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Join Date: Aug 2011
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What is the Aceptable Tolerance on 2:1 Semi Elipsoidal Dish Head

08/25/2011 12:19 PM

Hello Everyone,

I just want to ask those who have an idea what is the acceptable Tolerance on 2:1 Semi Ellipsoidal Dished Head for the Crown, Knuckle & Depth of the Dished Head before the Straight Face. Is there any Statement in the CODE for the tolerance of Crown & Knuckle acceptable by CODE ASME?

Because we receive Form Heads from our Vendor and the crown radius is greater than 5mm from the crown radius we specified, 4mm diff. on the knuckle radius and the depth of head is also changed.

we used crown & Knuckle formula as per UG-32 (e.i. R=0.9D for Crown and r=0.17D for knuckle)

for the Depth(height) of the dished head we consider H=D/4

My diameter (D) = 894mm

Thanks

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#1

Re: What is the Aceptable Tolerance on 2:1 Semi Elipsoidal Dish Head

08/25/2011 2:14 PM

yhinganetone, Please refer- UG-81-TOLERANCE FOR FORMED HEADS which mentions the following. (these are the exact words form the code) (a) The inner surface of a torispherical, toriconical, hemispherical, or ellipsoidal head shall not deviate outside of the specified shape by more than 11⁄4% of D nor inside the specified shape by more than 5⁄8% of D, where D is the nominal inside diameter of the vessel shell at point of attachment. Such deviations shall be measured perpendicular to the specified shape and shall not be abrupt. The knuckle radius shall not be less than that specified. Hope this helps!!!

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Guru
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#2

Re: What is the Aceptable Tolerance on 2:1 Semi Elipsoidal Dish Head

08/25/2011 5:01 PM

The shape with 0.9D crown radius and 0.17D knuckle radius is a torispherical head, rather than semi-ellipsoidal, which doesn't have a crown and knuckle. The difference is hard to see with an unpracticed eye. Make sure what the head actually is, and then check it via the appropriate standard.

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#3

Re: What is the Acceptable Tolerance on 2:1 Semi Ellipsoidal Dish Head

08/26/2011 1:45 PM

Formed Heads Tolerances, UG-81 (for torispherical, toriconical, hemispherical and ellipsoidal heads):

· Tolerance (deviation*) of inner diameter outside of the specified shape ≤ 1.25 % D

· Tolerance (dviation*) of inner diameter inside of the specified shape ≤ 5/8 % D

Where, D = Nominal inside diameter of the vessel shell at point of attachment.

* Such tolerance (deviation) shall be measured perpendicular to the specified shape and shall not be abrupt.

Very important to know the acceptance criteria and "How do you apply the tolerance for dished heads as per UG-81":

● For code items like dished heads (especially that intended to be used in vessels need to be certified U-stamp), it is mandatory by the code to prepare a material receiving report, and that report shall include dimensions checking/verification prepared by a specialist like as inspector or vessel engineer.

● And checking head dimensions (as detailed at the attached drawing) is very very important to satisfy yourself that all tolerances are within the limits of UG-81.

● The procedure depends on dividing the x-axis of the head into equally spaced intervals (may be 50 or 100 mm).

● The specialist who preparing the material receiving report must measure/calculate and/or draw/plot the 4 curves:

1. The standard ellipsoidal 2:1 curve using equation: y = SQRT [h2 - (h2/a2) x2],

where, SQRT = Square Root, √

a = D/2

h = inner depth of head at middle (in case of ellipsoidal head 2:1, h= D/4)

D = head inner diameter

x = axis x

y = axis y

2. The curve of allowed outer tolerance based on + 1.25 % D, based on standard curve,

3. The curve of allowed inner tolerance based on - 5/8 % D, based on standard curve, then

4. Plotting the actual curve of supplied head.

...................................

.....................................

● The actual locus of curve of supplied head must be located in between the two curves of allowed inner (black) and outer (red) tolerances, otherwise the head shall be rejected.

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Guru
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#4
In reply to #3

Re: What is the Acceptable Tolerance on 2:1 Semi Ellipsoidal Dish Head

08/27/2011 4:36 AM

GA.

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Active Contributor

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#5
In reply to #3

Re: What is the Acceptable Tolerance on 2:1 Semi Ellipsoidal Dish Head

08/27/2011 6:02 AM

Dear Sir Abdel Halim

Happy Eid.

Thank you very much for the elaboration of UG-81, this is how i suppose to ask with Mr. Mahesh on how i will exactly apply this tolerance.

I just have one request if possible, if you could post or send a clear copy of this sketch due to not clearly visible.

To everyone thanks for your support.

Thanks

Yhinganetone

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Guru
Engineering Fields - Piping Design Engineering - New Member Egypt - Member - Member since 02/18/2007

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#6
In reply to #5

Re: What is the Acceptable Tolerance on 2:1 Semi Ellipsoidal Dish Head

08/27/2011 8:03 AM

Dear yhinganetone,

Thank you very much.

The attached file is AutoCad drawing for the prescribed sketch: http://www.4shared.com/photo/uFiVZhRX/Ellipsoidal_Head_21_ID5640.html

With my best regards,,,

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Commentator

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#7
In reply to #6

Re: What is the Acceptable Tolerance on 2:1 Semi Ellipsoidal Dish Head

10/18/2012 2:50 AM

Dear Mr. Abdel,

Could you send an example calculation and drawing to my email, your cooperation are appreciated.

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#8

Re: What is the Aceptable Tolerance on 2:1 Semi Elipsoidal Dish Head

04/24/2015 8:45 AM

Try this to make 2:1 dish template very much close to accuracy instead of using formulas

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