Surely if its for liquid CO2 the vessel material would have been impact tested at -196 degrees C (-320 deg F). If the test average was above 27 Joules then this would be acceptable. And - 20F is only -29 Celsius.
You could use a portable hardness tester to see if the material has undergone any hardnes changes since fabrication.
Presumably you want to know if brittle fracture is imminent. Acoustic Emission (AE) may help. Sensors are applied at strategic locations around the vessel, a load (static or cyclic) applied and the resulting acoustic signals received reviewed for evidence of stresses within the material. Its basically monitoring complete structures for crack initiation. There are specialist companies who perform this as a service.
It depends on what is the minimum operating temperature of that vessel defined during the design.
If the temperature reached by the material is above the minimum design temperature i don't see problem.
Also the ASME VIII (if the vessel is build folowing ASME) allows to go to a temperature well below the minimum design temperature depending on the stress ratio, without the necessity to perform impact test. (see UCS-66 of ASME VIII)
MrsHAZOP;
If you indeed went below (how far below?) -20 deg F for this carbon steel vessel in-service, and nothing happened (in other words no signs of leakage or problems), I would perform a visual inspection and perform a wet fluorescent MT around several appurtenances or nozzles on this vessel just to be sure that no cracking was introduced at the time of the temperature excursion. This is what I would do for peace of mind.
Is it currently full? If not, then pressurize it with air. Set a Pressure Safety Valve near your operating pressure. If the PSV goes before you notice tank leaks, then you should be good to go. It's non-destructive, assuming your tank is not already destroyed.
Hi all, Thanks for your comments and suggestions. The vessel has gone down to -58F, vs. minimum design temp of -20F. It is currently in operation at 250 psig. PSV set to 300 psig. The temperature excursions have occurred due to failures in the heating system, or when attempting to open the vessel for inspections. Since it has been out of its design operating pressure range, I want to make sure it is still fit for service.
Hi MrsHAZOP,
Being that CO2 is highly corrosive to steel when in contact with the atmosphere (forms carbonic acid), I'd think that any cracks that would form will begin to rust fairly quickly.
Unfortunately, most CO2 vessels are insulated, making that rust impossible to locate unless you remove the insulation. You may consider doing this during your next outage, assuming there is one.
I would first perform some calculations to define actual stress vs specified minimum yield strength at the -58F temperature excursion. If they were less than 50% of the SMYS, I would recommend MT as metengr stated. If they were less than 20% of the SMYS, I wouldn't be concerned. If they were larger than 50%, I would also recommend volumetric examinations (UT) of highest stressed welds.
