Pipe Threads

A pipe thread is a spiral ridge on the end of a pipe that enables pipes to be joined together. For male fittings, pipe thread appears on the outer diameter of the pipe; if female, the pipe thread appears on the inner diameter. By rotating a male pipe end into a female pipe thread, the two fittings become joined. Since male and female pipe thread must align successfully to form a connection, manufacturers follow pipe thread industry standards.

The two main pipe thread standards are as follows:  

• ·         The British Standard Pipe thread (BSP)
• ·         The American National Pipe Thread (NPT)

Types of Pipe Threads Pipe threads are used to make not only a mechanical joint but also a leakproof liquid seal. This is accomplished by machining the thread form on a taper and using pipe sealant to fill any voids between the two threads which could cause a spiral leak.

In both thread standards there are 2 different types of threads:  

• ·    Parallel ('straight') threads, BSPP (British Standard Pipe Parallel Thread), which have a constant diameter; denoted by the letter G.  
• ·    Taper threads, BSPT (British Standard Pipe Taper Thread), whose diameter increases or decreases along the length of the thread; denoted by the letter R.

A parallel pipe thread only forms a mechanical seal and is not used for liquid seal applications. While a tapered pipe thread that can make a close-fitting wet seal.

Pipe threads used for liquid joints can be divided into two types:  

• ·     Jointing threads: These are pipe threads where pressure-tightness is  made through the mating of two threads together. They always use a taper male thread, but can have either parallel or taper female threads and the seal is usually secured with a sealant compound.
• ·     Fastening threads: These are parallel pipe threads used where a pressure-tight joint is achieved by the compression of a soft material (such as an o-ring seal, gasket or a washer) between the end face of the male thread and a socket or nipple face, with the tightening of a backnut.

British Standard Pipe (BSP) Thread

In the nineteenth century, many different types of screw threads were required for hydraulic and pneumatic circuits as well as fastening components. As a result, manufacturers started to devise their own fastening systems. This resulted in compatibility problems. The English mechanical engineer and inventor, Sir Joseph Whitworth devised a uniform threading system in 1841 to address the incompatibility problem. The Whitworth thread form is based on a 55 degree thread angle with rounded roots and crests. The joint is made self sealing by cutting at least one of the threads on a taper (usually the male thread). This became known as the British Standard Pipe thread has been adopted internationally for interconnecting and sealing pipe ends. The image below shows a BSPT (BSP Taper) male thread sealing in BSPP (BSP Parallel) female coupling. To achieve the taper the bottoms of the threads aren't on a cylinder, but on a cone; with a taper which is 1⁄16 inch in an inch, which is the same as 3/4 inch in a foot. The taper divided by a centre line yields an angle 1° 47' 24" or 1.7899° as measured from the centre axis. Commonly-used sizes are ⅛, ¼, ⅜, ½, ¾, 1, 1¼, 1½, and 2”, available at most suppliers. Larger sizes are used less frequently because other methods of joining are more practical for 3” and above in most applications. The most important point to understand about pipe threads is that their size refers to the diameter of the hole going through the fitting (i.e. where the media travels such as air or oil, etc), and not the diameter of the thread itself

National Pipe Thread (NPT)

In America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered Thread (NPT) in 1864. His 60 degree thread angle with flat crests and roots is the American standard for tapered threads used to join pipes and fittings. The ANSI/ASME standard B1.20.1 covers threads of 60-degree form in sizes from 1/16” to 24” Nominal Pipe Size (NPS).

The taper rate for NPT threads is the same as BSPT (3/4” per foot) measured by the change of diameter (of the pipe thread) over distance. Sometimes NPT threads are referred to as MPT ('Male Pipe Thread'), MNPT, or NPT(M) for male (external) threads; and FPT ('Female Pipe Thread'), FNPT, or NPT(F) for female (internal) threads.

Sealing a Tapered Thread

Because of the taper, a pipe thread can only screw into a fitting a certain distance before it jams. The standard specifies this distance as the length of hand tight engagement, the distance the pipe thread can be screwed in by hand. It also specifies another distance – the effective thread, this is the length of the thread which makes the seal on a conventional machined pipe thread. For pipefitters, instead of these distances, it is more convenient to know how many turns to make by hand and how many with a wrench. A simple rule of thumb for installing tapered pipe threads, both metal and plastic, is finger tight plus one to two turns with a wrench. Torque installation values can be determined per application, but due to the variations involved in pipe joints such as disimiliar materials of male and female threads, type of sealants used, and internal variations in product wall thickness, a standard torque specification cannot be generically applied. The table below compares the critical dimensions of BSP and NPT threads and gives the number of turns to hand tighten.