A steel pipe flange is a flat rim, collar or rib used to strengthen, guide or attach one pipe to another. Standard steel flanges are disc shaped and are bolted or welded in place. Each steel flange is bored to receive and secure a specific size of pipe. Flange sizes, materials and opening dimensions vary depending upon what application the flange is designed for. Slip-on, socket-wired and threaded flanges are standard styles.
A long weld neck flange, or LWN, is considered a specialty flange because of its protruding neck, also referred to as a hub. The neck extends from the flat surface of the flange and receives and guides the pipe into place — the neck serves as an extension of the boring. Tapering the neck provides extra reinforcement for use in high-pressure or extreme and fluctuating temperature applications. Tapering the flange reduces the steel's thickness so that the opening of the neck blends with the pipe it receives — the tapered neck appears to be an extension of the pipe.
LWN steel flanges are largely used in the petroleum and chemical industries but can also be used in residential and commercial buildings where fluctuating temperatures and high stress are expected, such as a main water or gas source. Long neck steel flanges are used under conditions where line or pipe expansion creates bending or repeated changes in the pipe. Because a long neck flange is used in pressurized applications, the flange may have additional pressure connections and may be referred to as an orifice flange. The flange supports free product flow when bored to match the diameter of the pipe it will receive. Long weld neck flanges are attached using V-shaped welds known as butt welds.
Manufacturers of steel flanges follow specific production standards when designing or machining a flange. Dimensional tolerances and dimensions for long neck orifice flanges are covered in the American Society of Mechanical Engineers section 36 of publication B16. AMSE publication B.16 covers pipes and fittings made of steel, cast iron, cast bronze and wrought copper.