MDMT for the Inservice Inspector

BY JOHN HOH, SENIOR STAFF ENGINEER

Minimum de­sign metal tem­perature (MDMT) of a pressure vessel is dependent on material toughness in regards to brittle fracture considerations. What does that mean? Some materials, including some common carbon steels, do not behave well at cold temperatures and may expe­rience brittle fracture while under stress. That is an over-simplified explanation but it helps summarize what we want to avoid – a catastrophic failure.

 

In addition to the general material specification, material thickness plays a large part in establishing MDMT dur­ing the vessel design process. Thicker material has a higher MDMT value. A typical pressure vessel has many com­ponents of different thicknesses and, sometimes, different materials. A vessel designer must evaluate the individual components and determine the MDMT for each. The highest MDMT becomes the governing MDMT for the completed vessel. If the MDMT determined through the evaluation is higher than required for the specified operating conditions, the designer can select a different mate­rial, perform post-weld heat treatment (when not required for other reasons), or perform impact testing on material and weld specimens. The process is de­tailed in American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME B&PVC), Section VIII. This ar­ticle is not intended to explain all of the design considerations involving MDMT, but rather, highlight the conditions inspectors find in the field. Inservice inspectors, after all, stand in front of an operating vessel and must deal with what is in front of them, not what could have been.

 

Looking at the value for MDMT on the vessel nameplate is just as important as the maximum allowable working pressure (MAWP) or maximum tem­perature. How many vessel owners pay attention to the MDMT value on the nameplate? We would like to believe vessels are designed with the operat­ing environment in mind, but there are many “stock” vessels (a common design for a common use – an air receiver is an example) operating in a wide range of environmental conditions. Consider the typical air receiver: the MDMT is usu­ally -20°F. Some owners may locate the air receiver and its attached compressor outside under a shed roof to keep the noise out of the enclosed shop. During the winter, some regions can easily dip below -20°F ambient temperature. If the metal temperature of the air receiver measures below that -20°F stamped on the nameplate, it must not be pressur­ized. Granted, the air receiver can benefit from the heat generated through the compression of the air, but that can never be used as justification to operate with a metal temperature lower than rated on the nameplate.

 

Some might say ASME B&PVC Section VIII, Division 1, UG-20(f)(3), al­lows lower operating temperatures due to lower atmospheric temperatures, but UG-20(f) begins by stating that impact testing is not mandatory for materials satisfying all of the listed conditions. That includes the material P-number and Group number, thickness, pres­sure testing, thermal or mechanical shock loadings, cyclical loading, and a maximum and minimum design tem­perature. UG-20(f)(3) states, “Occasional operating temperatures colder than -20°F are acceptable when due to lower seasonal atmospheric temperature.” That only ap­plies when taken with the other stated restrictions. What does “occasional” mean and how much colder than -20°F is ac­ceptable? Not seeing a definitive answer, it is this author’s opinion to never accept operation below the stated MDMT.

 

Consider this situation: the in service inspector is performing an inspection in the heat of summer. It’s 95°F in the shade. That -20°F on the nameplate is the furthest thing from the inspector’s mind at that point. The inspector has to consider where the vessel is located (heated or unheated) and the coldest winter temperature ex­pected for that area.

 

The idea for this article began sev­eral months ago as an offshoot from an investigation. The vessels investigated were relatively thick carbon steel mate­rial intended for use at pressures ranging from 5,000 to 10,000 psi at oil and gas well sites. I noticed the MDMT (which was not part of the investigation) for most of the vessels was 20°F and a few were 40°F. That’s above zero, not below. These ves­sels would be operating outside, subject to whatever temperatures winter would cast upon them. Last winter it got very cold, even in the southern part of the United States. Were some of those vessels still operating last winter? Probably.

As inspectors we owe it to the general public to keep all operating conditions in mind while performing in service inspec­tions. If you notice an MDMT value that could be exceeded (negatively) based upon environmental operating condi­tions, notify the vessel owner of those con­cerns and ensure the vessel is protected from cold temperatures or that it is not used during those situations.

 

 

Source: FALL 2014 NATIONAL BOARD BULLETIN