ASME Div. 2 saddles calculation, interpretation of Ri radius for formed heads 1

[m.bagattini]

Dear all,
calculation of σ5 in formula 4.15.18 (ASME Div. 2 2019) uses Ri for elliptical heads. Ri is defined as "inside radius of the spherical dome or a torispherical head", with no mention of elliptical heads: should I use Di/2 or what? And in the case of torispherical heads, is it Di/2 or the crown radius? I've searched the ASME interpretations with no success.

Thanks in advance!

EDIT: for sake of completeness, it seems that the ASME method was taken from CODAP; in that code, two separate formulae are available for torispherical and elliptical heads. In the case of torispherical heads, it uses the crown radius while for elliptical heads the inside diameter is used. ASME normalized the formula for elliptical heads, assuming that Ri=Di/2 which seems untrue.

 

[Geoff13]

Division 1 UG-32(c) states:

An acceptable approximation of a 2:1 ellipsoidal head is one with a knuckle radius of 0.17D and a spherical radius of 0.90D.​

You can draw both shapes to decide for yourself how good an approximation this is.

 

[m.bagattini]

@r6155 are you available, my Master?

Thank you @Geoff13 for the feedback, much appreciated

 

[Trestala]

@r6155 seriously, you need some training with a more humane way of communicating.

 

[r6155]

@ Trestala
There are many people who do not consider the great responsibility of working with pressure vessels. They think that a computer is enough to do the job. Is this human?

Regards

 

[Trestala]

@r6155, it is better to refrain from responding to each post if you're not going to provide any helpful response. You probably have other better things to do.

 

[saplanti]

In case of elliptical heads there iss no truee crown radius. If the ellips is based on internal diameter the height of head from the tangent line is Di/2 for 2:1 elliptical head.

If you use crown crawn and knuckle radius for manufacture the head is not going to be elliptical. If you have to use them for manufacture you are dealing with torispherical head and you need to manufacture it for the given conditions,

 

[saplanti]

Should be ....design and manufacture for given conditions.

 

[david339933]

Inside depth of SE head = ID/4 - SF.

 

[saplanti]

Thank you for correcting me. You are absolutely correct.

 

[Greenbee]

Spherical head is one of special elliptical types (head ratio=1.0), and the torispherical head can be approximated to elliptical head. Per my understanding, the formula 4.15.18 can be generally used to them per the definition of Ri, H2, th.

 

[Rob VE]

Dear all,

I also have found this error in the code and have transmitted this info to a pressure vessel software developing company.
formula in ASME VIII div2 code is incorrect, it uses Ri defined as "inside radius of the spherical dome or a torispherical head", for an ellipsoidal head. We all know a ellipsoidal head does not have a spherical or knuckle part.
σ5 combines the stresses in the head due to internal pressure with the stresses of the saddle. (only needed if saddle is placed close to the head)
If you then (as correctly stated above: acceptable approximation of a 2:1 ellipsoidal head a spherical radius of 0.90D.) use 0.9xD in the formula, stresses in head are too high.
Stresses only by the pressure term are then 3 to 4 times higher than allowable stress. This while the head thickness is already confirmed by its internal pressure calculation.

CODAP uses the exact same formula to calculate these stresses (not sure but I think they also call them σ5) with the only exception that they use Di/2 instead of Ri. However in codap Di is inside shell diameter.

- How can you handle this issue as vessel designer? I have not found a codecase yet about it.
- Are you mandatory to use the acceptable approximation of 0.90D?

Regards,
Rob

 

[Rob VE]

Dear all,
I have just got confirmation from our ASME U-Stamp notified body that until there is an Errata about this the code must be followed.
This means you need to use Ri=0.9xD for an 2:1 ellipsoidal head. For all other ellipsoidal heads formula of ASME VIII Div2 paragraph 4.3.7 must be used to calculate spherical part radius (Ri=L).

Because of this a lot of commercial software availble does not calculate the saddles correctly according to ASME VIII Div2! All packages that i tested who include Div2 calculations are doing this wrong. they all take Ri=0.5xD. *one remark: there is one package that i have informed about this and has already changed their software to calculate according to code.

On this point the formula of the code is incorrect, but still the code must be followed.

Best regards,
Rob

 

[m.bagattini]

Thanks @Rob VE for the heads-up