Piping in Blinds

[Tankman650]

I see a lot of instances where 10" piping is inserted into 20" blinds. The blind has a hole cut out and the 10" pipe is fillet welded in to the blind.

I am wondering how this is allowed in the piping codes.

If I do this for a sight glass in a blind for a presure vessel, ASME will require that the area cut out must be addressed and may (and usually does) require a repad. (UG-39)

Does the piping code not address this or other modifications to standard fittings???

 

[MJCronin]

tank,

I have also occasionally seen this practice and wondered....

According to my understanding of ASME B31.1/31.3, this practice violates both the intent and letter of the piping codes.

When piping flanges manufactured to ANSI/ASME B16.5 are modified in any way before use, the flanges then become unique and require additional detailed calculation based on the service intended. ASME VIII contains methodologies for evaluation of unique flange designs. This type of calculation is almost never done.....

This type of fabrication using smaller piping welded into a blind flange of larger piping usually is accomplished by fillet welds..... the use of fillet welding in piping joints larger than 3"NPS (I think) is also not permitted by the piping codes.

In my opinion, an acceptable alternate would be the use of a "flat" profile weldolet on the blind flange and the smaller pipe welded to the weldolet. This configuration would at least have all components pressure rated for service.

Has anybody else out there seen a justification for this practice ?

How about a comment from John Breen ???

My opinion only....

MJC

 

[StoneCold]

Tankman650
I am curious to hear other users input on this. We use this practice frequently. I cant't see why it would be different than a socket welded joint, or a slip on flange. Especially if you make the hole large enough for the pipe to fit throught the flange and weld it on both sides. Granted we are always in the #150 flange class.
The flange is significantly thicker than the pipe wall or a vessel wall why would you need to repad? I admitt I have never tried to calculate the stress when doing this procedure.

 

[aberta]

I cannot confirm it at the moment but I believe that stubs are permitted but you must derate the pressure rating by 50%. The maximun size of stub permitted is 0.5* main pipe nominal diameter. Like i said I cannot confirm this but this is our guide-line. The weld must be full penetration for stub thickness.
Hope this does not confuse the issue??

 

[MJCronin]

Stonecold,

Your analogy to a slip-on flange is a good one, however according to the piping codes,(B31.1/B31.3) slip-on flanges are not suitable for all types of service.

You should also be aware of the process used to qualify slip-on flanges for acceptability in the piping codes. Flanges must conform to the dimensional material and inspection requirements of B16.5....can you say the same thing for the stub-in application ?

aberta,

This sounds like a reasonable rule of thumb....but do you have any hard references, papers or code cases ? Opinions are cheap....

My thoughts only.....


MJC