Why does PD 5500 have a maximum Nozzle Thickness?

[Crompto29]

I am designing a manway for a Vessel. The vessel shell thickness is 10mm.

I would like to weld a 40mm thick 600mm diameter manway into the vessel. It will be 300mm deep. So essentially it is a set through nozzle.

The manway door is inside the chamber and self seals to the end of 40mm thick 'nozzle' using the vessels internal pressure. See attached pdf.

My question is: Why does PD 5500 limit the thickness of a nozzle to twice the shell thickness?

Does this only apply to Nozzles with connections? Does it exclude manways that have no load?

I am tempted to design the 300mm long set through 'nozzle' as a set through pad but it looks nothing like a pad.

Any advise/references would be appreciated?

 

[vesselfab]

Really?

They don't allow self reinforced forgings?

Enquiring minds want to know

 

[Crompto29]

PD 5500 allows studded pads of any size.

The only reason for limiting nozzle thickness that I can think of is the nozzle will be too rigid relative to the shell thickness and therefore could increase stress concentrations at the nozzle-Shell junction when loads on the nozzle are applied.

However manways don't have loads on them.

 

[IFRs]

Could it be due to material toughness at low temperatures which decreases as thickness increases?