MDMT Review and Low Temperature Resolution
The minimum design metal temperature (MDMT) for typical carbon steel ranges between -20°F and -50°F, yet many fluids used in upstream and midstream operations can reach temperatures below -90°F due to Joule-Thomson effects, when certain process upsets occur.
Examples of process upsets resulting in low fluid temperatures include:
- Blowdown or flaring of high pressure, low temperature, low molecular weight gases and volatile liquids
- Operation of non-emergency (maintenance) blowdown valves including those bypassing pressure relief devices
- Leaking valves to the effluent handling system
- Unusual flow paths due to valve misalignment
Effluent handling piping often sees fully depressured process fluids, allowing for the greatest J-T effects, and has the greatest potential to be cooled by low temperature fluids. Typical pressure relief analyses have potential gaps in identifying low temperature issues. Temperature excursions may also occur in process piping and equipment where material specification changes. Low temperatures can lead to brittle fracture with subsequent loss of containment. Inglenook Engineering has experience identifying locations where low temperatures may be present, even if they are not readily apparent.
It is important to note that having a fluid below the pipe’s MDMT will not automatically result in brittle fracture. Brittle fracture typically requires a few conditions:
- Wall temperature below the equipment’s MDMT, which is not the same as the fluid flowing temperature (see Shackelford, “Temperature Effects for High Velocity Gas Flow”, Chemical Engineering, January 2015; 122 (1): 47-51).
- Sufficient inventory of fluid for the metal to be cooled below the metal’s MDMT
- Stress on the pipe that exceeds what the pipe can withstand at the reduced temperature
Inglenook has developed certain evaluation and screening processes to identify potential instances where low metal temperature may occur and result in brittle fracture, and can provide mitigation steps. In many cases, pragmatic design options short of metallurgy upgrade are identified, resulting in substantial savings
(see Shackelford and Pack, “Overpressure Protection: Consider Low Temperature Effects inDesign”, Chemical Engineering, July 2012; 119(7): 45-48).
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