COMPRESS performs an analysis for stresses in the shell when external loads are specified on nozzles. The analysis is performed using WRC-107. Loads must be specified at the nozzle-shell junction following the conventions of WRC-107.

COMPRESS assumes that external loads are transferred through the nozzle neck to the shell and performs calculations for the bending stresses in the nozzle neck. These calculations are reported below the WRC-107 table.

Loads applied to the nozzle must also transfer through the flange. COMPRESS can optionally apply the loads to ASME/ANSI flanges if the user selects the check box on the nozzle flange dialog. COMPRESS determines equivalent pressures for the "P" load (WRC-107 nomenclature) and for the resultant moment due to "M1" plus "M2" or "MC" plus "ML" (note that the positive sense of P is such that it will counteract internal pressure). For design purposes, the flange MAWP rating is reduced by the liquid static head at the nozzle elevation plus the equivalent pressure due to external loads.

The equivalent pressure due to radial load (axial to flange) and bending moment is found using the method described in "Pressure Vessel Design Manual", Second Edition, by Dennis R. Moss. Appendix G, page 273.

COMPRESS provides the option to apply external loads to the flange design. This permits the designer to decide whether or not to apply external loads to the flange design. Historically, this analysis has rarely been done for Section VIII Division 1 vessels unless the vessel has been in a critical service. However, it is relatively simple to add these calculations to computer software so this feature has been added to COMPRESS.

Some designers or vessel operators may question the validity of applying external loads to the flange design, claiming that vessels have successfully operated for years. It is fundamentally clear that any structural loads are passed through a connection and the nozzle flange is a structural connection. Determining the true loads applied to the shell through the flange is very difficult and the specified loads may not reflect the true loads.

Vessel owners may ask, "Why haven't the flanges failed in service?". Some responses to this question are:

• The true loads may be much less than the calculated loads.
• ASME/ANSI flanges may have structural strength beyond that given by their pressure-temperature rating.

At best, overstressed flanges may be prone to requiring frequent maintenance. At worst, an overstressed flange may fail during service.

An example is shown below that illustrates the calculations involved: