Calculations

GE Digital APM calculates the values in the Highest Effective Inspection Level and Number of Highest Effective Inspections fields on the following datasheets:

• RBI 581 Cracking Damage Evaluation
• RBI 581 External Cracking Damage Evaluation
• RBI 581 Thinning and Lining Evaluation (only if you are evaluating 581-Atmospheric Tank Bottom Corrosion)

These values are calculated, however, only if the Use Combined Confidence check box is selected. They are calculated based on the values in the following fields:

• Number of A Level Inspections
• Number of B Level Inspections
• Number of C Level Inspections
• Number of D Level Inspections

They are calculated as follows:

• Two D level inspections equal one C level inspection
• Two C level inspections equal one B level inspection
• Two B level inspections equal one A level inspection

Flow Stress is a variable introduced by RBI 581, and is an important parameter used for API 581 Probability of Failure calculations. Flow Stress is calculated using the following formula.

Where:

• YS = Yield Strength at Design Temperature
• TS = Tensile Strength at Design Temperature
• E = Weld Joint Efficiency

Yield Strength and Tensile Strength values are stored in Stress tables (e.g., the PV Stress reference table). Meridum, Inc. only provides Yield Strength and Tensile Strength values for PV Stress, for the year 2010. For all other Stress records, you must ensure that Yield and Tensile Strength values exist and, if they do not, create values to be used with RBI 581 Analyses.

When you create an RBI 581 Analysis, you can override the Flow Stress value and enter your own value.

The Allowable Stress field on an RBI Analysis datasheet is populated based on the component type.

For Pressure Vessels and Piping Components

Values in the following fields on an RBI Criticality Analysis datasheet are used to calculate the value in the Allowable Stress field:

• Stress Lookup Table
• Construction Code
• Code Year
• Material Spec
• Material Grade

Depending on the value in the Stress Lookup Table field, the values in the rest of the aforementioned fields are populated with the values in the corresponding fields in the PV Stress, Piping Stress, and Tank Stress Reference Tables. For example, if the value in the Stress Lookup Table field is Piping, then the value in the Code Year field is populated with the value in the Code Year field in the Piping Stress Reference Table.

For Tanks

Values in the following fields on an RBI Criticality Analysis datasheet are used to calculate the value in the Allowable Stress field:

• Stress Lookup Table
• Construction Code
• Code Year
• Material Spec
• Material Grade
• Course Number
• Tensile Strength (from the Tank Stress record)
• Yield Strength (from the Tank Stress record)

Depending on the value in the Stress Lookup Table field, the values in the rest of the aforementioned fields are populated with the values in the corresponding fields in the PV Stress, Piping Stress, and Tank Stress Reference Tables. For example, if the value in the Stress Lookup Table field is Piping, then the value in the Code Year field is populated with the value in the Code Year field in the Piping Stress Reference Table.

Allowable Stress = Minimum (0.80 x Yield Strength, 0.429 x Tensile Strength)

Otherwise, Allowable Stress is calculated as follows:

Allowable Stress = Minimum (0.88 x Yield Strength, 0.422 x Tensile Strength)

The inventory component mass of a component is calculated using the values in the following fields in the Criticality Calculator RBI Component record:

• Operating Pressure (P)
• Operating Temperature (T)
• Diameter (d)
• Length (L)
  Note: If the component type is Piping, then the value in the Piping Length field is used instead of the value in the Length field.
• Percent Liquid Volume (pc)
• Molwt (This value is stored in the RepresentativeFluids reference table for the fluid specified in the Process Fluid field in the Criticality Calculator RBI Components record)
• Fluid Density (This value is specified in the RepresentativeFluids reference table for the fluid specified in the Process Fluid field in the Criticality Calculator RBI Components record)

The inventory component mass of a component is calculated as follows:

Inventory Component Mass = (Liquid Volume x Fluid Density) + (Vapor Volume x Vapor Density)

Where:

• Liquid Volume = (pc/100) x Volume of the cylinder
  Note: The volume of the cylinder is calculated from the values in the Diameter and Length fields in the Criticality Calculator RBI Component record.
• Vapor Volume = (1-(pc/100)) x Volume of the cylinder
• Vapor Density = (Molwt x (P + 14.7)) / (10.73 x (T + 459.15) x z)

This value is populated in the Inventory Component Mass field in the Common section of the associated RBI 581 Consequence Evaluation.

Example for Calculating Inventory Component Mass

Suppose a component has the following specifications:

• Component Type: Heat Exchanger (Shell)

• Process Fluid : C6-C8

• Operating Pressure: 101 Pounds/Sq Inch Gage

• Operating Temperature: 121 Degrees Fahrenheit

• Molwt of CO: 28

• Diameter: 97.625 inches

• Length: 360 inches

• Percent Liquid Volume: 50 percent

• Fluid Density: 42.704

Given these values:

• Volume of the cylinder = (pi x (d/12) x (d/12) x (L/12)) / 4

  Volume of the cylinder = ((22/7) x (97.625/12) x (97.625/12) x (360/12)) / 4

  Volume of the cylinder = 1560.075 Cubic feet

• Liquid volume = (pc/100) x Volume of the cylinder

  Liquid volume = (50/100) x 1560.075

  Liquid volume = 780.038 Cubic feet

• Vapor volume = (1 - (pc/100)) x volume of the cylinder

  Vapor volume = (1 - (50/100)) x 1560.075

  Vapor volume = 780.038  Cubic feet

• Vapor Density = (Molwt x (P + 14.7)) / (10.73 x (T + 459.15) x z)

  Vapor Density = (100 x (101 + 14.7)) / (10.73 x (121 + 459.15) x 1)

  Vapor Density = 1.859 Pounds per cubic feet

• Inventory Component Mass = (Liquid Volume x Fluid Density) + (Vapor Volume x Vapor Density)

  Inventory Component Mass = (780.038 x 42.704) + (780.038 x 1.859)

  Inventory Component Mass = 34760.526 Pounds

  Note: The value in the Inventory Component Mass field on the RBI Component datasheet may slightly differ from the value that is shown in the example. This happens because of rounding off some of the values.

The inventory group mass is calculated as the sum of the inventory component mass values of all the individual components in an inventory group. This value is populated in the Inventory Group Mass field in the Common section in the associated RBI 581 Consequence Evaluation record. If the component is not linked to an inventory group, then the value in the Inventory Group Mass field is the same as the value in the Inventory Component Mass field.

For example, suppose you have created an inventory group, Group 1, and linked it to three components, Component 1, Component 2, and Component 3. The inventory component mass values for each of these components are shown in parentheses.

In this case, the inventory group mass is calculated as follows:

Inventory Group Mass of Group 1 = Inventory Component Mass of Component 1 + Inventory Component Mass of Component 2 + inventory Component Mass of Component 3

Inventory Group Mass of Group 1 = 120 + 100 + 130

Inventory Group Mass of Group 1 = 350 Pounds

Suppose you have created an RBI 581 Risk Analysis, Analysis 1, for Component 1. GE Digital APM then calculates the inventory component mass of Component 1 based on the input values in Analysis 1. In the following diagram, this value is shown in parentheses next to Analysis1.

In this case, the inventory group mass is calculated as follows:

Inventory Group Mass of Group 1 = Inventory Component Mass calculated based on input values from Analysis 1 + Inventory Component Mass of Component 2 + Inventory Component Mass of Component 3

Inventory Group Mass of Group 1 = 150 + 100 + 130

Inventory Group Mass of Group 1 = 380 Pounds