API 650 nozzle reinforcement

[dslyecix]

Hello,

I have a customer who is insisting that the area provided by a repad be sufficient through all cross sections - the minimum cross section of course being the horizontal one.

I have tried to explain that per API 650, the area to be replaced is measured vertically. The definition in 5.7.2.1 states The cross-sectional area of the reinforcement shall be measured vertically, coincident with the diameter of the opening. This seems clear to me, that essentially we are only concerned with the vertical replacement of the material. None-the-less, they insist on reading "The minimum cross-sectional area of the required reinforcement" as "the minimum cross-section through any orientation".

Now, our manway repad details DO meet these requirements anyways, thanks to an arbitrarily thickened nozzle neck (for material ordering/plate layout reasons), but by principle I want to explain or prove to myself that it is not necessary that we do so.

I have had no luck convincing them that the only cross-section we are concerned with is the vertical plane. No explanations about hoop stress have worked to eliminate the concern.

What are people's thoughts here?

 

[IFRs]

Why is any cross section different? Isn't the hole in the tank shell round? Using D sub zero in the table for a round repad you should be good for any cross section. Are you including in your calculations the excess shell plate thickness at the centerline elevation of the opening?

 

[dslyecix]

Sorry, yes a detail I forgot to mention is that these are low-type reinforcing pads, so they are tombstone/rathole shaped. Thus the distance from the opening CL to the tank bottom is greater than that off the side (or above).

We are taking into account excess shell thickness, the neck material (within 4*t of the shell thickness). For the vertical cross-section this area isn't required, but we are barely squeaking by the horizontal check while including this extra area.

It's always been my understanding due to the way the definitions are laid out that the vertical cross section is the only one of concern. Obviously there needs to be some requirement horizontally, but API never says "maintain this required reinforcement area through the minimum cross section of the repad" or anything that is completely clear.

 

[dslyecix]

Sorry one last detail I thought I included originally. We are employing the Fig 5.7 note 9, regarding the use of the OD dimension from the table as the ID dimension. This stipulates that reinforcement area per 5.7.2 and 5.7.3 must be maintained, and is what this client is insisting means we must reevaluate the reinforcing area.

My thinking was that again, no matter what the API 650 standard repads are sized to handle this ID-sized opening, and then of course any extra thickness in the nozzle neck that translates to lost shell/repad width, is replaced 1:1 with the nozzle neck material anyways (assuming materials of course).

 

[IFRs]

I still don't understand your heartache. An API round repad would have equal reinforcement in all directions. You happen to have more than that in the vertical plane, but so what? It's what you need vs what you have. Any section through a round hole in the tank shell would have the same area removed and any section through a round repad would have enough area to cover you. The extra from the lower half pad is not even available for you to use since only 1/2 D above and below is effective.

The hydrostatic hoop stress tries to pull apart the tank in vertical planes. That is why reinforcement is needed in the vertical axis. There is little to no force pulling the tank apart top to bottom from hydrostatic pressure, hence there is no reason to check the reinforcement in the horizontal plane.

Maybe I'm missing something: What is the shell thickness, hole diameter in the shell, repad thickness, repad diameter and hole diameter in the repad? Are the shell and the repad the same material? Is this an atmospheric tank or is there pressure?

Neck thickness is not a zero sum game with the shell since neck thickness does not affect the neck OD or the hole in the shell diameter. Also, are you taking 4 times the neck thickness inside and outside as additional reinforcement supplied by the neck, as well as the neck thickness times the shell thickness? Also are you taking these areas twice as you are allowed because they occur twice in any plane?