fittings; sealing the connection with a pipe thread compound, Polytetrafluoroethylene (PTFE) Thread seal tape, oakum, or PTFE string, or by using a mechanical coupling. Process piping is usually joined by welding using a TIG or MIG process. The most common process pipe joint is the butt weld. The ends of pipe to be welded must have a certain weld preparation called an End Weld Prep (EWP) which is typically at an angle of 37.5 degrees to accommodate the filler weld metal. The most common pipe thread in North America is the National Pipe Thread (NPT) or the Dryseal (NPTF) version. Other pipe threads include the British standard pipe thread (BSPT), the garden hose thread (GHT), and the fire hose coupling (NST). Copper pipes are typically joined by soldering, brazing, compression fittings, flaring, or crimping. Plastic pipes may be joined by solvent welding, heat fusion, or elastomeric sealing. If frequent disconnection will be required, gasketed pipe flanges or union fittings provide better reliability than threads. Some thin-walled pipes of ductile material, such as the smaller copper or flexible plastic water pipes found in homes for ice makers and humidifiers, for example, may be joined with compression fittings. typically uses a "push-on" gasket style of pipe that compresses a gasket into a space formed between the two adjoining pieces. Push-on joints are available on most types of pipe. A pipe joint lubricant must be used in the assembly of the pipe. Under buried conditions, gasket-joint pipes allow for lateral movement due to soil shifting as well as expansion/contraction due to temperature differentials. Plastic MDPE and HDPE gas and water pipes are also often joined with Electrofusion fittings. Large above ground pipe typically uses a flanged joint, which is generally available in ductile iron pipe and some others. It is a gasket style where the flanges of the adjoining pipes are bolted together, compressing the gasket into a space between the pipe. Mechanical grooved couplings or Victaulic joints are also frequently used for frequent disassembly and assembly. Developed in the 1920s, these mechanical grooved couplings can operate up to working pressures and available in materials to match the pipe grade. Another type of mechanical coupling is a flareless tube fitting (Major brands include Swagelok, Ham-Let, Parker); this type of compression fitting is typically used on small tubing under in diameter. When pipes join in chambers where other components are needed for the management