Polythionic Acid stress corrosion cracking in heater pipes

[jonesey]

Anybody aware of any NDT method that can be used to test for the presence of polythionic acid stress corrosion cracking in heatr pipes in a fired heater ? Are there any methods that can be used to give a 100% picture of the status of the pipes whilst they are mounted in the heater? I know you can use PT, but we can only come to visually inspect half of the surface area of the pipe due to access problems.

 

[jrjones]

Jonesey,
We have used LPI as you stated and have also used a modified eddy current technique with reasonable success. Your damage is on the OD I presume? Are you sure it is polythionic?
Anyway, we used eddy current and cut a few tube samples out to proof up the method through cutting up cross sections.

 

[jonesey]

Cracks have been confirmed as resulting from polythionic SCC. All tubes are now scrapped and new tubes are being sourced. Cracks are on the OD of the tubes. We have always prevented PASCC on the inside by soda washing, as we knew the situation could arise. We were unfortunately unaware of a problem in the combustion section of the heater, which on shutdown resulted in PASCC.

 

[jrjones]

Jonesey,
I am trying to find the report for one of our furnaces to get more details...

We were surprise to find PASCC on our tube OD's generally because it is in an oxidizing environment on the tube OD. We attributed the potential for PASCC to "dirty" fuel gas for the furnace.

One interesting not that I remember from the report (arg, I wish I could find it) was that we had a sample of tube from the same batch that operates in our furnace that had never seen service (left over from the last retube). This sample showed what looked like PASCC!! So we are trying to determine what in the manufacturing process for the tube could have caused this...more to come if I can find the report...

jrjones

 

[jonesey]

we now know we have had a reducing (oxidising) atmosphere in the heater under operation which has given rise to the formation of the oxide layer on the tube surface. Which, after we shut the heater down, has resulted in PASCC.........if only we had known before we shitdown, we could have prevented it. Hindsight - a wonderful thing !!