Properly Calculate Vessel and Piping Wall Temperatures During Depressuring and Relief

Determining if and when a vessel and/or piping component is going to fail under fire exposure and/or from cold temperature embrittlement is an important factor in consequence analysis and risk assessment. This paper describes detailed methods for establishing the conditions for vessel/piping failure and whether the material of construction for vessels and piping is properly selected for fire exposure and/or cold depressuring/relief.

Several case studies presented in this work were modeled using our software Process Safety Office^® SuperChems™ and the new fire flux equation presented in API-521. The calculations illustrate important concepts dealing with how wall temperatures should be calculated for single and multiphase systems in order to establish if a vessel and/or a piping component is going to fail.

Introduction

Loss of containment scenarios caused by catastrophic vessel and piping failures can lead to severe consequences including fire, explosion, blast wave damage, or a toxic cloud moving across the property and into the surroundings. The proper quantification and understanding of scenario frequencies, actual vessel and piping failure potential, and any associated consequences can provide for better risk management and allocation of resources for risk reduction.

Vessels and piping components can fail because of excessive deviations in internal or external pressure and/or wall temperatures. Other causes of vessel and piping component failures not considered in this article include corrosion under insulation, manufacturing defects, external impact, etc.

A classic scenario that is almost always considered in risk assessments is the exposure of a process/storage vessel and piping components to a pool fire or flame jet. The heat input from the fire causes the pressure and temperature of the contents as well as the temperature of the metal walls to increase. As the wall temperature increases, the metal strength decreases, and if the internal pressure is high enough, loss of containment will occur. If the contents are flammable, and depending on the size of the vessel contents, a spectacular fireball and/or vapor cloud explosion can follow. There is a substantial difference in the likelihood of vessel failure, depending on whether the exposure is caused by a pool fire or a flame jet and whether the dry wall is exposed to fire.

Flame jet impingement causes high intensity localized heating. Because jets are efficient mixers, the wall impinged area can receive a time average fire flux as high as 350 kW/m2. The intense heating of the jet fire causes the exposed metal to heat up, which reduces its tensile strength. If the heating continues, the wall temperature may eventually reach the vessel’s ultimate tensile strength (UTS) and rupture may take place. Failures caused by flame jet impingements on the vapor space (no liquid) typically occur within a few minutes, for example, on the order of 5 min.

 

---