Low Temperature Carbon Steel (LTCS) is used in piping system when there is a possibility of process fluid temperature falling below -29 degree centigrade during operation. In typical refinery A 106 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 in between these two materials in composition. Also no non-destructive testing is available which can ensure these two materials. So if by mistake CS and LTCS mixes up with one another and someone installs CS in place of LTCS without knowing the major operational impact whats the solution?
Reason for Mix up:
In construction sites sometimes the 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 which can arise in a construction site like:
1. Common surface preparation & painting yard for all materials.
2. In many cases, the colour coding of pipes (Positive Material Identification-PMI) was not followed due to schedule pressure.
3. On fittings, identification marks are written by low stress punch which was not visible after painting.
4. Common store/contractor/fabrication area in construction site.
5. Ignorance of all concerned about the criticality & impact of mix-up with normal carbon steel.
So what will you do in such situation?
Process piping code ASME B 31.3 provides a guidelines in such situations. There is a provision in ASME B 31.3 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 whats this stress ratio?
Stress ratio can be defined as maximum of the
a) Nominal pressure stress divided by allowable stress at design minimum temperature.
b) For piping component with pressure ratings, the operating pressure divided by 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 stress ratio as per above guidelines.
Now the code provides a graph (reproduced above) from where you can calculate the amount of temperature reduction with respect to 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 not to be worried. In case the stress ratio exceeds 0.65 provide additional support to reduce 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.