A threaded pipe is a pipe with screw-threaded ends for assembly.
The threaded pipes used in some plumbing installations for the delivery of gases or fluids under pressure have a threaded section that is slightly conical (in contrast to the cylindrical section commonly found on bolts and
power screws). This is called a "tapered thread". The seal provided by a threaded pipe joint depends upon the labyrinth sealcreated by the threads; upon a positive seal between the threads created by the deformation of the threads when they are tightened to the proper torque; and sometimes on the presence of a sealing coatingsuch as Teflon tape, or a liquid or paste pipe sealant.. Tapered thread joints typically do not include a gasket.
Especially precise threads are known as "dry fit" or "dry seal" meaning that no sealant is required for a gas-tight seal. Such threads are needed where the sealant would contaminate or react with the media inside the piping, e.g., oxygen service.
Due to the taper (cone) of the thread, when tapered threads are cut by a tap or a die, the amount of material being cut increases as the length of the thread is increased, and the force required to cut the thread therefore increases. It is therefore common to use powered tools when cutting tapered threads in pipes with diameters greater than one inch.
Tapered threaded fittings are sometimes used on plastic piping. Due to the wedging effect of the tapered thread, extreme care must be used to avoid overstressing the female component of the joint. Such fittings may split days, weeks, or even years after initial installation, and therefore many municipal plumbing codes restrict the use of threaded plastic pipe fittings.
Pipes may also be threaded with cylindrical threaded sections, in which case the threads do not themselves provide any sealing function other than some
labyrinth sealeffect, which may not be enough to satisfy either functional or code requirements. In these cases the seal may be provided by an O-ring seated between the shoulder of the male pipe section and an interior surface on the female part.
British standard pipe thread(BSP)
National pipe thread(NPT)
* O-Ring Boss Seal
RIDGID, notable manufacturer of pipe threaders
Taps and dies
Buttress Thread Form
* [http://www.cutting-tool-supply.com/TechTips/Tapping/NPTvsNPTF/NPTVsNPTF.htm NPT Vs. NPTF Taper Pipe Threads]
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