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U. S. Department of Labor
Occupational Safety and Health Administration
Directorate of Science, Technology and Medicine
Office of Science and Technology Assessment
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U. S. Environmental Protection Agency
Chemical Emergency Preparedness and Prevention Office
EPA 550-F-03-001
August 2003
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Hazards of Delayed Coker Unit (DCU) Operations
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Safety and Health Information Bulletins |
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SHIB 08-29-03 (C) |
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The statements in this document are intended solely as guidance. This document does not substitute for EPA’s or other agency regulations, nor is
it a regulation itself. Site-specific application of the guidance may vary depending on process activities, and may not apply to a given situation.
EPA may revoke, modify, or suspend this guidance in the future, as appropriate.
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| This Safety and Health Information Bulletin is not a standard or regulation, and it creates no new legal obligations. Likewise, it cannot and does
not diminish any obligations established by statute, rule, or standard. The Bulletin is advisory in nature, informational in content, and is intended
to assist employers in providing a safe and healthful workplace. The Occupational Safety and Health Act requires employers to comply with
hazard-specific safety and health standards. In addition, pursuant to Section 5(a)(1), the General Duty Clause of the Act, employers must
provide their employees with a workplace free from recognized hazards likely to cause death or serious physical harm. Employers can be cited
for violating the General Duty Clause if there is a recognized hazard and they do not take reasonable steps to prevent or abate the hazard.
However, failure to implement any recommendations in this bulletin is not, in itself, a violation of the General Duty Clause. Citations can only
be based on standards, regulations, and the General Duty Clause.
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The Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration
(OSHA) are jointly issuing this Chemical Safety Alert/Safety and Health Information Bulletin (CSA/SHIB)
as part of ongoing efforts to protect human health and the environment by preventing chemical accidents.
We are striving to better understand the causes and contributing factors associated with chemical
accidents, to prevent their recurrence, and to provide information about occupational hazards and
noteworthy, innovative, or specialized procedures, practices, and research that relate to occupational safety
and health and environmental protection. Major chemical accidents cannot be prevented solely through
regulatory requirements. Rather, understanding the fundamental root causes, widely disseminating the
lessons learned, and integrating these lessons into safe operations are also required. EPA and OSHA
jointly publish this CSA/SHIB to increase awareness of possible hazards. This joint document supplements
active industry efforts to exchange fire and safety technology and to increase awareness of environmental
and occupational hazards associated with DCU operations. It is important that facilities, State Emergency
Response Commissions (SERCs), Local Emergency Planning Committees (LEPCs), emergency
responders, and others review this information and take appropriate steps to minimize risk. This document
does not substitute for EPA or OSHA regulations, nor is it a regulation itself. It cannot and does not
impose legally binding requirements on EPA, OSHA, states, or the regulated community, and the
measures it describes may not apply to a particular situation based upon the circumstances. This guidance
does not represent final agency action and may change in the future, as appropriate.
Purpose
The batch portion of DCU
operations (drum switching and
coke cutting) creates unique
hazards, resulting in relatively
frequent and serious accidents.
The increasingly limited supply of
higher quality crude oils has resulted in
greater reliance on more intensive
refining techniques. Current crude oils
tend to have more long chain
molecules, known as “heavy ends” or
“bottom of the barrel” than the lighter
crude oils that were more readily
available in the past. These heavy ends
can be extracted and sold as a relatively
low value industrial fuel or as a
feedstock for asphalt-based products,
such as roofing tile, or they may be
further processed to yield higher value
products. One of the most popular
processes for upgrading heavy ends is
the DCU, a severe form of thermal
cracking requiring high temperatures
for an extended period of time.
This process yields higher value liquid
products and creates a solid carbonaceous
residue called “coke.” As the supply of
lighter crude oils has diminished, refiners
have relied increasingly on DCUs.
Unlike other petroleum refinery
operations, the DCU is a semi-batch
operation, involving both batch and
continuous stages. The batch stage of the
operation (drum switching and coke
cutting) presents unique hazards and is
responsible for most of the serious
accidents attributed to DCUs. The
continuous stage (drum charge, heating,
and fractionation) is generally similar to
other refinery operations and is not further
discussed in this document. About 53
DCUs were in operation in the United
States in 2003, in about one third of the
refineries.
In recent years, DCU operations have
resulted in a number of serious accidents
despite efforts among many refiners to
share information regarding best practices
for DCU safety and reliability. EPA and
OSHA believe that addressing the hazards
of DCU operations is necessary given the increasing
importance of DCUs in meeting energy demands, the
array of hazards associated with DCU operations, and
the frequency and severity of serious incidents
involving DCUs.
