Minimum Design Metal Temperature (MDMT)
Presentation: Thai Truong Apr 17, 2014
Brittle Fracture Phenomena Definition of Brittle Fracture Brittle fracture is a sudden, often catastrophic ca tastrophic failure which is inherent to brittle materials. It involves little or no deformation, and has been experienced in pressure vessels, tanks, and pipes.
Brittle Fracture Phenomena In some reactions and processes, loss of process control can result in a significant change in temperature and/or pressure. The result can exceed the intended limits of the materials selected. Thus, where cryogenic fluids are being processed, a reduction in pressure could lower the temperature of the fluids to a level below the minimum allowable design temperature of the equipment, with the attendant risk of a low-temperature brittle fracture. An example of this phenomena is when high levels of pressurized gases are released into a piping system. The quick depressurization creates the possibility for auto-regeneration of the piping materials that may cause brittle fracture. Below are typical depressurization times for high pressure systems:
Brittle Fracture Phenomena •
At normal or high temperatures, a warning is normally given by plastic deformation as signs of potential vessel failure:
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Bulging Stretching Yielding Cracking Leaking
At low temperature condition, no such warnings of plastic deformation are given. An abrupt fracture at low temperature condition can cause a catastrophic event due to fragmentation of the structure and fast release of energy. It is a major concern in failure of pressure vessels, heat exchangers, and piping.
Brittle Fracture Phenomena As the metal temperature decreases, many metals loose their ductility and toughness. They become susceptible to brittle fracture. “Brittleness” indicates that the material is prone to failure without deformation. Examples of brittle materials: - Chalk Brick - Glass Hardened steels Brittle materials are prone to fracture when they are stressed in the vicinity of a notch or stress concentration. Brittle fractures are infrequent, most occur during hydrotest rather than in operation.
Brittle Fracture Phenomena “Toughness” is the opposite of “Brittleness”, is the material’s ability to resist brittle fracture. Toughness depends on: - material strength - thickness - temperature. To resist brittle fracture, higher strength materials and thick materials require greater toughness than low strength and thin materials. Steel lose toughness as temperature decreases.
Brittle Fracture Phenomena Four main factors, in combination, can cause brittle fracture of steel vessels: - low temperature - loading - susceptible steel - crack / stress risers
Brittle Fracture Phenomena Low Temperature Factor A metal depending on its toughness property has a transition temperature range within which it is in a semi-brittle condition – ductile to brittle transition. Although the transition occurs over a temperature range, a point within this range is selected as the “transition temperature” to delineate the boundaries of ductile and brittle zone. The transition temperature is usually taken as the point where 50% of the fracture is brittle. One of the ways to determine this temperature is by performing a series of Charpy impact tests on materials. Above the transition range, brittle fracture will not happen, even a notch exists. Below the transition range, brittle fracture can happen, even no notch exists.
Brittle Fracture Phenomena The absorbed energy (Joule) is plotted against testing temperature, giving a ductile to brittle transition temperature curve. Test temperature range shall be wide enough to establish the upper and lower shelf energies, with sufficient testing at intermediate temperatures to permit plotting a reasonable smooth curve. The curve represents a change in fracture behavior from ductile at high temperature to brittle at lower temperature.
Brittle Fracture Phenomena
Brittle Fracture Phenomena Loading Factor Dynamic loading associated with mechanical/thermal , impact loading, cyclic loading, or rapid decreases in equipment temperatures is a contributing factor. Susceptible Steel Factor • Steel composition. Steel with lower carbon content (C %) have higher toughness.
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Phosphorus (P %) has a strong effect in raising the transition temperature and improves weldability. Steel transition temperature is a function of carbon content plus 20 times the percent of P. Steel structure. Grain size has a strong effect on transition temperature. Decreasing the grain diameter from ASTM grain size 5 to fine grain size 10 can change the 10 ft/lb Charpy V-notch transition temperature from about 39°C to -33°C (70°F to -60°F). Hydrogen cracks. When hydrogen atoms diffuse into the metal during material manufacturing operations such as forming, forging and welding or when hydrogen is introduced to the metal through a galvanic or hydrogen sulfide (H2S) corrosion process, the metal is prone to hydrogen cracks.
Brittle Fracture Phenomena Crack/Stress risers • Steel vessels with thicker walls have a greater probability potential for brittle fracture due to the larger thermal gradient across the wall thickness. Thicker metal walls can result in differential expansion of material across the wall thickness and could possibly lead to a crack occurrence and eventually brittle fracture. • Stress raisers such as sharp or abrupt transitions or changes of sections, corners or notches (as may be found in weld defects) as a result of design or fabrication processes are all stress risers, which can cause stress intensification. The weak points are prone to brittle fracture when other susceptible conditions exist.
