What will you do if Carbon Steel pipe is installed in place of LTCS

Introduction:

Low-Temperature Carbon Steel (LTCS) is used in piping systems when there is a possibility of process fluid temperature falling below -29 degree centigrade during operation. In a typical refinery, A106 Gr B material is used for carbon steel and A 333 Gr 6 is used for LTCS in normal operation. There is no major difference between these two materials in the composition. Also, no non-destructive testing is available which can ensure these two materials. So if by mistake CS and LTCS mix up with one another and someone installs CS in place of LTCS without knowing the major operational impact what’s the solution?

Reason for Mix-up:

In construction sites sometimes material mix-up may occur. Someone may be wondering how Carbon steel (CS) and Low-Temperature Carbon Steel (LTCS) material could mix up. The main reason is that no physical identification is possible and There are various possibilities that can arise at a construction site like:

• Common surface preparation & painting yard for all materials.
• In many cases, the color coding of pipes (Positive Material Identification-PMI) was not followed due to schedule pressure.
• On fittings, identification marks are written by a low-stress punch which was not visible after painting.
• Common store/contractor/fabrication area on the construction site.
• Ignorance of all concerned about the criticality & impact of a mix-up with normal carbon steel.

So what will you do in such a situation?

Process piping code ASME B31.3 provides guidelines in such situations. There is a provision in ASME B31.3 codes that is based on the stress ratio, the minimum allowable temperature of carbon steel materials can be further lowered without any impact testing.

So what’s this stress ratio? 

The stress ratio can be defined as the maximum of the

• a) Nominal pressure stress divided by allowable stress at design minimum temperature.
• b) For piping components with pressure ratings, the operating pressure is divided by the rated pressure at the design minimum temperature.
• c) Combined longitudinal stress, without stress intensification factor, due to pressure, deadweight, and displacement strain divided by allowable stress at the design minimum temperature. (Coincident Conditions)

Note that Stress Ratio requires computed stresses at minimum temperature and coincident pressures and No stress intensification factor to be used for stress calculations.

So, calculate the stress ratio as per the above guidelines.

Now the code provides a graph (reproduced above in Fig. 1) from where you can calculate the amount of temperature reduction with respect to the stress ratio. From the figure for a temperature reduction of -17 degree centigrade (to make it -29-17=-46 degree centigrade) the required stress ratio is 0.65. So if the calculated stress ratio is within 0.65 then do not be worried. In case the stress ratio exceeds 0.65 provide additional support to reduce the stress ratio within that limit.

In this way you can reduce the probable huge impact of material changing (thereby cost) after the erection of piping is already over.