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What about MAP?

What about MAP?

What about MAP?

Here's sort of a philosophical question for you all ("uh-oh"). What is the point of calculating MAP? Do you calculate this value in most cases? If so, why? Is it more useful for vessel owners? fabricators? etc?

In my elementary COMPRESS training classes I've asked attendees the mostly rhetorical question of "why bother calculating the MAP?" I'd like to get an answer but I don't really expect to get one.
I had a long discussion recently with a very experienced vessel engineer about calculation of MAP. ASME Section VIII Division 1 does not define "MAP" in Appendix 3. Some people refer to "new & cold" condition and that is pretty much the definition that COMPRESS goes by: Find the maximum pressure that the component (or pressure chamber) can sustain in the new (uncorroded) condition, at ambient temperature (so the highest allowable stresses apply), with no operating liquid applied (because we're looking for the "maximum" allowable pressure).
Actually, what COMPRESS applies as the MAP is pretty much what ASME defines in Appendix 3 as the basis for the "calculated test pressure" for the UG-99(c) test.
For most vessel components it is trivial to determine the MAP: just use the Code formula relating pressure and thickness that is applicable for the component. In most cases of shell components only a single formula is necessary. For nozzles you must investigate all of the applicable nozzle requirements for reinforcing area, weld path strength, etc, so this is more complicated but still straightforward.
The big question is what about discontinuity stresses such as at cone-cylinder junctions? These must be considered because there is the possibility they can limit the pressure capacity of the transition rather than circumferential stress being the governing factor. Consequently, the orientation of the vessel during calculation of the MAP must be considered; this is because the stress at the junction is a function of the external load applied to the junction in direction parallel to the vessel axis. For example, if the vessel is in a horizontal orientation then no "weight" load acts on the junction, resulting in one value for pressure rating of the junction. But if the vessel is in a vertical orientation then a definite "weight" load acts on the junction, which results in a different value for pressure rating of the junction.
COMPRESS takes this into account when determining the MAP. If the vessel orientation during test is horizontal, then the MAP is also based on a horizontal orientation. But if the vessel orientation during test is vertical (such as by selecting option for vertical shop test, or a field test for vertical vessel), then the MAP is also based on a vertical orientation.
This is what the engineer ran into yesterday: the MAWP was reported as 90.77 psi and the MAP as 277.11 psi. He couldn't understand this in light of the fact that the tabulated allowable tensile stresses were the same at the two temperatures. In his case, the MAWP was governed by the stresses due to seismic code acting on the cone-cylinder junction. This accounted for about 95% of the difference between the MAWP and MAP values. Once the wind and seismic loads were removed the values were MAWP = 283.85 psi versus MAP = 277.11 psi. In this case the test orientation was horizontal and so the weight load was not available to counteract the force due to internal pressure, resulting in less pressure capacity. When the test was changed to vertical the MAWP and MAP were identical.
My suggestion, unless you have a good need to calculate the MAP, don't select the "Calculate MAP" option. This will save some trees. If the vessel owner has specified a UG-99(c) test for "calculated test pressure", then you will need the MAP.

  Tom Barsh
  Codeware Technical Support

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