What are the industrial practices being followed in refineries to mitigate polythionic acid corrosion in furnaces?
Answers
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22/02/2017
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RAMAKRISHNA DADI, HMEL, dadi2rama@gmail.com
if the fired heater tubes are used stabilization grade stainless steel material is best like SA 312 TP 347H and your fluid temperature is not more than 400 deg C is not sensitization. before opening the tube inlet piping for blinding / pigging arrangement for launcher receiver / pusher do with positive nitrogen purge with less than 5% of oxygen. Then you need to use alkaline solution like soda ash 2-5% and DM water to get the neutralization solution and inject the tubes to loosen the coke and to maintain the alkaline filming to protect the layer.
Iy any stainless steel equipment before opening positive nitrogen and use the neutralization solution.
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21/11/2016
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Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
Polythionic acid is formed by the reaction of sulphide scale, oxygen and water. It can cause corrosion of austenitic stainless steel radiant tubes during furnace cooldown if the furnace tubes have become sensitised in operation. The best mitigation is to specify thermally stabilised grades of stainless steel (eg. 347H SS) for radiant tubes as these have greater resistance to sensitisation. Another mitigation for the fire-side of the radiant tubes is to ensure only low-sulphur fuel gas is fired so the scale on the outer surfaced is oxide rather than sulphide scale and therefore precludes polythionic acid formation. Continuous dry nitrogen purging of the process-side of the tubes once the furnace has been taken off-line prevents polythionic acid formation on the inner surfaces of the tubes by excluding oxygen and water. If the austenitic stainless steel tubes have become sensitised and need to be mechanically pigged to remove coke and/or internal sulphide scale, they should be washed with soda ash solution first.
Refer also to NACE RP-0170, "Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking during Shutdown of Refinery Equipment."
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21/11/2016
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Sridhar Balakrishnan, Bharat Oman Refineries Limited , laksrid@yahoo.com
Around five percent soda acid wash of the equipment before or right after it is exposed to air and moisture. It can also be prevented by removing oxygen from the environment by performing a dry nitrogen purge or removing moisture by performing a dry air purge.
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