Auto-Refrigeration & Flashing and Cold Ambient Exposures
We see around a half dozen pressure vessels in a “service” they were not initially designed for. There are literally hundreds of excuses as to why we find these issues, but they are simply excuses. It was not uncommon for us to find a PV in an NH3 refrigeration process with an MDMT of -20°F being operated in the -40°F range, and the facility would say, “We’ve been operating like that for X years, and nothing has failed.” So let’s talk about what the Minimum Design Metal Temperature (MDMT) means and how it plays a crucial role in process safety when the chemical we’re covering is a gas at standard temperature and pressure.
Auto-Refrigeration & Flashing: When volatile liquids (such as anhydrous ammonia, propane, or carbon dioxide) experience a sudden drop in pressure, they flash to a gas. This rapid phase change causes the temperature of the remaining liquid and the vessel wall to plummet instantly. If the temperature drops below the MDMT while the vessel is still under pressure, brittle fracture is a serious threat.
Cold Environmental Exposures: Vessels located outdoors in cold climates (like northern regions) must have an MDMT that accounts for the lowest expected ambient atmospheric temperatures.
For an industrial LPG (liquefied petroleum gas/propane) pressure vessel designed under ASME Section VIII, Division 1, the absolute minimum MDMT rating is determined by two (2) independent factors:
- environmental conditions (ambient temperature) and
- process safety hazards (auto-refrigeration).
The standard operational benchmark for a bare-minimum MDMT rating for a standard domestic or commercial LPG bulk tank is -20°F (-29°C). However, depending on geographic location and engineering specifications, it frequently drops to -50°F(-46°C).
For the vast majority of standard LPG/propane tanks constructed from common carbon steels (such as SA-516 Gr. 70 or SA-106 Gr. B), -20°F is the baseline minimum design rating.
The Heavy-Duty/Cold Climate Limit: -50°F(-46°C)
In cold climates (such as Canada or the northern United States) or for heavy industrial applications, the industry standard shifts down to -50°F.
- The Physics of Propane: Pure propane boils at -44°F (-42°C) at atmospheric pressure.
- The Auto-Refrigeration Risk: If an LPG tank suffers a rapid, uncontrolled loss of containment—such as a sheared liquid nozzle, a wide-open liquid dump valve, or a catastrophic piping failure—the liquid propane will rapidly flash into its gas state. This rapid evaporation absorbs a large amount of heat, causing the remaining liquid and the adjacent tank steel to instantly self-refrigerate to the liquid’s boiling point of -44°F.
- The -50°F Rating Requirement: To prevent the steel from experiencing a catastrophic brittle fracture under the residual pressure from an auto-refrigeration event, industrial owners (especially in refineries, chemical plants, and oil & gas) mandate an MDMT of -50°F.
It is worth noting that an LPG tank with a common maximum allowable working pressure (MAWP) of 250 psig at 125°F almost never reaches 250 psig when it is cold.
As the temperature drops, the vapor pressure of propane drops drastically:
- At 70°F, propane vapor pressure is roughly 127 psig.
- At 0°F, it drops to 31 psig.
- At -44°F and below, vapor pressure is 0 psig (atmospheric or slight vacuum).
Because the stress in steel decreases with temperature, engineers often use the coincident ratio reduction rules in ASME Section VIII (Paragraph UCS-66.1). This allows a vessel to be code-stamped with a dual rating:
Example Matrix on an LPG Nameplate:
- MAWP: 250 psig at 125°F with an MDMT of -20°F at the full 250 psig.
- Coincident MDMT: An allowable lower MDMT of -50°F, provided the internal pressure does not exceed 115 psig.