(74d) Calculation of Flare Header Pipe Temperatures during Depressuring

Abstract:  LNG facilities have wet and dry flare systems that need to handle cold relief flows for short time periods.  While most of the main flare header lines are stainless steel the use of stainless throughout the flare system is very expensive especially if there are lines leading from one train to another.  It is customary to use stainless steel in cryogenic lines and transition to impact or non-impact tested carbon steel after the temperature in the lines have been established to be higher than the minimum tested temperatures the LTCS (Low Temperature Carbon Steel) has been tested for.  This paper discusses the calculations required to evaluate the use of LTCS piping in cold intermittent service and the associated stresses due to differential thermal expansion of the materials in the event of a HAZOP analysis review by process safety.  Transient and steady state heat transfer calculations are used to estimate the distance over which the cryogenic relief flows impact the piping and assess the suitability of transitioning to the LTCS material.