Base Hoop Stress On (ID/OD/Mean/Lam?) indicates how the value of hoop stress should be calculated.
The default is to use the ID of the pipe.
Most piping codes consider the effects of pressure in the longitudinal component of the CODE stress.
Usually, the value of the hoop stress has no bearing on the CODE stress, so changing this directive does not affect the acceptability of the piping system.
If necessary, you may change the way CAESAR II computes the hoop stress value. Available options are:
ID - Compute hoop stress according to Pd/2t, where d is the internal diameter of the pipe.
OD - Compute hoop stress according to Pd/2t, where d is the outer diameter of the pipe.
Mean - Compute hoop stress according to Pd/2t, where d is the average or mean diameter of the pipe.
Lam? - Compute maximum hoop stress according to Lam?'s solution: s = P(Ro2+Ri2)/(Ro2-Ri2).
In pressure design of piping, Y is the correction factor for temperature. So, it is not simple just considering the failure criterion based on elastic stress behavior on the cylindrical shell due to internal pressure.
The following is the another story regarding the establishment how Y is determined.
For thick wall cylindrical shell,
Shoop,average=P(D/2t − 1)
Sradial,average=-0.5P
The maximum shear stress failure criterion, Tresca's is as follows:
Saverage=Shoop,average-Sradial,average=P(D/2t − 0.5P)
In 1943, a factor of 0.4 was recommended in place of 0.5 in the stress formula [Boardman].
Resulting from burst test data available, ASME task group judged that the choice of Y being 0.4 while adequate for temperatures up to 900°F. However, it would result in excessively thick pipe for service above 900°F.
This would in turn lead to unnecessarily heavy and costly pipe, less flexibility to absorb thermal expansion and also larger through-wall thermal gradients. To avoid these difficulties, and based on burst test results, it was decided that between 900°F and 1150°F the 0.4 factor is gradually increased to 0.7.
This was the origin of the Y factor in today’s ASME B31.1 and ASME B31.3 pressure design equations.