The Different Extension or Joining Fittings

The Different Extension or Joining Fittings

There are six main types of extension fittings for heating and air conditioning pipes. Nipples, locknuts, couplings, offsets, joints and unions are all examples of extension or joining fittings. The fittings are designed to join and extend a length of a pipe for different situations. In this article, we have compiled a detailed description of the six different fittings.


Nipples can be classified into three types; close, short or long. The length of close nipples can vary with the size of the pipe. The length of the thread determines the size of the close nipple. There is only one size of short nipple for each size of pipe. Long nipples are made in many lengths up to a foot long. Anything over 12 inches is known as a cut pipe.


Locknuts are commonly used on long screw nipples that have couplings. A recessed of grooved end on the locknut fitting is used to keep up packing when a particularly tight joins is required. If at all possible, a union is preferred to a locknut because the join is much tighter. The normal length of all sized of locknut is six inches. They are made from a standard-weight pipe threaded four inches long on one end with a regular pipe thread on the other.


A join is a pipe fitting designed to adjust to the linear expansion and contraction of the pipe metal caused by the temperature differences between the water or steam inside the pipe and the air on the outside of it.


A coupling is a pipe fitting used to connect two lengths of pipe together. They are obtainable in various shapes and sizes. The ‘standard coupling’ is threaded together with right-handed threads. There are other couplings obtainable with both right-handed and left-handed threads. There are already couplings that have a male thread on one end and a female thread on the other end.


Another form of fitting to connect two pipes is a union. The most shared types of unions are the ground-joint and the plain/gasket unions.

The ground-joint union consists of a composition ring pressing against iron or both contact surfaces of composition. A join using a ground-join union is characterized by spherical contact. Because no gasket is used, perfect alignment of the two pipes is not as important in making up the join as it is when using a plain/gasket union.

The plain or gasket union alignment must be good to obtain a tight join. If both ends are in line and firmly pressed together against the gasket by the ring, the gasket will bear uniformly over the complete contact surface and the joint will be tight. If the two ends are out of alignment when the ring is screwed tight, it will cause great pressure on the gasket and the surfaces of the unions will not come together causing a leak.


An offset fitting is used when it is necessary to have a pipe bypass an obstruction that blocks its path. The new path will be similar to the old one but not exactly aligned with it.

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