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Member

Join Date: Jan 2012
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01/10/2012 1:30 AM

Hai

I was checking a design calculation of flat head pressure vessel.The shell thickness is 12 mm.Made of normal carbon steel.Design internal pressure is very less(20 PSI).the inside dia is 3500mm.when i did calculation using ASME Sec 8 Div 1 UG-34.I was getting a minimum thickness of around 35 mm.Does it make sennse?Please help me to identify where i am making the mistake if any? why, a pressure vessel with bigger inside dia but with lesser internal pressure need a high thickness unstayed flat head?

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#1

01/10/2012 2:09 AM

What is the formula in ASME Sec 8 Div 1 UG-34?

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#2

01/10/2012 4:21 AM

t min= (d √(CP/ SE)

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#3

01/10/2012 5:53 AM

I forget what C means in that formula. What value are you using for E? If you illlustrate with the numbers you are using, we can see if 35mm makes sense for the head thickness. (Off the top of my head, I think it does.)

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#7

01/10/2012 11:39 PM

Thanks guys for all reply.....But still....

The contactor need a flat head design.the vessel is not a high pressure vessel.As i told earlier the design pressure is 20 PSI.My concern is for a vessel with less internal pressure but with a bigger internal dia (like 3.5m) why we require a high thickness flat head.when we use a high thickness flat head will there be any buckling effect on the shell(since it is of only 12 mm thick)

Valuses i have been using is C factor=0.2

inside dia of unstayed flat head=146.61 inch(3724mm)

Joint efficiency E =0.85

Maxi allowable stress=20000 PSI

thats how i got a value of 48 mm thick

Looks very surprising that 48mm thick plate to cover a low pressure pressure vessel

with a shell thickness of 12mm

Guru

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#9

01/11/2012 3:12 AM

Antony, using joint Efficiency E=1 (Increasing NDE´s like RT or UT),

Significantly reduce the thickness required for

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#4

01/10/2012 8:04 AM

Hai

It is not unusual for an ASME pressure vessel flat head thickness to be 3 to 5 times the calcculated thickness of the shell.

Flat head closures only make economic sense for small diameter vessels.

You should not be surprised at your result of 35 mm.

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Member

Join Date: Jan 2012
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#6

01/10/2012 11:39 PM

Thanks guys for all reply.....But still....

The contactor need a flat head design.the vessel is not a high pressure vessel.As i told earlier the design pressure is 20 PSI.My concern is for a vessel with less internal pressure but with a bigger internal dia (like 3.5m) why we require a high thickness flat head.when we use a high thickness flat head will there be any buckling effect on the shell(since it is of only 12 mm thick)

Valuses i have been using is C factor=0.2

inside dia of unstayed flat head=146.61 inch(3724mm)

Joint efficiency E =0.85

Maxi allowable stress=20000 PSI

thats how i got a value of 48 mm thick

Looks very surprising that 48mm thick plate to cover a low pressure pressure vessel

with a shell thickness of 12mm

Guru

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Location: Placerville, CA (38° 45N, 120° 47'W)
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#8

01/11/2012 12:51 AM

Have you considered that 146.61" Diameter (73.3" radius) has (∏R^2)=16,900 square inches of area, which means 338,000 pounds, or 169 tons of force at only 20 psi?

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#10

01/11/2012 3:31 AM

thats what i am also asking?

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#12

01/11/2012 11:42 PM

A flat head without the stiffeners will be very thick and uneconomical. Use Ms Flat stiffeners which reduces the span. The exact design details can be found in a book by Timoshenko, which I have used during my haydays.

Hope it helps

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#13

01/12/2012 10:21 AM

looks like a good idea.can you expain further...i didnt get completly

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#14

01/12/2012 11:24 AM

I can try to explain more. You have to weld MS Flats of a adequate size (50 X 6 should be fine) at a spacing of 700 mm. This means you will have 4 nos. Longitudinal Stffeners. Pls ensure that the 50 mm dimension is kept vertical to the plane of the Flat Plate. In between the adjacent rows of these stiffeners, weld another set of stiffeners perpendicular to the previously welded rows. You can keep the same distance. It will form a Grid like structure with the middle grids of size 700 mm x 700 mm. The required plate thickness with this kind of arrangement will be much less.

