Sour corrosion in refinery overheads
Sour corrosion can be very severe and is often associated with hydrogen damage risk. It is usually more episodic than is realised, occurring only, for example, immediately after equipment installation, internal inspection, or during process changes which change the corrosivity of the process.
Sour corrosion in refineries depends in part on the sulfur content of oil blends used. Most refineries now take oil from global sources, and blend in lower cost higher sulfur crudes to increase profitability. Sulfur content may vary from day to day with changes of feedstock, rendering flux measurements especially informative.
The sulfur is present in part as mercaptans and dissolved H2S, which release sour gas in distillation units (crude distillation units, CDU, and vacuum distillation units, VDU) . The gas is released from the top of the distillation columns, into overheads, together with water and acids such a HCl which may dissolve scale which otherwise protects steel from sustained corrosion. Sour corrosion may also arise in downstream equipment in which the sour water condensers out, such as overhead condensers, coolers and fin-fans. At such locations or Hydrosteel 7000 TL may be viable for identifying episodes of severe corrosion and their effective control. Spot measurements using Hydrosteel 6000 with AT-S probes and continuous measurements using Hydrosteel 7000 have been used in defining the extent and severity of an issue, particularly at locations where corrosion can be managed by better process or chemical control (F.Al-Faqeer, F.Addington,‘Hydrogen permeation application to crude unit overhead corrosion monitoring’, Corrosion 2008 , Paper 08548, NACE Conference Series, NACE, Houston 2008).