Pipe Stress Analysis

In the world of process piping and industrial systems, safety, reliability and performance are essential. One of the most important technical tasks that ensures these characteristics is Pipe Stress Analysis.

It is essential for the design, construction, and maintenance of piping systems that transport liquids and gases at different temperatures and pressure levels. This article discusses the concept of Pipe stress analysis and its various types.

What is Pipe Stress Analysis?

Pipe stress analysis examines piping systems to verify their strength, safety, and efficiency. This involves using computer software and human calculations to determine the stresses, deformations, and displacements of pipes under various load conditions.

Such conditions include internal pressure, thermal expansion, the weight of the pipes and fluid flow, seismic events, and other external forces in various industries, including oil and gas, petrochemicals, and energy production.

The goal is to identify potential causes of defects, such as excessive stress, vibration, or thermal expansion, and then devise ways to mitigate those risks.

Main types of stresses in pipes

Certain pressure, temperature, and vibration conditions, as well as incidental loads, all affect piping systems. The main stresses in pipes can be divided into the following categories:

Fatigue stress: caused by the combination of continuous stresses that can affect certain piping systems.

Ring stress: a type of uniform pressure exerted internally or externally that can affect the diameter and wall thickness of the pipe.

Axial stress: caused by factors such as thermal or pressure expansion, as well as applied forces that result in limited axial growth of the pipe. Since different materials react differently to this type of stress, pipe stress analysis remains crucial to detect this problem.

Torsional stress: caused by body forces that generate rotational moments around the axis of the pipe.

Bending stress: caused by certain body forces that can be concentrated (such as those from valves) or incidental (such as those caused by atmospheric forces, including seismic movements or extreme wind). Bending stress can also be detected as forced displacements generated by the growth of other equipment and pipes, which ultimately affects the analyzed pipe.

Three categories of stresses according to ASME B31.3 and B31.1 standards, which affect the stress in the pipe

Displacement stresses: these must remain within a certain range in accordance with the permitted standards and can be reduced by adding flexibility to the piping system.

Primary or sustained stresses: these account for approximately 55% of the standard permissible stress according to the standards.

Occasional stresses: caused by one-off events that are usually related to seismic movements, extreme wind events, or upward pressure loads. The standards allow for certain increases in the case of these stresses.

Stress analysis of piping for industrial applications

Stress analysis of piping is crucial for numerous sectors, each with its own requirements and challenges.

Oil and gas: This sector uses enormous pipeline networks to deliver crude oil, natural gas, and refined products. The pipelines operate under high pressure and temperatures, making stress analysis crucial to prevent leaks and malfunctions.

Chemicals and petrochemicals: This industry relies on piping systems to deliver potentially hazardous substances. Stress analysis is therefore essential to ensure the integrity of these systems, particularly given the corrosive nature of many chemicals.

Power generation: The steam piping systems of power plants are exposed to extreme temperatures and pressures. This contributes to the reliability of the above-mentioned systems, which are crucial to the operation of the plant.

Pharmaceutical industry: Stress analysis is crucial for the safety, purity, and quality of pharmaceutical products, as piping systems transport sterile and non-sterile fluids.

Food industry: Stress analysis of pipes helps in designing systems that are easy to clean and maintain, thereby preventing contamination.

Water and wastewater treatment: Plants in this industry use piping systems to transport water. Stress analysis ensures that these systems can withstand the pressure and flow required for treatment procedures such as transferring sludge and other materials.

When is stress analysis of pipes required?

Below are some basic rules for stress analysis of pipes, which serve as a reference to help less experienced professionals understand important considerations and facilitate discussions with experienced professionals. The application of stress analysis can vary significantly by industry, with some industries employing more stringent or simpler approaches.

In addition to legal or contractual requirements, here are some general situations in which stress analysis typically applies.

• This is particularly dependent on the operating temperature and the pipe diameter.
• Pipes of 2 inches or larger connected to rotating equipment or heat exchangers.
• Pipes of 6 inches and larger connected to pressure vessels.
• Cryogenic piping systems.
• Pipes that transport hazardous chemicals.
• Double-walled pipes where there is a significant temperature difference between the inner and outer pipes.
• Short sections of hot pipe that are anchored at both ends.
• Systems that, after professional assessment, prove to be insufficiently flexible.
• Complex systems with, for example, multiple branches.
• When seismic analysis is necessary.

It is also crucial to remember that thermal stress can occur in both hot and cold piping systems. The greater the difference between the operating temperature of the pipe and the ambient temperature during installation, the more likely it is that a stress analysis will be necessary.

Some codes and standards for stress analysis of pipes

• ASME 31.1 - Power Piping
• ASME B 31.3 - Process Piping
• ASME 31.4 - Pipeline Transportation Systems for Liquids and Slurries
• ASME 31.8 - Gas Transmission and Distribution Piping Systems
• ASME 31.12 - Hydrogen Piping and Pipelines
• ASME Section VIII - Pressure Vessels
• API 610 - Centrifugal Pumps for Petroleum, Petrochemical, and Natural Gas Industries
• API 676 - Positive Displacement Pumps
• API 617 - Axial and Centrifugal Compressors and Expandercompressors
• API 618 - Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services

Objectives of pipeline stress calculations

The primary objective of pipeline stress analysis is to ensure the structural integrity of the pipeline system, taking into account the fluid it transports and potential failures during the pipeline's lifecycle. To achieve this, stresses must remain below the above-mentioned standards.

Secondly, pipeline stress calculations can ensure the operational efficiency of the pipeline network. This includes avoiding problems such as leakage, sagging, or unwanted shifts, but also certain actions that lead to design optimization, such as unnecessary excessive flexibility and choosing the right support structure, pipe connections, and joints.

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