Piping Low Temp Materials

[Sharik]

If a piping system is to have a design minimum temperature of -50F, when is it necessary to use low temp, impact tested materials? The way I read ASME B31.3, we could just use SA-106 B with impacted tested WPS's.

When would it be mandatory to use SA-333 or something similar?

 

[bvi]

You can be exempt from impact testing due to material/thickness/temperature considerations, or due to a condition of low stress. However, if you are not exempt from impact testing, then you need both impact tested material, and welding procedures that have been qualified by impact testing the weld and heat affected zone as part of the weld procedure qualification testing.

 

[Cockroach]

Actually, Cold Weather Engineering kicks in for thermal conditions lower than -25C. For long soak times in surpressed themal environments, ASTM A333 Grade 6 has proven to be superior to ASTM A106 Grade B.

The only purpose of the Charpy Notch Test, or impact test as you have refered to it, is to determine the minimum energies required for a steel specimen to propagate a crack. Often but not necessarily always, cracks are generated from "notches" or areas of razor stresses that can be induced by loading. The depressed temperatures assist the phenonema greatly. It has been the case for many years that some carbon components are better than others so building on experience of various industries, requirements have been stipulated.

Of other important issues is the chemistry for various corrosion mechanisms. Note that in the cold, fluid environments are typically "dry" since the presence of water vapor has long been removed through frosting. Therefore the gas fluid for sour environments has an important component removed. It has been the experience in the oil & gas energy sector that most of the steels specified for cold weather are also excellent against degragation from galvanic, crevice, sulfide cracking mechanisms associated with corrosion.

So if you are in Texas (Gulf Coast States), Charpy Testing is typically not the issue. However, northern climates such as Canada, Russia and Alaska, crack propagation can be an "explosive" issue! I have seen Slave Lake, Northern Alberta Canada at -58C, dropped a Grade 8 Cap Screw and watched it crack like glass. This has seem to leave a very marked impression on my engineering experience!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[DrillerNic]

One way of using the charpy test to to find out the Charpy transition temperature, when the failure mode in the charpy test moves from ductile or plastic (ie higher energy) to brittle (lower energy).

If the transition temperature of your material from these tests, and the weld and the HAZ etc is above your design temperature, then you are surer that the material won't shatter the way Cockroach described iof you whack it.

This type of testing may be more expensive: standard Charpy tests are often just 3 specimens at a stated temperature, recording the energy absorbed; the tests I'm describing would be three specimens (say) at each temperature point down to and below your design temperature.