Anhydrous hydrogen fluoride (AHF) is used as a chemical catalyst in the reaction that combines C3 and C4 olefins and isobutane to produce alkylates, an important component of gasoline. AHF readily reacts with any available moisture to create the corrosive and toxic hydrofluoric acid (HF). So hazardous is this material to humans that OSHA lists the Permissible Exposure Limit (PEL) as 3 ppm averaged over an 8-hour shift, while NIOSH lists the IDLH (Immediately Dangerous to Life or Health) concentration as 30 ppm. In and of itself, this hazard level arguably makes HF acid alkylation units the single most dangerous sealing application in modern refining.
In view of this hazard, the first requirement of a gasket for HF units must be the ability to provide a tight, positive seal to prevent leakage of HF.
HF units periodically must be cleaned, serviced and inspected. At such times, water or other aqueous solutions may be used for cleaning up piping or hydrotesting prior to returning the line to service. Any of this moisture that makes it past the inside of a flange gasket and sits trapped between the flange and gasket becomes a potential source of corrosion. When the system is charged with AHF, these pockets of moisture can quickly become pockets of aggressive HF acid, resulting in flange pitting and significant corrosion, especially in the flange's inner diameter.
As a result of the prevalence of flange corrosion, flange replacement in HF acid alky units is common. Some facilities report they replace one-third of all flanges opened in plant turnarounds.
The second requirement, then, of gaskets for HF units is that they seal against intrusion of liquids between the flange faces. They must have sufficient compression and suppleness to conform to existing imperfections in the flange faces while being able to work in an aggressive chemical environment.
For more information and to learn more about advancements in gaskets for HF service through a technical webinar, email koswald@eriksna.Com or call (724) 213-1211.