I am working on vacuum rating of an old pressure vessel (23 feet diameter) which uses 15 stiffening rings throughout its length. As per the drawings, one of the stiffening rings passes through five different nozzles one of which is a 24" ID Manway. Section UG- 29 and Figure UG-29.2 provides the maximum arc of shell left unsupported because of gap in stiffening ring.

However, I am still unclear about the following points.

1) Number of gaps permitted in one stiffening ring.
2) The distance between two gaps.

Also, UG-29-(b) states that "Stiffening rings shall extend completely around the circumference of the cylinder except as permitted in (c) below. Any joints between the ends or sections of such rings, such as shown in Figure UG-29.1(A) and (B), and any connection between adjacent portions of a stiffening ring lying inside or outside the shell as shown in Figure UG-29.1(C) shall be made so that the required moment of inertia of the combined ring-shell section is maintained."

I am using PV Elite to evaluate the gaps on the stiffening rings. PVElite calculates the required moment of inertia in the ring plus shell and compares it with the available moment of inertia. However, I could not figure out if PVElite considers the gaps due to the openings while calculating the available moment of inertia

Hi Krishna

Have you considered welding the rings to the nozzles. To the best of my knowledge, this would not be a violation of the code.

Hello Ray,

This is an existing vessel built in 1950s, and I am vacuum rating it for my client. As per my client drawings, the stiffening rings intersect with some of the nozzles. I do not know if they are welded to the nozzles.

So, if the rings are welded to the nozzles, it's not considered a gap? If that is true, it would make my life a lot easier.

Hell Krishna

Just think about it. The nozzles are very stiff compared to the rings. In effect, you would have no gaps, as the nozzles would be treated as part of the rings.

Thanks Ray.

What if the stiffener ends at the OD of the reinforcing pad and thus is not welded to the nozzle ? Is that still considered stiff enough because of the pad?

Let's say there is a gap of 10". How do we input this information in the PVElite?

Regards,
Krishna Pokharel

Hello Krishna

You don't specify the gaps in PV Elite. You have to go to that graph to determine the gap size to put on the drawing of the vessel.