ISO 228 and ISO 7 understanding pressure joint meaning 2

[shussain786]

Hi,

ISO 228 is where pressure tight joints are not made on the thread whereas ISO 7 does make for pressure tight joints on the thread.

But what exactly is a pressure tight joint? I want to know when to use which of the two?

Or does it just mean the ISO 7 thread relieves pressure at the threads, if so where would the pressure be relieved for a ISO 228 thread fitting?

Thanks,
Sadik

 

[Tmoose]

What project are you looking at the standards for?

ISO 228 and ISO 7 are for pipe threads.
If you don't want the stuff inside to leak out = pressure tight. Think the water or gas plumbing in your house/apartment or on your car's engine.

If you are making a handrail for your front steps it is convenient to make it from pipe fittings and flanges as "structural" joints. Leakage is not much of a consideration there.

 

[MintJulep]

https://www.iso.org/standard/33777.html

"... Appropriate seal..."

e.g o-ring, gasket, crush washer

 

[shussain786]

Thanks Tmoose/MinJulep,

It is an adaptor for nitrogen gas application at max 40 bar in 1inch pipe.

So my understanding is that if I don't want leaks, then ISO 7 or ISO 228 with an appropriate seal.

To further understand which standard I could use when;
1. Is the female ISO 228 the same as a female ISO 7? If not, does a male ISO 228 physically screw inside a female ISO 7? If it does, is this safe?
2. I understand male ISO 7 can be screwed inside female ISO 7 or ISO 228 and is safe. Have I understood this correctly?

Curiosity, if male tapered ISO 7 screws into a female parallel ISO 7/228, its thread engagement will be fewer than when both mating are tapered or parallel. With this reduced thread engagement, does it make it weaker and prone to thread stripping? Is this ok for pressure applications, i.e. does longitudinal stress have an effect on the threads?

Thanks again

 

[Tmoose]

it looks like commercial NPT (tapered!) fittings can be rated over 1000 psi for nitrogen.

http://www.mcmaster.com

https://www.oasisngv.com/vdb/document/1179

Forget the straight pipe threads for sealing.

ISO 228-1:2000 - "Pipe threads where pressure-tight joints are not made on the threads" —

"Both internal and external threads are parallel threads,intended for the mechanical assembly of the component parts of fittings, cocks and valves, accessories, etc. These threads are not suitable as jointing threads where a pressure-tight joint is made on the thread. If assemblies
with such threads must be made pressure-tight, this should be effected by compressing two tightening surfaces
outside the threads, and by interposing an appropriate seal.
NOTE 1 For pipe threads where pressure-tight joints are made on the threads, see ISO 7-1."

" 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, needs 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 leaktight joint."

 

[XL83NL]

Shussain. You need to actually buy both standards and review them. This is not a free consulting firm here under the name of ent-tips.
That being said, the main difference is that when there’s mention of a pressure tight joint, it actually means the pressure tight sealing is achieved by the threads themselves (usually by means of a sealant like PTFE tape, on the threads).
We use ISO 228 for pressure joints as wel, but then through for example ISO 1179, which uses a chamber for a sealing ring that’s compressed by the threads.

Hope this helps.

Huub
- You never get what you expect, you only get what you inspect.

 

[Tmoose]

ISO 7 tapered threads should not be combined with features like gaskets or o-rings.

Synchronizing the tapered sealing thread stopping point AND correct compression of the seal requires mighty precise machining.
Not desirable for easy manufacturing and assembly, and rarely necessary.

Reference HSK tool holding (hollow, to provide slight flexibility) with machining tolerances with many zeroes to the right of the decimal point.
Reference "Big Plus" dual contact tool holders.