was higher, other conditions being equal, from the fact of there being less water at a high temperature ready to flash into steam at the moment of liberation.
Testing new boilers under steam pressure is both dangerous and unwise—the hot water expansion test is just as efficient, less costly and safe in every respect—hence, there is no occasion for a steam test. A manufacturer was testing a boiler in the way mentioned when a rivet in a brace blew out and the contents of the boiler rushed out, striking a man in the face, and parboiling him from head to foot. Another who was inspecting the boiler, was struck on the head and enveloped in steam and water; another was also scalded from the shoulders down; another was injured about the arms; a fifth man was scalded and severely injured about the back. The apartment was so filled with steam that the victims could not be rescued until all the damage mentioned had been done to them.
Danger from exploding steam pipes is greater than supposed. An inspector in a pipe works was testing a tube by means of a double-action hydraulic pump; the pipe suddenly burst with the pressure of 5,000 pounds to the square inch, and the water striking the unfortunate man on his face, he was killed on the spot.
There is a tendency on the part of engineers to trust too implicitly in their steam gauges. These are usually the only resort for determining the steam pressure under which the boiler may be working. But the best gauges are liable to err, and after long use to require a readjustment. It is fortunate, however, that the error is usually upon the safe side of indicating more than the actual pressure.
Any boiler that has been standing idle for a few weeks or months is a dangerous thing to enter, and no one should attempt it until it has been thoroughly ventilated by taking off all the man hole and hand-hole plates and throwing water into it. This is due to the presence of a gas which is generated from the refuse and mud, or scale, which, to a greater or less degree, remains in all boilers. Contact with fire is certain to result in an explosion. Not long since a locomotive was in a roundhouse, where it had been waiting some weeks for repairs. Some of the tubes were split and a man was pulling them out. He had only removed one or two when, putting in his lamp to see what remained, there was a fearful explosion which shook the shop. There are many other places which are unsafe to enter when they have been long closed, such as wells, pits of any kind, and tanks. Precisely what the nature of the gas is no one seems to know, but it is assuredly settled that a man who goes into it with a light seldom comes out unharmed.