【 Weak current College 】 on bellows compensator failure---Power By 【 China power house network 】

Introduction: the failure of the bellows are corrosion leaks and instability in two forms. By correctly selecting bellows construction materials and structures, reasonable design waveform parameters and fatigue life, and we assure the installation of quality measures that can improve the bellows type safety and reliability.

Keywords: bellows compensator failure analysis
1 preface
Bellows compensator can in many industry-wide, in addition to excellent compensation ability, high reliability, is the main reason. Its reliability is through the design, manufacture, installation, operation and management, and other links to guarantee that any one link of runaway will result in lower life expectancy of compensator or even failure. Author after years of statistics found that caused the bellows failure: design of 10%, manufacturers of shoddy work accounted for 50%, the installation does not meet the requirements of device descriptions, and the remaining 20% by running management caused by wrong.
2 bellows compensator failure types and causes
2.1 failure types
Bellows pressure test failure in the pipeline, and there are during a run. Pipeline pressure problems mainly has three types: as the pipe system temporary support improperly, or piping attachment bracket set unreasonable, lead to excessive damage, corrugated pipe bracket deformation and failure; as the bellows design by pressure or displacement safety rich enough, pipeline pressure of time bellows produce unstable deformation; compensator manufacturing quality, manufacturing plant, Jerry, 5-layer stainless steel without changed to layer 3 or less.
Corrugated pipe failure during run mainly corrosion leaks and buckling deformation in two forms, with most of the corrosion. From the anatomy of the bellows corrosion failure analysis found that corrosion is often divided into point corrosion perforation and stress corrosion cracking, including chloride stress corrosion cracking accounted for the entire corrosion failure of 95%. Bellows instability intensity instability and structural instability of two types, intensity instability include internal and external pressure bellows plane instability and external pressure bellows circumferential instability; structural instability is the internal pressure bellows compensator column buckling.
2.2 design fatigue life and stability and the relationship between stress corrosion
Bellows design considering compressive strength, stability and fatigue properties, such as the three factors. Although the United States National Institute of standards and standard on the few EJMA calculations and evaluation are clearly defined, but from many years of practice and corrugated pipe failure analysis found that the standards for calculating on stability and evaluation methods not comprehensive enough, and the fatigue life and only give a rough comparison of the limits of the range (average fatigue life at 103-105). Sometimes a fully compliant with standards based products actually used some problems also occur. As internal pressure axial compensator pre-State pressure testing of the bellows is easy to produce surface instability, large diameter external pressure axial compensator full displacement working bellows easy week to instability, small diameter dual lever-type compensator, hinge-type compensator full displacement working state is easy to produce column buckling. Bellows excessive deformation, not only for its impact on the stability, but also to stress corrosion with a favourable environmental conditions.
2.2.1 bellows fatigue life with its comprehensive stress bellows compensates the quantity depends on its fatigue life and fatigue life, bellows single wave, the smaller the amount of compensation. In order to reduce costs, increase the amount of compensation for a single wave, some manufacturers will bellows allowable fatigue life drops very low, it will result in the displacement caused by the corrugated pipe bending stress ebb to the great, comprehensive stress is very high, reduced the stability of the bellows. Table 1 gives no strengthening of U-shaped bellows allowable fatigue life and Meridian to comprehensive stress and single wave in the relationship between the amount of compensation.


Table 1 allowable stress fatigue life and comprehensive and single wave compensates the quantity relationships

Allowable fatigue life [N]/time 5000300020001000500200100
Comprehensive stress σ t/M P a 90298810661224141717412051
Single wave displacement e/% 708090100120150175


