Threads

The Basics on Threads of stainless steel Fittings, Valves and Flanges:

There are various types of connection designs for pipes, fittings, valves and flanges. The most popular and common type is the thread end design, due to its simple installation properties. The following table provides an overview of the main thread standards used for threaded fittings, pipe fittings, valves and threaded flanges:

ANSI/ASME B1.20.1- Pipe Threads, General Purpose (Inch)

ANSI/ASME B1.20.1:2013- Pipe Threads, General Purpose (Inch)

DIN 2999 - Pipe threads for tubes and fittings; parallel internal thread and taper external thread

DIN 2999-1:1983 Pipe Threaded for Fittings and Tubes; Parallel

DIN 2999-2:1973 Whitworth Pipe Threads; Gauging Systems, use of Gauges

DIN 2999-3:1973 Whitworth Pipe Threads; Gauge Dimensions

DIN 2999-4:1986 Whitworth Pipe Threads; Conical Screw Limit Plug

DIN 2999-5:1973 Whitworth Pipe Threads; Cylindrical Screw Limit Ring

DIN 2999-6:1975 Whitworth Pipe Threads; Conical Screw Check Plugs

DIN 259 -

DIN 259-1: 1979  Whitworth Pipe Threads - Parallel Internal and Parallel External Threads - Basic Sizes

ISO 7 - Pipe threads where pressure-tight joints are made on the threads 

ISO 7-1:1994 Dimensions, tolerances and designation
ISO 7-2:2000 Verification by means of limit gauges

ISO 228 - Pipe threads where pressure-tight joints are not made on the threads 

ISO 228-1:2000 Dimensions, tolerances and designation

ISO 228-2:1987 Verification by means of limit gauges

 

EN 10226 - Pipe Thread where Pressure Tight Joints are made on the Threads

EN 10226-1:2004 Taper external threads and parallel internal threads — dimensions, tolerances and designation

EN 10226-2:2005 Taper, external threads and taper internal threads — Dimensions, tolerances and designation

EN 10226-3:2005 Verification by means of limit gauges

 What is the main difference between the American (NPT) and British/European Thread?

Threads can be grouped into two main categories. There are threads that have a 60° angle or a 55° angle. Threads according to American NPT standard have an angle of 60° while those of British/European standard have an angle of 55°. The two kinds of thread are incompatible due to the differences in their thread degrees. Because of the difference in thread angle there will also be a difference in thread per inch. Threads can be tested and classified with either a plug gauge (Female Threads) or a ring gauge (Male Threads) to determine of the thread has a 60 degree or a 55 degree angle.

What is the difference between Thread Standard DIN 2999, EN10226, ISO7, ISO228, and BS21? 

There are several different standards within the British standard, therefore causing a lot of industry wide confusion. Installers and engineers are easily confused on threads according to DIN 2999, EN10226, ISO7, ISO228, and BS21. The German standard DIN2999 was obsolete and is replaced by the ISO7 Standard. The standard DIN 259 is the same standard as the thread standard ISO228. The European standard EN10226 is based with editorial modifications on ISO7, giving the threads the same gauging system. The BS standard threads are the BS 2799 and BS 21. The BS 2799 has been withdrawn and replaced by ISO 228. BS 21:1985 is technically equal to ISO 7, however the gauges design for the tow standards are different. When using a ISO 7 gauge to test a BS21 thread, the gauge will fit, while the other way around, the fit might not be perfect. Due to the different gauge design, some factory will have different machining tolerances on the two standards, to fit both standards. This makes further argument on whether BS 21 is equal to ISO 7 or not. Threads can further differ in being either taper or parallel. Tapered threads have walls that would, if continued, eventually meet, while parallel threads have thread walls which are straight.

Threads SWISS FITTINGS Online Shpo

Withworth Pipe Thread DIN EN 10226, ISO 7-1 (old: DIN 2999, BS 21) 
ISO 7-1              
Internal thread      
  - Parallel Thread Rp
  - Taper Thread Rc 

External thread
  - Taper Thread R

The internal parallel thread Rp can be screwed together by hand with the taper external thread R. The pressure tight seal is made on the steel thread, however it is always recommended to also use additional sealing, such as silicon band, and Teflon band. A male thread taper R according to DIN EN 10226 (ISO7-1) can be installed with a parallel female G thread according to ISO 228. Rp female threads have similar nominal sizes to ISO 228 G female threads, however both thread standards have different tolerances. ISO 228 G male threads should not be connected with a parallel female thread according to DIN EN 10226 (ISO7-1) as the DIN EN 10226 (ISO7-1) female thread will be too tight. If possible, same standards of thread should always be used together. Examples of fittings with DIN EN 10226 (ISO7-1) threads are stainless steel unions, seamless sockets, tees, and barrel nipples in 1 meter length.

Withworth Pipe Thread DIN/ISO 228

ISO 228-1
Internal thread     
  - Parallel thread G

External thread    
  - Parallel thread G

Threads according to ISO 228 do not create a pressure tight seal on the thread. A sustainable seal is created through the force pressed on a sealing surface, such as when installed with an O-ring. Example for fittings with an ISO 228 thread are stainless steel running nipples, hexagon nuts, and plugs according to DIN 910

NPT, American ANSI/ASME Thread Standard
NPT Internal Thread Parallel (Female)
NPT External Thread Parallel (Male)

Both the female and male thread in the NPT thread standard are parallel and the pressure tight joint is created on the thread. To create a pressure tight connection, it is recommended to use silicon tape or Teflon tape. Example of fittings with an NPT thread are welding nipple, barrel nipples, elbow, and hexagon nipples.

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