BP tackles corrosion threat at UK ion exchange plant

London - BP had to respond sharply when a water treatment plant at its Hull site began to corrode. The situation threatened the continuous supply of water to the steam generators at the operation, which houses Europe’s largest manufacturing facility for acetic acid.

The water treatment plant, which comprises three 140m3/h streams each with weak and strong acid cation exchangers, a common degasser and weak and strong base anion exchangers, was installed in 1997 to provide feed water to the factory’s 64barg and 45barg steam boilers. Without steam, production stops therefore the reliability of the water treatment plant is critical to the site.

The problem occurred in the plant's 2.4m diameter strong acid cation exchangers. The units are co-counter current flow regenerated using a technique in which 25% of the sulphuric acid regenerant flows simultaneously downwards through the top part of the resin bed with the remaining 75% flowing upwards through the bottom section and is collected in a central header and lateral system.

The forces on the collector system during the occasional backwash and possibly during regeneration can be very high and, if it is not properly designed and supported, it can move leading to eventual mechanical failure. Within a few years of installation the units were suffering from loss of resin and inefficient regeneration.

“The strong acid cation exchange units were backwashed on an infrequent basis, however the collector systems were not adequately supported and there was enough movement to damage the protective rubber coating," said BP. "This allowed the regenerant acid to attack the carbon steel underneath resulting in corrosion and failure of the collector header to vessel manifold joint with a resultant escape of resin, ineffective regeneration and eventually leading to total failure of the units ."

BP Works Engineering planned to retrofit new collectors. They were able to design and install the necessary supports inside the pressure vessels and fitted new stainless steel nozzles on the SAC shell for connection to the collector system. The design of the collector system itself was a job for specialists with BP selecting ELGA Process Water to design and fabricate a complete new stainless steel header with laterals fabricated from stainless steel wedge wire.

The modifications were carried out in three stages over a period of four months, BP stating: “The modified internals have performed well, with good distribution of the regenerant acid, greatly improved reliability, with no further problems.”