Four Types Of Thread Process Connections Your Need Know - Part 1/2

Back in the 19th century, screw Threads Process Connections come in a great variety and are incompatible. 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: 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 MALE	1/2"G EN837 DIN16288

1/4"G EN837 DIN16288	1/4"G DIN3852

BSPP (G)				BSPT (R/Rp)
Thread size (inch)	Major Diameter (mm)	Minor Diameter (mm)	TPI	Male Thread size (inch)	Female Thread size (inch)	Major Diameter (mm)	Minor Female Diameter (mm)	TPI
G 1/16”	7,723	6,561	28	R 1/16”	Rp 1/16”	7,723	6,490	28
G 1/8”	9,728	8,566	28	R 1/8”	Rp 1/8”	9,728	8,495	28
G 1/4”	13,157	11,445	19	R 1/4”	Rp 1/4”	13,157	11,341	19
G 3/8”	16,662	14,950	19	R 3/8”	Rp 3/8”	16,662	14,846	19
G 1/2”	20,955	18,631	14	R 1/2”	Rp 1/2”	20,955	18,489	14
G 3/4”	26,441	24,117	14	R 3/4”	Rp 3/4”	26,441	23,975	14
G 1”	33,249	30,291	11	R 1”	Rp 1”	33,249	30,111	11
G 2”	59,614	56,656	11	R 2”	Rp 2”	59,614	56,476	11

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 MALE	1/4"NPT MALE

1/4"NPT FEMALE	1"NPT SEALED

Thread size	Major Diameter (mm)	TPI
1/16” – 27 NPT	7,938	27
1/8” – 27 NPT	10,287	27
1/4” – 18 NPT	13,716	18
3/8” – 18 NPT	17,145	18
1/2” – 14 NPT	21,336	14
3/4” – 14 NPT	26,670	14
1” – 11 ½ NPT	33,401	11,5
2” – 11 ½ NPT	60,325	11,5

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 Diameter	Major Diameter (mm)	Minor Diameter (mm)	TPI	Nominal Diameter	Major Diameter (mm)	Minor Diameter (mm)	TPI
1/4” x 20 UNC	6,322	4,978	20	1/4” x 28 UNF	6,325	5,360	28
5/16” x 18 UNC	7,907	6,401	18	5/16” x 24 UNF	7,910	6,782	24
3/8” x 16 UNC	9,491	7,798	16	3/8” x 24 UNF	9,497	8,382	24
7/16” x 14 UNC	11,076	9,144	14	7/16” x 20 UNF	11,079	9,728	20
1/2” x 13 UNC	12,661	10,592	13	1/2” x 20 UNF	12,667	11,328	20
5/8” x 11 UNC	15,834	13,386	11	5/8” x 18 UNF	15,839	14,351	18
3/4” x 10 UNC	19,004	16,307	10	3/4” x 16 UNF	19,012	17,323	16
7/8” x 9 UNC	22,176	19,177	9	7/8” x 14 UNF	22,184	20,269	14
1” x 8 UNC	25,349	21,971	8	1” x 12 UNF	25,354	23,114	12
2” x 4,5 UNC	50,726	44,679	4.5

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 choice	2nd choice	Coarse	Fine	1st choice	2nd choice	Coarse	Fine
1	0.25	0.2	16	2	1.5
1.2	0.25	0.2	18	2.5	2 or 1.5
1.4	0.3	0.2	20	2.5	2 or 1.5
1.6	0.35	0.2	22	2.5	2 or 1.5
1.8	0.35	0.2	24	3	2
2	0.4	0.25	27	3	2
2.5	0.45	0.35	30	3.5	2
3	0.5	0.35	33	3.5	2
3.5	0.6	0.35	36	4	3
4	0.7	0.5	39	4	3
5	0.8	0.5	42	4.5	3
6	1	0.75	45	4.5	3
7	1	0.75	48	5	3
8	1.25	1 or 0.75	52	5	4
10	1.5	1.25 or 1	56	5.5	4
12	1.75	1.5 or 1.25	60	5.5	4
14	2	1.5	64	6	4

Comparison for Thread Process Connections:

COMPARISON SHEET (M – BSPP – BSPT – NPT – UNC - UNF)
UNC (2A)
Metric: Standard/Fine	Inch: BSPP (G)	BSPT (R)	NPT	UNC/UNF
Type	Parallel	Parallel	Tapered	Tapered	Parallel
Flank angle	60°	55°	55°	60°	60°
Thread angle	0°	0°	1°47’	1°47’	0°
Seal location	O-ring/ Gasket/Cone	O-ring/ Gasket/Cone	On threads	On threads	O-ring/ Gasket/Cone

