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jprashant31
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Engineering Fields - Mechanical Engineering - New Member

Join Date: Oct 2007
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Elongation of 2:1 Dish End

12/08/2008 8:24 AM

Hi,

I am designing pressure vessel as per ASME Section VIII Div 1.

UCS-79 gives the imperical formula to calculate the elongation.

Can anybody explain me how to calculate Ro & Rf which are required to calculate elongation?

I have to calculate elongation of 2:1 Dish end having Crown Radius as 2250. Dish nominal thickness is 16mm & Minimum thickness is 12mm. How to calculate Ro & Rf.

Thanks

Prashant
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Guest.
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#1

Re: Elongation of 2:1 Dish End

12/08/2008 12:28 PM

According to ASME Interpretation VIII-1-83-81: Rf applies to the knuckle radius in the case of double curvature.

A 2:1 ellipsoidal head may be approximated with a head containing a knuckle radius of 0.17D (D = inside diameter of head skirt).

Therefore:

Rf = final centerline radius, in. (mm) = 0.17D

Ro = original centerline radius (equals infinity for flat plate), in. (mm) = ∞
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Abdel Halim Galala
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Engineering Fields - Piping Design Engineering - New Member Egypt - Member - Member since 02/18/2007

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#2
In reply to #1

Re: Elongation of 2:1 Dish End

12/09/2008 1:34 PM

As per ASME Code UG-32, the ellipsoidal heads 2:1 have two radii Rf1 and Rf2, where Rf1 = Knuckle Radius = 0.17 D & Rf2 = Spherical Radius = 0.90 D.

And the % extreme fiber elongation for head = 75 ( t / Rf )(1 - Rf / Ro) must be calculated for both radii Rf1 and Rf2, where Ro = Original Centerline Radius and Ro = infinity (where the original radius of plate used in fabrication of ellipsoidal head was ∞, hence the value of Rf / Ro = Rf / ∞ = 0).

So the final equation shall be: % extreme fiber elongation = 75 ( t / Rf )(1-0)

So the calculation based on t = 16 mm and D = 2250 mm:

Rf1 = Knuckle Radius = 0.17 D = 382.5 mm

Rf2 = Spherical Radius = 0.90 D = 2025 mm

% extreme fiber elongation (knuckle radius) = 75 ( t / Rf1)(1 - Rf1 / Ro) = 3.13725

% extreme fiber elongation (spherical radius) = 75 ( t / Rf2 )(1 - Rf2 / Ro) = 0.59259

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Guest.
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#3
In reply to #2

Re: Elongation of 2:1 Dish End

12/09/2008 2:32 PM

As always Abdel - I admire your ability and patience to fully explain/articulate your answer.
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Guest.
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#4
In reply to #3

Re: Elongation of 2:1 Dish End

12/09/2008 3:29 PM

............ just as a side note - the controlling factor will be the knuckle radius to determine the fibre elongation / heat treating requirement (if you don't have a problem in the knuckle you won't have a problem with the head)
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Abdel Halim Galala
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Engineering Fields - Piping Design Engineering - New Member Egypt - Member - Member since 02/18/2007

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#6
In reply to #4

Re: Elongation of 2:1 Dish End

12/14/2008 1:49 AM

I agree with you, but the precise calculations must including both calculations for both radii (knuckle and spherical) to see -at least- the limits of both. In certain cases, may be you have a problem with knuckle radius, in this case you are not sure that there is no problem related with the spherical radius.

That is looks like ASME calculations for both stresses in circumferential and longitudinal directions in a pressure vessel as per (UG-27)(c)(1) & (2). Even we know that the hoop stress (circumf. stress at longit. joints) is the governing stress due to internal pressure only, but in certain cases and due to additional of external loads, the sum of stresses in longit. direction (at circumf. joints) must be taken into consideration to be within the allowable limits.

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jprashant31
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#5
In reply to #2

Re: Elongation of 2:1 Dish End

12/13/2008 11:52 PM

Thank you all for your reply.
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