Also, Pls check and confirm whether a vessel with 20 psi Internal pressure falls under ASME SECT VIII. Your vessel looks like a storage vessel and .

We used to design the stoarge vessels as per API codes.

I have tried to explain and I hope it is conveyed in the true sense.

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#15

01/12/2012 11:54 PM

thanks a lot....But i cant corelate your suggetion to to code...if possible please tell where this option is mentioned in the code.i read through Sec 8.Vessels with internal pressure greater than 15 psi can comes under sec 8.

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#16

01/13/2012 1:06 AM

Unfortunately the ASME Code did not mention anything on this type of designs. I am not aware of any amendments that may have been added later on. Although, the use of Stiffeners for External Pressure for Shell design is very much covered in the codes.

Basically, the design formulae given in the codes is derived from the STRENGTH OF MATERIAL CONCEPTS, which are based on the assumption of thin shells. The code mainly defines the allowable stress values of the material being used, Welding parameters, Joint Efficiencies, Stress correction factor due to thick shells and so on.

Also, the custom design can be submitted to ASME divisions for their approval.

Hope it explains.

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#5

01/10/2012 7:31 PM

Flat heads aren't good for much. Beer bottles even fall off.

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#11

01/11/2012 10:24 AM

Flat heads are the weakest type of head covered under Section VIII. You should get a head thickness greater than the shell thickness. I think that's what you were asking for right?

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#17

01/21/2012 2:43 AM

Your calculation is OK, see the attached example:

As per ASME VIII-1, UG-34 c(2), The minimum required thickness of flat unstayed circular heads, covers and blind flanges shall be calculated by the following formula:

Example:

 Given: . . . . . . . . . . . . . . . . . . . . d Vessel Inside Diameter . . 137.7953 inch 3500 mm . c Constant From Fig.UG34(f) . 0.33 . . . . P Internal Design Pressure . 20 psi . . . S Allowable Tensile Strength of Material 20000 psi . . . E Joint Efficiency from Table UW-12 . 0.85 . . . . . . . . . . . . . Find: Min. Required Thickness of Flat Head, t . . . . . . . . . . . . . . . . . . . . . . . . . Solution: . t = d * SQRT( CP / SE ) . 2.715075 inch 68.96291 mm

Fig. UG-34 Some Acceptable Types of Unstayed Flat Heads and Covers

For the same conditions, except the vessel diameter d = 350 mm instead of 3500 mm:

 Given: . . . . . . . . . . . . . . . . . . . . d Vessel Inside Diameter . . 13.77953 inch 350 mm . c Constant From Fig.UG34(f) . 0.33 . . . . P Internal Design Pressure . 20 psi . . . S Allowable Tensile Strength of Material 20000 psi . . . E Joint Efficiency from Table UW-12 . 0.85 . . . . . . . . . . . . . Find: Min. Required Thickness of Flat Head, t . . . . . . . . . . . . . . . . . . . . . . . . . Solution: . t = d * SQRT( CP / SE ) . 0.271508 inch 6.896291 mm

Conclusion: Flat head thickness is about 7 mm for vessel diameter 350 mm, where it was about 70 mm in case of vessel diameter 3500 mm.

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#18

11/17/2013 10:37 PM

Because flat head with larger diameter will experiences greater normal stress due to bending moment. For the body of your pressure vessel, hoop stress is dominant, but for flat end, bending stress is dominant. Remember, larger the diameter, greater the bending stress. Therefore requires higher thickness.

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#19

02/10/2015 5:54 AM

Dear All,

Means Design of flat head under external pressure. (In ASME CODE Sec 8 Div 1 UG 34 is only for Internal Pressure Only?)

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