Pipeline systems are important components in many industries such as refineries, power plants, and chemical plants, where their prime purpose is the transport of fluid from one piece of equipment to another. Normally, the content fluid of the pipe is hot, and science the piping system is initially designed at reference temperature, the temperature change cause thermal expansion. If the ends of the piping system are restricted, which is usually the case, forces and moments are produced at the supports of the pipes causing thermal stresses in the system. The art of piping flexibility analysis is to give enough flexibility to the piping system so that the resulting stresses at all points of the system remain under a safe limit [1]. Usually, this flexibility is designed with an engineering pipe loops or through the use of flexible expansion joint at the ends.
Pipe loops
Natural flexibility is generally the cheapest way to accommodate thermal expansion in piping system. Offsets in the piping system are required for providing natural flexibility. Offsets can exist due to the routing of the lines, or can be created in the form of Z-bends (figure 1) or U-loops (figure 2).
Locations of anchor and supports for the system are important for designing of pipelines with natural flexibility. Stress analysis should be performed to determine the location in the piping system where the least amount of thermal displacement occurs. Locating the anchors at these points will result in the lowest anchor point loads. Advantages of hard loops are same material as the rest of the piping, medium anchor loads and construction on site. But using of pipe loops require lots of room. Also fabrication, hanging and insulation are costly, and high lateral loads are acting on moment guides. [2].
Expansion joints
When there are long straight runs of pipe with no room to add expansion loops, or adding them would result in excessive pressure drop, or existing offsets are too short or too few to provide sufficient natural flexibility, then expansion joint can be used.
There are two different types of expansion joints (unrestrained type and restrained type), and it is very important to know differences between them. They have different applications, different acting forces along the pipeline, and must be installed in a different ways.
Unrestrained type joints are usually used to accommodate axial movement in piping systems. Bellows joints and packed slip type expansion joints are examples (figure 3).
Restrained type joints are used to take up offset and angular displacements. Hinge joints, gimbal joints, tied bellows fall into this category (figure 4).
Unrestrained bellows expansion joints can be applied when axial displacement is presented and only small amount of torsional movements. Because angular displacement decreases ability of bellows expansion joints to take in movement and hold pressure. In cases when torsion in pipe system is significant, than using of restrained expansion joints are necessary.
The thrust load which tends to force the expansion joint open is referred to as pressure thrust and is equal to the cross sectional area multiplied by the system design pressure. The pressure thrust load acts on the system anchors.
Unrestrained expansion joints are not provided with attachments such as tie rods or hinges to restrain pressure thrust. Therefore, they can be used only in piping systems that incorporate correctly designed anchors and pipe alignment guides. It is essential that the pipeline be properly guided before using unrestrained expansion joints. Failure to provide proper guiding can result in untimely failure of the expansion joint.
Restrained expansion joints typically require offsets in the system piping and absorb movement by lateral bending and/or rotation. Restrained expansion joints have the advantage of eliminating pressure thrust forces and providing flexibility with minimal forces. Guiding requirements with restrained expansion joints are far less stringent. Restrained expansion joints provide piping systems with increased flexibility by allowing relatively free lateral and bending movements. Pressure thrust loads are contained within the joint, resulting in lower anchor point loads. Because restrained joints are designed to accommodate lateral and bending movements, the guiding requirements are far less stringent [3].
Conclusion
- Thermal expansion can downfall piping system, but it can be prevented with the proper design and pipe system supplies;
- When choosing an expansion joint you must know the magnitudes, directions and types of movements that it needs to accommodate;
- Selecting the right type of expansion joint is a key to having a problem free system.