Dynamic Model for a Heat Exchanger Tube Rupture Discharging High-Pressure Natural Gas Vapor into Low-Pressure Liquid-Filled Shell in LNG Processing Industry

The rapid growth in global demand for natural gas as a fuel has led to expansion of the production capacity of existing gas processing trains and the design of new process trains. Liquefied Natural Gas (LNG) has emerged as the main source to cater for the global fuel demand. The unique properties of LNG such as its cryogenic temperature, requires a series of cooling processes using heat exchangers. The increasing complexity of high performance processing systems leads to more complex failure modes and new safety issues [1]. Kettle type shell-tube heat exchangers are often used to exchange heat between a high-pressure fluid and a low-pressure fluid. A significantly high pressure difference between these two fluids are normally observed. In a tube rupture contingency, a transient pressure rise singularity could occur due to the expansive forces created by the high pressure vapor in the tube egressing into the low pressure liquid filled shell, which may cause the shell to exceed its excursion limit and Safe Operating Envelope (SOE) with subsequent Loss of Primary Containment (LOPC) [2].

A dynamic model is presented in the paper to describe the transient phenomenon occurring on the shell side following various parameters of tube rupture, ranging from fluid pressure, temperature and composition differences. The paper will briefly describe the various phases of the phenomenon, however the paper focuses on the beginning phase of the contingency during which shell overpressure may be encountered [2]. The model is applied to two circuits involved in LNG processing; the first uses methane on the tube side to be cold down by propane on the shell side, while the second uses ethane, also on the tube side. The ratio between the tube design pressures to the shell design pressure in both these systems are 0.188, respectively. The practical aspects and discussion around techniques to alleviate potential overpressure scenarios due to tube rupture are emphasized throughout the paper.

[1] S. Rathnayaka, F. Khan, and P. Amyotte, J. Loss Prev. Process Ind., vol. 25, no. 2, pp. 414–423, Mar. 2012.

[2] C. J. Ennis, K. K. Botros, and C. Patel, J. Loss Prev. Process Ind., vol. 24, no. 1, pp. 111–121, Jan. 2011.