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After fabrication, multilayer vessels are pressurized to 1.5 times the design pressure to reduce the gaps between layers which result from the fabrication technology of multilayer vessels [1]. The vessels are kept under this pressure for three hours, resulting in uniform plastic deformation of the central tube and part of the inner layers. After the pressure is removed the vessel has a residual deformation and the gap dimensions between layers are changed. This requires the design of multilayer vessels for elastoplastic conditions. The stress conditions of a multilayer vessel in subsequent elastic operation can be calculated more accurately knowing the residual stresses and the size of the new gaps.Let us examine a multilayer cylinder of n layers with gaps 5j (j = 1, 2 ..... n-1) between layers. The design is based on the assumption that all gaps are eliminated during elastic operation of the cylinder, so that pressure p, under which the multilayer cylinder operates in an elastoplastic regime, should be higher than the pressure for gap elimination. During operation in the elastic regime, the circumferential at, radial (rr, and axial (~z stresses at the radius r of layer j are determined by formulas given in [2].Calculating the stress intensity on the inner surface of the vessel by the formula we determine whether the vessel is operating in the elastic or plastic regime. If at the given pressure p the stress on the inner surface of the vessel is higher than the yield point of the central tube material, the vessel is operating in the elastoplastic regime and it is necessary to find the radius of plasticity, which determines the boundary between the plastic and elastic zones.Let us assume that the radius of plasticity r t has already been determined. Using approximate relations [3] to simplify calculations ~t + ~r + ~z = 0; ~) where et, st, and az are the circumferential, radial, and axial deformations, respectively, we get an expression for radial stress in the plastic zone [4] s~where e and e 0 are the deformations intensities at the radius under consideration and at the inner surface, respectively.Using Eq. (2) we can write an expression to determine deformation intensity in the form For any layer j located in the plastic zone of a multilayer cylinder, a simple relationship exists between the deformation intensities ~j_l and aj at radii rj_i and rj [41 = 87 4 (5) i where rj _ 1 -< r -< rj, aj_ 1 is the deformation intensity on the inner surface of layer j with radius rj_ t, and e~is the deformation intensity on the outer surface of layer j with radius rj.

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Existing methods of determining residual working life for vessels and plant usually deal with only some of the relevant factors [1]. Experience over many years of examining hundreds of them in the chemical and petrochemical industries has enabled the Irkutsk Chemical Engineering Research Institute to formulate basic principles for calculating residual life [2] and methods for the purpose, which incorporate the requirements of the regulatory documentation [3].The residual working life is determined from engineering diagnosis (examination). This extends to all the main components of a vessel: cylindrical and spherical bodies, planar and convex end sections, flanges and connections, convex and planar lids, compensators, and reinforcement around holes, for all of which calculations on the strength are performed in accordance with the documentation for the purpose and on the test pressures to be used as appropriate to the actual properties of the material, the wall thicknesses, and the corrosion rates. Calculations are also performed on the stability and on the wind and seismic loadings to be expected, the external pressure, and the supporting loads. If positive results are obtained from the examination and calculations, one can calculate the residual life.The methods of determining the life involve identifying the major damaging factors and evaluating the state of the vessel as affected by them. The working life of a vessel is considered as exhausted when it attains the limiting state defined by the standard criteria.Basic damaging factors:-corrosion and erosion; -cyclic load;-changes in metal characteristics; -creep; -cyclic loading under creep conditions; and -the danger of brittle failure. There may also be other factors such as hydrogen corrosion. We consider methods of calculating the working life subject to those factors. Forecasting Residual Working Life for Plant Subject to Corrosion and Wear (Erosion). The following defines the residual working life of plant subject to corrosion (erosion):Tres.c (Tres.e) = Sa -Sc KI K2"( 1 ) a where S a and S c are the actual and calculated thicknesses in mm of any wall; a is the rate of uniform corrosion (erosion wear) in mm/year; K 1 is a factor that incorporates the difference between the mean expected residual working life and the guaranteed life (with probability y = 0.7-0.95); and K 2 is a factor that incorporates the difference between the residual life defined with linear change in the relevant parameter (wall thickness) and the residual life calculated from a more exact (nonlinear) law for variation in that parameter.

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