Thickness Due to Internal Pressure

The appropriate formula from ASME Section VIII is referenced, and the formula and substitutions are shown. The diameter or crown radius is adjusted to take into account the corrosion allowance. If your shell design includes hydrostatic head components, the additional pressure due to the height of the liquid column and the operating liquid density will be included with the basic design pressure. The hydrostatic head will be subtracted in order to properly determine the MAWP for the vessel part that is being analyzed. Remember, when pressures are being read from the pressure gauge, the gauge is usually at the high point of the vessel. The pressure registered by the gauge would be different if were at the bottom of the liquid filled vessel.

For elliptical heads, the K factor is (2 + Ar * Ar) / 6, per App. 1-4 (c). For torispherical heads, the factor M is (1/4) * (3 + SQRT (L / R)), where "L" (the crown radius) and "R" (the knuckle radius) were entered by the user.

CodeCalc does not replace the given thickness with this calculated minimum. If you are choosing the thickness for a component, compare the values shown under "Summary of Internal Pressure Results" (required vs. actual) and adjust the actual thickness up or down accordingly.

Maximum Allowable Working Pressure at Given Thickness

This value is calculated as described above, using the given thickness minus corrosion allowance and the operating allowable stress. The hydrostatic head component is subtracted from this value. The pressure gauge is assumed to be at the top of the vessel.

Maximum Allowable Working Pressure, New & Cold

This value is calculated as described above, using the uncorroded thickness and the ambient allowable stress.

Actual Stress at Given Pressure and Thickness

Note that the joint efficiency is included in this value, so this can be considered as the stress at the welded joint rather than in the base metal.

Summary of Internal Pressure Results

Either of two conditions can indicate a problem in your design. First, if the required thickness plus corrosion allowance is greater than the given thickness, then you must increase the given thickness. Second, if the MAWP is less than the design pressure, then you must either decrease the design pressure or increase the given thickness to achieve an acceptable design.

The hydrotest pressure is calculated as the maximum allowable working pressure times 1.5 or 1.3 (depending on the material database selection) times the ratio of the allowable stress at ambient temperature to the allowable stress at design temperature.

The hydrotest pressure may not be appropriate for the entire vessel for three reasons. First, some other component may have a lower maximum allowable working pressure, which may govern the hydrotest pressure. Second, you may choose to base hydrotest pressure on design pressure rather than maximum allowable working pressure. Third, if the vessel is tested in the vertical position you may have to adjust the hydrotest pressure for the head of water in the vessel.

For the UG99-C hydrotest, the liquid head is subtracted from the basic result.

Minimum Metal Temperatures

For carbon steels, these temperatures represent the minimum design metal temperature for the given thickness and, in the second case, the given pressure. The first temperature is interpolated directly from chart UCS-66. The second temperature is reduced if the actual stress is lower than the allowable stress, using figure UCS-66.1. The program also checks for materials, which qualify for the -20 minimum design temperature per UG-20 and prints it in the output. See the input notes above to enter normalized or non-normalized materials.

Weight & Volume Results, No Corrosion Allowance

CodeCalc computes the volume and weight of the shell component. Additionally, the inside volume for a 2.00 inch straight flange is computed and used in the computation of the total volume for the head and the flange. The dimensions used in the volume and weight calculations are non-corroded dimensions.

Results for Max. Allowable External Pressure

For the given diameter, thickness, and length, the maximum allowable external pressure is computed per UG--28.

Results for Required Thickness for External Pressure

Required thickness results are calculated iteratively using the rules of UG-28. Items such as the length and outside diameter are held constant, and the software calculates the required thickness based on the entered value for external pressure.

Summary of External Pressure Results

Summary listing displaying external pressure results for the user-entered thickness and the calculated required thickness.