On improving the fatigue life of U-form bellows

Abstract: There were three consecutive occurrences of bellows failure in a particular pressure safety valve (PSV) of a petroleum refinery within a time span of one week. The bellows were made of 316L grade austenitic stainless steel, and the PSV was mounted on one of the vessels of vacuum gas oil service in a hydrocracker unit. Metallurgical analysis of the failed bellows revealed that the failure had occurred by stress corrosion cracking (SCC). It was found that the SCC was promoted in the bellows due to the presence of high amount of chloride ions in the operating environment. Studies confirmed that SCC had initiated from the outer surface of the bellows and propagated inwards, resulting in leakage of hydrocarbon from the PSV. The source of chlorine in the environment was identified. It was discovered that SCC in the bellows was caused due to a previous failure in the heavy polynuclear aromatics (HPNA) absorption bed located upstream the process flow line. This failure was due to the presence of high concentrations of chlorine in the granulated activated carbon that was used in the HPNA absorption bed. During the repair of the HPNA bed, there was deposition of carbon soot on the body of the PSV. This carbon soot was the source of chloride ions for SCC to occur in the bellows. Generally, in chloride SCC, crack propagation in 316L SS takes place by transgranular mode. In the present case, however, the crack propagation was predominantly by intergranular mode. The metallurgical factors responsible for this change in micro-mechanism of crack propagation during SCC have been discussed.

Abstract: Bellows are complex shaped components which are widely used in pipe lines where the expansion and compression of the pipe occurs and absorbs the mechanical vibrations too, in high corrosion and high temperature environments. The constant movement and load which acts on the bellows leads to failure of the bellows. Thus, predictions of the fatigue life of the bellows are necessary. This paper investigates the fatigue behaviour of U shaped hydroformed bellows. The developments of meridional stress in bellows were found using the finite element method. The fatigue tests were conducted at room and elevated (650°C) temperatures. The experimental results of the fatigue test correlates well with the numerical results.

Abstract: Tubular Bellows are generally formed by hydroforming process is to achieve the uniformity in structure with adequate dimensional accuracy. This research investigation focused on evaluating the performance of the hydroformed tubular bellows expansion joints made up of inconel 625 (superalloy). The optical microscopic image has been used to find the variation in thickness of bellows. The flexibility and the fatigue property of the bellows were evaluated by using the spring rate test and life cycle test. The cross-sectional SEM (Scanning Electron Microscope) images were perfectly identifying the defects which presence on the surface of the convolutions of the bellows expansion joints.

Abstract: Current assisted superplastic forming combines electric heating technology and superplastic forming technology, and can overcome some shortcomings of traditional superplastic forming effectively, such as slow heating rate, large energy loss, low production efficiency, etc. Since formability of titanium alloy at room temperature is poor, current assisted superplastic forming is suitable for titanium alloy. This paper mainly introduces the application of current assisted superplastic forming in the field of titanium alloy, including forming technology of double-hemisphere structure and bellows.

Abstract: Titanium alloy bellows has advantages of light weight, excellent mechanical property, good heat resistance and corrosion resistance, etc. But the cold formability of titanium alloy is poor, so it is difficult to manufacture titanium alloy bellows by traditional mechanical forming and hydroforming. In this paper, current assisted superplastic forming technology was used to process titanium alloy bellows, which could overcome some shortcomings of traditional superplastic forming effectively, such as slow heating rate, high energy loss and low production efficiency. And titanium alloy bellows formed by this technology is of good quality with uniform wall thickness, light oxidation.

Abstract: Methods are described for constructing a fatigue curve based on strain- fatigue data for use in pressure vessel design. When this curve is used, the same fatigue strengthreduction factor should be used for low-cycle as for highcycle conditions. When evaluating the effects of combined mean and alternating stress, the fatigue strength-reduction factor should be applied to both the mean and the alternating component, but then account must be taken of the reduction in mean stress which can be produced by yielding. The complete fatigue evaluation of a pressure vessel can be a major task for the designer, but it can be omitted, or at least drastically reduced, if certain requirements can be met regarding design details, inspection, and magnitude of transients. Although the emphasis is on pressure vessel design, the same principles could be applied to any structure made of ductile metal and subjected to limited numbers of load cycles.

Abstract: In this paper the fatigue strength of U-shaped bellows under axial loading is studied. Fatigue tests of bellows in the elastic and plastic ranges are carried out by using the fatigue testing machine designed for this research, and the results are compared with the fatigue strength of test pieces made of the same material as bellows. From experimental data a formula for estimation of the life of U-shaped bellows made of stainless steel is proposed, i.e. [numerical formula] where N is cycles to failure and σ is the maximum stress value calculated from the design formula or charts in the 1st or 2nd report. This formula is compared with M.W.Kellogg's formula which is nowadays usually employed.