A ship’s piping system is subjected to multiple loads during operation: internal pressure, thermal expansion due to temperature changes, displacement caused by hull deformation, and dynamic loads transmitted from equipment vibration. If the piping design does not fully consider these loads, rigidly connected pipes will develop cracks at stress concentration points, or even rupture. Jiangsu Haizhongzhou Shipping Industry Co., Ltd. introduces stress analysis technology into piping design and extensively uses flexible elements to absorb displacement and vibration.
Pipe stress analysis is typically performed using finite element software. Haizhongzhou’s designers build a three‑dimensional model of the piping and set boundary conditions – including pipe material, wall thickness, medium temperature, pressure, support positions and hull deformation. The software calculates primary stresses (generated by pressure and self‑weight) and secondary stresses (generated by thermal expansion and displacement constraints) at each node. According to code requirements, primary stress must not exceed two‑thirds of the material’s yield strength, and secondary stress must not exceed the allowable stress range. For pipe sections exceeding the limits, the designer reduces the stress level by adding expansion loops, adjusting support positions or changing the pipe routing.
Expansion loops are the most common structure for absorbing thermal displacement. On long straight pipe runs, if the temperature change is significant, the pipe will undergo substantial axial expansion and contraction. Haizhongzhou installs U-shaped or Z-shaped expansion loops on the main steam lines and exhaust lines in the engine room. The dimensions of the expansion loop (arm length and span) are calculated based on pipe diameter, wall thickness and temperature difference. In areas where there is insufficient space for an expansion loop, a bellows expansion joint is used instead. The expansion joint consists of multiple layers of stainless steel bellows and can absorb axial, lateral and angular displacement, but its selection and installation require special attention–guide supports and fixed supports must be provided at both ends of the expansion joint to prevent instability.
Flexible connections are also an important means of pipe stress control. Haizhongzhou installs rubber expansion joints or metal hoses at the interfaces between piping and vibrating equipment such as pumps and compressors. Rubber expansion joints can absorb vibration and small displacements, suitable for low-pressure systems; metal hoses (typically stainless steel bellows covered with a braided mesh sleeve) are suitable for high-temperature, high-pressure systems, and their bending radius should not be less than ten times the hose outer diameter to prevent fatigue fracture of the inner bellows due to excessive bending.
The design of pipe supports is equally important. Fixed supports fully restrain the pipe, suitable for locations where displacement needs to be limited; guide supports allow axial sliding of the pipe but restrict lateral displacement; spring supports are used for locations subject to vertical displacement, maintaining a constant supporting force through spring compression and extension. All of Haizhongzhou’s pipe supports are prefabricated and installed according to the design drawings, and rubber or PTFE pads are added at the contact surfaces between supports and pipes to prevent galvanic corrosion and wear. It is this “rigid-flexible combined” design philosophy that has kept the piping systems on Haizhongzhou-built vessels at a very low failure rate during long-term operation.
