5 replies to this topic

[ski]

Apologies if this idiotic!

 

I'm designing a vessel which will be located outside where temperatures could drop as low as -20C in the winter. Although the vessel will have insulation, technically the vessel could experience this temperature since eventually if this low temperature persists, the interior wall and insulation will come to equilibrium with the exterior temperature - is this correct?  

 

If it is then this seems to be below the -10C that is often cited as the temperature below which  metals can fail in a brittle manner, therefore would I have to consider metal (such as austentic stainless steel or aluminium alloys) used in low-temperature equipment.... 

 

However I know that other vessels with the same process are made of 316 SS, but perhaps they are located in warmer climates....

 

In summary, should I use the -20C as my Critical Exposure Temperature (knowing that temperatures are unlikely to reach below this inside the vessel)?

[ankur2061]

ski,

 

What do you think 316 SS is? (some diifferent kind of steel from austenitic stainless steel)

 

Even Carbons Steel can be used for -20°C, the kind that is referred to as "Low Temperature Carbon Steel" abbreviated as LTCS as long as it is chemically compatible with the fluid stored in the vessel.

 

The right terminology for the minimum temperature that the vessel needs to be designed is "Minimum Design Metal Temperature" abbreviated as MDMT.

 

Refer the link below:

 

http://www.eng-tips.....cfm?qid=239736

 

Regards,

Ankur

[ski]

Thanks Ankur.

Apologies, I didnt realise that 316 SS was an austenitic steel.

 

However even knowing this, I still don't know whether I should be specifying a material that could deal with these low temperatures.   I think it would be ridiculous to put a heat tracer on the vessel since the reaction inside is highly exothermic and removing heat is the main problem.   I think I might assume that even if the temperature outside gets very low, the reactor will probably remain running (haven't looked into a full evaluation of low temperatures on the full plant), so that the metal will never reach this low temperature, and that the temperature gradient across the metal wont be too high due to the insulation around the vessel.   Other factors will probably call for austenitic stainless steel anyway!

 

Yes I did read up on plenty of MDMT posts over the past while here, but I thought that the MDMT was a specified temperature which is less than or equal to the CET, but greater than or equal to the MDMT_M, the lowest temperature allowed by the material of construction of the vessel.   Therefore I thought I would specify the CET, and then just pick a slightly lower temperature for the MDMT to give as a specification for equipment design with regards to temperature. Then the final choice of material of construction (dependent on these specified temperatures among other factors) would give the MDMT_M, the temperature which would be stamped on the vessel.

 

Is this not correct? 

[Steve Hall]

316 SS is used in cryogenic service. You don't have to worry about -20C.

[Lai.CY]

Hello Ski,

 

Theoretically, carbon steel can go down to -27C. Low Temp Carbon Steel can go down to -40C, and Stainless Steel -101C.

These are just some general figures I use as rule of thumb, since you have different types and grades of steels that will

give you different MDMT.

 

So, -20C for stainless steel is definitely not a concern.

[ski]

Thanks to everyone for their input.