2 replies to this topic

[Andree]

Does anybody know if small threadolets (G 1/4" BSPT female) can be welded on 1 1/2" pipe?
I read somewhere that threadolets are recommended for "big" pipes or vessels (however it wasn't explained what notion "big" means), hence my concern.
And I also wonder if this is good engineering practice to use threadolets fitted to small piping or other solution is recommended to install for example pressure transmitter on pipe.
Regards,
A.

[Art Montemayor]

Andree:

A “threadolet” is a quick and easy access to a flow stream. But it has its inherent liabilities.

By design, it is intended to be welded onto a flow pipe by means of using a fillet-lap weld. This is the weakest weld available in the boilermaker / pipefitter industry. As a welder, I always have always stayed away from this type of weak welding technique when dealing with process (or utility piping or vessels). I always insist on a “full-penetration” weld technique – which is a type of “butt-weld”. This type of weld ensures that the introduced joint is not a “weak link” in the chain – so to speak.

The technique of applying a threadolet is by simply laying it onto the main pipe and welding a fillet bead around its outer periphery and subsequently drilling a hole through the threadolet nozzle, attaching a pressure gauge or whatever to the threaded nozzle. This works for low pressure piping applications – for a while. What you have done, when you follow the above procedure, is that you have created a much weaker pipe wall at that junction and, as if that wasn’t enough, you have also introduced a stagnant, corrosive site directly under the threadolet’s “saddle”, where it rests on the outer pipe’s surface. This site is subject to static fluid and is a candidate for increased corrosion – at a point in the pipe where you have purposely weakened it! That, in my opinion is not too swift. I have always out-lawed the use of threadolets in all plants and operations that I have operated or managed.

If you are dealing with a 1-1/2” line, simply cut it with a band saw and butt-weld a reducer tee at that junction.

[Andree]

Art,

Thank you for your comments.

I fully agree with your suggestions about advantages of butt welding for high pressure application.
However, I will operate at not very high pressure (max. 7 bar) and just wondered if this is possible to weld 1/4" threadolet (which has about 1" at the "base") on 1 1/2" pipe (about 1 7/8" OD). I think that it should be some special shape threadolets suitable to given size of a "run pipe".
I didn't find standard reducer tee 1 1/2" to 1/4" to fit my pressure sensor (max. reduction 1 1/2" to 1/2"). Hence, I started thinking about threadolets - it is quick, easy and for low pressure application should provide reasonable streangth... The increased corrosion problem is however inavoidable in this case...

Regards,
A.