Vessel Freezing 1

[James01]

I have a 30 year old pressure vessel constructed from BS-150-151-28A plate. The vessel is in good condition and is still at nominal wall thickness of 19.0mm.

During an operational upset the vessel was subjected to a temperature of -14 degrees C, it was only at this temperature for 10 minutes and was below freezing for a maximum of 1 hour 30 mins.

It did not fail during this stage and subsequent visual inspection shows no anomalies - NDT of welds being planned.

Would the low temperatures and timescale cause problems in this instance.

Thanks
Jim

 

[metengr]

Jim;
Was the vessel under pressure during this operational event, and what actually caused the decrease in temperature? Your visual inspection and nondestructive testing statements are acceptable to evaluate structural integrity. As far as low temperature effects on material properties, I would say if the visual inspection and NDT results are acceptable, the vessel should be acceptable for continued service. This assumes that the vessel was manufactured from carbon or low alloy steel plate.

 

[dcasto]

. I'm not familiar with the grade, but if it is a mild carbon steel, there should be no problem with temperatures down to -30 C for any time period. As a matter of fact there is no concern for a -50C, but I would not subject the vessle to any sharp blows or shock as it may become brittle and fracture. The NDT should be looking for cracks around welds and nozzels. If you think this could happen again, get a CHARPY test on it and rerate to -40C

 

[JStephen]

I was under the impression that the main problem with low temps would be with brittleness while at that temperature- so once it's brought up to normal, it should be fine. Unlike overheating it, which could potentially cause metallurgical changes, distortion, or yielding while hot.

 

[James01]

The vessel contained gas condensate at the time, not normal conditions and was at 5barg and ambient temp. This vessel should not have contained the condensate, this was introduced by error!! the process system tripped and the system blew down and in a sudden depressurisation the vessel temperature freezes rapidly, the pressure went from 5 barg to atmospheric in a matter of minutes.

This particular grade of steel is a low carbon steel.

Like JStephen I was under the impression that once returned to normal conditions the grain structure would be unaffected and that the risk was at the time of low temperatures, I am having the NDT done as confirmation that no internal cracking has resulted.

 

[metengr]

Yes, I concur.

 

[stanweld]

I would not be concerned. The Joules-Thompson cooling occurs with decreasing pressure, so the stress on the vessel is slight at the minimum calculated temperature. Depending on the heat mass transfer, the vessel most likely did not experience the gas temperature, unless the -14 C temperature was actually recorded and not calculated.

 

[robsalv]

Conventional wisdom says that if the vessel didn't go bang, i.e. brittly fail, when below it's CET/MDMT/MSOT <insert definition of choice>, it can be considered to have survived those EXACT conditions ONLY. If the vessel warmed up slowly and survived, you can breathe a sigh of relief and remain unconcerned about the vessel in its design operational window.

Semi cluey operational folks will try to grandfather a vessel for these new low temperatures, but resist this at ALL COSTS!