During operation of an air separation unit (ASU) a plate fin heat exchanger developed a leak. This apparently simple heat exchanger, exchanging heat between two gaseous streams of air versus nitrogen, was not expected to suffer significant stress levels for producing a damage.
During the analysis of the damage which occurred at the connection welds of three adjacent modules — module construction is widely used in the process industry — an operating situation was discovered leading to high thermal gradients inside the exchanger. It occurred during a stop of the plants nitrogen cycle compressor. The warm flow continued and heated up the exchanger in a very fast way. This led to unacceptably high stresses in the top part of the exchanger. But a more severe situation occurred when the cycle compressor has been restarted. Then cold nitrogen entered the lower end of the exchanger which had been warming up in the meantime.
A finite element analysis based on a modeling technique  for brazed plate fin heat exchangers was performed for this dynamic incident showing extreme stress levels at the location of the damage. A fatigue evaluation of the calculated stresses showed that only a few cycles were acceptable for this scenario. As a consequence operational measures have been implemented to protect the exchanger against this severe condition.