OSHA Hazard Information Bulletins
July 29, 1994
MEMORANDUM FOR: |
REGIONAL ADMINISTRATORS |
FROM: |
CHARLES E. ADKINS, CIH
Acting Director
Directorate of Technical Support |
SUBJECT: |
Hazard Information Bulletin(1) - Corrosion of
Piping in Hydroprocessing Units |
___________
FOOTNOTE(1) The Directorate of Technical Support issues Hazard
Information Bulletins (HIB) in accordance with OSHA Instruction CPL 2.65
to provide relevant information regarding unrecognized or misunderstood
health hazards, inadequacies of materials, devices, techniques, and safety
engineering controls. HIBs are initiated based on information provided by
the field staff, studies, reports and concerns expressed by safety and health
professionals, employers, and the public. Information is compiled based on a
thorough evaluation of available facts and in coordination with appropriate
parties.
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 (NH(4)HS), 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:
- All cases of failure were associated with carbon steel
piping.
- All the units had non-symmetrical outlet piping. - The
units' velocities were outside the recommended guideline.
- Failures were associated with erosion, or corrosion of a dead
leg area.
- Corrosion was localized. Inspections of adjacent areas showed
very little loss while the failed areas had experienced very high corrosion
rates.
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:
- Make all REAC inlet and outlet piping symmetrical. -
Restrict linear velocity to 20 ft/sec maximum and 10ft/sec minimum.
- Restrict ammonium bisulfide concentration in the separator
water to maximum of 8% by weight.
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.
Attachment
Figure 1. Reactor Effluent Air Coolers (REACS)
Partial Flow Diagram. Arrows indicate internal tube corrosion: and header
errosion and fouling occurred mainly at the outer bay bends (cutaway shows
internal tubes). Flow is shown from reactors through four (4) fin fan product
cooler inlets. The flow exits the outlets to the separator. The diagram also
illustrates the area most vulnerable to corrosion.
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