PV ELITE - allowable stress lower for shell than heads.. How and why?

[Minamtungekar]

I am designing a pressure vessel of 1400 diameter and in the output results, internal calculations tab, the allowable stress for the shell is less than that for the ellipsoidal heads. How is that possible? Please help

 

[MrPDes]

Have you set the design temperature of the head to be different to the shell?
Have you set the material of the head to be different to the shell?
Have you set the design Pressure of the head to be different to the shell?

 

[pressvessel]

Assuming your design is to ASME VIII Div.1 and your ellipsoidal head has a K factor >1.0, it could possibly be due to End Note 90 which is referenced in App. 1-4.

 

[Minamtungekar]

I got the answer. I had given a joint efficiency of 0.85 for the shell welding. That's the reason it showed a lower allowable stress for the shell than the head. For which I had a joint efficiency of 1

 

[jte]

Minamtungekar-

Thanks for closing the loop on your thread! That's a good practice that E-T could use more of!

Now... it took you two and a half months for figure that out?

 

[aida2011]

Dear Sir

I am not an expert in boiler design but could it be different way during design? You select your allowable stress first, then determine the material and the thickness required.

I think allowable stress is dependent on the temperature. If you chose different design temperature for the shell and the head, the allowable stress will be different.

For example, when selecting the material and thickness of the material of superheater, you cannot base on design pressure because superheater can have different temperatures. In this case, allowable stress is based on creep strength, you select the material which exhibits a strain of 0.01% in 1,000 hrs.

Aida

 

[Minamtungekar]

@jte... like you said .. it's a good practise to close the loop which I did. I guess I should have done it earlier.
@aidaa2011 .. dear sir, what I believe is that the material is first selected by the process department depending on the service. We have to check the mechanical adequacy of the material. But they too have an idea of the strengths of different materials. It's done by a materials engineer I think.
Please correct me if I am wrong in all this