Hot Tapping Hydrotest Buckling Calculation 3

[Jinunim]

I am working on an external pressure calculation for a hot tap during hydrotest. To derive Factor B&A I need L/Do and Do/t according to equation is from ASME Sec. VIII, Div. 1, UG-28.
In case of vessel, the "L" value is specified as length between stiffening ring but what about for pipe? I assume that the "L" value would be branch OD but generally for pipe (specially hot tapping hydrotest), how do you apply "L" value?

 

[SnTMan]

Jinunim, hard to say w/o any geometry, but (see Fig UG-28, 28.1) L is taken between lines of support. Your pipe run likely has none. Not to worry, as L/Do tops out at 50, and changes little above about 25, and even lower, depending on Do/t. See Sec II, Part D, Subpart 3, Fig G.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[DSB123]

If you are trying to check the pipe for hydrotesting the branch before cutting out the blank then you are approaching it the wrong way!!!

 

[LittleInch]

That's what I was thinking as well. Pressure testing a hot tap connection before you actually do the cutting of the main pipe is very difficult and the maximum differential pressure from inside to outside is quite low. It really needs an FEA analysis as it is not a pressure vessel and trying to use a PV calc looks like the wrong idea to me.

Also depends a lot on what your hot tap tee looks like and where it is welded.

Many people don't pressurize greater than the internal pressure to avoid all these issues as the max DP often comes out only 5-10 bar more (10% more than internal operating pressure).

Try typing "hot tap pressure test" into the search box at the top - a few good answers.

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

 

[mk3223]

For the hot tap on the pipe with a split tee or saddle, the "L" is the axial distance between two welds of the split tee or saddle on the header, and "Do" is the OD of the header. And then, use L/Do and Do/t to find the values A and B for calculating the allowable external pressure.

 

[Jinunim]

#mk3223
Thank you for your answer.
The problem is that normal saddle type split tee has been rejected by Company because of small annular gap between the tee and the run pipe which may lead to corrosion problem.
I will apply Reinforced Set-On Branch type for hot tapping.
So, welding line will not be linear, but Circle. The size is 42"x 36" Flare.

 

[mk3223]

The reinforced set-on branch with the circular base, the "L" is the "maximum" axial distance between the welds on the header.
Hope it helps.

 

[DSB123]

You cannot take "L" (the length of the pipe under external pressure as the distance between the welds for a reinforced set on branch to check for external pressure. That's stupid!!! The pressure is not applied around the pipe.