Understanding the Hazards
Safe DCU operations require an understanding
of the situations and conditions that are most
prone to frequent or serious accidents.
Process Description
Each DCU module contains a fired heater, two (in
some cases three) coking drums, and a fractionation
tower.
This document focuses on the coke drums, which are
large cylindrical metal vessels that can be up to 120
feet tall and 29 feet in diameter.
In delayed coking, the feed material is typically the
residuum from vacuum distillation towers and
frequently includes other heavy oils. The feed is
heated by a fired heater (furnace) as it is sent to one
of the coke drums. The feed arrives at the coke drum
with a temperature ranging from 870 to 910°F.
Typical drum overhead pressure ranges from 15 to 35
psig. Under these conditions, cracking proceeds and
lighter fractions produced are sent to a fractionation
tower where they are separated into gas, gasoline,
and other higher value liquid products. A solid
residuum of coke is also produced and remains
within the drum.
After the coke has reached a
predetermined level within the
“on oil” drum, the feed is
diverted to the second coke
drum. This use of multiple
coke drums enables the
refinery to operate the fired
heater and fractionation tower
continuously. Once the feed
has been diverted, the original
drum is isolated from the
process flow and is referred to
as the “off oil” drum. Steam is
introduced to strip out any
remaining oil, and the drum is
cooled (quenched) with water,
drained, and opened
(unheaded) in preparation for
decoking. Decoking involves
using high pressure water jets
from a rotating cutter to
fracture the coke bed and
allow it to fall into the
receiving area below. Once it
is decoked, the “off oil” drum
is closed (re-headed), purged
of air, leak tested, warmed-up,
and placed on stand-by, ready
to repeat the cycle. Drum
switching frequency ranges
from 10 to 24 hours. DCU
filling and decoking operations
are illustrated in Figure 1.
Equipment used in coke
cutting (hydroblasting)
operations is illustrated in
Figure 2.
Figure 1. Delayed Coker Unit -
Cutaway to Depict Drum In Filling and Migration Mode (Left) and Drum In Cutting Mode (Right). |
Figure 2. Delayed Coker Unit -
Coke Drums and Hydroblast Systems.
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Accident Investigation
Once removed from the coke drums, the coke is
transported away from the receiving area. From here,
the coke is either exported from the refinery or
crushed, washed, and stored prior to export.
The following specific operations and more general
situations and conditions contribute most
significantly to the hazards associated with DCU
operations:
Specific operation hazards
- Coke drum switching
- Coke drum head removal
- Coke cutting (hydroblasting operation)
Emergency and general operational hazards
- Coke transfer, processing, and storage
- Emergency evacuation
- Toxic exposures, dust irritants, and burn
trauma
The hazards associated with these specific operations
and DCU operations, in general, are explained below
to share lessons learned and increase awareness of
the situations and conditions that are most prone to
serious accidents. Following this section, the joint
CSA/SHIB describes actions that can be taken to help
minimize the risks associated with these situations
and conditions.
Specific Operation Hazards
Coke Drum Switching
Most DCU operations consist of several DCU
modules, each typically alternating between two coke
drums in the coking/decoking sequence. Some DCU
modules include a third drum in this sequence. Each
drum includes a set of valving, and each module
includes a separate set of valving. Differences in
valving among drums and among modules may be
difficult to distinguish and can lead to unintended
drum inlet or outlet stream routing. Similarly, valve
control stations, for remotely activated valves, may
not always clearly identify the operating status of
different drums and modules. Activating the wrong
valve because of mistakes in identifying the
operational status of different drums and modules has
led to serious incidents.
Coke Drum Head Removal
Conditions within the drum, during and after
charging, can be unpredictable. Under abnormal
conditions, workers can be exposed to the release of
hot water, steam and coke, toxic fumes, and physical
hazards during removal of the top and bottom drum
heads. The most frequent and/or severe hazards
associated with this operation are described below:
- Geysers/eruptions - Under abnormal situations,
such as feed interruption or anomalous shortcircuiting
during steaming or quenching, hot
spots can persist in the drum. Steam, followed by
water, introduced to the coke drum in
preparation for head removal can follow
established channels rather than permeate
throughout the coke mass. Because coke is an
excellent insulator, this can leave isolated hot
areas within the coke. Although infrequent, if the
coke within the drum is improperly drained and
the coke bed shifts or partially collapses, residual
water can contact the isolated pockets of hot
coke, resulting in a geyser of steam, hot water,
coke particles, and hydrocarbon from either or
both drum openings after the heads have been
removed.
