Investigation of a Shell and Tube Exchanger in Liquefied Natural Gas Vaporization Service

Abstract

Liquefied natural gas (LNG) is commonly converted from liquid to vapor for gas distribution. One of the methods for vaporizing LNG is to use a shell and tube heat exchanger. Water is used on the shell side to provide the heat source and LNG is then vaporized through the tube side passages of the exchanger. In many of these applications, the LNG is at a high pressure on the tube side while the water is at a lower pressure than the LNG as it flows through the shell side. The industry consensus document API 521 [1] “Guide to Pressure Relieving and Depressuring Systems,” Fourth Edition, paragraph 3.18 “Heat Transfer Equipment Failure” states that a complete tube rupture is to be considered for the possible overpressure of the equipment. The typical shell and tube exchanger application described above has rupture discs on the shell body to protect the shell from being over-pressured due to a tube rupture scenario. The possible freezing of the water in the shell due to mixing with cryogenic LNG is a concern. The issue to consider is whether freezing will occur before the rupture discs can safely relieve a possible over-pressure condition of the shell. A numerical analysis of the condition was performed using Computational Fluid Dynamics (CFD) software. The exchanger service, the analysis procedure and the conclusions found are detailed in this paper.