Tubesheet - Why are some of my results showing as NC
In this scenario, NC stands for Not Calculated. There are different reasons why NC may be showing up in different parts of your report. Look through the items below for a detailed explanation.
Shear Stress
The 2013 edition of the Section VIII, Division 1 incorporated a code case that allows for the shear stress calculation to be based on a step wise perimeter at the outer tubes instead of the more conservative assumption that used the perimeter based on a diameter at the outer tube limit (OTL). With this change in 2013, a basic screening criteria was put in place that set the original OTL based shear calculation to the allowable shear stress (80% of S). If this screening criteria is met, assuming the conservative smaller perimeter, then we already know it will pass with the more favorable step-wise based perimeter.
Channel Stress Calculations
Channel stress calculations only apply to tubesheet configurations that have an integral channel. Tubesheet configurations a, e, f, and A have integral channels. All others will show NC for these calculations.
Shell Stress Calculations
Shell stress calculations only apply to tubesheet configurations that have an integral shell. Tubesheet configurations a, b, and c have integral shells. All others will show NC for these calculations.
Tube Buckling Stress Calculations
Tube buckling stress calculations only come into play when sigma(tmin) is less than zero. In the example below, Loading Case 1 and 4 meet this criteria and so the grid shows a value for the Maximum Allowable Buckling Stress. The other two cases have NC because the value is not less than zero.
Simply Supported Tubesheet Calculations
The simply supported calculations apply to tubesheet configurations that have an integral channel, an integral shell, or both. These are configurations a, b, c, e, f, and A. This calculation allows the allowable stress in the integral member (shell / channel) to increase for the design conditions if an additional set of calculations is run showing the tubesheet can pass being simply supported or not taking advantage of the rigidity of the integral member(s). Below is an example of the additional simply supported calculation for a fixed tubesheet and NC is listed for the operating cases as the calculation only applies to the design cases.
Elastic Plastic Tubesheet Calculations
The elastic plastic calculations apply to tubesheet configurations that have an integral channel, an integral shell, or both. These are configurations a, b, c, e, f, and A. Like the simply supported option, the elastic plastic option only applies to design loading conditions. In addition, they only apply for cases where the integral shell, channel, or both is overstressed before doing the EP adjustment. In the example below, only loading cases 2 and 4, corroded, for the Design Loading Cases trigger the calculation.
Radial Differential Thermal Calculations
Radial differential thermal calculations are optional calculations for both Fixed Tubesheet and Floating Tubesheet designs. They apply only to thermal loading cases and only if they are flagged to run by the user selecting "Yes" in the conditions grid. If these calculations are not run, due to the settings or loading case, then several values will be set as NC.
