The reduction of fatigue life of bellows compensator can greatly increase the single wave displacement of bellows, but at the same time, the comprehensive stress of bellows is also greatly increased, which will have a great impact on the strength and stability of bellows. The comprehensive stress and compressive strength of bellows reflect the strength problem. When the allowable service life of bellows is low, not only the comprehensive stress in meridian direction is higher, but also the circumferential stress is relatively high, which makes the local part of bellows enter into plastic deformation quickly, leading to the instability of bellows.

　　For the bellows under internal pressure, the displacement stress forms plastic hinge at the wave crest and trough of the bellows. In addition to the pressure stress, the plane instability of the bellows quickly occurs. This is the fundamental reason why the plane instability pressure of bellows with low fatigue life is much lower than that of bellows with high fatigue life. For example, when the displacement of the bellows is 1 / 2 of the allowable value, a bellows with a allowable fatigue life of 200 times has not yet reached its allowable design pressure, resulting in plane instability; When the allowable fatigue life of the bellows is 1000 times, the bellows is in plane stable state when the design pressure reaches 1.5 times of the design pressure, and the bellows is still in the plane stable state when the allowable fatigue life of the bellows reaches 1.5 times of the design pressure.

　　From the longitudinal section of the external pressure bellows, it is equivalent to an arch beam under pressure. When the stainless steel bellows, metal hose and stainless steel compensator work, the bellows is in the tensile state, which is equivalent to that the arch beam reduces the arch height, and its ability to resist instability is naturally reduced. When the single wave displacement of the bellows is too large, the straight part of the corrugated pipe inclines, which makes the diameter of the wave crest of the bellows decrease. However, the diameter of the wave crest ring is determined. In order to coordinate the deformation, the peak collapse will occur and the circumferential instability of the bellows will occur. In the corresponding standards at home and abroad, the influence of displacement on the circumferential stability of bellows under external pressure is not involved, which needs to be further discussed.

　　To sum up, although the failure caused by fatigue has not been found in the application of thermal pipe network, the low design fatigue life of bellows will lead to disastrous consequences.

　　The displacement of the compensator and the stability of the column. For the compound tie rod type and hinge type compensator, the transverse displacement is realized by the inclination of the middle pipe section caused by the angular displacement of the bellows. When the bellows has angular displacement, the bearing area of the protruding side is larger than that of the concave side, which leads to the additional lateral force of the compensator, which is more likely to cause column instability than the axial type compensator. Obviously, the larger the single wave displacement of bellows is, the greater the lateral displacement of compensator is, and the more likely to cause column instability.

　　The reliability of bellows compensator is composed of design, manufacture, installation and operation management. Reliability should also be considered from these aspects

　　In addition to the working medium, working temperature and external environment, the possibility of stress corrosion, the influence of water treatment agent and pipeline cleaning agent on the material should be considered in the material selection of corrugated pipe used for heat supply network. On this basis, the economical and practical bellows material is optimized by combining the welding, forming and performance price ratio of the material.

　　In general, the material of bellows should meet the following conditions: (1) good plasticity, which is convenient for the processing of bellows, and can obtain enough hardness and strength through subsequent treatment process (cold work hardening, heat treatment, etc.). (2) High elastic limit, tensile strength and fatigue strength ensure normal operation of bellows. (3) Good welding performance, meet the welding process requirements of bellows in the production process. (4) Good corrosion resistance, meet the requirements of bellows in different environments. Most manufacturers use austenitic stainless steel, such as 0Cr18Ni9 (equivalent to 304), 00Cr19Ni10 (equivalent to 304L), 0cr17nimo2 (equivalent to 316), 00cr17ni4mo2 (equivalent to 316L). In order to improve the corrosion resistance of corrugated pipes, 316 or 316L are mostly used for corrugated pipes in heating pipe network. These two materials should be better materials with better performance and price when used in heat supply network.

　　For the heat pipe network laid in the trench, when the pipeline where the compensator is located is at a low terrain, the corrugated pipe will be soaked by rainwater or accidental sewage. Therefore, materials with stronger corrosion resistance, such as iron nickel alloy and high nickel alloy, should be selected. Due to the high price of this kind of material, only a layer of corrosion-resistant alloy can be added to the surface contacting with corrosive medium when manufacturing bellows.

　　The fatigue life design shows that the plane stability, circumferential stability and corrosion resistance of bellows are related to the displacement, i.e. fatigue life. If the fatigue life is too low, the stability and corrosion resistance of bellows will be reduced. According to the test and application experience, the fatigue life of bellows used in heating engineering should not be less than 1000 times.

　　Most of the failure of bellows is caused by corrosion of external environment, so the contact between external corrosive medium and bellows can be considered in the structural design of compensator. For example, for the outward type compensator, a packing sealing device can be added between the outlet end ring and the outlet pipe, which is equivalent to a sleeve compensator, which can not only resist the invasion of external corrosive medium, but also add a safety barrier to the bellows compensator. Even if the bellows is damaged, the compensator can also play a compensation role and avoid the failure of the bellows.

　　Ensure installation quality

　　The bellows can not bear load and should be lifted separately; except for the pre deformation required by pre stretching or cold tightening, it is strictly forbidden to adjust the installation deviation of the pipe by deforming the bellows; welding slag is not allowed to splash on the surface of the bellows and suffer other mechanical damage during the installation process; all moving components of the bellows shall not be blocked by external components or restrict their normal operation; The water for hydrostatic test must be clean and non corrosive. For austenitic stainless steel, the chloride ion content in water shall not exceed 25 × 10-6, and the accumulated water in the ripple shall be drained out in time