There are many methods to solve the problem of heat expansion and cold contraction of heating pipelines, such as pipeline preheating installation, elbow natural tension compensation, compensator, etc., but all methods have only one purpose: to make the pipeline operate safely within the allowable stress range of materials. In the direct buried pipeline, the application of elbow natural tension compensation is very limited. The compensator can most effectively reduce the stress of the pipeline during operation, but at the same time, the compensator has become the weakest link in the safety of thermal pipeline. Ordinary pipeline preheating has low installation cost and high pipeline safety, but it is subject to many restrictions during installation, such as the need to preheat the whole line in open ditch, difficult pipeline heating, and short operation time. One time compensator is produced by combining the advantages of compensator and pipeline preheating installation.
1. Disposable compensator structure
The structure of disposable compensator is shown in the figure. Because the expansion element is only used once and the medium pressure is low during preheating, the bellows can have a large amount of compression, which can greatly reduce the cost compared with ordinary compensator. The casing of disposable compensator will bear medium pressure during pipeline operation, so its wall thickness shall not be less than the wall thickness of standard pipes with the same diameter. As the stress of the whole compensator shell is relatively large, there are many stiffeners. After the one-time compensator absorbs the pipeline elongation, seal and weld the secondary welded junction firmly. Since the secondary welded junction is subject to shear stress, it is recommended to use two layers and three times of welding, as shown in the figure.
2. Installation of disposable compensator
As shown in the figure, the pipe section where one-time compensator is installed shall be separated from other pipe sections with fixed piers to ensure absorption of preheating elongation of fixed length pipe. To ensure its accuracy, the theoretical elongation of the fixed length pipe can be calculated before preheating, and then marks can be made on each one-time compensator, so that the sum of the compression amount of each one-time compensator is equal to the calculated elongation of the fixed length pipe. When the preheating elongation is in place, the secondary welded junction of the compensator will be welded and fixed. The pipeline no longer has expansion and contraction movement in operation, and the expansion and contraction of the pipeline length caused by its temperature change will be converted into the internal stress of the pipeline.
