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Back in the 19th century, screw Threads Process Connections come in a great variety and go incompatible with each other. Nowadays, thanks to the efforts made by people committing to standardizing process connections threads, choices are nailed down to a few when choosing a pressure transmitter. In what follows, three types of thread are introduced, namely, BSP, NPT, and UNF.

British Standard Pipe

BSP, British Standard Pipe, was created by Joseph Whitworth in the middle of 19th century and is now widely accepted from Europe to Asia, in particular in the UK. BSP is a type of parallel thread and the pressure tight seal is enabled with a sealing washer, which is made of different materials depending on the pressure and the medium being used. The most common sizes are ¼ or ½ inch BSP.

1/2"BSP MALE1/2"G EN837 DIN162881/4"G EN837 DIN162881/4"G DIN3852
1_2'BSP MALEEN83701 G_2B G1_2A DIN 16288 FORM B1_4'BSP EN837 DIN162881_4'G DIN3852-2
BSPP (G)BSPT (R/Rp)
Thread size (inch)Major Diameter (mm)Minor Diameter (mm)TPIMale Thread size
(inch)
Female Thread size
(inch)
Major Diameter (mm)Minor Female
Diameter (mm)
TPI
G 1/16”7,7236,56128R 1/16”Rp 1/16”7,7236,49028
G 1/8”9,7288,56628R 1/8”Rp 1/8”9,7288,49528
G 1/4”13,15711,44519R 1/4”Rp 1/4”13,15711,34119
G 3/8”16,66214,95019R 3/8”Rp 3/8”16,66214,84619
G 1/2”20,95518,63114R 1/2”Rp 1/2”20,95518,48914
G 3/4”26,44124,11714R 3/4”Rp 3/4”26,44123,97514
G 1”33,24930,29111R 1”Rp 1”33,24930,11111
G 2”59,61456,65611R 2”Rp 2”59,61456,47611

National Pipe Taper

In the USA, NPT (National Pipe Tapered) is the most popular process connection, especially in businesses of oil and petroleum. Compared with BSP, NPT is easier to cut and user-friendly, and is considered a booster to American Industrial Revolution. Being a kind of screw thread system, NPT is tapered thread with the pressure tight seal being made on the thread itself. The most common sizes are 1/4, or 1/2 inch NPT.

1/2"NPT MALE1/4"NPT MALE1/4"NPT FEMALE1"NPT SEALED
1_2'NPT MALE1_4'NPT MALE1_4'NPT FEMALE1'NPT SEALED
Thread sizeMajor Diameter (mm)TPI
1/16” – 27 NPT7,93827
1/8” – 27 NPT10,28727
1/4” – 18 NPT13,71618
3/8” – 18 NPT17,14518
1/2” – 14 NPT21,33614
3/4” – 14 NPT26,67014
1” – 11 ½ NPT33,40111,5
2” – 11 ½ NPT60,32511,5

Unified Thread Standard

William Sellers also developed what became the Unified Thread Standard. Straight thread Process Connections known as SAE are now called UNF (Unified National Fine) under the Unified Thread Standard.

A common variation of this Process Connections is the M/F-250, or the autoclave fitting. This fitting has a cone at the end of it for pressure ranges above 10,000 psi. It is also commonly referred to as a Sno Trik® fitting as well – though that is a brand name.

The most common types of UN (Unified National) thread are:

  1. UNC – Unified National Coarse Thread, comparable with the ISO metric thread.
  2. UNF – Unified National Fine Thread.

*Compared to standard threads (coarse thread), a fine thread has a smaller pitch.

Unified threads come in three different classes:

  1. 1A (external) & 1B (internal): for applications where a liberal tolerance is required to permit easy assembly even with slightly nicked threads.
  2. 2A (external) & 2B (internal): most commonly used class for general applications
  3. 3A (external) & 3B (internal): for applications where closeness of fit and/or accuracy of thread elements are important.
UNC (2A)UNF (2A)
Nominal DiameterMajor Diameter (mm)Minor Diameter (mm)TPINominal DiameterMajor Diameter (mm)Minor Diameter (mm)TPI
1/4” x 20 UNC6,3224,978201/4” x 28 UNF6,3255,36028
5/16” x 18 UNC7,9076,401185/16” x 24 UNF7,9106,78224
3/8” x 16 UNC9,4917,798163/8” x 24 UNF9,4978,38224
7/16” x 14 UNC11,0769,144147/16” x 20 UNF11,0799,72820
1/2” x 13 UNC12,66110,592131/2” x 20 UNF12,66711,32820
5/8” x 11 UNC15,83413,386115/8” x 18 UNF15,83914,35118
3/4” x 10 UNC19,00416,307103/4” x 16 UNF19,01217,32316
7/8” x 9 UNC22,17619,17797/8” x 14 UNF22,18420,26914
1” x 8 UNC25,34921,97181” x 12 UNF25,35423,11412
2” x 4,5 UNC50,72644,6794.5

ISO Metric Screw Thread

The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.

The “M” designation for metric screws indicates the nominal outer diameter of the screw, in millimeters (e.g., an M6 screw has a nominal outer diameter of 6 millimeters). (Source from Wikipedia)

ISO 262 selected sizes for screws, bolts and nuts
Nominal diameter D (mm)Pitch P (mm)Nominal diameter D (mm)Pitch P (mm)
1st choice2nd choiceCoarseFine1st choice2nd choiceCoarseFine
10.250.21621.5
1.20.250.2182.52 or 1.5
1.40.30.2202.52 or 1.5
1.60.350.2222.52 or 1.5
1.80.350.22432
20.40.252732
2.50.450.35303.52
30.50.35333.52
3.50.60.353643
40.70.53943
50.80.5424.53
610.75454.53
710.754853
81.251 or 0.755254
101.51.25 or 1565.54
121.751.5 or 1.25605.54
1421.56464

Please refer to Part 2 for further reading.