Fiitings fabrication (internal threads)

[Nukie]

My question is regarding ANSI B1.20.1, article 4 ''SPECIFICATIONS FOR INTERNAL STRAIGHT THREADS IN PIPE COUPLING' NPSC.''

- Does that means coupling internal threads are straight threads (NPSC)?

- Or that means it can be possible for some applications?

- If it is used on some applications I would appreciate to have some examples and the pros & cons about it.

- Does coupling stands for all fittings (el, tee,couplings etc...)?

- When internal NPT taper threads are used if so ?

Thanks

Marc

 

[unclesyd]

Taper pipe threads are self sealing where straight pipe threads are not. Straight pipe threads are much stronger than taper pipe threads. This has it's advantages in cases where added strength and fatigue resistance is needed. Unless the joint is carefully designed it will usually require a secondary seal.

We started using straight pipe threads to repair a connection between two components in our manufacturing process after the original all welded connection failed. We could use 1" Sch 160 or XX pipe to handle 2500 psi at 600°F. This application required the pipe to go through one piece and screw into the other. We would thread the pipe and feed it through on piece and screw it into the second piece. Then a single pass seal weld was applied.

You can have straight pipe threads on any fitting though some are not readily available as standards.

When I worked at a fitting company we made a lot of malleable iron straight pipe fittings of a major oil company. No idea of the end use.

Taper pipe threads are standards for nearly all use in domestic and industrial water, oil, and gas. This also means any other thing that is compatible with material of construct and pressure temperature ratings.

Some electrical conduit use to have straight pipe threads. I don't know if that's still true.

 

[RiBeneke]

Taper threads can also have different thread tooth shapes. One type (I think it is called 'dryseal') is more resistant to weeping leakage.

 

[KimBellingrath]

Dryseal has a more complete thread contact (and is identified by adding the letter 'F' as in: 1/4 NPTF). Even within the dryseal type there are two grades. Grade 2 requires gage-type inspection.