Prefabricated piping and stress relief: How to address pipeline thermal expansion in modular design?

Prefabricated piping and stress relief: How to address pipeline thermal expansion in modular design?

1. The Invisible Force: Understanding Thermal Expansion in Constrained Modules

In modular skid design, piping systems are rigidly framed within a structural steel base. When operational temperatures cause pipes to expand or contract, this natural movement is restricted by the fixed endpoints of the skid itself. This restriction generates significant internal forces, known as thermal stress. If not properly addressed, these stresses can lead to catastrophic failures, including deformation of pipe supports, leaks at flanged joints, damage to connected equipment like pumps and valves, and ultimately, fatigue cracking. The challenge is magnified in prefabricated modules because the compact, space-efficient layout often results in pipe runs with limited flexibility. Therefore, the first and most critical step is a thorough stress analysis during the design phase, using specialized software to simulate temperature changes and predict the resulting forces, displacements, and stresses on the piping and its restraints.

2. The Designer's Toolkit: Strategic Flexibility through Layout and Hardware

To safely absorb thermal expansion, designers employ a combination of strategic layout and specialized components. The most effective fundamental approach is to incorporate inherent flexibility directly into the piping geometry. This is achieved by intentionally designing expansion loops or offsets (Z-shaped or U-shaped bends) into the system. These features allow the pipe to flex and absorb its own thermal movement elastically within the skid boundaries. Where space is too limited for loops, metal bellows expansion joints serve as a compact solution, accommodating movement through their corrugated design. Furthermore, the correct selection and placement of pipe supports are paramount. Instead of rigid restraints, guides and spring hangers are used. Guides restrict movement in certain directions while allowing it in others, and spring hangers support the pipe's weight while permitting vertical movement, preventing the transfer of excessive stress to nozzle connections on sensitive equipment.

3. The Final Assurance: Pre-Commisioning Verification and Cold Springing

The theoretical design must be validated with physical verification before the skid is shipped. A critical procedure is the pre-insulation visual walkdown. Before installing insulation, the entire piping system is inspected to ensure all supports, guides, and expansion joints are installed correctly according to the design drawings, with no unintended restraints. For critical systems, a technique known as cold springing or "cut short" may be employed. This involves intentionally misaligning the pipe during cold installation by a calculated amount, so that when it expands to its operating temperature, it settles into a neutral, low-stress state rather than an over-stressed one. Finally, during the Factory Acceptance Test (FAT), the system is subjected to thermal cycles—heated to, or near, its operating temperature—while monitoring movements and stresses at key points. This practical testing confirms the effectiveness of the design and ensures the module arrives on-site ready for safe, long-term operation.

Summary 

Effectively managing thermal expansion is not an afterthought but a foundational aspect of successful skid-mounted module engineering. It requires a proactive, systematic approach that integrates sophisticated stress analysis, intelligent layout design with flexible components, and rigorous physical validation. By mastering this discipline, manufacturers like Shenqi deliver modules that are not only compact and efficient but also inherently safe, reliable, and durable, capable of withstanding the rigors of thermal cycling throughout their operational life.