How to perform a pipe stress analysis

Understanding the various types of pipe stresses, the process, and best practices are necessary to perform effective pipe stress analyses.

09/21/2017


This article has been peer-reviewed.Learning objectives

  • Define and evaluate the pipe stress analysis process.
  • Understand pipe stress analysis.
  • Learn how to model a piping system and pressure design basics.

Pipe stress analysis is an analytical method to determine how a piping system behaves based on its material, pressure, temperature, fluid, and support. Pipe stress analysis is not an accurate depiction of the piping behavior, but it is a good approximation. 

The analytical method can be by inspection, simple to complex hand calculations, or a computer model. The computer models can vary from 1-D beam elements to complex, finite element models. For instance, if it is a water system with no outside forces applied to the piping system, inspection or hand calculations are usually sufficient. If it is a high-pressure, high-temperature, hazardous-fluids system, and/or large outside forces are applied to the piping system, a computer-aided model may be required. 

Understanding pipe stress analysis software does not make for a solid foundation of pipe stress analysis. It’s important to understand the various types of pipe stresses, the process, and other items related to pipe stress analysis for best practices in performing a pipe stress analysis.In this lateral restraint on a hot reheat steam system pipe, the bottom-left inset shows a 3-D piping model. The top-right inset is of a 3-D pipe stress analysis model. Courtesy: Stanley Consultants

There are many piping codes and standards that could be used during a pipe stress analysis depending on the application (power, process chemical, gas distribution) and location (country or local jurisdiction). However, to keep things simple, this discussion is based on American Society of Mechanical Engineers (ASME) B31.1 Power Piping. The physics of pipe stress analysis does not change with piping code.

Pipe stress analysis should be done primarily to provide safety to the public, whether you are designing a building heating system or a high-pressure gas line in a refinery. Public safety is paramount. The National Society of Professional Engineers (NSPE) Code of Ethics’ first cannon is: “Hold paramount the safety, health, and welfare of the public.” 

On a good day, a pipe failure is only a broken support that the owner does not call the designer/engineer about. On a bad day, the owner requires the designer/engineer to pay for the damage and the engineer to provide a solution for free. On a horrible day, someone is killed.

Another reason a pipe stress analysis is performed is to increase the life of piping. Most engineers won’t consider a piece of pipe to be equipment, but it is no different than a pump. Both have moving parts and must be designed and maintained properly to ensure a proper life. Pipe stress analysis also is used to protect equipment, because a pipe is nothing more than a big lever arm connected to a delicate piece of equipment. If not properly supported and designed, it can have devastating effects on that equipment.

There are several common reasons that could warrant a pipe stress analysis, in addition to those above. They include:

  • Elevated temperatures (>250°F).
  • Pressure mandated (300 psig).
  • Sensitive equipment connections.
  • Large D/t ratio (>50).
  • Piping subject to external pressures.
  • Critical services.

The key when performing a pipe stress analysis is determining the required level of detail.

How to model the piping system

Pipe stress analysis computer models are a series of 3-D beam elements that create a depiction of the piping geometry. Three-dimensional beam elements are the most efficient way to model the piping system, but not necessarily the most accurate; and without complex finite element models, it is nearly impossible to account for everything. However, it is known from historical empirical testing that these methods and 3-D beam computer models demonstrate enough behavior that they are a good approximation. In addition, piping codes, such as ASME B31, have safety margins that allow for approximation. That being said, there are some pitfalls with modeling piping systems that one should avoid:

The computer models are only as good as the information entered into them. It is important when developing a pipe stress analysis, as with any finite element analysis (FEA) model, to also understand the physics and boundary conditions of the model.

Elements used to model the piping system have their limitations. One-dimensional beam elements are great for straight pieces of piping, but not so good with pipe fittings (elbows, tees, reducers, etc.). Therefore, ASME has developed stress-intensification factors (SIFs) for piping fittings through empirical testing. They allow for greater approximation without using complex FEA models with shells, plates, and brick elements.

It is important to make sure these limitations are considered when developing a pipe stress analysis. Most pipe stress analyses do not perform like a high-powered FEA software package.

Three-dimensional beam element

The 3-D beam element behaviors are dominated by bending moments. As mentioned above, it is efficient for most analyses and sufficient for system analysis. However, there are downsides to using a 3-D beam element:

  • No localized effects will be seen on the pipe wall.
  • No second-order effects.
  • No large rotation.
  • No accounting for a large shear load.

    • Wall deflection occurs before bending failure.
    • Short, fat cantilever versus long and skinny.

  • No shell/wall effects can be seen.

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