The development of fracture mechanics has defined one.; Crack tip opening displacement; (Crack Tip Opening Displacement, CTOD), which can accurately evaluate the toughness of welded joints. In 1991, the British Welding Research Institute put forward the standard BS7448 Part 1, which gives the test methods of critical CTOD, J integral and KIC of metal materials. In 1997, the second part of the bid, BS7448 Part II, Method for Determining KIC, Critical CTOD and J-integral of Weld Metal Materials, was put forward. In view of the characteristics such as non-uniformity of performance and residual stress in each area of welded joints, BS7448 Part 1 was supplemented, which is generally recognized by the international engineering community, Test standard for determining the CTOD fracture toughness value of welded joints.

Based on years of operation experience, five golden guidelines are summarized for the layout design of bellows compensator:
4.1 Only one bellows compensator is set between any two adjacent fixed supports.
4.2 Various fixed supports shall have sufficient strength and rigidity.
4.3 During installation, it is not allowed to forcibly adjust the deviation of pipeline device by the deformation of bellows compensator.
4.4 The diameter of any two adjacent fixed points shall be the same and in a straight line. The spacing between the first guide, the second guide and the third guide of the compensator shall be reasonably arranged. 4.5 The flow direction of the medium in the process of the device shall meet the requirements of the manufacturer, and the pre stretching shall be stopped correctly to reduce the elastic thrust of the bellows compensator on the fixed support. The pre stretching amount is usually 50% of the thermal compensation amount, The compensator manufacturer shall pre stretch it before leaving the factory. Of course, compared with other compensators, the elastic force of bellows deformation and the thrust of blind plate formed by internal pressure are very large. After calculation, the thrust of a DN200 heavy load fixed point in hot state is about 13 tons, and the comprehensive axial thrust of cold water pressure test is 16 tons. If it is an overhead fixed pier, its root size is 3 m × 2.5 m × 0.8 m larger; However, the bellows compensator is used on the directly buried steam pipeline. Although the thrust received by the fixed point is also 16 tons, it has a proprietary internal fixed support product. The strength and stiffness of the external protective casing are very large, and it is directly buried below 0.8m depth. The soil itself exerts a certain pressure on the steel casing, and the soil itself becomes the huge foundation of the directly buried steam pipeline. The arrangement of bellows compensator in the directly buried steam pipeline is the same as that in the overhead pipeline, but also follows five golden guidelines, which is more stringent than the overhead arrangement in a certain level.
With the long-term implementation of the national energy conservation and emission reduction policy, the direct burial of steam in urban heat supply network will become the main laying method of urban heat supply network, and the bellows compensator will become the main thermal compensation method of urban direct burial heat supply network. Therefore, the correct layout of bellows compensator has become the key to ensure the design quality and safe operation of urban heat supply network.