Strength analyses of high-temperature reactors with consideration of contact pressure due to facing

Abstract

In high-temperature reactors fabricated from alloy steels with an austenitic facing, contact pressure develops when the wall of the cylindrical shell is heated above 200°C in the zone where the built-up layer abuts the base metal due to the difference in their coefficients of linear expansion. A procedure is proposed for analysis of the contact pressure and evaluation of the cyclic strength of the housing and facing with consideration of the internal pressure and pressure within the zone of contact. As an example, results are presented for an R-201 reactor in a plant used for hydrotreating fuels. Calculations indicated that the allowable number of cycles for the facing is an order lower than that for the housing of the reactor.High-temperature reactors, for example, the R-201 reactor of a plant employed for the hydrotreating of diesel fuels, or an R-1 reactor of a complex designed for the hydrotreating and dewaxing of lube oil, are fabricated from alloy steels with an austenitic facing on the inner surface of the housing. When the wall of the cylindrical shell is heated, a contact pressure P c develops in the zone where the built-up austenitic layer and base metal (steel SA-336 F22V in the R-201 reactor) abut due to the difference in their coefficients of linear expansion. In conformity with the standards [1], the thickness of the built-up protective austenitic layer is disregarded in determining the computed wall thickness of the components of vessels or equipment operating at a temperature in excess of 200°C. Here, however, strength analysis may be necessary for low-cyclic loads even when the number of load cycles is less than 1000 due to the effect of the internal and contact pressures.Let us examine the analysis of the contact pressure using the R-201 reactor as an example. The built-up metal is a thinwall shell, and the wall thickness of the cylindrical housing formed from the base metal is substantially greater than the thickness of the facing; the base-metal housing can therefore be considered a nondeformable body in determining the contact pressure.The relative strains (hoop ε h and axial ε a ) in the cylindrical shell (facing) under consideration are:where Δα is the difference in the coefficients of linear expansion of the facing material α 1 and base material α 2 , T = 455°C is the working temperature, α 1 = 1.8·10 -5 /°C is the coefficient of linear expansion of the facing material [2], and α 2 = 1.38·10 -5 /°C is the coefficient of linear expansion of the base material [2]. Let us assume that the facing material resides in the plastic region, as confirmed by calculations. The condition of material incompressibility: ε r = ε h + ε a = 0; µ = 0.5,where μ is Poisson's ratio.