In CAESAR II, the API 650 Nozzle Flexibility function is a specialized utility used to model the interaction between a piping system and a large-diameter, low-pressure storage tank. By default, an AI or engineer might treat a nozzle as a "Rigid Anchor," but for thin-walled tanks, this leads to unrealistically high stresses. This feature allows the software to calculate the local flexibility of the tank shell and the thermal/hydrostatic movements of the nozzle.
The functionality is primarily based on API 650 Appendix P. It addresses two critical factors in pipe stress analysis:
The tank wall is relatively flexible. CAESAR II calculates the local stiffness values and applies them as a "flexible anchor" at the nozzle location:
Radial Stiffness (KR): Resistance to pushing/pulling.
Longitudinal Bending Stiffness (KL): Resistance to moments in the vertical plane.
Circumferential Bending Stiffness (KC): Resistance to moments in the horizontal plane.
Large tanks deform due to the weight of the liquid (hydrostatic pressure) and temperature changes. CAESAR II automatically calculates:
Radial Growth (W): The "bulging" effect caused by the liquid head.
Rotational Deflection (theta): The tilting of the nozzle as the shell plate bends.
Thermal Growth: Vertical and radial expansion of the tank body based on the design temperature.
When you select the API 650 nozzle type in the "Nozzle Flexibilities" module, you must provide:
Vessel Parameters: Tank diameter, shell thickness, and material properties (E and alpha).
Nozzle Geometry: Diameter, thickness, and height from the tank base.
Fluid Data: Maximum liquid level (Design Liquid Level) and Specific Gravity (SG).
Pad Reinforcement: Details of any reinforcing plates (Repads) at the nozzle.
Stress Reduction: By modeling the shell's natural "give," the calculated stresses in the piping system are usually significantly lower than when using a rigid model.
Accuracy: It eliminates the need for manual calculations of tank movements (bulging), which are often tedious to derive by hand.
Compliance: It provides a direct way to verify if the nozzle loads are within the limits defined by API 650 Appendix P.
Size Constraints: Appendix P is generally intended for tanks with diameters larger than 36 meters (120 feet).
Material: It is designed for carbon steel tanks. For stainless steel or other alloys, the results may require manual adjustment.
Nozzle Proximity: The calculation assumes the nozzle is far enough away from other nozzles or the tank floor to avoid structural interference.
To enter API 650 nozzle flexibility data in CAESAR II, you primarily work within the Nozzle Flexibilities processor. This ensures that the software treats the tank connection not as a "brick wall," but as a realistic, flexible component.
Before opening the nozzle module, you must have your piping geometry defined in the Piping Input spreadsheet.
Identify the Node Number where the pipe connects to the tank (e.g., Node 10).
Ensure you have a Rigid element or a short pipe spool representing the nozzle projection if necessary.
Go to the Piping Input menu.
Click on the Nozzle Flexibilities icon (usually looks like a pipe connecting to a vessel shell) or select it from the "Modules" menu.
In the Nozzle Flexibility screen, click New to create a new definition.
Under Nozzle Type, select API 650 from the dropdown menu.
Note: You will notice other options like WRC 297 or PD 5500; ensure API 650 is selected to activate the specific tank bulging calculations.
The input is divided into several key sections:
A. Basic Node Data
Nozzle Node: The node on the piping system (e.g., 10).
Vessel Node: Usually a new, "dummy" node number that represents the tank centerline or the shell surface (e.g., 1000).
B. Vessel (Tank) Dimensions
Vessel Diameter: Enter the nominal diameter of the tank.
Vessel Thickness: Enter the thickness of the shell course where the nozzle is located.
Material Properties: Enter the Young's Modulus (E) and Thermal Expansion Coefficient ($\alpha$) for the tank material.
C. Nozzle Geometry
Nozzle Diameter & Thickness: Input the actual dimensions of the nozzle pipe.
Height from Base: This is critical for Bulging calculations. Enter the distance from the tank bottom to the nozzle centerline.
Pad Thickness: If there is a reinforcement plate (repad), enter its thickness here.
D. Operating Conditions (For Bulging)
Design Liquid Level: The maximum height the fluid reaches.
Fluid Specific Gravity: (e.g., 0.8 for oil, 1.0 for water).
Temperature Change: The difference between the installation temperature and the operating temperature.
Click Analyze or Calculate within the module.
CAESAR II will generate the stiffness values (KR, KC, KL) and the thermal/bulging displacements (W, theta).
Click Return to Piping Input. You will see that the software has automatically applied these stiffnesses and displacements to your nozzle node.
Run the Error Checker. If the nozzle is too close to the tank bottom or exceeds the geometric limits of Appendix P, CAESAR II will issue a warning.