Pipe stress analysis is a crucial operation in both pipe design and maintenance operations.
As pipe failures might lead to critical situations, the performance of correct pipe stress calculations becomes essential to ensure maximum safety and efficiency while verifying pipe design has been correct in order to extend the product’s life cycle.
Take a look at the basics of pipe stress analysis and learn how it’s performed and why it’s important in cryogenic applications and beyond.
What is pipe stress analysis
Pipe stress analysis is a testing method that examines a piping system’s behavior under different loading situations.
As such, it’s able to analyze how the material responds to pressure, temperatures, fluid and supports, thus helping engineers:
- Observe the pipe’s flexibility and stiffness
- Determine values such as maximum stresses, forces, displacements and restraints
- Monitor the limits of stress in piping components and their correspondence to applicable standards
- Decide on the right support systems to ensure their loads and movements are correct and safe avoiding unsuitable materials that do not support the necessary loads and pressures
- Notice potential disengagements from support structures and pipes
- Foresee how mechanical vibrations, seismic loads or acoustic vibrations might influence pipe operations
- Guarantee pipes are leak-proof to prevent leakage
- Select appropriate materials that meet strength and durability requirements
All in all, the main reason to perform pipe stress analysis is to guarantee maximum safety wherever pipe systems are installed, so that pipe failures can be minimized. The right pipe analysis can also extend the pipe’s life cycle and ensure the quality and integrity of the transported product.
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Main types of piping stresses
Certain pressure, temperature and vibration conditions, as well as occasional loads, all have an impact on pipe systems. As such, the main piping stresses can be divided in 5 categories:
- Hoop stress: a type of uniform pressure applied internally or externally, it can have an impact on the pipe’s diameter and wall thickness
- Axial stress: caused by factors such as thermal or pressure expansions, as well as applied forces that result in the pipe’s restrained axial growth. As different materials react differently to this type of stress, pipe stress analysis remains crucial to detect this issue.
- Bending stress: it originates by certain body forces that can be concentrated (such as those related to valves) or occasional (such as the ones created by atmospheric forces, including seismic movements or extreme wind events). Bending stress can also be detected as forced displacements that are generated by the growth of other equipment and piping that ultimately impacts the analyzed pipe.
- Torsional stress: caused by body forces that bring about rotational moments around the pipe axis.
- Fatigue stress: this is created by the combination of continuous stresses that may impact certain pipe systems.
Additionally, it’s also important to understand the three categories of loads that influence pipe stress:
- Primary or sustained stresses, which account for 55% of the standard allowable stress following ASME standards
- Displacement stresses, which should be kept between 80% to 90% of allowed ASME requirements and can be reduced by adding flexibility to the piping system
- Occasional stresses, originated by one-time events (typically related to seismic movements, extreme wind events or relief-thrust loads). ASME codes allow for certain increases in the event of these stresses, including allowing a 15% increase if the event lasts less than 8 hours and less than 800 hours per year (wind-related) and a 20% increase if the event lasts less than 1 hour and less than 80 hours per year (seismic movements and relief thrust).
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How to perform a pipe stress analysis
When is pipe stress analysis recommended
- Operating temperatures are 150F or higher
- Pipe or line sizes are 4’ or above.
- When rotating equipment is considered, the analysis should take care when line size is 2 ½” and above
- If pressure vessels are connected, analysis are performed when line size is 6″ and above
- Cryogenic pipe systems and those carrying hazardous chemicals must be subject to pipe stress analysis
- When the system is complex (branches are multiple)
- When a lack of flexibility is detected
- When the pipe is subject to vibrations (for instance, when suction or discharging operations occur)
- If the plant is in an area of high seismic activity
Options to perform pipe stress analysis operations
Inspection and hand calculations are accepted as techniques to perform pipe stress analysis. These methods are based on simplifying assumptions and mathematical models, and use theoretical equations and formulas. This can be enough for water systems that aren’t influenced by outside forces, as part of buried pipe stress analysis.
However, computer models today allow the piping stress engineer to access more information and to efficiently monitor multiple potential scenarios thanks to specialized software programs. This is the right approach for more complex piping systems, including high-pressure pipes, cryogenic pipes or hazardous-fluids systems because they allow input pipe geometry, material properties and loading conditions..
In the latter case, it’s important that the pipe stress analysis goes beyond understanding the software: the piping stress engineer must employ a holistic perspective where all the types of potential pipe stresses are considered, as well as following the applicable standards and best practices.
Inputs to be considered
- Governing code
- Design and operating pressure and temperature
- Material of construction and its specifications
- The fluid’s gravity and density
- Type of insulation technique used in the piping system
- The line size
- Corrosion values
- Schedule number
- Flange ratings and weight
- Weight of valves
Some tips and best practices
- The process must be guided by quality data that is guided by real-life scenarios that may influence pipe behavior
- Normal operations are not the only contexts to be included: start-up and shutdown scenarios must also be considered, in order to obtain realistic pipe stress calculations
- Even if carried out by computer models, pipe stress analysis should be verified and examined by expert human teams
- Records and documentation should be kept so that other evaluations in the future can compare values in an easy and more efficient manner
- ASME 31.1 – Power Piping, B 31.3 (Process Piping), 31.4 (Hydrocarbon pipeline), 31.8 (Gas Pipeline) 31.12 (Hydrogen piping and pipeline)
- ASME Section VIII – Pressure Vessels
- API 610 (Centrifugal Pumps), API 676 (Positive Displacement Pumps), API 617 (Centrifugal Compressors), API 618 (Reciprocating Compressors)
Objectives of Piping Stress Calculations
The first goal of piping stress analysis is to be able to guarantee the piping system presents structural integrity, considering the carrying fluid and potential likely failures in the piping’s life cycle. In order to do so, the stresses should be kept below the codes’ recommendations mentioned above.
Secondly, pipe stress calculations can guarantee operational efficiency for the pipe network. This includes avoiding issues such as leaking, sagging or undesired displacements, but also certain operations that lead to optimizing design such as unnecessary excess flexibility, choosing the right supporting structure, piping connections and joints.
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Cryospain, a company specialized in pipe stress analysis
At Cryospain we offer state-of-the-art super-insulated cryogenic pipe systems.
As part of our engineering expertise, our team provides our clients with the best pipe stress analysis operations, in order to guarantee our equipment is safe and optimized, and its life cycle extended. Learn more about pipe stress analysis in cryogenics and how we can help you access the right piping system for your business by getting in touch with us.