Boiler Accessories Manholes and Handholes a

BOILER ACCESSORIES: MANHOLES AND HANDHOLES A manhole allows access to the boiler for cleaning and repairs.

It is usually elliptical in form and large enough to admit a man. About 16 inches for the major axis, and 12 for the minor axis, is a good size. The manhole is closed by a plate or cover made of cast or wrought iron. This plate is held to the seat by a yoke or yokes, and bolts. Fig. 24 shows one form, Y being the yoke, L the cover, and N the bolt. The joint between the cover and the shell is made steam tight by packing.

The strength of the boiler should always remain unimpaired; so, whenever a large hole is cut in the plate, the edge should be strengthened, for the tension is concentrated there, and the plates are, moreover, likely to become weak by corrosion. The strain put upon the plate by screwing up the cover, if no packing is used, is considerable, especially if a piece of scale gets between the faces and the joint is then made tight.

Fig. 25 shows the section of a strong and simple manhole. The edge of the plate is strengthened by a broad ring of steel, which is flanged and riveted to the shell, its edge forming the seat. The cover as shown in the figure is shaped for strength. The edge of the ring which forms the seat, and the cover, are machined to make a tight joint without packing. The strengthening ring should be at least inch thick and 4 inches wide, that the rivet-holes may not be too near the edge.

Handholes and mudholes are more commonly placed in boilers, which are so constructed that a man cannot enter—in a vertical boiler, for example. They are used to some extent in other boilers; in horizontal return-tube boilers there is usually a handhole in each end, near the bottom. Handholes are very convenient to admit hose for washing out the boiler, also for removing scale and sediment. Handholes are similar to manholes in construction, but require only one yoke and one bolt to keep them in place. Mudholes should be provided in order that the sediment and detached scale can be removed without lifting the accumulated mass to the top manhole. Mudholes and handholes greatly facilitate cleaning the fire-box water-leg of locomotive and small vertical boilers.