Thermal Hazards Laboratory

Computer Modelling of Pressure Vessels in Fire

Computer modelling of presure vessels in fires is an integral part of any fire testing program. We do modelling to better understand what we see from the testing. Ideally we want to develop our models so that we can do less of the very expensive testing. Regulators would like to have models so that they can evaluate the designs of vessels and their thermal protection systems.

Thermal models are developed to consider different fire scenarios such as:

  • engulfing or partially engullfing pool fires
  • jet or torch fires
  • remote fires

The models also consider various types of thermal protection including:

  • pressure relief valves (PRV)
  • fusible plug pressure relief devices
  • thermal barriers such as insulation, radiation shielding, etc.
  • internal protection devices (expanded metalic mesh, baffles, etc)

The thermal model must consider the following processes to accurately model a tank in a fire.

  • fire heat transfer (radiation and convection)
  • heat conduction through the tank wall and thermal barriers
  • degradation of thermal barriers
  • heat convection and radiation in the vapour space
  • convection and boiling in the liquid space
  • thermodynamic condition of liquid and vapour (temperatures and pressures)
  • PRV action and mass flow
  • tank material properties (stress and strength)
  • stress-rupture failure prediction

The following figures show some colour contours for fire view factor and wall temperature for the case of a large vertical cylinder heated by a remote burning wall (i.e. the tank is heated by thermal radiation from a nearby fire).

The following animation shows the heating up of another vertical cylinder (this one has hemi heads). As you will see the animation shows the movement of the liquid level inside the tank. The liquid level first rises due to heating of the liquid. The level then drops once the pressure relief valve opens. As the liquid level drops the wall temperatures rise.

To view this animation (2.8 Mb Windows video file) click here

The following figures show typical model results for a full scale (33000 gallon) non-thermally protected tank-car filled to 94% with propane engulfed in a hydrocarbon pool fire. The plots show the following:

  • time varying average temperature of fire (input)
  • predicted tank pressure
  • predicted average liquid and vapour temperature
  • predicted wall temperatures
  • predicted tank strength and stress
  • predicted PRV mass flow
  • predicted tank fill
  • predicted thermal energy stored in liquid and vapour
  • predicted internal convection for vapour space

Based on this model run tank failure is predicted at about 24 minutes. At that time the tank was still more than 50% full of liquid and a very powerful BLEVE would be expected. This simulation agrees well with results from full scale tests.

for more information contact A. M. Birk at