Head M.A.F.

[ElCidCampeador]

Code: ASME VIII Div.I
Case: Elliptical Head 2:1, Mat. ASTM 516 Gr.70
From calculation, considering only corrosion (not plate tolerances), M.A.F. thk results 25,41mm.
Is it enough to ask to supplier of this head (which could be hot/cold formed+PFHT) to respect 25,5mm as minimum thickness? Or is it better to add something (like +0,3mm), in the unlucky case of plate is under tolerance?

I know that heads' suppliers start from higher thicknesses in order to obtain the M.A.F. you ask, but I don't understand if I have to consider the undertolerance of the thickness of the plate in the calculation or it's a problem only of the suppliers.
Thank you

 

[david339933]

If you request Min. after forming....that's what they will satisfy.

UG-16(c) allows an undertolerance of .01" for plate....same as SA-516-70 allows (SA-20).

Standard Designation: BPV Section VIII Div 1
Edition/Addenda:
Para./Fig./Table No:
Subject Description: Section VIII - Division 1, UG-16(c) and UG-32(a), Tolerance on Plate Thickness
Date Issued: 08/08/1980
Record Number: BC-79-322
Interpretation Number : VIII-80-70
Question(s) and Reply(ies):
Question: Assume that in accordance with footnote 12 of UG-32, Section VIII - Division 1, a plate which is to be formed into a head is ordered with a forming allowance considered appropriate for the application but without an additional allowance for mill under tolerances on the plate. After forming, the head is found to have a minimum thickness less than required by UG-32. May the undertolerance permitted by UG-16(c) be applied to the minimum thickness calculations, and if the actual minimum thickness is within this limit, may the head be used at the design pressure corresponding to the calculated minimum thickness?
Reply: Yes.

 

[JStephen]

How I've usually seen this done- inquire from the head supplier what the forming allowance is- order a nominal thickness that includes that forming allowance and also specify a minimum thickness after forming.
So, if minimum thickness is 1.00" and forming allowance is 0.125", order 1.125" nominal thickness with minimum thickness to be no less than 1.00".
The concern is that if you just order a 1.00" thick head, the order might be taken as a 1.00" nominal head with 0.875" minimum thickness.