Figure 3-58. - ypes of iron pipe couplings, nipples, plugs, caps, and bushings.

At times, you may use the DIELECTRIC or INSULATING type of fitting. These fittings connect underground tanks or water heater tanks. They are also used when pipes of dissimilar metals are joined. The purpose of dielectric fittings is to curtail galvanic or electrolytic action. The most common dielectric fittings are the union, the coupling, and the bushing.

READING THE SIZE OF FITTINGS. - To help you read the designations of fittings, you should know that each opening of the fitting is identified with a letter that indicates the sequence in reading the size of the fitting. As shown in figure 3-59, a cross with one end of run and one outlet reduced is designated as 2 1/2(A) x 1 1/2(B) x 2 1/2(C) x 1 1/2(D). This is done simply by naming the largest opening first and then naming the other openings in the order indicated.

Elbows and crosses are always identified by designating the size of the largest opening first, following with the size of the other openings in proper order. Tees, 45-degree Y-branches, and double-branch elbows are identified by designating the size of the largest opening on the run first, the opposite opening of the run second, and the size of the outlet last; for example, a 3 x 2 x 1 l/2 size tee is one that has openings 3(A) x 2(B) x 1 1/2(C). In designating the outlets of side outlet reducing fittings, the size of the side outlet is named last. Refer to figure 3-59. The rules for reading screwed fittings also apply to reading other reduced fittings.

MEASURING. - Galvanized steel, galvanized wrought-iron, and black iron pipe are measured for size by the INSIDE diameter of the pipe. The outside diameter of the pipe remains constant for the different weights; whereas the inside diameter of the pipe varies because of the wall thickness of the pipe. The outside diameter is held constant because the pipe is normally joined by threaded joints, and one die can be used to thread any weight of one size. Also, fittings of a uniform size on the inside fit all the different weights 3-46