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COMMERCIAL SAFETY PRODUCTS |
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Corrosion of Piping in Hydroprocessing Units.
- INFORMATION DATE : 19940729
- RECORD TYPE : Hazard Information Bulletin
- SUBJECT : Corrosion of Piping in Hydroprocessing Units.
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
(For Figure REACS, Click Here)
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