Elliptical nozzle cut outs and repads API 650

[Legoman92]

Hi there,

We have a pipe between tanks (DN350) that is on quite an acute angle so the vertical hole in the tank is roughly 700mm long in the vertical. Would I treat this like a Dn700 pipe repad and specify a 1440 diameter repad? Or is there scope to adjust the width to a narrower repad and still qualify it as API 650?

 

[JStephen]

See 5.7.6.3. It's not clear to me if the standard actually prohibits this kind of construction or not, but at the least, it would have the repad sized based on the vertical dimension of the hole cut in the shell.
It would be preferable to put 45 degree or 90 degree ells in the line so it could enter the tanks in a horizontal plane.
Also see Note 7 in Table 5.17, which is not normally used for piping attachments, but the concern there would be something to consider.

 

[Legoman92]

Thanks for the response. In my 2018 edition note 7 in table 5.17 is regarding handrail...?

 

[IFRs]

You are correct - use the vertical dimension, in accordance with 5.7.2.1.

Per 5.7.2.1 The minimum cross-sectional area of the required reinforcement shall not be less than the product of the vertical diameter of the hole cut in the shell and the nominal plate thickness...

Per 5.7.6.3 Nozzles over NPS 3 and used for piping attachment need modified repads.

Per 5.7.6.3 ...nozzles not larger than NPS 3—for the insertion of thermometer wells, for sampling connections, or for other purposes not involving the attachment of extended piping—may be installed at an angle of 15 degrees or less off perpendicular in a vertical plane without modification of the nozzle reinforcing plate.

If you want to make an oval repad, use the vertical and horizontal dimensions of your opening and the repad sizes that correspond to those.

 

[JStephen]

It may be in Table 5.16 in that edition then. Anyway, the note:
"A tank runway that extends from one part of a tank to any part of an adjacent tank, to the ground, or to another structure shall be supported so that free relative movement of the structures joined by the runway is permitted. This may be accomplished by firm attachment of the runway to one tank and the use of a slip joint at the point of contact between the runway and the other tank." IE, don't put a rigid connection between two tanks where major movement in one messes up the other.

I said I didn't know if it was the intent of the code to prohibit the arrangement. What I had in mind was 5.7.1.2, "The shell opening designs described in this standard are required, except for alternative design allowed in 5.7.1.8." So 5.7.6.3 allows nozzles of any size, any angle when in the horizontal plane, provided the repad is increased, but only 3" and less 15 degrees and less in the vertical. The issue I see is that as the opening gets taller and taller relative to width, you no longer have a "round" opening as normally used, and potentially more issues with stress concentration, etc. It'd be a similar situation if you were to use, say, a square nozzle.

 

[Legoman92]

I guess my thinking is pretending it's like a DN700 pipe and using a 1440mm circular repad would be conservative. I take the point on API650 not outlawing this vertical pipe, but not specifically allowing it either.

 

[IFRs]

Legoman - using a 14400mm circular repad shold be conservative, as I think you could use a 1440mm high by 700mm wide oval pad and be in compliance with the standard. Additional reinforcement in the horizontal plane where there is little shell stress to begin with is extra metal and weld. In other words, I think you could make the repad contribute as much area as was removed by the hole in all directions.

JStephen - I read section 5.7.6.3 to say that only over 3" and over 15 degrees you have to change the repad to oval. I agree that stress on the shell needs to be kept to a minimum, using creative piping design and support. I would avoid Annex P type calculations if possible.

 

[r6155]

Please see API 650 EDITION MARCH 2020
5.7.6.3.......... In addition, nozzles NOT larger than NPS 3—for the insertion of thermometer wells, for sampling connections, or for other purposes not involving the attachment of extended piping—may be installed at an angle of 15 degrees or less off perpendicular in a vertical plane without modification of the nozzle reinforcing plate.

Regards

 

[IFRs]

I agree - read whe entire section, which only says that small nozzles don't need a moidfied repad under certain circumstances.