- Hot tar ball ejection - Feed interruption and
steam or quenching water short-circuiting can
also cause “hot tar balls,” a mass of hot (over
800°F) tar-like material, to form in the drum.
Under certain circumstances, these tar balls can
be rapidly ejected from the bottom head opening.
- Undrained water release - Undrained hot water
can be released during bottom head removal,
creating a scalding hazard.
- Shot coke avalanche - Sometimes, the coke
forms into a multitude of individual, various
sized, spherical shaped chunks known as “shot
coke,” rather than a single large mass. In this
situation, the drum contents are flowable and
may dump from the drum when the bottom head
is removed, creating an avalanche of shot coke.
- Platform removal/falling hazard - Some DCUs
require the removal of platform sections to
accommodate unheading the bottom of the drum.
This can introduce a falling hazard.
Coke Cutting (Hydroblasting
Operation)
Coke-cutting or -hydroblasting involves lowering
from an overhead gantry a rotating cutter that uses
high pressure (2000 to 5000 psig) water jets. The
cutter is first set to drill a bore hole through the coke
bed. It is then reset to cut the coke away from the
drum interior walls. Workers around the gantry and
top head can be exposed to serious physical hazards,
and serious incidents have occurred in connection
with hydroblasting operations. Some of the most
frequent and/or severe hazards are described below:
- If the system is not shut off before the cutting
nozzle is raised out of the top drum opening, a
high pressure water jet can be exposed and
seriously injure, even dismember a nearby
worker.
- Fugitive mists and vapors from the cutting and
the quench water can contain contaminants that
pose a health hazard (see section on Toxic
Exposures, Dust Irritants and Burn Trauma,
below).
- The water hose can burst while under high
pressure, resulting in whipping action that can
seriously injure nearby workers.
- The wire rope supporting the drill stem and
water hose can fail (part), allowing the drill stem,
water hose, and wire rope to fall onto work areas.
- Gantry damage can occur, exposing workers to
falling structural members and equipment.
Emergency and General Operational
Hazards
Coke Transfer, Processing, and
Storage
The following coke conveyance, processing, and
storage operations have presented safety and health
hazards for DCU workers:
- The repositioning of rail cars by small
locomotives or cable tuggers to receive coke
being cut from a drum can create physical
hazards for workers in the rail car movement
area.
- Mechanical conveyors and coke crushers may
contain exposed moving parts that can cause
fracture or crush type injuries at pinch points.
- Fires are common in coke piles and rail cars.
Large chunks of coke can contain pockets of
unquenched material at temperatures well above
the ignition point. When fractured and exposed
to air, this material can ignite. Fires have also
been attributed, although less frequently, to
reactions that lead to spontaneous combustion.
- Combustion products and/or oxygen depletion
resulting from spontaneous fires can create
hazardous conditions for workers in confined
spaces.
- Wet coke in an enclosed area has been reported
to have absorbed oxygen from the surrounding
air under certain circumstances. This can make
the area oxygen deficient and cause
asphyxiation.
Emergency Evacuation
The delayed coking process is very labor intensive.
Each batch process cycle requires 25 or more manual
operations (valve, winch operation, drum heading,
etc.), and many DCUs operate with three or more sets
of drums. Tasks are performed at several levels on
the coke drum structure. The upper working platform
(frequently called the “cutting deck”) is generally
well over 120 feet above ground. During an
emergency, evacuation from the structure can be
difficult.
In addition, moisture escaping from drum openings
during cold weather can produce fog. This can
obscure vision and make walkways, and hand rails
wet and slippery, creating additional difficulties
during emergency evacuation.
Toxic Exposures, Dust Irritants, and
Burn Trauma
DCU workers can be exposed to coke dust and toxic
substances in gases and process water around DCU
operations. Workers can also be exposed to physical
stress and other hazardous conditions. The following
exposures to toxic substances, irritants, and
hazardous conditions have been associated with DCU
operations, in general:
- Hot water, steam, and liquid hydrocarbon (black
oil) can escape from a coke drum and cause
serious burn trauma. Contact with black oil can
cause second or third degree burns. In addition,
liquid hydrocarbon escaped from a coke drum
can be well above its ignition temperature,
presenting a fire hazard.
