DISHED HEADS

Does anybody know what the allowance for thinning is when carbon steel plates are cold spun into heads.
I have a feeling it is 10% but am not sure.
Please help as I need to order them soon.

 

[ume153]

Hi

The best method is to get a chart from your manufacturer based on the thickness. As a thumb rule 15% is sufficient.

 

[abeltio]

The allowance depends on the thickness and the radius of curvature... I've seen 25% allowance for thick (>1.5&quot plates.
It also depends whether it is a one piece or a fabricated head.
HTH
Saludos.
a.

 

[rdf]

UKBASE,
The allowance for thin-out will depend on the material of construction for the spun heads. In general the more expensive the material, the more time and effort the head spinner can afford to put into the proccess and Q.A. of the heads.
Case in point: I have some on order at Brand X, where I needed (4) 0.310" Min. Hastelloy C-276 Heads at approx. 10-ft diameter, this required mill run material. The mill order for this thickness was in the range of 0.45".

Had these same heads been SA-516 Gr. 70, the nominal required might have been 0.625" (Stock not mill order). And the head manufacturer would spend considerably less time and care in spinning. The "extra" material costs less than the manufacturer's time it would take to control thin-out.

If there was 3/4" plate in stock in the required width, I might receive that instead of the head-manufacturer welding together the 5/8" thick.

As to Nominal Thickness required, it is best to specify your minimum (including corrosion and grinding allowances, if required) to the head fabricator/manufacturer and let them tell you what they can offer, and adjust your support/seismic design accordingly.

 

[Merzi]

If the head is designed as per ASME Sec VIII , UCS79 mentions the allowable thinning as 10%

 

[juancito]

I fully agree with RDF. Just specify XYZ [IN]as "mín. wall thickness after forming" and let the head fabricator to take care and select the proper initial thicknes to comply with what you specified as minimun.
Regards

 

[Shmulik]

Please note that you have to consider UG-79 (Forming Shell Sections and Heads), as well as UG-85 (Heat Treatment) and UCS-56 (Requirements for Postweld Heat Treatment) Due to extreme fiber elongation you will probably have. See definitions on UCS-79 (d) – Forming Shell Sections and Heads.