The 21st Ethylene Producers Conference
Shell-tube type heat exchangers are often used to exchange heat between a high pressure fluid and a low pressure fluid, and the pressure difference between the two fluids could be significantly high. In the event of a partial- or full-rupture of a tube, a problem may arise in that a transient pressure rise phenomenon could occur due to the flashing of the sub-cooled fluid into the low pressure shell, which may cause the shell to rupture with subsequent damage to equipment. This paper presents a dynamic model to describe the transient pressure occurring on the shell side following various scenarios of tube rupture. Secondly, the spatial and temporal aspects of the flow transients along the pressure safety valve riser are accounted for by solving the one-dimensional hyperbolic continuity and momentum partial differential equations as applied to the liquid filled riser. The dynamics of the attached piping system are also accounted for via two mechanistic models; the first is based on an inertial-resistive assumption of the fluids in this system, while the other is based on the assumption of anechoic perturbations passing through a long section of the attached piping. The latter is justified in cases where the attached piping is long enough such that reflections from the downstream end do not interfere with transients occurring in the shell during the initial phase of fluid flashing into the shell side following rupture. The various phases of this phenomenon are described, however the paper focuses on the initial phases of the phenomenon during which shell overpressure may be encountered. The model is applied to two ethylene heaters in tandem; the first uses propylene on the shell side to heat the ethylene on the tube side, while the second uses methanol, also on the shell side. The pressure ratio between the shell design pressure to the tube design pressure in these two heaters are 0.169 and 0.154, respectively, hence the motivation to accurately model the transients involved in this phenomenon. The practical aspects and discussion around techniques to alleviate potential overpressure scenarios due to tube rupture are emphasized throughout the paper.
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