Straight thread for Category D Fluid 1

[McDermott1711]

Hi engineers,
In para. d of 314.2.1 of ASME B31.3, it is written that "A coupling having straight thread may be used only for Category D Fluid Service, and only with taper-threaded mating components.". Why not straight mating thread? As I know straight threads are straight for both mating threads.
Any reply will be highly appreciated!

 

[brasol]

I am face same problem with here if You have any solution provide me.

 

[McDermott1711]

Hi Brasol,
Is that you handshaking with Barak?

 

[LittleInch]

The reason is that without the taper thread, the sealing surface is the end of the pipe, not the threads. The end of the pipe could be a small area, insufficient to bear on any gasket or seal. In addition all the sealing force is on the pipe end and gasket and a small movement of the coupling would start a leak.

Figure 335.3.3 provides for accepted use of straight threads, but note the stub on the piping which actually bears on the sealing surface.

A straight thread onto a straight threaded coupling could just screw right over it or leave one part with only a small portion of the coupling if the coupling has no inside lip.



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[McDermott1711]

Thanks littleinch for your reply.
I've no problem with straight-to-straight threads, but straight-to-tapered is my problem. Why not tapared-to-tapered or straight-to-straigh with gasket (for category D)?

 

[LittleInch]

Tapered to tapered isn't an issue and isn't noted in section 314.

Straight to straight (specifically for a coupling) is clearly seen as something by the committee as historically something having a higher failure rate or even fundamentally unsafe / unsecure. The difficulty of getting a gasket to seal on the end of a threaded pipe section is clearly open to error and misalignment of the gasket or coupling and too small a bearing area to properly seat a gasket.

Just because it is category D doesn't mean it's not dangerous if it leaks or the coupling fails. Cat D can be up to 10 bar / 150 psi and up to 186 C / 366F. The fluid might not be harmful to human tissues, but I wouldn't want to be hit by a jet of fluid at 180C coming at me with 10 bar behind it....

Personally I distrust all things whereby the thought process is "Oh it's only water / cat D / temporary " therefore I can "get away" with doing something that I wouldn't do if it was "Normal Fluid Service". But that's just me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[McDermott1711]

Got it!