Pipe stress analysis

From a mechanical standpoint, stress analysis ensures that a piping system, whether connected to equipment or not, remains safe under all possible conditions:

• During startup

• During shutdown

• Under normal operating conditions

• Under abnormal (upset) conditions

• During an earthquake

• Under wind loads

To ensure piping systems operate safely, stress analysis considers several key factors:

• Comply with piping code constraints under which the system is designed, manufactured and tested.

• Adhere to the maximum forces allowable on connected equipment.

• Comply with stress limits for structural piping supports.

• Ensure proper support for all load conditions to ensure piping remains in place.

• Limit vertical deflection between supports, especially for sloped pipes used for drainage.

• Prevent interference between pipes under all operating scenarios.

CAESAR II software

BBA uses CAESAR II software for stress analysis. This tool simulates how a piping system behaves under various conditions, including:

• Internal fluid pressure

• The weight of piping system components and external elements, such as ice

• Thermal expansion or contraction caused by fluid temperature differences relative to the installation temperature

• Displacements imposed by external elements, such as structural displacements

• Forces from wind and seismic activity

• Loads induced by activating safety devices in the system, such as relief valves and rupture disks

The software calculates stresses based on the equations of the relevant piping code for different loading conditions. It then compares these calculated stresses with the allowable limits specified in the code.

Moreover, built-in subroutines help users verify that forces and moments at the connection points of rotating and non-rotating equipment remain within acceptable limits. These limits vary depending on the equipment type and applicable industry standards.

Not all piping in a project needs to be analyzed using software such as CAESAR II.

Below is an overview of the types of piping that are typically analyzed across various industries:

Piping connected to:

• Steam turbines
• Reciprocating compressors or pumps
• Reactors
• Steam generators

Piping that is 3” in diameter and larger connected to:

• Rotating equipment, such as centrifugal pumps

Piping that is 4” in diameter and larger connected to:

• Air coolers

• Pressure vessels

• Heat exchangers

Piping that:

• Is connected to equipment or piping supports that are subject to significant ground settlement or differential settlement between the piping equipment and supports

• Operates at temperatures below -6°C (20°F)

• Is used in cryogenic service

• Has a very high design pressure (i.e., exceeding the pressure rating of ASME B16.5 Class 2500 flanges)

• Is subject to severe cyclic service (as defined by ASME B31.3)

• Has a pipe diameter-to-wall thickness ratio exceeding 90

• Has thin walls (Schedule 10 or lower) and a diameter of 12 inches or more

• Requires an expansion joint

• Is buried (with high temperature or pressure conditions)

• Is jacketed (double wall)

• Contains toxic liquids or gases that may pose risks to humans or the environment (ASME B31.3 Category M)

• Is equipped with safety valves of 2 inches or more in diameter

• Has two-phase flow

The software follows three main steps to perform a pipe stress analysis:

Before starting the modelling process in the software, you need to gather the following information about the piping system:

• Piping routing drawings

• Piping and instrumentation diagrams (P&IDs)

• Piping specifications, including:
  • Piping material
  • Pipe wall thickness
  • Corrosion allowance
  • Type of valves and fittings
  • Flange rating
  • Insulation type and thickness

• Piping list, including:
  • Design and hydrostatic test pressure
  • Operating, design and upset temperatures
  • Fluid type

• Structural drawings to determine possible locations for piping supports

• Equipment drawings

• Existing piping drawings, if the new piping connects to an existing system

• Seismic coefficient, if a seismic analysis is required

• Wind pressure as a function of elevation, if a wind analysis is required

This step involves entering all the required loading cases into the software to perform an accurate analysis and comply with piping code requirements.

This step involves verifying the piping system for all defined load cases.

Piping systems consist of many components. If a single component fails, it can lead to a full plant shutdown, or worse, serious accidents that endanger the health of workers and the public. Recognizing that system reliability and safety are top priorities for industry, BBA has developed its own expertise in piping stress analysis.