BLOWING OFF BOILERS.
The sudden cooling down of boilers, by blowing them off while hot, is a most pernicious practice, which is responsible for many cracked sheets and broken stay-bolts. It also tends to make a boiler scale the heating surfaces rapidly. Every time a boiler is blown out hot, if the water contains calcareous solution, a coat of mud is left on the heating surfaces, which dries hard while the steel is hot. If a piece of scale taken from a boiler periodically subjected to this blowing-out process be closely examined, it will be found to consist of thin layers, every one representing a period of blowing off, just as plainly as the laminæ of our rocks indicate the method of their formation. When a boiler must be cooled down quickly for washing out or other purposes, the steam should be blown off, and the boiler gradually filled up with water. Then open the blow-off cock, and keep water running in about as fast as it runs out until the temperature gets even with the atmosphere. The boiler may now be emptied without injury. Or another good plan is to blow off about two gauges of water under a pressure of forty or fifty pounds of steam, then cool down the boiler gradually, to prepare for washing.
Although the dangers of blowing off hot boilers, and then rushing in cold water to wash out, are well known and acknowledged, yet the practice is still followed on many roads where more intelligent action might be expected.