Nozzle Safety With Welding Ribs

Nozzle ribs are the wrong way to go, except maybe if there some plan to use the nozzle as a rigging attachment point. But even then, it looks fishy.

Instead, use a nozzle of greater wall thickness, and/or a reinforcment pad on the vessel shell. There is a Code calculation for this that involves five cross-sectional areas, but I don't have a copy at hand.

SHELL OD 812 mm - 6 thk , Nozzl dia is 600nb - 6 thk, also Pad plate of outer Dia 915 & 6 thk is given.

Nozzle fails at edge of pad. Maximum calculated stress ratio is > 1.0

Increasing nozzle thk is not affecting the value of maximum calculated stress ratio at edge of pad.

It is not possible to increase PAD od & Shell thk.

The ASME opening reinforcement calculations (detailed by Abdel) and WRC-107 calculations are two independent analyses of the same area. The ASME opening reinforcement calculation is not really stress analysis, but an approximate method for guaranteeing that pressure will not overstress the shell at an opening. The WRC-107 is an imperical stress analysis method for determining stresses in the shell due to externally applied piping loads.

One limitation of WRC-107 is that it does not deal with reinforcing pads explicitly, but most people (including myself) have ways of analyzing nozzle with reinforcing pads using WRC-107. The most common way is to consider two cases: (1) nozzle to pad junction - nozzle outside radius & shell thickness equal to the shell plus pad; and (2) pad to shell juction - pad outside radius & shell thickness by itself. This is most likely conservative, as in reality, there is some flexibility in the reinforcing pad, which distributes or mitigates the stresses in the shell at the pad od.

The upshot is that if you have stress issues at the pad od, you should evaluate these by another method - FEA is the most likely candiate. With this you can see if removing the conservatism in the WRC-107 pad od assumptions gets you to where you need or you can investigate larger pads or actual effect of ribs and gussets.

Other directions:

1. Increase the thickness of the shell. It may just be too thin.
2. Reduce the loads - piping loads from standard tables are often much higher than the loads which actually occur. They are useful tools for architect-engineers as they tell piping designers to keep loads under these values and they tell vessel designers to make it capable of withstanding these loads. This will take discussion with the AE and piping designers.