Note: (1) comprehensive stress caused by displacement and stress of bellows Meridian to integrated nominal stress; (2) bellows average fatigue life n = 10, [N]; (3) single wave displacement is given to allowable life 1000 times as the reference value.
As can be seen from table 1, lower fatigue life can significantly improve the bellows single wave displacement, but at the same time there is also a comprehensive stress bellows significantly increase, this is bound to the strength and stability of the bellows caused a greater impact.
2.2.2 comprehensive stress and bellows pressure resistance intensity by standard for stability and Zhou in corrugated plane to the stability of the calculation method and standard, we can see that they reflected the strength of both. When the bellows design of allowable life is low, not only its child afternoon to comprehensive stress is high, the ring to relatively high stresses, the bellows into local soon, cause plastic deformation of the bellows instability.
For internal pressure bellows, displacement and stress in the corrugated pipe wave crest and the trough Department formed plastic hinge, plus the pressure stress, bellows quickly produce plane instability. This is the lower fatigue life bellows in displacement under plane buckling stress much lower than high fatigue life of bellows. For example, in the pre-State, i.e. the amount ofallowable bellows displacement values of 1/2, a permissible fatigue life for 200 of bellows, has not yet reached its allowed design pressure, have generated plane instability; permissible fatigue life is 1000 times the design pressure bellows, reach, the bellows in plane steady state, 1.5 times the design pressure, corrugated pipe in a critical state of instability; permissible fatigue life is 2000 times the design pressure bellows reached 1.5 times, bellows is still flat and stable state.
From the longitudinal profile of external pressure bellows, equivalent to a pressure of arch beams, work the bellows is stretched, the equivalent of arch beam reduces its resistance to high arch instability of the ability of natural decrease. When the bellows single wave offset too large, Corrugated straight section inclined so that the diameter of a narrowing of the bellows crest of the trend, but the wave crest circle diameter is determined, in order to coordinate and deformation, wave collapse, the instability of the bellows. Appropriate standards at home and abroad, about the displacement of the corrugated pipe external pressure to influence the stability of the week are not involved, to be explored.
To sum up, although so far the application of thermal networks have not yet been discovered in the process caused by fatigue damage, but the bellows is too low for fatigue life, will lead to disastrous consequences.
2.2.3 compensator displacement and its column stability for penthouse lever-type and hinge-type compensator, lateral displacement is determined by the corrugated pipe angle displacement caused by the intermediate segment tilt. When the bellows produce angular deflection, bellows out side pressure area greater than depression side bearing area, causing the Compensator attached a lateral force, compared with axial compensator easier to produce column buckling. Obviously, the greater sin wave displacement of the corrugated pipe, compensator, the greater the morelateral displacement, easy to produce column buckling.
3 the reliability of the bellows Compensator
The reliability of the bellows is determined by the design, manufacture, installation and operation management, multiple link. Reliability should be several aspects from this.
3.1 reliability design
3.1.1 material selection for the corrugated pipe heating network, with the exception of selection shall take into account the work of the media, working temperature and external environment, you should also consider the possibility of stress corrosion, water treatment agent and pipe cleaning effect of the material and, on that basis, combined with bellows material welding, molding and material cost performance, optimizing the economic and practical bellows manufacture materials.
In General, the selection of materials should meet the bellows to the following conditions: (1) good plasticity, easy to bellows of forming and through subsequent processing technology (cold-hardening, heat treatment and so on) have sufficient hardness and strength. (2) high elastic limit, tensile strength and fatigue strength, guarantee the normal work the bellows. (3) excellent welding performance, meet the bellows in the production process of welding process requirements. (4) the better corrosion resistance, meet the bellows work in different environments. Most manufacturers are made of austenitic stainless steels, such as material designation as 0Cr18Ni9 (equivalent to 304), 00Cr19Ni10 (equivalent to 304L), 0Cr17NiMo2 (equivalent to 316), 00Cr17Ni4Mo2 (equivalent to 316L). In order to improve corrosion resistance, the corrugated pipe heating network bellows material lot selection 316 or 316 l, both materials used for thermal networks should be performance price comparison for fine materials.
For the laying of heat pipe trench, when the pipeline compensator low-lying, rainwater or accident of sewage will soak bellows, consideration should be given to the selection of corrosion-resistance and stronger materials such as iron-nickel alloys, nickel alloys, etc. As a result of such material, higher prices in manufacture of corrugated pipes, can be considered only in contact with corrosive surface increase a layer of corrosion resistant alloys.
3.1.2 fatigue life design by bellows compensator failure type and cause analysis shows that the stability of the plane of the bellows, the stability and corrosion resistance are its displacement quantity that is related to fatigue life. Low-fatigue life will result in the bellows corrosion resistance and stability. According to the trial and experience, for heating project of bellows fatigue life should be not less than 1000 times.
Most of the corrugated pipe failure is caused by corrosion of the external environment, so the design of the structure of the Compensator, consider isolating external corrosion medium and bellows contacts. As for external pressure axial compensator can be exported into and export increase packing seal between tube device, its role is equivalent to sleeve compensator to resist external corrosion medium of intrusive, bellows to increase the secure barrier, even if the damage to the bellows compensator can also serve to compensate and avoid corrugated pipe failure.
3.2 guarantee quality installation
Bellows cannot load hoisting should be separate; in addition to the design requirements of pretension or cold tight predeformation, forbidden to corrugated pipe deformation method to adjust the pipeline installation deviation; the installation process does not allow welding slag splash onto the corrugated pipe surface and other mechanical damage; all bellow the active component shall not be an external component card dead or limit their activities to the site to work properly; the hydraulic test water shall be clean, non-corrosive to stainless material should strictly control the chloride ion in water should not exceed 25 × 10-6, and should be timely discharge of water and other corrugate.
4 closing
Compensator problems mainly has the stability and corrosion of the bellows. Through reasonable design bellows waveform parameters and fatigue life, installed correctly and tube stress analysis improvement measures to address the issue of stability of the bellows. For corrosion problems can be solved in two ways: (1) reasonable bellows material selection and design of the Compensator, blocking the corrosion source. (2) strengthen small room water management, and fundamentally solve the problem of corrosion.