July 29, 1994
The San Francisco Regional Office has brought to our attention a potentially serious safety hazard associated with hydrocracking (hydroprocessing) units. During an investigation of an explosion and fire at a refinery, the region discovered that severe corrosion resulted in the failure of the hydrocracking Reactor Effluent Air Coolers (REAC) and adjacent piping. The exact cause of the explosion has not been determined.
Hydrocracking, or hydroprocessing, is a two stage process combining catalytic cracking and hydrogenation. Sulfur and nitrogen compounds are converted by a catalyst in the first stage reactor to hydrogen sulfide and ammonia. As the effluent stream from the reactor cools down, the ammonia and hydrogen sulfide combine to form solid ammonium bisulfide. Ammonium bisulfide, also called ammonium hydrogen sulfide (NH4HS), is a metal reagent.
Concentrated ammonium bisulfide is highly corrosive to carbon steel. To reduce corrosion, wash water is introduced in the system since ammonium bisulfide is highly soluble in water. The corrosion rate is accelerated, however, if the flow velocity of the ammonium sulfide solution is increased.
The piping areas most vulnerable to corrosion are upstream of the REAC unit and between the REAC and the separator. The REAC unit itself also is vulnerable to corrosion (see diagram). The licensor of this particular unit advised users of the system to use appropriate alloy piping since it is highly resistant to ammonium bisulfide corrosion. The previous industry practice specified carbon steel piping upstream and downstream of the REAC.
Prior to the recent explosion, in the San Francisco region, the hydroprocessing unit manufacturer was aware of three separate cases of failure related to the systems REAC piping. It is our understanding that the manufacturer of the hydroprocessing unit has mailed more than one hundred letters of warning, containing a strongly worded recommendation regarding the need to monitor corrosion of the unit's carbon steel pipes.
The following general observations about REAC Unit failings were noted:
According to the Science and Technology research division of a major oil company, to minimize ammonium bisulfide corrosion of the REAC and associated carbon steel piping, the following actions are recommended:
Any systems operating outside these guidelines may be subject to severe corrosion.
It is recommended that companies establish a corrosion monitoring program for the REAC piping, especially for units that operate continuously or intermittently outside these guideline limits. The inspections should include coverage for all potential turbulent or dead zones. REAC facilities need to identify critical points to be inspected for corrosion and monitor the integrity of the piping on a predetermined inspection schedule.
Companies can use straight beam conventional ultrasonic, B-scan ultrasonic, or radiography to establish corrosion patterns and thicknesses. Additionally, this testing can be done when the unit is operating, minimizing production loss.
Your attention is called to this matter because of the explosion potential resulting from failure to follow the PSM requirements for mechanical integrity and monitoring [29 CFR 1910.119(j)].
Please distribute this bulletin to all Area Offices, State Plan States, Consultation Project Managers and to appropriate local labor and industry associations.
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