Pipe-Fitting Materials

American Standard Taper Pipe Threads - NPT

The tapering pipe thread has a standard taper of 3/4 inch per foot measured on the diameter and along the axis. Although These threads are designed to be self-holding and self-sealing when properly assembled pipe dope should be applied to the middle of external threads to assure a good seal. When pipe or a male fitting is properly screwed into a tapped hole or a female fitting, it will give the correct fit when all but two or three threads are covered.

Dryseal Threads

Dryseal pipe threads are based on the USA (American) pipe thread, however, they differ from the USA (American) pipe thread in that they are designed to seal pressure tight joints without the necessity of using sealing compounds. To accomplish this some modification of thread from and greater accuracy in manufacture is required. The roots of both the external and internal threads are truncated slightly more than the crests, i.e. roots have wider flats than crest, so that metal-to-metal contact occurs at the crests and roots

FITTINGS

Common Pipe Fittings

Shipboard piping systems include a variety of fittings made from a number of different metals and alloys. The layout and function of a system determine the composition and the number of fittings employed. The use of fittings is held to a minimum, since each fitting is a possible source of leakage. However, some fittings are necessary in any piping system. The fittings most commonly used in shipboard piping systems are described in this section.

Illustrated are a number of common fittings. These fittings vary widely in design. For example the return bend has the centers of the openings as close together as possible. Elbows are available in 90° and 45°. Most fittings are available threaded, weld, or braze. This variety in design provides appropriate fittings for any system.

Pipe plugs are used to close openings in fittings or equipment. Caps are used to close the pipe ends. Reducing bushings and couplings are used to connect pipe, fittings and tubing of different diameters or to connect pipe or tubing to machinery. Nipples are used for connections to machinery, and are also frequently used in plumbing systems.

Shock resistant piping nipples with straight threads are available and are used for root connections to machinery.

Union are used To connect two lengths of pipe previously set in place. Unions and union end valves are provided at terminal connections to facilitate repairs, altercations or cleaning. Unions are available in bronze, iron, and steel. They are designed to withstand a wide range of temperatures and pressures.

Unions are available in bronze, iron, and steel. They are designed to withstand a wide range of temperatures and pressures.

Flanges can be attached to pipes and tubing by welding, brazing, rolling, flaring, and in low pressure systems with pipe threads. The method will depend on the size, service, and the construction period of the ship. Flanged joints are sealed with gaskets.

Malleable iron flange - is another type of union. Its intended use is 150 PSI steam lines and is available in sizes up to 6 inches.

Socket Welding flange - is suitable for various service, pressures, and temperatures. This type of flange is slipped onto the pipe and fillet-welded.

Van Stone Flange - is used in high pressure steam lines that are subject to high temperature and expansion strains. It consists of a regular flanged upper portion and a ground lower portion, allowing for a self aligning leakproof joint when the flange bolts are tightened.

Expansion Joints

Metals expand when heated and contract when cooled, piping systems are subject to change in dimension with changes in temperature. Unless suitable provision is made for expansion and contraction stress will set up and systems fail.

Systems that carry steam at high temperature require expansion joints. At left is corrugated type expansion joint, used in some medium pressure steam lines. Expansion and contraction is taken up by the corrugations in the joint and the internal sleeve serves to eliminate the noise which would occur from the passage of steam over the corrugations. For high pressure steam lines a bellows type expansion joint is used. Expansion and contraction is taken up by stainless steel bellows.

In some systems expansion and contraction are allowed for by the use of directional changes in the runs of piping or by the use of U-bends or horseshoe bends. In very small tubing such as gauge lines, loops are sometimes used, these loops also absorb vibration.