Pipe stress analysis provides an empirical tool to evaluate how a piping system behaves based on its material, pressure, temperature, fluid, and structure. Pipe stress analysis gives a good estimate of piping behavior in its operating conditions. It is a key component for designing a stable pressurized piping system, system configuration, support structure, flexible joint specifications, flange design, and guiding piping specification.
In this article, you will learn the basics of pipe stress analysis, analytical methods, objectives, the various stresses a pipe receives, and finally how to mitigate stress and ensure safety.
Basics of Pipe Stress Analysis
Piping systems are essential for most manufacturing plants or facilities, and they need to be engineered and installed with precision and care. The productivity and operational flexibility of many plants and facilities are highly dependent on the capacity, flexibility, and efficiency of transporting fluid so that equipment and machinery work effectively. Because of its vital role in the transport of fluids, the piping system design involves a systematic approach and requires various engineering tools and methods in its design before installation and commissioning.
A piping system undergoes different operational conditions, such as weight, pressure, temperature differentials, and dynamic loads. Thus, piping designed must comply to these considerations to avoid failure. When pipes routing complies with design specifications, piping stress analysis checks all requirements to ensure smooth functioning during its operational life. Piping stress analysis is the most significant practice in piping design.
In any process facility, pipes are not only the most critical components, they are also the busiest. Piping systems undergo nearly all types of loading, whether deliberate or accidental. It is very important to take note of all possible loads that a piping system will experience throughout the life cycle of a process plant during service as well as during other periods. Ignoring any such load during construction, testing, start-up, operation, maintenance, etc. will lead to a piping system being inadequately constructed. Upon the occurrence of such an ignored load, the pipes will fail. A piping system failure can result in a domino effect and can cause catastrophic damage to the entire facility.
The analytical approach for design may be by inspection, simple manual calculations, or complex computation by using a computer model. There are various types of computer models from simple 1-D beam elements to complex finite element models. For instance, if designing a simple water system with no external forces expected on the piping, then a visual inspection or manual simple calculations may be sufficient. On the other hand, a computer-aided model may be essential when designing a high-pressure, high-temperature, hazardous-fluid system with significant external forces expected on the piping system.
Depending on the application, many piping codes and standards may be applicable during a pipe stress analysis. This analysis evaluates the static and dynamic loads on the pipe imposed by:
- Internal and external pressures
- Fluid flow rates
- Seismic activity
- Wind activity
Objectives of Pipe Stress Analysis
To ensure structural integrity design, engineers perform stress analysis of critical piping systems. This practice protects against failure from different loads in the life cycle. An appropriate analysis ensures operational integrity by minimizing pipe displacement and their supporting structures while maintaining economic viability.
Proper pipe stress analysis guarantees safe operation and increases the piping life. Pipes are a piece of equipment that is no different from any other engineering equipment, like a pump, and must be properly designed and maintained to ensure its proper life. A pipe works like a large lever arm attached to other pieces of machinery. Catastrophic effects occur without proper design and installation.
Stresses in Pipes
The stresses on pipes can originate from various sources. To assure safe operation, it is necessary to evaluate the combined effects of all the forces involved on the pipes. These forces include the following:
- The bending moment applied between supports by the weight of the pipe and the fluid it carries.
- Stress applied by the internal pressure in the pipe.
- Linear or torsional bending displacement resulting from thermal expansion.
- The bending moment applied by vibrating displacement.
Pipe stresses can be categorized into three groups: primary, secondary, and occasional.
Due to weight and strain, primary stress most frequently occurs from applied mechanical loading. Extreme primary stress generates gross plastic deformation and rupture. There are no self-limiting primary stresses.
Once plastic deformation starts, it continues until force balance occurs, or until cross-section failure results. Only removal of the loading or strain hardening in the material may avoid failure. Limits for sustained stresses are proportional to the material yield stress, the ultimate strength, or time-dependent stress rupture properties.
Local primary stresses can surpass yield; however, they will act as secondary stresses under this state and redistribute themselves as the distortion of the local pipe wall occurs. The failing point would be when the entire cross-section of the pipe, undergoes plastic behavior.
In a piping system, secondary stress develops due to the system’s resistance against displacement, whether by thermal expansion or forced anchor and restraint movements. Distortion of the piping system appears to mitigate the stresses due to forced displacements, so it can be said that these stresses are self-limiting.
On the other hand, primary stresses are not self-limiting because they are not reduced by local yields when they increase. For instance, as the internal pressure rises the pipe wall stresses will continue to increase until the pipe ruptures.
Secondary stresses can also result in catastrophic failure after sustained application of a high number of loads. Just because a system has been operating for many years does not mean that the system has been sufficiently designed for fatigue failure.
Short-term events, such as seismic, wind, and relief-thrust loads, cause occasional stresses within the piping system. Some of the potential occasional load combinations include these three types of events. Most piping codes allow for increased pipe stresses occurring for a short time.
Reducing Pipe Stress
The pipe expands or contracts as its temperature varies from what it was at the installation time to when it comes into operation. Thermal expansion in a pipe can produce tremendous force and stress in the system. However, the expansion can be absorbed without causing excessive force or stress if the piping is flexible enough. One of the main considerations in the design of piping systems is to have sufficient flexibility built in the system to avoid failure.
In the case of primary stresses, there have been two major problems. Considering the wall thickness of the piping materials, including any reinforcement, the hoop stresses due to internal pressure should be within defined limits. Owing to strain, weight, and other sustained loads, the total longitudinal stresses should be less than the limit specified. Operating temperature is also a significant factor in determining these limits.
It is obvious that the shorter the pipe system, the lower the capital cost. Long pipe runs can also cause an undue drop in pressure, rendering it unsuitable for safe operation. However, the shortest direct layout is normally not suitable for the absorption of thermal expansion. To determine the behavior of the pipe when its temperature varies from ambient to operational, flexibility analysis provides the most economical configuration with a sufficient safety margin.
Pipes bend under the weight of their own. The longer the pipe, the easier it is to bend. If a pipe bends beyond its elastic limit after the load is removed, it behaves like a spring and returns to its original shape. The forces are far less than in a straight run if the elbows and anchors are incorporated to allow displacement. Some flexibility is also provided by the turns and offsets required to run the pipe from one point to another. Additional flexibility may be provided by adding expansion loops or joints.
Piping system stress analysis is an integral part of the design process. Many piping systems are designed and installed using rules of thumb and experience with similar systems. However, even a moderately complex piping system must use pipe stress analysis. Optimizing the design will ensure reliability and safety while keeping the installation and operational costs low.