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What are the two standards for pipe threads?

Introduction

As we know, there are two sets of standards for pipe threads under the ISO international standard, they are ISO7 and ISO228, which represent the standard of sealed thread and the standard of non-sealed thread, respectively.

Usually, when making 150lbs pipe fittings, we generally choose sealing threads to achieve quick sealing of the two. Because in this thread state, there is no need for other substances such as O-ring or PTFE pad filling, and the sealing can be achieved only by wrapping the raw material tape.

In the pipe connection process, the sealing thread is highly convenient for disassembly and reinstallation. If you don’t need sealing, you can choose ISO228 straight thread. Of course, it can also achieve a sealing effect when used with an O-ring.

Pipe Threads where pressure-tight joints are made on the threads

The expression of the sealing thread is as follows, the internal thread is a parallel thread, which is represented by “Rp”, the internal thread is a tapered thread, which is characterised by “Rc”; the external thread is meant by “R”.

Examples: The complete designation for a right-hand thread size 1 1/4″.

Internal thread: (parallel) Pipe thread ISO 7-Rp 1 1/4″ (taper) Pipe thread ISO 7-Rc 1 1/4″ ;

External thread: Pipe thread ISO 7-R 1 1/4″]

It is worth noting that the external threads under the ISO7 standard are all tapered threads, which means that there are two thread combinations under ISO7: the first collocation is internal parallel thread & external taper thread, and it is expressed according to the above example: 1 1/4″(Rp, R); The second collocation is internal taper thread & external taper thread, and it is also expressed according to the above example That is: 1 1/4″(Rc, R).

R = BSPT = PT = …

  • The letter R comes from the German Rohr, which means “tube”. R thread is a series, and the common ones are Rp, Rc, R1 and R2. Details are as follows:
  • Rp – British Standard Pipe Parallel Female Thread
  • Rc – British Standard Pipe Taper Female Thread
  • R1 – British Standard Pipe Taper external thread, used in conjunction with Rp, that is, Rp/R1 “parallel/taper” cooperation
  • R2 – British Standard Pipe Taper external thread, used in conjunction with Rc, that is, Rc/R2 “taper/taper” cooperation

Parallel thread

The basic form of the parallel pipe thread shall be as shown in figure 1. The angle between the flanks, measured in axial plane section, is 55°. The thread profiles are rounded equally at crests and roots by circular arcs blending tangentially with the flanks.

Taper thread

The basic form of the taper pipe thread shall be as shown in figure 2. The taper is 1 to 16, measured on the diameter. The angle between the flanks, measured in an axial plane section, is 55°, the flanks making equal angles with the axis. The thread profiles are rounded off equally at crests and roots by circular arcs blending tangentially with the flanks in such a manner as to give the same thread height “h” as for parallel threads.

Combination with fastening thread

The combination of an external parallel thread G, tolerance class A or B in accordance with ISO 228-1, with an internal parallel thread Rp in accordance with ISO 7-1 needs special consideration.

When it is necessary to have this combination, the positive or negative tolerance of the internal thread to ISO 7-1 shall be considered in the relevant product standards, where external parallel threads G are used.

Such a combination of threads may not necessarily achieve a leak-tight joint.

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

The representation of the non-sealed thread is like this, the internal thread and the external thread are both parallel threads, represented by “G”. 【Examples: The complete designation for a right-hand thread size 1 1/4″. Internal thread: Pipe thread ISO 228-G 1 1/4″; External thread: (tolerance class A)Pipe thread ISO 228-G 1 1 /4″A (tolerance class B)Pipe thread ISO 228-G 1 1/4″B】

The letter G is the abbreviation of Gas because it was initially used on gas pipelines. G thread is divided into internal and external threads used for non-sealed connections. The executive standard is BS EN ISO 228-1, and the old standard is BS 2779, which is now invalid.

Also, about the meaning of several standard versions of the standard:

  • ISO-xxx = International Standard
  • EN-ISO-xxx = European version of the International Standard
  • BS-EN-ISO-xxx = British version of the European version of the International Standard

Combination with jointing thread

Combining an external parallel thread G, tolerance class A or B, in accordance with ISO 228-1, with an internal parallel thread Rp, in accordance with ISO 7-1, need special consideration.

When this combination is necessary, the tolerance of the internal thread in accordance with ISO 7-1 shall be considered in the relevant product standards, where external parallel threads G are used.

NOTE Such a combination of threads does not necessarily achieve a leak-tight.

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