Storage spheres, also known as spherical tanks, are pressure vessels designed to store liquefied gases and other industrial products under controlled conditions. This type of installation is used in sectors such as Oil & Gas, petrochemicals, energy, and steelmaking, although it is particularly common in refineries, petrochemical complexes, storage terminals, and gas distribution centers.
During the manufacture and assembly of a sphere, welding operations generate localized heating and cooling cycles. As a result, residual stresses may develop in the material and must be controlled to ensure the vessel’s stability and reliability.
To reduce and redistribute these stresses, post-weld heat treatment, or PWHT, is recommended. In large spheres, heating can be carried out using gas burners installed inside the vessel, following a carefully planned internal combustion procedure.
Therefore, the heat treatment of a storage sphere does not simply involve heating the structure. It is a controlled industrial process in which parameters such as temperatures, holding times, heating and cooling rates, heat distribution, and suitable monitoring systems must be defined for each specific situation or project.
What Are Industrial Storage Spheres?
Storage spheres are pressure vessels whose geometry allows the stresses generated by internal pressure to be distributed uniformly. This feature makes them a common solution for storing gaseous or liquefied products.
Products that may be stored in this type of vessel include LPG, propane, butane, ammonia, and other gases or products used in petrochemical and industrial processes.
Although they can be found in different settings, their most common applications include:
- Refineries and hydrocarbon processing plants.
- Petrochemical complexes.
- Storage and distribution terminals.
- Gas production and treatment plants.
- Energy facilities.
- Certain steelmaking and industrial processes.
In the Oil & Gas and petrochemical sectors, these structures form part of critical facilities. Their manufacture, welding, inspection, and heat treatment must therefore be carried out in accordance with defined technical procedures and with full traceability.
What Is the Heat Treatment of a Storage Sphere?
The heat treatment of a sphere is a controlled process involving heating, temperature holding, and cooling. It is applied to the vessel after certain manufacturing or welding operations have been completed.
In this type of structure, the main objective is usually to relieve residual stresses caused by welding. These stresses occur because the areas close to the welded joint heat up and cool down differently from the rest of the material.
Post-weld heat treatment reduces or redistributes these stresses through a predefined thermal cycle. Depending on the material, thickness, design, and other requirements, it may also help to:
- Stabilize the welded assembly.
- Improve the material’s performance in service.
- Reduce the risk of subsequent deformation.
- Promote a more uniform distribution of stresses.
- Bring the vessel into compliance with the applicable design or manufacturing code.
- Provide thermal records confirming that the process was carried out correctly.
The need for treatment, as well as the parameters used, must be determined from the technical documentation and the specific characteristics of the sphere.
Why Do Residual Stresses Develop During Welding?
During the construction of a sphere, the different shell courses, segments, or metal sections are joined by welding. Each weld bead introduces heat locally into the material.
The welded area reaches high temperatures and subsequently cools. However, cooling does not occur completely uniformly throughout the structure. While one area expands due to heat, other parts of the vessel remain at a lower temperature and restrict that movement.
These differences in temperature and contraction may generate internal stresses even before the sphere is subjected to operating pressure.
The presence of residual stresses does not necessarily indicate a defect, since they are a common consequence of welding processes. Nevertheless, they must be assessed and controlled, especially in large vessels, thick sections, or applications subject to demanding technical requirements, in order to prevent cracking and deformation.
Post-weld heat treatment is used precisely to bring the structure to a specified temperature for a controlled period, thereby allowing these stresses to relax and redistribute.
Intec-Heat’s Role in the Heat Treatment of Spheres
The heat treatment of a storage sphere requires experience, suitable equipment, and the ability to coordinate a complex on-site operation.
Intec-Heat performs industrial heat treatments on large vessels and structures, with particular experience in Oil & Gas and petrochemical projects.
For storage spheres, the work may include defining the heating system, installing burners, applying insulation, positioning thermocouples, monitoring the thermal cycle, and preparing the corresponding records.
Proper execution of PWHT on this type of equipment requires not only technical expertise and specialized resources, but also rigorous planning, continuous coordination among the various teams involved, and the ability to adapt to actual site conditions.
Intec-Heat provides the flexibility needed to tailor each operation to the project requirements while maintaining precise process control and high safety standards, which are especially critical in refineries, petrochemical complexes, and other highly demanding industrial facilities.
The objective is to carry out the thermal procedure defined for the equipment while maintaining continuous control over the vessel’s temperatures, holding times, and temperature gradients.
When Is Post-Weld Heat Treatment Required?
The need to perform PWHT does not depend solely on the vessel having a spherical shape. It must be determined according to the material, thicknesses, welding procedure, intended service, and applicable manufacturing code.
It may be required when:
- It is specified by the applicable code or the project’s technical specification.
- The material thickness exceeds certain limits.
- Materials requiring specific post-weld control are used.
- It is necessary to reduce residual stresses in the assembly.
- The vessel’s service conditions are particularly demanding.
- The manufacturer or responsible engineering company includes it in the manufacturing process.
The decision must be supported by technical documentation. Likewise, the cycle parameters must not be improvised; they must be defined before treatment begins.
Frequently Asked Questions
Is a storage sphere the same as a spherical tank?
Yes. Both terms are used to describe spherical pressure vessels intended for the storage of liquefied gases or other industrial products.
What is post-weld heat treatment used for?
Its main purpose is to reduce and redistribute the residual stresses generated during welding. It also helps stabilize the structure and ensure compliance with the technical requirements established for the vessel.
Why is the treatment carried out inside the sphere itself?
The size of these structures usually prevents them from being treated in a conventional furnace. Internal combustion makes it possible to heat them directly at their location using burners installed inside the vessel.