• In: Training | On: Oct 16, 2024

Managing Residual Stresses in Pressure Equipment

Residual stresses are an often-overlooked but critical factor in the long-term reliability and safety of pressure equipment such as vessels, piping, and storage tanks. These stresses, which remain within a material after fabrication processes like welding, forming, or heat treatment, can significantly influence equipment’s structural integrity under high-pressure and high-temperature conditions.

In this article, we explore residual stresses, where they tend to accumulate in pressure equipment, the problems they can cause, and how they should be carefully managed and assessed, particularly around welds. This is especially important in industries like oil and gas, petrochemicals, and power generation, where equipment failure can have catastrophic consequences.

What Are Residual Stresses?

Residual stresses are the internal stresses that remain in a material after it has undergone plastic deformation or thermal cycling, such as during welding, forming, or machining processes. These stresses are independent of any external loads and can be either tensile or compressive in nature.

During welding, for example, localised heating and subsequent cooling of the weld metal and the heat-affected zone (HAZ) cause non-uniform expansion and contraction of the material. This results in residual tensile stresses in the weld area and the HAZ, potentially leading to cracking, while compressive stresses may develop in adjacent areas.

Residual stresses are classified into:

 

Macro-Residual Stresses:  Stresses that act over larger sections of the material, such as across welds, nozzles, and flanges.

Micro-Residual Stresses: Localised stresses that exist within the microstructure of the material, particularly around grain boundaries.

These stresses can significantly affect the equipment’s performance and longevity, making them a key consideration in the design, inspection, and maintenance of pressure equipment.

 

Where Are Residual Stresses Found in Pressure Equipment?

 

Residual stresses tend to accumulate in areas that experience significant temperature gradients or plastic deformation during manufacturing or repair processes. Key locations include:

1. Weld Zones

The weld metal and heat-affected zones (HAZ) around weld seams experience the highest levels of residual tensile stress due to the localised heating and rapid cooling cycles during welding. This is particularly true for thick-walled components, where temperature differentials between the weld and the base metal are more pronounced.

 

Macro test 

 

2. Nozzles and Attachments

Nozzles, flanges, and other attachments that are welded onto the main body of a pressure vessel or piping system are critical areas for residual stresses. The combination of differing material thicknesses, geometries, and welding processes can lead to significant stress concentration in these regions, making them more susceptible to cracking and fatigue failure.

3. Cold-Formed and Bent Sections

During cold bending or forming processes, such as in the fabrication of pipe bends or rolled plates, the outer layers of the material experience tensile residual stresses, while the inner layers experience compressive stresses. These stress imbalances can cause distortions or buckling, especially under operating loads.

 

Why Are Residual Stresses a Concern?

 

Residual stresses, though often invisible to the naked eye, can significantly affect the performance, safety, and lifespan of pressure equipment. Key concerns include:

1. Crack Initiation and Propagation

Residual tensile stresses, especially in welded components, promote crack initiation. When combined with operating stresses, such as internal pressure or external loading, these cracks can propagate rapidly, leading to fatigue failures or sudden fractures. This is particularly concerning in high-pressure applications, where even small cracks can lead to catastrophic equipment failure.

2. Stress Corrosion Cracking (SCC)

Residual tensile stresses are a major contributor to stress corrosion cracking (SCC). SCC occurs when a material is exposed to a corrosive environment under tensile stress, leading to cracks that can grow over time. This is a common problem in equipment exposed to hydrogen sulphide (H₂S) or chloride-containing environments, such as refineries and chemical plants.

3. Fatigue Failure

When combined with cyclic operational loads, residual stresses can significantly accelerate fatigue damage in pressure equipment. Welds, nozzles, and flange connections are particularly vulnerable to this form of degradation, as these areas are often subjected to both operational stresses and residual stresses from fabrication.

4. Distortion and Warping

Residual stresses in welded or formed components can lead to distortion or warping, affecting the dimensional tolerances of pressure equipment. This can result in misalignment of flanges, leaking gaskets, or difficulties in assembling or repairing the equipment. In severe cases, distortion can lead to operational inefficiencies or structural instability.

