vacuum rating and stiffener rings

[plantguyjj]

Can someone please explain why the results for a stiffener ring give a different external pressure than the MAEP allowed for the cylindrical shell that it is attached to? I am modelling different scenarious for a standard cylindrical shell with stiffener rings. For some courses of the shell, the calculated MAEP of the shell is equal to the external pressure displayed in the ring calcs. For other courses, the MAEP of the shell is equal to the external pressure used in the ring calcs. Why are the ring calculations using a different external pressure than the MAEP of the shell?

thanks

 

[plantguyjj]

Sorry, my first post should read:


For some courses of the shell, the calculated MAEP of the shell is equal to the external pressure displayed in the ring calcs.

For other courses, the MAEP of the shell is NOT equal to the external pressure used in the ring calcs. Why are the ring calculations using a different external pressure than the MAEP of the shell?

thanks

 

[cycguy]

I believe it is telling you the Maximum Allowable External Pressure (MAEP)for each shell course. The courses were the MAEP is equal to the ring is probably because the shell is just suitable for the external pressure probably because of the ring spacing. If you make the rings closer together the MAEP will go up.

 

[plantguyjj]

What value for "t" in Do/t is being used for the ring calculation? For situations where the available moment of interia is much larger than the required area, the value of t is equal to the actual shell thickness.

For smaller rings when I required is close the I available, the program is using some value for "t" that does not equal the shell thickness, the ring thickness, or the combined thickness. So where is it getting the value for "t", and why does it change depending on the ring dimensions, and why does it sometimes but not always use the shell thickness?

 

[TomBarsh]

In "Design" calculation mode, COMPRESS does not rate the MAEP of the vacuum rings above the specified design external pressure, the rating sub-routine stops at the design external pressure. The thickness used when determining the required moment of inertia of the combined ring and shell cross-section for external pressure is the required thickness of the shell for this pressure (the reported ring MAEP, which should be the design external pressure); this thickness can be found in the report for the cylindrical shell to which the ring is attached.

In "Rating" calculation mode, COMPRESS will rate the vacuum ring to its full MAEP even if this is greater than the specified design external pressure. The thickness used when determining the required moment of inertia of the combined ring and shell cross-section is the required thickness of the shell for the ring's MAEP. This thickness will be equal to or less than the actual corroded shell thickness. If the ring's MAEP is limited by the available shell thickness then the thickness used in the required moment of inertia calculation will be the actual shell thickness, otherwise it will be a lesser thickness.


Tom Barsh
Codeware Technical Support

 

[plantguyjj]

Tom,

Thanks for the reply. If I understand this correctly, the thickness used for moment of intertia in the ring calculation is the required shell thickness at the MAEP of ring?

If that is the case, this thickness would not be known until after the MAEP for the ring is determined, so what thickness is assumed to begin this calculation?

Also, can you please explain in the reporting why a ring may have a MAEP rating that is less than the shell course that it is attached to? Wouldn't the shell default to the lower MAEP rating of the ring? The summary report also does not give me a "deficiency error" whenever the ring MAEP rating is below full vacuum but the shell rating that the ring is attached to is okay for a full vacuum and the summary report is limiting the MAEP of the entire chamber to the lower MAEP of the ring? Since the entire chamber MAEP rating is below the design point, why doesn't it flag this as a deficiency?

Thanks for your help, I'm a new user (obviously) and just want to make sure that I understand the results and reporting on the vacuum calcs. The manual just doesn't get into this level of detail.

 

[TomBarsh]

The ASME Code requirements for the shell and stiffener ring are loosely related through the required shell thickness, "t". However, there is no strict relationship when investigating the MAEP of each component.

The requirement for the shell (UG-28) based on external pressure is for a minimum thickness based on the parameters L, Do, and t (length between lines of support, outside diameter, and nominal thickness of the shell). (And, of course, the material and design temperature.)

The requirement for the stiffener ring (UG-29) is for a minimum moment of inertia based on the shell thickness required for the given pressure.

The MAEP of the shell can be found using the Code rules of UG-28. Typically when doing these calculations by hand a nominal thickness is assumed, then the maximum pressure rating is determined by rules of this paragraph, thus obtaining the shell MAEP. Given enough time and paper, one could "back calculate" the required thickness to just meet the design pressure, thus coming up with the required thickness. COMPRESS does this automatically and reports both the thickness required for the design pressure and the MAEP based on the nominal thickness.

As discussed previously, in "Design" calculation mode COMPRESS calculates the required stiffener ring moment of inertia based on the required thickness of shell for the design external pressure.

In "Rating" calculation mode COMPRESS determines the MAEP of the combined ring-shell section. This can be limited by either of the following:

a) The available shell thickness may limit the ring MAEP. In this case, the thickness required for the ring MAEP is equal to the full shell thickness; this is the thickness applied in the calculation for required moment of inertia.

b) The shell thickness does not limit the ring MAEP. In this case, the required thickness applied in the calculation for required moment of inertia is the thickness required for the ring MAEP.




Tom Barsh
Codeware Technical Support

 

[TomBarsh]

Note, for questions on a specific design I suggest you send your COMPRESS file to our support e-mail address for a response.

Tom Barsh
Codeware Technical Support

 

[unclesyd]

May not be relevant and has not been mentioned is that decreasing the distance between rings has a very definite minimum where any further decrease doesn't help. I don't know how this is handled now, but this was a problem in the early releases of several of the PV softwares.