It is quite an odd situation that you require to reinforce the head to vacuum, but of course it may be possible.
The best method for reinforcing depends on actual conditions, such as how much you are far from the required thickness, if there are many nozzles on the head, and perhaps something more.
If there are no nozzles or a central one only, the simplest way may be to cover the spherical portion with a dish of sufficient thickness, assuming the knuckle is of sufficient thickness. Otherwise you will need to weld radial ribs, and possibly also circumferential ones, with spacings that depend on the required thickness and available thickness.
The most difficult part, in my opinion, will be how to calculate the repaired head per code, if you need to do so.
PREX,
Thanks for your quick response. Yes, you are right about the calculation for the rib reinforced head. I’m thinking what approach I should take to analysis it, without using FEA. Got any idea, any suggestion is welcome.
Thanks again.
just a further thought wrt to prex's reply. If money is no consideration, you could assume that the dished end is a flat plate, and design the stiffening around that assumption. This should be a conservative approch for the spherical (crown) section. One thing you should also consider might be buckling of the knuckle region, depending on how highly stressed it is. Also, are there any high inwardly acting radial loads on the nozzles, these may also have an effect on the "effective" vacuum that will collapse the head. I'd guess that the most accurate way to solve your problem would be to make use of FEA, that way you could model in all pressures, loads and the shape of the head and see what happens under the vacuum load. Doing an FEA model and including stiffeners and such will be pretty costly, though. If you are wanting to form another head and attach it to the outside of the existing one, after the forming, site work to confirm dimensions of the existing head, calculations, site work to attach the new head, etc, it may be cheaper to remove the old head and replace it with one of the correct thickness for your application.
We once used stay rods on the bottom heads of (2) 12'x50' columns to reinforce them for continual operation at 150mm. This also help with the flooded condition. We used beams, actually parallel plate plates with welded spacers, attached to the skirt to anchor the stay rods. If I remember the heads, ASME F&D, were calculated as a flat plate. I don’t think you can get by with plate on plate. I have never seen stiffening ribs on heads, only stiffening rings on columns.
Have you done Section VIII, Div. 1 calcs. for vacuum? I assume you have the design information on the head configuration and material. If so the first step is to do regular code calculations. If the head itself is not good for the vacuum conditions to my knowledge there is no Section VIII procedure that will reinforce it. If the head is good for vacuum then you can check for nozzle reinforcement. If the head itself is not good for vacuum or you cannot meet reinforcement requirements for nozzles using pads only then you can go to FEA to see if there is a scenario that will meet your operating conditions.