 

 

Assessing and Managing Residual Stresses in Pressure Equipment

 

Given the critical role that residual stresses play in the reliability of pressure equipment, it is essential to assess and manage these stresses, particularly around welds and other critical areas. Here are some of the most effective methods for doing so:

 

1. Residual Stress Measurement Techniques

Residual stresses cannot be directly detected by traditional non-destructive testing (NDT) methods like X-ray or ultrasonic testing (UT). However, more advanced techniques are available for measuring residual stress fields:

 

X-Ray Diffraction (XRD): XRD is widely used to measure surface residual stresses by analysing the diffraction patterns of X-rays passing through the crystalline structure of the material. It is particularly useful for detecting residual stresses in welds and the heat-affected zone.

Neutron Diffraction: This technique is used to measure residual stresses deeper within the material. Neutron diffraction is highly effective for thick-walled pressure vessels and provides a detailed stress profile across welded sections.

Hole-Drilling Method: This is a semi-destructive method where a small hole is drilled into the material, and the deformation (strain) around the hole is measured. The residual stresses can then be calculated from these strain measurements. This method is useful for mapping stress fields in welded components.

Contour Method: In this technique, the material is cut along a plane of interest, and the resulting deformations are measured. The residual stress distribution is then back-calculated from the deformation data. This method is highly effective for mapping residual stresses in welds and thick-walled sections.

 

2. Post-Weld Heat Treatment (PWHT)

 

Post-weld heat Treatment (PWHT) is one of the most effective methods for reducing residual tensile stresses, especially in thick-walled pressure vessels and piping systems. By reheating the material to a temperature below its transformation point and holding it for a set period, residual stresses can be relieved through thermal relaxation. PWHT reduces residual stresses and improves resistance to stress corrosion cracking (SCC) and fatigue failure.

According to ASME Section VIII, PWHT is typically required for pressure vessels made of certain materials or those that exceed specific thickness limits. The temperature and duration of PWHT must be carefully controlled to achieve optimal results without compromising the material’s mechanical properties.

 

 

3. Mechanical Stress Relief Methods

 

Mechanical methods, such as shot peening or hammer peening, introduce compressive stresses on the material’s surface, which can counteract the detrimental tensile residual stresses present in welds. Shot peening, for example, can be used in pressure vessels and piping systems to improve fatigue resistance, especially in high-cycle service environments.

 

4. Finite Element Analysis (FEA)

 

Finite Element Analysis (FEA) is a powerful tool for modelling the distribution of residual stresses in pressure equipment. FEA simulations can predict stress concentrations in complex geometries, such as nozzle-to-shell connections or dished heads, helping engineers identify areas requiring frequent inspections or targeted maintenance. By simulating different operational conditions and fabrication methods, FEA can also assist in optimising design to minimise residual stresses.

FEA Model

 

 

Best Practices for Managing Residual Stresses in Pressure Equipment

 

Residual stress monitoring is not typically part of routine inspections due to the complexity and cost of these methods (XRD, neutron diffraction, etc.). Monitoring is generally reserved for critical components or research purposes. It is crucial to account for residual stresses in relevant Fitness For Service Analysis.

However, if it is required, the following techniques may be utilised;

 

Residual Stress Monitoring: Use advanced residual stress measurement techniques, such as XRD or neutron diffraction, to monitor residual stresses in critical areas like welds, nozzles, and attachments where defects are present.

Implement PWHT: For pressure vessels and piping systems, apply post-weld heat treatment to relieve tensile residual stresses and reduce the risk of SCC, fatigue, and cracking.

Use Mechanical Stress Relief: In components exposed to cyclic loading, use shot peening or similar techniques to introduce compressive surface stresses and enhance fatigue resistance.

Leverage FEA for Design Optimisation: Use FEA simulations to predict residual stress distribution and to optimise the design and fabrication of pressure equipment. This can help identify high-risk areas and reduce inspection intervals.

 

In summary…

Residual stresses are a critical factor in the long-term performance and safety of pressure equipment. By understanding where these stresses are likely to occur, assessing them using the right techniques, and managing them through methods like PWHT, mechanical stress relief, and FEA, engineers can significantly reduce the risk of failure and ensure the continued reliability of pressure vessels, piping, and other equipment. The management of residual stress must be tailored to the material in question, as not all materials benefit from the PWHT.

Proactively addressing residual stresses in pressure equipment ensures safer operations, extends the service life of critical assets, and reduces maintenance costs. For more detailed guidance on managing residual stresses and other aspects of pressure equipment integrity, visit Wilkinson Coutts Engineering Training.

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