local stiffeners for external pressure 3

[PMCap]

I have a Section VIII, Div. 1 pressure vessel with a small area of shell wall that is thinner than the surrounding area. Thin area thickness is acceptable for internal pressure, including sufficient margin for corrosion. Thin area is thinner than thickness required for external pressure. Does anyone have any insight or experience regarding acceptability of locally stiffening the thin area only as opposed to adding complete stiffeners around the vessel so that the thin area thickness meets Code tmin to satisfy external pressure requirements.

The Repair/Alteration needs to be 100% ASME Code and NBIC compliant.

Thank you in advance.

Paul

 

[jte]

Paul-

You didn't explicitly state it, but by stating that the repair/alteration must be NBIC compliant you are implying that you are dealing with an inservice vessel. Off the top of my head, I see a few potential options for you:

* Evaluate the local thin area (LTA) per Code Case 2286. This is not a simple code case, but there are folks who can handle it and chances are that the analysis will show that your situation is ok as is. This would be a fitness for service approach and the AI would not necesserely be involved, though his technical resources may be useful.

* Not sure whether API-579 directly deals with this, but there may be something there for you.

* If you are already thinking about welding rings (partial or whole) to the vessel, then consider clearing the vessel for internal hot work and weld build-up the required thickness. Depending on the corrosion/erosion mechanism consider topping the weld buildup (by definition this would be the same basic material as the shell) with a weld overlay of stainless or other alloy which would resist future damage.

* Cutting out the LTA and butt welding in a flush patch may be possible, but sounds like overkill. Depending on your actual LTA dimensions this may be an option.

* Consider evaluating the LTA as a gap in a stiffening ring such as shown in VIII-1 Fig. UG-29.1(D).

jt

 

[PMCap]

jt

Thanks for the response.

I've performed the Fitness for Service calculations and can obtain some, but not all, the required additional service life, especially when considereing requisite corrosion allowance. However, I would like to perform a vessel restoration by alteration. Accordingly, I was wondering if anyone had run across AI or ASME or NBIC acceptance of using local stiffeners to restore the external pressure capability of a local thin area. I'm confident that the calculations can show the addition of local stiffeners will provide me with the code required safety factor against collapse failure (assuming calculations can be performed under the allowances for which U-2(g)applies and its use and applicability allowance for performing detailed evaluations where the code does not have specific rules.

These local thin area conditions occure frequently and often external pressure is the only limitation on acceptabiliy when performin Fitness for Service evaluations. The addition of full stiffening rings can often be expensive to install when compare to local stiffening.
I guess the real questions is"

"Does anyone have knowledge or experience regarding application of local stiffeners to remedy shell locally degraded below tmin and additionally, has this been done with the acceptance of the AI. Does anyone have knowledge of either an ASME Code Case or Interpretation (for new construction) or an NBIC Interpretation for an in-service vessel restoration using this method?

Thanks again

Paul

 

[metengr]

The NBIC will not get into the level of detail regarding stiffeners associated with pressure vessel design, and in my opinion the NBIC does not prohibit their use. By the way, I would treat this as a repair not an alteration.

Under the guidance of the NBIC, you are restoring the integrity of the pressure boundary in accordance with rules of the original code of construction. This is not an alteration.

You need to run this by your AI. Show them your calcs, and explain your rationale in detail. In essence you are using a mechanical repair method versus complete weld restoration of the pressure boundary. The only issue with regards to the NBIC would be the welding of stiffeners to the pressure boundary (wall). If the AI says no, your only other alternative is to plead your case to the Jurisdiction or regulatory body.

 

[unclesyd]

We have done this many times on SS distillation vessels designed for FV and 15-60 psig. These vessel are 12 ft in dia. We get a varying and non uniform internal corrosion from the process below the first complete tray.

The vessels originally had no stiffening rings or initial rings at 48" centers. We have added additional fully encircling stiffening rings as the corrosion progressed in the lower section and as we would loose a tray, in other words the corrosion progress further up the column.
The limiting factor on doing this is the diminishing return for additional rings. If I remember correctly we fell out at 14". On one column we actually put one ring at 24" and then later due to some scheduling problems we stuck two more at 12" and 36" just to gain a few months operation. This procedure was accepted by all concerned as a viable option.