Min. thickness of head is given because that is what is required to be listed per Table W-3.

It is always a good idea to retain the original design drawings for a vessel which contains the design nominal thickness ...

MJCronin
Sr. Process Engineer

The answer to your first question is that ASME only requires the supplier to specify the minimum thickness of the heads on the Manufacturer's Data Report because that is the only information that's pertinent to the pressure-containing capability of the equipment. The head nominal thickness is really only useful for determining the weight of the vessel.


-Christine

@david339933

Is there a compelling reason why the code doesn't require the nominal thickness to be stated for the heads like they are for the shell?

@MJCronin
I completely agree. Every single plant I worked at though there is always at least a handful of equipment that was built in the 50s with little of the original documentation available.

@Christine74

Why is specifying the nominal shell thickness required then?

Shell cylinders are made from pipe or rolled plate. Pipe already comes with a 12.5% mill tolerance, and rolled plate typically does not experience significant thinning as a result of the forming process.

The process used for forming heads from plate is very different and can result in substantial thinning, perhaps as much as 100X the amount of thinning seen in the cylinder.


-Christine

@Christine74
Thank you!
That makes sense!