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British Standard Pipe

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The British Standard Pipe thread (BSP thread) is a family of standard screw thread types that has been adopted internationally for interconnecting and sealing pipe ends by mating an external (male) with an internal (female) thread.

Types

Two types of threads are distinguished:

  • 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.

These can be combined into two types of joints:

  • 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. (In Europe, taper female pipe threads are not commonly used.)
  • Longscrew 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 or a washer) between the end face of the male thread and a socket or nipple face, with the tightening of a backnut.

Threadform

For both the taper and the parallel pipe threads, the Whitworth threadform is used, which has the following characteristics:

  • symmetrical V-thread in which the angle between the flanks is 55° (measured in an axial plane)
  • one-sixth of this sharp V is truncated at the top and the bottom
  • the threads are rounded equally at crests and roots by circular arcs ending tangentially with the flanks
  • the theoretical depth of the thread is therefore 0.64 times the nominal pitch

Pipe thread sizes

A list of 16 thread sizes are defined by the standards, ranging from 1/16 to 6. The size number was originally based on the inner diameter measured in inches (25.4 mm) of a steel tube for which the thread was intended, but is in the modern metric version of the standard simply a size number.

Thread
size 		Threads
per inch 		Pitch
[mm] 		Major diameter
of the thread 		Gauge
length
[mm] 		Corresponding pipe
[mm] [in] DN OD
[mm] 		OD
[in] 		Thick-
ness
[mm]
1/16 28 0.907 7.723 0.304 4
1/8 28 0.907 9.728 0.383 4 6 10.2 0.40 2
1/4 19 1.337 13.157 0.518 6 8 13.5 0.53 2.3
3/8 19 1.337 16.662 0.656 6.4 10 17.2 0.68 2.3
1/2 14 1.814 20.995 0.825 8.2 12 or 15 21.3 0.84 2.6
5/8 14 1.814 22.911 0.902 16 2.6
3/4 14 1.814 26.441 1.041 9.5 20 26.9 1.06 2.6
1 11 2.309 33.249 1.309 10.4 25 33.7 1.33 3.2
1¼ 11 2.309 41.910 1.650 12.7 32 42.4 1.67 3.2
1½ 11 2.309 47.803 1.882 12.7 40 48.3 1.90 3.2
2 11 2.309 59.614 2.347 15.9 50 60.3 2.37 3.6
2½ 11 2.309 75.184 2.960 17.5 65 76.1 3.00 3.6
3 11 2.309 87.884 3.460 20.6 80 88.9 3.50 4
4 11 2.309 113.030 4.450 25.5 100 114.3 4.50 4.5
5 11 2.309 138.430 5.450 28.6 125 139.7 5.50 5
6 11 2.309 163.830 6.450 28.6 150 165.1 6.50 5

The major diameter listed is the outer diameter of the external thread. For a taper thread, it is the diameter at the "gauge length" from the small end of the thread. The taper is "1 to 16", meaning that for each 16 units of measurement increase in the distance from the end, the diameter increases by 1 unit of measurement.

Pipe thread designations

These standard pipe threads are formally referred to by the following sequence of blocks:

  • the words "Pipe thread",
  • the document number of the standard (e.g., "ISO 7" or "EN 10226")
  • the symbol for the pipe thread type:
    • G = external + internal parallel (ISO 228)
    • R = external taper (ISO 7)
    • Rp = internal parallel (ISO 7)
    • Rc = internal taper (ISO 7)
    • Rs = external parallel
  • the thread size

Threads are normally right-hand. For left-hand threads, the letters "LH" are appended.

Example: Pipe thread EN 10226 Rp 2½

The terminology for the use of G and R originated from Germany (G for Gas, as it was originally designed for use on gas pipes ; R for Rohr (meaning pipe).)

References

  • International standard ISO 7-1: Pipe threads where pressure-tight joints are made on the threads — Part 1: Dimensions, tolerances and designation. International Organization for Standardization, Geneva.
  • BS EN 10226: Pipe threads where pressure tight joints are made on the threads. (The European version of ISO 7.)
    • Part 1: Taper external threads and parallel internal threads — Dimensions, tolerances and designation.
    • Part 2: Taper external threads and taper internal threads — Dimensions, tolerances and designation.
  • BS 21: Pipe threads for tubes and fittings where pressure-tight joints are made on the threads (metric dimensions). British Standards Institute, 1985. (Superseded by BS EN 10226:2004).
  • International standard ISO 228-1: Pipe threads where pressure-tight joints are not made on the threads — Part 1: Dimensions, tolerances and designation.
  • BS 2779: Specification for pipe threads for tubes and fittings where pressure-tight joints are not made on the threads (metric dimensions), 1986.

BS EN 10226-1:2004

See also

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