Here is something that confuses many people. There is a difference between the MAEP of the cylindrical shell and the MAEP of any vacuum stiffeners that may be present. This seems so self-evident when you understand the calculations, but years ago I had a hard time grasping the concept. Based on our calls and letters over the years I know that many people run into the same conceptual problem. Or sometimes it is their customer, maybe a vessel owner/operator, that doesn't understand this.

For example, the vessel owner may want to re-rate a vessel for higher vacuum and wants to use some existing insulation rings as vacuum rings. What may happen is that the cylindrical shell will check out okay, meaning that based on the new (shorter) value of unsupported length L it has an MAEP sufficient for the desired pressure, but the "vacuum rings" (formerly insulation support rings) are woefully inadequate...meaning that their MAEP is less than the design external pressure and far less than that of the shell. How can this be? How can the shell be adequate with a high MAEP but the rings are no good?

ASME Code provides different rules for determining MAEP of the cylindrical shell and the vacuum rings. The MAEP of the cylindrical shell is based on the length between the "lines of support", and the shell's diameter and thickness. This calculation assumes that the ends...those "lines of support"...are capable of supporting the force imposed on them. Further, the vacuum rings must meet a minimum required moment of inertia that is based on the required thickness (for the given external pressure) of the cylinder and the length of shell that is supported by the stiffener. Thus the required moment of inertia of the vacuum ring is related to the force imposed on the ring from external pressure. The rings act as the "foundation" to support the cylindrical shell. It is entirely possible to have a cylindrical shell with sufficient MAEP rating to be supported by a weak "foundation" that would limit the MAEP of the vessel. In this case the vacuum rings would have inadequate moment of inertia to support the shell at the given pressure.

An interesting thought experiment: consider a cylindrical shell with heads, and a very large stiffener ring at mid-span. Find the MAEP of the cylinder. Now consider the effect on the stiffener ring as it is further and further decreased in size. Eventually a point will be reached at which the ring cannot support the load from the external pressure and will either collapse or buckle.

 

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