Issue 38

F. Majid et alii, Frattura ed Integrità Strutturale, 38 (2016) 273-280; DOI: 10.3221/IGF-ESIS.38.37 274 interacting with the stored product and the external environment such as climate conditions and earthquakes. The higher number of PVs accidents [1] oblige us to be careful when using such equipments and to go beyond the codes and the standards for further detailed engineering design, develop new concepts in the performance framework and create a more dynamic vision and methodology as part of predictive and autonomous maintenance. The designers do usually a routine design of the PVs, but they don’t take into consideration the fatigue and the cumulative damage calculations for lifetime prediction. A lot of researchers are usually dealing with the uniaxial fatigue. But many other researchers tried to deal with the multiaxial fatigue to show the complexity of the phenomenon. A PV is always subjected to multiaxial loadings and multiaxial stresses. Meanwhile, the prediction of industrial equipments’ reliability and availability still a difficult task for final clients and engineers. Thus five approaches dealing with multiaxial fatigue exist in the literature. The first approach is the stress or strain invariant approach leaded by many authors [2- 8]. The second one is the critical plan approach leaded by Brown and Miller [9-28]. The third one is the integral approach leaded by [29-32]. The fourth one is the energetic approach leaded by [33, 34]. The fifth and the last one are the empiric formulas leaded by [35-38] for high cycle fatigue et Mowbray [39], Manson and Halford Kalluri and Bonacuse for low cycle fatigue. The metal’s damage due to fatigue has a well-known cycle, going through micro crack initiation, then its propagation until the rupture at the end. The fatigue rupture causes 50% to 90% of all the mechanical failures. According to many researches as Fatemi, 2010 and NASA, 1994 for metal, micro cracks of about 10 to 100 micrometers uses 60 to 80% of the fatigue resistance, in other words the metal life time. That’s why it is very interesting to study the small cracks in progress ie the first stage (Stage I) of crack. One of the major PV’s failures is the fatigue’s cracking. For that reason, we have to predict and analyze the cracks behavior, and specifically the crack propagation, in order to ensure the correct maintenance of PV. Many studies have been developed to face this kind of failures . P RESSURE VESSELS DESIGN he tanks are classified into three groups according to the operating pressure The atmospheric storage tanks for operating pressure of less than 18 kPa which are managed by the API 650 standard, The low-pressure storage tanks 18 kPa <P <100 kPa which are managed by the API 620 standard and PVs whose operating pressure P> 100 kPa which are managed by ASME Sec VIII [40]. In this part of work, we developed a standard methodology for PVs design. We start by defining the design assumptions through the PV’s geometry, the site conditions, the service conditions, the test conditions and the design conditions. Then, the material choice is done through the clients recommendations and the international standards CODAP, ASME II, EN13345 or EN 10222-4 or standards for materials choice EN-10025, EN 10028, ISO 9327-4: 1999, JIS G 3202: 1988 and ASTM. In the next step, we define the codes for PV calculation, figure (a), such as ASME, CODAP or API. Next, we define earthquake, safety elements, metallic construction codes such as CM66, and the regulations for the stored product. After that, we start the PV element calculation through the shell’s thickness calculation, figure (a), head’s thickness calculation, figure (b), nozzles calculation, figure (c), saddles calculation, seismic through UBC 1997 ground supported code and wind through the building code ASCE 7-05 verifications, calculation of lifting lugs, figure (d), and finally the calculation of fire circuit tanks through NFPA or other recognized standard [50]. P RESSURE VESSELS MULTIAXIAL FATIGUE DESIGN V is subjected to repeated loading that could cause failure by the development of progressive fracture, ASME Section VIII Division 2, API 579-1 and EN 13445-3 Annex B has detailed procedures for determining if a vessel in cyclic service requires a detailed fatigue analysis or not, and how to conduct the analysis. The ASME code is taking into consideration non conservative approaches, which are dealing combined load sources, rather than the other codes. The exemption of fatigue calculation is given by 3 screening procedure. The first one is based on successful experience and the second one, method A, uses a simple six step procedure for material with tensile strength of 550 MPa maximum. The third one, method B, is the most important one and it is developed in the table below according to the Section VIII Division 2 Paragraph 5.5.2.4. We start by determining the history of the loading given by the specs (step1) and then we determine screening criteria factors, C1 and C2 (step2). Then, we proceed directly to fatigue analysis if any step inequation is false, else if we pass to the next step. The fatigue life is predicted from the S-N curve, results of fatigue tests on smooth bar, based on fatigue strength reduction factors (K f ). T P