COMPARISON SHEET (M – BSPP – BSPT – NPT – UNC - UNF)
Major Diameter (mm):
BSPP (G)	BSPT (R/Rp)	NPT	UNC	UNF
1/16”	7,723	7,723	7,938
1/8”	9,728	9,728	10,287
1/4”	13,157	13,157	13,716	6,322	6,325
3/8”	16,662	16,662	17,145	9,491	9,497
1/2”	20,955	20,955	21,336	12,661	12,667
3/4”	26,441	26,441	26,670	19,004	19,012
1”	33,249	33,249	33,401	25,349	25,354
2”	59,614	59,614	60,325	50,726	-
Minor Diameter (mm):
BSPP (G)	BSPT (R/Rp)	UNC	UNF
1/16”	6,561	6,490
1/8”	8,566	8,495
1/4”	11,445	11,341	4,978	5,360
3/8”	14,950	14,846	7,798	8,382
1/2”	18,631	18,489	10,592	11,328
3/4”	24,117	23,975	16,307	17,323
1”	30,291	30,111	21,971	23,114
2”	56,656	56,476	44,679	-
TPI (Treads per inch):
BSPP (G)	BSPT (R/Rp)	NPT	UNC	UNF
1/16”	28	28	27
1/8”	28	28	27
1/4”	19	19	18	20	28
3/8”	19	19	18	16	24
1/2”	14	14	14	13	20
3/4”	14	14	14	10	16
1”	11	11	11,5	8	12
2”	11	11	11,5	4,5	-

How to Choose Thread Process Connections

There are no criteria to judge which Thread Process Connections mentioned above is better. Actually, each of them appears at a proper time with a proper reason. When it comes to choose a process connection, what really matters is the pipe fitting your pressure transducer is to be used.  It is true that different country has its own standards and it is also a fact that any thread standard is available worldwide.

If you need help in finding what your need. Contact Us! We are always ready to give your support in choosing pressure sensor Thread Process Connections.

Thread	Permissible maximum pressure
	Cu-alloy		Stainless steel		Monel®
	bar	psi	bar	psi	bar	psi
BSP ⅛ (G⅛)	400	6,000	400	6000	400	6,000
BSP ¼ (G¼)	600	8,600	1,000	15,000	1,000	15,000
BSP ⅜(G⅜)	600	8,600	1,000	15,000	1,000	15,000
BSP ½ (G½)	1,000	15,000	2,500	36,000	2,500	36,000
M10 x 1	400	6,000	400	6,000	400	6,000
M12 x 1.5	400	6,000	400	6,000	400	6,000
M20 x 1.5	1,000	15,000	2,500	36,000	2,500	36,000
⅛ NPT, R ⅛	400	6,000	400	6,000	400	6,000
¼ NPT, R ¼	600	8,600	1,000	15,000	1,000	15,000
⅜ NPT, R ⅜	600	8,600	1,000	15,000	1,000	15,000
½ NPT, R ½	1,000	15,000	1,600	23,000	1,600	23,000
7/16-20 UNF	400	6,000	800	12,000	800	12,000
The specified values for the maximum pressure are rounded values and are assigned to the nearest standard scale range.

Thread Glossary
ASME	The American Society of Mechanical Engineers (ASME)
BSPP	British Standard Pipe Parallel per ISO 228/1
BSPT	British Standard Pipe Tapered per EN 10226-1. See ISO 7/1.
DIN	Deutsche Institut für Normung e.V.
ISO 228/1	International Standards Organization Specification 228/1, straight threads, reference specification: BSPP, DIN 259, JIS B0202.
JIS	Japanese Industrial Standard
Metric	ISO Metric thread is a globally standardized thread
NPT	National Pipe Tapered.
SAE	Society of Automotive Engineers
UN	Unified Constant-Pitch Thread Series
UNC/UNRC	Unified Coarse Thread Series
UNEF/UNREF	Unified Extra-Fine Thread Series
UNF/UNRF	Unified Fine Thread Series
UNR	Male Screw Thread only
UNS/UNRS	Selected special combinations of diameter, pitch, and length of engagement
Pitch	For the purposes of this guide, pitch refers to threads per inch, instead of the distance between the threads, for fractional screw threads and pipe threads. For all metric screw threads, pitch refers to the distance between adjacent threads.