The Importance of MDMT in Preventing Brittle Fracture Failures

The rules of UCS-66 are some of the most important and yet most misunderstood provisions of the ASME Code.  Because of its importance, minimum design metal temperature (MDMT) is one of just a few critical parameters stamped onto vessel nameplates.  The rules of UCS-66 guard against vessel failure by brittle fracture, a low probability high consequence event.  Brittle fracture failures exhibit “break before leak” behavior and can be catastrophic in that they entail high risks and occur without warning.  Carbon steel pressure vessels are most susceptible to brittle fracture during start up, shut down, hydro test (test water that is too cold) and rapid depressurization (auto-refrigeration).  Given the consequences of this type of failure, all relevant factors listed in UCS-66 need to be properly addressed.  These include consideration of the coldest temperature expected, metallurgy, material thickness, post weld heat treatment, impact testing, applied stress and weld pass size.

COMPRESS helps engineers prevent pressure vessel brittle fracture failures by properly applying the impact test and MDMT rules of UCS-66
COMPRESS dialog showing the impact test settings that affect the UCS-66 MDMT rating performed

How COMPRESS Determines MDMT

COMPRESS assists the vessel designer by automatically finding the coldest impact test exempt MDMT allowed by Code.  If this rated MDMT is warmer than the required MDMT, the designer is alerted and the limiting component(s) highlighted.  At this point the designer can either change one of the component’s input variables or specify impact testing be performed.  As impact testing is expensive, it is usually called for only when it is a Code or local jurisdictional (CSA) requirement.  COMPRESS automatically:

  • Determines if the selected material falls under the provisions of UCS-66.

  • Accounts for different materials by using the correct impact test exemption curve from Figure UCS-66.  This includes consideration of Figure UCS-66 notes.

  • Determines the governing thickness to be used.

  • Calculates the coincident stress ratio and determines MDMT reductions from Figure UCS-66.1.  All MAWP calculations performed by COMPRESS take into account any stress ratio reductions taken.

  • Applies the additional MDMT reduction from UCS-68(c) if allowed.  COMPRESS determines eligibility for the UCS-68(c) reduction based on the PWHT requirements of UCS-56.

  • Considers other MDMT limits and impact test exemptions, such as UG-20(f), that may apply.

  • Finds the recommended minimum hydrotest temperature per UG-99(h).

  • Passes the relevant impact test information to Shopfloor so it can be included in the appropriate welding procedures.  Note that not all impact test requirements are covered by UCS-66; see UCS-67 for more information.

A Brief UCS-66 Technical Background

It is widely acknowledged that it is impractical to build equipment that does not contain at least some flaws.  In the discussion above, it was also stated that UCS-66 considers temperature, metallurgy, material thickness, post weld heat treatment, applied stress and weld pass size.  Roughly speaking, UCS-66 takes these factors in combination with an assumed critical flaw size to arrive at its MDMT ratings.  Let’s take a look at the factors involved and hopefully shed a little light on why they are used and how they affect the toughness of carbon and low alloy steels.

  • Temperature and Applied Stress – If a vessel is exposed to a temperature colder than its brittle-to-ductile transition temperature it can, depending on the applied stress, become susceptible to brittle fracture.  This behavior is accounted for in Figure UCS-66.1.  To reduce the likelihood of brittle fracture during start-up it is often standard procedure to warm vessels up before subjecting them to full operating pressure.  In addition, the Code equations for maximum allowable working pressure (MAWP) assume a stress reduction ratio of 1.0.  Consequently, the designer must ensure that the allowable stress used to calculate MAWP is consistent with the stress reduction ratio used to determine the coincident vessel MDMT.

  • Metallurgy – Different steels exhibit various degrees of toughness.  Generally, large grain sizes and steel impurities are associated with reductions in steel toughness and an increase in susceptibility to brittle fracture.  In other words, materials produced to fine grain practice are generally tougher.  The toughness properties of various steels versus temperature are reflected in the curves and notes of Figure UCS-66.

  • Thickness – The thicker the vessel, the more susceptible it is to brittle fracture.  To a first approximation, this is because thin (low pressure) vessel walls are subject to bi-axial stress whereas thick (high pressure) vessel walls experience a tri-axial stress.  This tri-axial stress state includes an out-of-plane shear acting through the vessel wall which increases the stress intensity acting on flaws at or near the inside surface.  Generally, thicker sections experience higher crack tip tri-axiality as well.  This behavior is captured in Figure UCS-66 through the concept of governing thickness.  It also comes into play when using the thickness exemptions of UG-20(f).  The UG-20(f) exemption does not depend on vessel diameter.  This means that small diameter vessels falling under UG-20(f) may actually experience a tri-axial stress state.  For this reason COMPRESS includes an option that will not allow UG-20(f) exemptions in these special cases if desired.

  • PWHT – Post weld heat treatment improves the weld heat affected zone (HAZ) grain structure and increases weld metal toughness.  If applied to the entire vessel it also reduces residual cold forming stresses.  Brittle fracture initiation flaws are most often encountered in welds in or near high stress regions such as nozzles or head to shell junctures.  Reducing residual weld stresses via PWHT reduces the likelihood of brittle fracture originating from these flaws.  For cases where PWHT is not required by some other Code rule, the provisions of UCS-68(c) can be used to take advantage of this behavior.

  • Weld Pass Size – An easily overlooked UCS-66 assumption is the maximum size of the individual weld passes used to create the joint.  The impact test exemptions and rated MDMT determined by UCS-66 are generally limited to weld pass sizes that do not exceed 1/2″.  For MDMT’s colder than -20 degrees F, this may be as small as 1/4″ depending on which impact test exemption was used. See UCS-67 for more details.

Vessel failure by brittle fracture. Note the fracture initiation point on the inner surface.
INSPECT plots Minimum Allowable Temperature (MAT) vs. Pressure to quickly establish the MAT Integrity Operating Window

MDMT Ratings and Vessels Built Before 1987

UCS-66 Code rules apply to new construction.  But what if the vessels in your plant were fabricated before 1987, the year the MDMT rules of UCS-66 were put in place?  This is where our API 579-1/ASME FFS-1 Fitness-For-Service software, INSPECT, comes into the picture.  INSPECT includes API 579-1 Part 3 brittle fracture assessments and the ability to produce Minimum Allowable Temperature (MAT) curves . INSPECT’s MAT feature automatically runs Part 3 assessments over a range of pressures to simplify and speed up the creation of equipment integrity operating windows.

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