Hello,

When dish ends are formed, some amount of thinning occurring.

1. What is the value of thinning allowance as per ASME Sec VIII Div. 1. Is it 10% of nominal thickness.
2. Is it the same for all types of dish ends.

Sharjeel,
You need to refer to the forming specification. Polish norm allows not more than 10%. The others- similar. Make sure that your order refers to something with a specific number or you write something down in the PO. And that somebody checks this before releasing the head to production (plus the shape, of course).

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Konrad Anikiel

Thinning allowance?

No, there is no such allowance in the Code.
The Foreword says that the vessel manufacturer shall specify tolerances in such manner as to assure that the finished vessel has got the required thickness.

The only applicable paragraphs could be UG-32 (a) and Note 18, and Appendix 32.

Forget the 10%, please.
DK

Dirk, I'm not going to say any revelation here, but let's systematise this.
In everyday life heads are bought off the shelf. It's not about the design engineer's wish. It's about the manufacturer's standards. And in general they follow some national standard, which has some requirement. Like... 10% allowable thinning! It's not a rule of thumb, it's not a suggestion. It's just a maximum value you can anticipate before you buy and measue actual values. It's nothing bad when people want to make technical design before they start procurement
But, as I said: one has to make sure that some number is specified either in PO or in a referred spec.

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Konrad Anikiel

I think Dirk is correct forget 10 % As a designer i will not allow even 10 % also IF it is crossing my MINIMUM REQUIRED thickness limit.
Designer and manufacturer are Important FACTORS-
Designer has to also ensure the process of manufacturing - Point press or Spining method.
So taking 10% is little bit risk in some cases.

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Niraj

Niraj, how do you design things then? Do you assume (guess) the actual thickness after forming and put this number with -0 tolerance? Send such a drawing to any head formers- they will die from laughter!

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Konrad Anikiel

Great misunderstanding, people.
Probably for thinning tolerance you meant "thin out" which is quoted on computer programs. It is the reduction of thickness that you may expect during thinning. In my experience this reduction is in the crown (centre) and it is approx. 12%.
The thin out tolerance that Dirk meant is the tolerance below the nominal thickness which is better to consider as -0.
Or, if you wish, the "Mill undertolerance" is prescribed in UG-16(c) as -0,25 mm or 6%, whichever is the less.
Hope issue is closed.
Mauro

Probably by saying forget 10%, Dirk means that better contact your dish/ head manufacturer to obtain a correct thinning allowance as this will be based on the forming process followed by them.

Relying on just 10% thinning allowance as a rule of thumb may be disastrous some time. For the same nominal thickness thinning will vary based on diameter, head geometry and forming process.

For higher thickness and smaller diameter thinning may be more than 10% also. I have experienced maximum thinning in knuckle portion of the dish.

If heads are formed in petals, thinning may be very less also.

Regards