2. Valve body inspection Every six months, check whether the valve body appearance is good, whether the installation flange has leakage, if it is convenient to check whether the valve body seal is good, whether there is wear, whether the valve plate operates flexibly, and whether the valve has foreign matters stuck.

Analysis of failure types and causes of compensators
1、 Failure type
The failure of the compensator occurs during the pipeline pressure test and operation. There are mainly three types of problems in pipeline pressure test: support damage and failure of compensator due to improper temporary support of pipeline system or unreasonable setting of fixed support of pipeline system; Due to insufficient safety margin of pressure or displacement considered in the compensator design, the compensator will lose its stability and deformation during pipeline pressure test; The manufacturing quality of the compensator was poor. The manufacturer cut corners and changed the 5 layers of stainless steel to 3 layers or less without permission. The failure of the compensator during operation is mainly manifested by corrosion leakage and instability deformation, of which corrosion failure is the most. From the anatomy of corrosion failure compensator, it is found that corrosion failure is usually divided into pitting corrosion perforation and stress corrosion cracking, of which chloride ion stress corrosion cracking accounts for about 95% of the total corrosion failure. There are two types of compensator instability: strength instability and structural instability. Strength instability includes plane instability and circumferential instability of external pressure compensator; The structural instability is the column instability of the internal pressure compensator.
2、 Relationship between design fatigue life, stability and stress corrosion
The design of the compensator mainly considers the three factors of pressure strength, stability and fatigue performance. Although the national code and the American EJMA code have clear rules for the calculation and assessment of these aspects, it is found from years of application theory and failure analysis of compensators that the calculation and assessment methods for stability given in the code are not comprehensive enough, and fatigue life only gives a relatively rough boundary range (uniform fatigue life is applicable in 103~105). Sometimes a product that fully meets the requirements of the specification will also present some problems in practical application. For example, in the pre displacement state of the internal pressure axial compensator, the compensator is prone to plane instability during the pressure test, in the full displacement working state of the large diameter external pressure axial compensator, the compensator is prone to circumferential instability, and in the full displacement working state of the small diameter composite rod type compensator and hinge type compensator, the column instability is prone to occur. The excessive deformation of the compensator not only affects its stability, but also provides favorable environmental conditions for stress corrosion. The fatigue life of the compensator and the compensation amount of its comprehensive stress compensator depend on its fatigue life. The higher the fatigue life, the smaller the single wave compensation amount of the compensator. In order to reduce the cost and improve the single wave compensation, some consumer manufacturers have reduced the allowable fatigue life of the compensator to a very low level, which will lead to a large meridional bending stress of the compensator caused by displacement, and a high comprehensive stress, which greatly reduces the stability of the compensator. Table 1 shows the relationship between the allowable fatigue life of the unreinforced U-shaped compensator and the meridian comprehensive stress and the single wave compensation amount.