[Fig. 3412] represents the “Martin” boiler for marine engines. In the return flue there are a number of vertical water tubes which are very effective in promoting circulation as well as in generating steam. These boilers are used largely in the United States navy for moderate pressures.
The following upon the testing and examining of a boiler of this class is from Modern Steam Boilers:
“Every new boiler should, when complete, be tested by water pressure to double the amount of the intended working pressure; for while the wisdom of applying as high a pressure as three times the working pressure, which is sometimes done, may be questionable, experience has shown that a test by hydraulic pressure will reveal defects that would otherwise be apt to pass unnoticed.
“For instance, when the top plate of a combustion box is stayed against the pressure by girder stays that are not stayed to the boiler shell, the girder stay merely acts to stiffen the top plate, and as a result the whole pressure on the area of the top plate falls on the walls of the combustion box. The back tube plate therefore springs down and transfers part of this pressure to the furnace, causing it to become elliptical, as may generally be found by the application of rod gauges fitted to it before testing and tried while the pressure is on.
“This flattening under test naturally drew attention to the defectiveness of girder stays. Another instance may be given with reference to gusset stays, which, if fitted so as to support too large an area of back plate, in proportion to the area of combustion box it supports, may cause the combustion box to distort from its natural shape, pulling the tube sheet back and flattening the furnace. The amount of distortion may be only 1⁄16 inch in some cases, but that is sufficient to show the existence of unequal strains which require attention in boiler designing.
“This brings us to the important fact that in almost every instance where the furnaces of marine boilers collapse, they come down at the sides, notwithstanding that when collapse occurs from overheating, the crown of the furnace must have been left bare of water first, and should therefore come down first, flattening the furnace at the top. This points to the conclusion, that the top of the furnace received some extraneous support.
“When a furnace collapses from corrosion, it naturally gives way at the most corroded part. An hydraulic test to twice the working pressures is recommended for new boilers only, unless it be small vertical cylindrical and steam launch boilers, which may always be subjected to the same test as new main boilers.
“In the case of old main boilers, however, and particularly rectangular ones, an hydraulic test of less than twice the working pressure may be employed, the amount being governed by the circumstances of the case. If, for instance, a boiler has undergone a thorough repair and received new furnaces, then every part of the boiler should have received proportionate consideration and an hydraulic test depending upon the judgment of the responsible engineer, but not less than one and one-half times the working pressure should be made, while one of one and three-quarter times could scarcely be objected to. This, however, is a subject upon which there is some controversy, especially in the case of old boilers having a good foundation of strength, but patched or local weak spots, such as combustion chamber backs and sides, these patches having been, perhaps, made with a view to a more extensive repair in the near future.
“In such a case as this an hydraulic test sufficient to prove the tightness of the seams and joints may, perhaps, be all that is absolutely essential.
“After a boiler has been tested by hydraulic pressure it should be examined internally, as it sometimes occurs that a stay may break under the test (especially if gusset stays are employed), and the extra strain thrown on the adjacent parts may cause them to fail, and thus cause the destruction of the boiler when under strain.