unions or threaded couplins for galvanized steel?

[Educv87]

Hi, I am a mechanical engineer working for an EPC company located in venezuela. Right now there is a discussion because another company we are working with, did not include threaded couplings in the piping line class for galvanized steel < 2". According to piping designers, unions should be used between two pipes on places where you need to disconnect the spool to do maintenance or so on, and threaded cuplings should be used on any other place to connect two threaded pipes. I would like to know if this is true and I can´t place unions along the entire line between pipe segments and I have to request threaded couplings to be included into the line class.

I would highly appreciate if you could help me with this matter.
Sorry for my English, as Spanish is my native language.

 

[Duwe6]

Your English is excellent

I would put a union at every change of direction, and one about every three joints of straight pipe; 2 couplings, 1 union, 2 couplings, etc.

I have a strong dislike for couplings with straight threads. The pipe is much stronger and seals a little better if the couplings have tapered threads, just like the tees, ells, and unions have.

 

[Educv87]

Great! Thank you very much, it sounds extremely logic.

I already requested threaded couplings with tapered threads, based on your recommendation.

 

[miya2014]

In my facility, we try to avoid threaded couplings at all times, its always a source of a leak. so it depends on the regulations and maintenance practices. here any leaks to atmosphere, even to containement is unacceptable, we have ambitious goals about leaks, we document every drop of every drop from every flange and find ways to avoid it.
when it comes to maintenance, I prefer flanges and isolation ball valves. but like I said before, depends on the regulations ad maintenance practices in the area.

 

[europipe]

Miya,
For air and nitrogen most pipespecs are threaded <2".
I know companies without unions in pipespec, they use thr. flanges.

 

[moltenmetal]

If the unions you use are the ones with the brass seating rings to act as a gasket between the metal seating surfaces, then by all means use unions.

If you do not use the brass seating rings, expect the unions to LEAK. If they are not available with brass seating rings (because you have specified 3000# class fittings where 150# class fittings will be adequate, or because pressure is beyond 150# class), then I suggest you use threaded flanges rather than threaded unions.

Threaded full couplings are NOT a useful disassembly or joint repair point, any more than an elbow or tee or any other threaded joint is a disassembly point. You cannot tighten a leaking joint unless you have a point of free rotation. The ones you can use are flanges, unions, Victaulic couplings, or perhaps the assembly flanges of a 3-piece ball valve. As noted in previous posts, having one of those on each change of direction or approximately every X joints is a good idea. What X will be depends on your piping design- X can be larger or smaller depending on whether your line lengths are short (i.e. fitting to fitting) or long.