Cleaning of Oil Refinery Equipment

The maximum temperature for inhibited hydrochloric acid used for cleaning cast iron in petroleum refinery equipment is 51°C (125 °F), while other metals can be cleaned safely at 77 °C (170 °F). Using 7.5% acid at 77 °C (170 °F) causes graphitization of some cast irons, particularly those with combined carbon. 

Because ferric ions accelerate corrosion by cleaning solutions, a limit of 0.4% by weight usually is the accepted maximum. A 1% solution of ferric ions has been known to increase the corrosion rate by a factor of nine. 

Inhibitor Low-carbon steel Free machining steel FeS FeSj FeO Fe3 04 

Note (-) Sign indicates slight acceleration of dissolution. In the case of Fe304 in inhibitor C, acceleration of dissoiution as compared with plain 5% HCI was by 1 70%. 

Spent acid from one cleaning operation should not be used in another because of possible bad effects from concentrations of cupric or ferric ions in the used solutions. All stainless steel systems can be cleaned effectively using sulfuric or nitric acid solutions. Stainless steels that have reached their sensitization temperature are likely to suffer intergranular attack. 

---