Tank Load Case 1 Calculations

Two Compliance Codes are implemented in IMS for the LC1 calculations: The one is based on the API specifications and the other on internal Shell (DEP) specifications. Each calculation is further split up into specific calculations for each part of the Tank. This is currently divided into Roof, Shell, and Floor. The Nozzle is not calculated. LC1 is calculate during the RBI Analysis, on the Calculator tab (see Tanks - Step 6).

LC1 is calculate in the RBI Analysis, on the Calculator tab. 

API Calculations

Allowable Wall Thickness (AWT) Calculations

Calculated AWT:

Roof: Fixed values, either 1 mm or 2 mm, depending on the Pitting mode. If the tank is in Pitting mode 1 mm is used, if not 2 mm is used. Note: If any analysis of the tank has Pitting set (on the RBI Specs tab - see Tanks - Step 6), the whole tank will be considered to be Pitting.

Shell: The Calculated Thickness (Minimum Wall Thickness (TMin)) value with a threshold of at least 2.54 mm [0.100 in.]. Thus, the TMin value, or 2.54 mm, whichever is highest.

Floor: If a Leak detection system is installed, a fixed value of 1.27 mm [0.050 in.] is used. If no Leak detection system is installed, a fixed value of either 1.27 or 2.54 is used, depending on the Internal bottom lining: If there is internal bottom lining, use 1.27, else use 2.54 mm.

Allowable Wall Thickness:

Roof and Floor: This is the Renewal Thickness (Sub-Component Spec). If this spec is not filled-in, this value will be empty.

Shell: Same as Calculated AWT.

Corrosion Rate (CR) Calculations = Calculated CR

In case of a Shell, the Product Height (PH) is calculated. If that is 0, the CR is also 0. Otherwise, it will be calculated in the same way as for Roof and Floor.

The basic calculation is thickness degradation over time (installation date vs last inspection date). The thickness degradation is the Thickness as built (TAB) minus the last Measured Thickness (MT), i.e., Actual Wall Thickness. The time factor is the difference between the “install date” and the “last measured date” (in years).

Note: The time difference is calculated in whole years and not in fractions.

Minimum Wall Thickness (TMin) = Calculated Thickness

Shell: This is a compound formula involving the calculated Product Height (PH), tank Diameter (D), Design Pressure (DP) (Equipment Spec), Joint Efficiency Factor (JEF), Spec Gravity @ Op Temp (Equipment Sepc), and the calculated Maximum Allowable Stress (MAS). MAS is the effective Yield or the Effective Tensile Strength, whichever it the lowest. For Courses 1 and 2, MAS is the smaller of 0.8 * Yield or 0.429 * Tensile Strength. For the remaining Courses, MAS is the smaller of 0.88 * Yield or 0.472 * Tensile Strength. Note: The Tensile Strength is capped at 560 N/mm2 [80,000 lbf/in2].

For this calculation, the following defaults are used in case the values are 0:

• Joint Efficiency Factor: 1
• Tensile Strength: 385 N/mm2 [55,800 lbf/in2]
• Yield: 210 N/mm2 [30,470 lbf/in2]

Roof: A fixed value of 2.3 mm [0.09 in.] is used, unless the tank is in Pitting mode, in which case 0 is used.

The general tank specs are used in the TMin calculations.

Floor: Not calculated.

DEP Calculations

Allowable Wall Thickness (AWT) Calculations

Calculated AWT:

Roof: The Roof calculation is the same as for API. The fixed numbers 1 mm or 2 mm are used, depending on the Pitting mode. If the tank is in Pitting mode 1 mm is used, else 2 mm is used.

Shell: Only applies if the tank is not in Pitting mode. If it is, the result will be 0, if it is not, the highest value of half the Thickness as built (TAB) and the Calculated Thickness (TMin) is used.

Floor: Only applies if not in Pitting mode. If it is, the result will be 0. If it is not, the result depends on the Sub-Component Type:

• For Sub-Component “Bottom Annular” half the TAB is used.
• For Sub-Component “Bottom Shell Annular” the TMin of Course 1 is used.
• For Sub-Component “Sketch Plates” or “Sump” a fixed value of 2.5 mm [0.100 in.] is used.
• For all others, 0.32 * TAB is used.

Allowable Wall Thickness:

Roof and Floor: This is the Renewal Thickness (Sub-Component Spec). If this spec is not filled-in, this value will be empty.

Shell: The highest value: half the Thickness as built (TAB) or the Calculated Thickness (TMin).

Corrosion Rate (CR) Calculations = Calculated CR

Roof: The ratio of the Thickness Measurements of the last two inspections over time. If there are not at least two inspections available, the result will be 0. This is the slope of the line when measured thickness (x axis) is plotted against inspection date (y axis).

Shell and Floor: The ratio of the thickness decrease over time, when looking at the time of installation and the last inspection.

Note: The time difference is calculated in whole years and not in fractions.

Minimum Wall Thickness (TMin) Calculations = Calculated Thickness

Shell: This is a compound calculation depending on several parameters, such as the Diameter (D) (Equipment Spec), Yield, Joint Efficiency Factor (JEF), Spec Gravity @ Op Temp (Equipment spec), Design Pressure (Equipment spec), and the calculated Product Height (PH).

Roof and Floor: Not Calculated.

Shared Calculations

The following calculations are the same for both compliance codes:

Design Corrosion Allowance (DCA) Calculations = DCA

The DCA suggestion is the difference between the Thickness as built (TAB) and the calculated Allowable Wall Thickness (AWT).

Remaining Corrosion Allowance (RCA) Suggestion = Calculated RCA

The RCA Suggestion is the difference between the last Measured Thickness (MT) at the latest inspection, i.e., the Actual Wall Thickness, and the AWT.

Product Height (PH)

PH only applies to Courses. For each Course, the PH of that Course, is the Maximum Filling Height (MFH) of the tank minus the cumulative Course Height (CH) of the previous Courses below it.

For example, if we have 4 Courses, with the top Course as Course 4, and bottom Course as Course 1, then the Course PHs are:

• PH_{Course 1} = MFH
• PH_{Course 2} = MFH – CH_{Course 1}
• PH_{Course 3} = MFH - CH _{Course 1} - CH _{Course 2}
• PH_{Course 4} = MFH - CH _{Course 1} – CH _{Course 2} - CH _{Course 3}

MFH is part of the tank Equipment specs.