ASTM A106 Gr. B Low Ambient Temperature Applications

Due to many piping failures, which were believed to be partly caused by brittle fracture of material goverment regulatory agencies are becoming more inclined to check with engineers and owners about whether relevant code requirements have been met.

For many years, both Canadian and American engineers have used ASTM A106 Gr.B for flare piping, instrument air and miscellaneous uninsulated piping systems which are exposed to cold ambient temperatures. The cold ambient temperature in Canada, however, is significantly lower than that in most parts of the U.S.

Because many older plants in northern regions are originally built without the consideration of the effects of extreme temperatures, the code requirement has never been met and failures were resulted.

Note (2) of ASME B31.3 Table 323.2.2 stipulates the following: "Impact testing is not required if design temperature is below -29 C but at or above -50 C and the maximum operating pressure of the manufactured components will not exceed 25% of the maximum allowable design pressure at ambient temperature and the combined longitudinal stress due to pressure, dead weight, and discplacement strain does not exceed 6 ksi."

In the northern country, the calculation of longitudinal stresses due to displacement strain can be based on a warm summer installation temperature of 30 C while the cold ambient temperature is -50 C.

This cold ambient temperature is now more commonly known as the CET, the critical exposure temperature. By definition, the CET is the minimum metal temperature at which the comonent is subject to a pressure greater than 25% of design pressure, or a stress greater than 25% of the material design stress in the case of non-pressure components such as supports, or the combined longitudinal stress due to pressure, dead weight, and displacement strain exceeds 6 ksi.

Flare headers in gas plants and refineries tend to run a very long distance with minimal flexibility. The combined longitudinal stress, by displacement strain itself, could very easily exceed 6 ksi (41 Mpa).

If an existing system with ASTM A106 Gr.B is to be checked for compliance with this code requirement, or to satisfy the responsible govenment agencies, acceptable methods are:

Check to see if the batch of carbon steel pipe was impact tested. Perform a computer analysis to insure combined longitudinal stresses are below 6 ksi. Insulate & steam trace. Replace certain components of the piping system low temperature material.

Although verification of impact testing of existing piping is the best solution, it is very often impractical.

Failing impact test verification, the most economical approach is to analyze the system numerically to determine whether components in the system experience more than 6 ksi between installation and cold ambient temperatures.

For existing facilities, performing computer stress evaluation is the most economical way to achieve code compliance.

If the computer analysis shows that stresses are unacceptable, the next best alternative is often just a matter of economics.

Insulating and heat tracing can be expensive and impractical. Replacing a few fittings with low temperature materials is a more economical solution than insulating and heat tracing, redesigning, constructing new routing, or even adding loops or flexible expansion legs.

For new flare and instrument air system designs, meeting the 6 ksi requirement can be a simple selection of the right material, but if you are considering the use of ASTM A333 Gr.6 or stainless steel, it may be that the use of ASTM A106 Gr.B with computer analysis is a much lesscostly way to meet this code requirement.