Alternate Sustained Case
On CAESAR II, the Alternative Sustained Case accounts for a more accurate representation of sustained stresses in nonlinear boundary conditions, such as one-way restraints, lift-off supports, and gaps.
The "Alternative Sustained Case" in CAESAR II is a specialized load case used in piping stress analysis to evaluate the effects of sustained loads under specific conditions that differ from the standard sustained load case. This case is particularly useful when the piping system has unique configurations or operating scenarios that require a different approach to assessing sustained stresses.
Below is a detailed explanation of the Alternative Sustained Case, including its purpose and why it is needed:
What is the Alternative Sustained Case?
The Alternative Sustained Case is a load case that considers sustained loads (such as weight and pressure) but under conditions that are different from the standard sustained load case. For example:
It may include different boundary conditions (e.g., fixed or restrained displacements).
It may account for specific operating scenarios where the standard sustained case does not fully capture the system's behavior.
Purpose of the Alternative Sustained Case
The primary purpose of this load case is to:
Evaluate Sustained Stresses Under Unique Conditions:
Assess the piping system's response to sustained loads in scenarios where the standard sustained case may not be applicable or sufficient.
Comply with Code Requirements:
Ensure that the piping system meets the stress limits specified by codes like ASME B31.3 for sustained loads, even under non-standard conditions.
Optimize Support Design:
Identify potential overstressing in specific areas of the piping system due to unique configurations or boundary conditions.
Why is the Alternative Sustained Case Needed?
The Alternative Sustained Case is required in situations where the standard sustained load case does not adequately represent the system's behavior. Here are some common reasons for using this case:
1. Different Boundary Conditions
In some systems, certain supports or anchors may behave differently under sustained loads due to thermal expansion, settlement, or other factors. The alternative sustained case allows for the evaluation of sustained stresses under these unique boundary conditions.
2. Multi-Purpose Systems
For piping systems that operate under multiple scenarios (e.g., different operating temperatures or pressures), the alternative sustained case can be used to evaluate sustained stresses for each scenario separately.
3. Complex Configurations
In systems with complex geometries or configurations (e.g., pipe racks, multi-planar bends, or loops), the standard sustained case may not capture localized stresses accurately. The alternative sustained case provides a more detailed analysis.
4. Compliance with Codes
Some codes or project specifications may require an alternative sustained case to ensure that the piping system meets stress limits under all possible operating conditions.
5. Special Operating Conditions
For systems that experience unique operating conditions (e.g., partial loading, temporary supports, or staged construction), the alternative sustained case helps evaluate the system's behavior under these conditions.
How is the Alternative Sustained Case Defined in CAESAR II?
In CAESAR II, the Alternative Sustained Case is typically defined as a combination of:
Weight Loads: Includes the weight of the pipe, fluid, insulation, and other components.
Pressure Loads: Includes internal or external pressure acting on the piping system.
Boundary Conditions: May include fixed or restrained displacements, depending on the scenario being evaluated.
The load case is defined in the Load Case Editor by combining the relevant load components (e.g., W+P1 for weight and pressure) and applying the appropriate boundary conditions.
Example Scenario
Consider a piping system with a temporary support that will be removed during operation. The standard sustained case evaluates the system with the temporary support in place, but the Alternative Sustained Case evaluates the system without the temporary support to ensure that the stresses remain within allowable limits after its removal.
Conclusion
The Alternative Sustained Case in CAESAR II is a valuable tool for evaluating sustained stresses under unique or non-standard conditions. It ensures that the piping system remains safe and compliant with code requirements, even in complex or specialized scenarios. By using this load case, engineers can identify potential issues early in the design process and optimize the system for reliability and longevity.