ASME CODE - MDMT MDMT of Vessels. The Minimum Design Metal Temperature (MDMT) of a vessel is the minimum metal temperature in which the vessel can sustain its full design pressure without having to be impact tested. When the vessel operates at pressures less than its full design pressure, concessions on MDMT are allowed based on ASME Section VIII. Minimum Allowable Temperature (MAT), as defined in API 579, is “the lowest (coldest) permissible metal temperature for a given material and thickness based on its resistance to brittle fracture. It may be a single temperature or an envelope of allowable operating temperatures as a function of pressure. The MAT is derived from mechanical design information and material specification. MAT at design pressure is MDMT.
ASME CODE - MDMT Lowest Metal Temperature (LMT) is the lowest metal temperature due to the operating condition and minimum ambient temperature. LMT may be a single temperature at an operating pressure or an envelope of temperatures and coincident pressures. LMT is derived from the calculated inner wall temperature due to the contained process fluid temperature and also the minimum ambient temperature. Determine the MDMT – API 510 Welded Storage Tank
ASME CODE - MDMT Determine the MDMT – ASME To establish a minimum design metal temperature for new equipment, startup temperature and reasonably expected abnormal operating temperatures, including autorefrigeration should be considered, as well as normal operation. The best available local weather data should be used to establish startup temperatures if the equipment is not normally preheated. If local temperature data are not available, the lowest 1-day mean temperature shown in Figure 4-2 of API-650 can be used.
ASME CODE - MDMT
ASME CODE - MDMT Fracture initiation in steels was found to be difficult above a transition temperature corresponding to a CVN impact energy of 10 ft-lb. Crack propagation was found to be difficult above a temperature corresponding to 15 to 25 ft-lb. From these findings, a 15 ft-lb CVN requirement at the minimum loading temperatures became a widely used fracture criterion.
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ASME CODE - MDMT Materials Selection Requirement. One of two methods is used to assure steels are used above their transition temperature: • Impact test exemption curves • Charpy V-Notch impact testing
Application point
15F
The application point is the point corresponding to the thickness and minimum pressurizing or design metal temperature. A steel has adequate toughness if the application point is above the steel’s curve. To use a steel at an application point below that steel’s curve, CVN impact testing is required to prove adequate toughness.
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ASME CODE - MDMT The impact-test exemption curves are preferred to CVN impact testing where prior data or service experience are available. CVN tests increase materials costs substantially and complicate delivery.
Minimum Pressurizing Temperature (MPT) ASME Code, Section VIII, Division 1 The MPT is the lowest temperature at which a pressure greater than 35% of the maximum allowable working pressure should be applied to the vessel. Below 35% of the maximum allowable working pressure, stresses are considered low enough to essentially eliminate the risk of brittle fracture in the absence of significant other stresses (such as those due to weight and differential thermal expansion). Due to increases in ASME code allowable stresses for Division 1 vessels built in 1999 and later, MPT is the lowest temperature at which a pressure >40% of MAWP should be applied to vessels built in before 1999.
AUTOREFRIGERATION Autorefrigeration occurs on adiabatic expansion of gasses and boiling of liquids. The resulting low temperature can bring materials like carbon steel below their ductilebrittle transition temperature resulting in metal embrittlement. Autorefrigeration temperature is defined as the temperature that the contents of the vessel would reach if the vessel is depressured to 35% of its maximum allowable working pressure. If the autorefrigeration temperature is less than 20oF, then the vessel should be treated as subject to autorefrigeration, and this used as a design basis to avoid brittle fracture. Due to an increase in ASME code allowable stresses in 1999, vessels built before 1999 will have autorefrigeration temperatures equal to what the contents would reach if the vessel is depressured to 40% of its MAWP. Vessels that are subject to autorefrigeration require additional consideration as follows: 1. Steels from Curve D of Code, Division 1, Figure UCS-66, should be used. Typically, carbon steel plate steel should be normalized SA 516. Forgings may be SA 350-LF2 and pipe SA 333 Gr. 1 or 6. 2. Impact testing is not required for autorefrigeration. SA 350 and SA 333 materials are impact tested in accordance with their respective specifications.
ASME CODE - MDMT Note: Code Paragraph UCS-68(c) allows a 30°F reduction in impact testing exemption temperature for P-1 materials (carbon steel) that are given postweld heat treatment (PWHT) and PWHT is not otherwise required by Code.
