Auto-Refrigeration: When Bad Things Happen to Good Pressure Vessels
Francis Brown, P.E.
Category : Incidents
Summary: The following article is a part of the National Board Technical Series. This article was originally published in the Fall 2002 National Board BULLETIN.(2 printed pages)
Auto-refrigeration is a phenomenon common to liquefied compressed gases. Liquefied compressed gases exist in both the liquid and gaseous phases at ambient temperatures with pressures ranging from 2 psig up to 2,500 psig. That is, there is a gaseous layer over the liquefied gas within the pressure vessel. Some common liquefied gases are shown in the following table:
Liquefied petroleum gases*
* Too numerous to list
An example of auto-refrigeration can often be seen when using an LPG (Liquid Propane Gas) grill. On a warm, humid day, moisture in the air condenses on the lower part of the propane tank when the burners are in operation. The withdrawal of propane gas from the tank reduces the temperature of the liquid propane and the tank itself below the dew point temperature, causing the moisture in the air to condense on the surface of the tank. Cooling occurs at very modest rates of gas withdrawal, with the temperature decreasing more as the gas withdrawal rate increases.
Withdrawing gas from the pressure vessel reduces the pressure as well as the temperature within the vessel. The gas that is withdrawn is replaced as the liquid vaporizes by absorbing heat from the remaining liquid and the vessel itself. Auto-refrigeration occurs when the gas is withdrawn at a rate so that cooling exceeds the heat available from ambient sources.
The cooling, if excessive, may lower the vessel metal temperature to the point where failure from brittle fracture is possible. Flaws (cracks) in the welds or the pressure boundary materials that are located in areas of high stress are subject to rapid crack growth when vessel temperatures reach the Nil Ductility Temperature (NDT). The NDT is that temperature at which the behavior of the vessel material (steel) changes from ductile to brittle. Fortunately, pressure decreases as temperature decreases. For example, for a vessel containing liquefied carbon dioxide, a decrease in vessel temperature from 20°F to -20°F (-7°C to -29°C) decreases the pressure from 400 psig to 200 psig. The decrease in pressure associated with the decrease in vessel temperature reduces the stresses from pressure in the vessel material, thus reducing the energy available to produce crack growth. Cracks will not propagate if the total stresses are sufficiently small, even though the vessel material is at or below the NDT.
Total stresses include residual stresses, pressure stresses, and thermal stresses. Residual stresses are the stresses remaining in the vessel from the manufacturing process, and are constant. Pressure stresses decrease with decreasing temperatures, but the thermal stresses induced by the rapid cooling may be increasing. The more rapid the cooling, the higher the thermal stresses. It is very difficult to determine the total stress in a vessel during auto-refrigeration. With the possibility of vessel failure by brittle fracture, appropriate measures should be taken to prevent auto-refrigeration of vessels that were not designed for low operating temperatures.
However, vessels that were not designed for low operating temperatures may be cooled to a temperature below the NDT with no apparent damage. Damage will not occur until the total stresses increase to a critical value. To minimize the possibility of damage, the vessel should be very slowly warmed to ambient temperatures in the non-pressurized condition. This will keep the thermal and pressure stresses low, thus minimizing the total stresses in the vessel.
Vessels not designed for low operating temperatures but which have been subjected to auto-refrigeration should be thoroughly inspected for cracks before the vessel is returned to service. This inspection should include a thorough examination of all nozzles (especially the outlet nozzle) and the major weld joints, including the heat-affected zone, of the vessel. A visual inspection of the vessel is inadequate because small cracks may not be detected. The vessel should be inspected by the magnetic particle, liquid penetrant, or ultrasonic method, whichever is most appropriate and compatible with vessel contents and materials.
Compliance with all OSHA requirements for safety of personnel, including entry into a confined space, is essential. Also, knowledge of the vessel contents is required because many of the gases are combustible and may explode when exposed to an ignition source. The vessel interior must be well ventilated and caution exercised when using sources of electrical energy where these gases may be found.
In summary, auto-refrigeration of a pressure vessel not designed for low-temperature operation places the safety of the vessel in question. During auto-refrigeration, a pressure vessel may be cooled to temperatures at which vessel failure by brittle fracture may occur. The thorough inspection required to ensure the vessel can be safely returned to service is both time consuming and costly. Therefore, auto-refrigeration of pressure vessels not designed for low-temperature operation should be avoided.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.