- Heat stress can be a health hazard during warm
weather, particularly for those required to wear
protective clothing while performing tasks on the
coke drum structure.
- Hazardous gases associated with coking
operations, such as hydrogen sulfide, carbon
monoxide, and trace amounts of polynuclear
aromatics (PNAs), can be emitted from the coke
through an opened drum or during processing
operations.
- If allowed to accumulate and become airborne,
dust around a DCU may exceed acceptable
exposure limits and become a hazard.
Controlling the Hazards
Evaluating hazardous conditions, modifying
operations to control hazards, actively
maintaining an effective emergency response
program, and familiarizing workers about risks
and emergency procedures will help reduce the
frequency and severity of serious incidents
associated with DCU operations.
Specific Operation Hazards
Coke Drum Switching
No one system has proven effective in eliminating all
incidents associated with incorrect valve activation
due to mistaken coke drum or module identification;
however, the following actions have been reported as
beneficial:
- Conduct human factors analyses to identify,
evaluate, and address potential operator actions
that could compromise the safe operation of the
coke drum system.
- Provide interlocks for automated or remotely
activated valve switching systems.
- Provide interlocks for valves that are manually
operated as part of the switching/decoking cycle
to avoid unanticipated valve movement.
- Color code and clearly label valves and control
points to guard against incorrect identification.
- Provide indicator lights at valve and valve
control stations to help the operator determine
which is the correct valve station for the intended
operator action.
- Use the “buddy system” (employees working in
pairs) to help verify accurate valve or switch
identification.
- Conduct periodic and documented training
focusing on the importance of activating the
correct valve or switch and the consequence of
incorrect activation.
Coke Drum Head Removal
It can be difficult to anticipate the presence of either
a hot spot or a hot tar ball in the coke drum prior to
drum head removal. In light of this possibility and the
potential for serious incidents, it is prudent to:
- Be alert to any operating abnormalities or
variations during charging, steaming, or
quenching that may forewarn a hot spot or tar
ball. Have a contingency plan to deal with such
issues before proceeding with coke drum head
removal and coke cutting.
- Always assume the possibility of a hot-spot
induced geyser or the release of hot tar balls or
undrained hot water, and incorporate protective
operational measures in drum unheading
operations. Further control the hazard by
establishing restricted areas; minimizing the
number of workers in restricted areas;
minimizing the time spent by essential workers
in restricted areas; and maintaining readiness for
a rapid evacuation.
- Consider equipment upgrades to further control
the hazards associated with geysers and release
of hot tar balls and undrained hot water during
drum head removal, such as installing protective
shrouds and automating both top and bottom
head removal operations to keep workers away
from these unprotected areas.
- Consider emergency steam/cooling water
sources in the event of loss of primary
steam/cooling water supply or because of drum
inlet flow path obstruction.
- Provide temporary guardrails to prevent
employees from falling while platform plating is
removed for bottom head removal.
- Consider installation of vapor ejectors to draw
vapors away from the open top head area.
Coke Cutting (Hydroblasting
Operation)
The following actions could help control hazards
associated with coke cutting operations:
- Install an enclosed cutter’s shack for worker
protection--preferably supplied with air from a
remote source to maintain slight positive
pressure.
- Ensure that personnel who must be on the coke
drum structure when a drum is open wear
prescribed personal protective equipment.
- Conduct training in recognition and prevention
of worker heat stress.
- Make sure the interlocks will work to shut off
and prevent restart of the cutting water pump any
time that the cutting head is raised above a
predetermined point within the coke drum.
Consider installing redundant switches to
provide an additional level of protection against
extracting a cutting head that is under pressure.
- Verify the adequacy of the inspection and
maintenance program for cutting water hoses,
wire ropes, and hoists.
- Establish a gantry structure inspection and
maintenance program. Periodically verify that
gantry structures have not been weakened due to
corrosive conditions, such as mist exiting from
the top nozzle, that could lead to gantry collapse.
- Install drill stem free fall arresters.
Emergency and General Operational
Hazards
Coke Transfer, Processing, and
Storage
The following actions could help control hazards
associated with coke conveyance, processing, and
storage operations:
- Establish and enforce restricted areas (e.g., areas
where heavy equipment movement and possible
lash path of a wire rope from failed equipment
may occur) to prevent personnel entry and,
ultimately, injury.
- Establish and periodically verify the operability
of an alarm system that activates immediately
before and during heavy equipment (rail car,
bridge crane, or conveyor) movement.
- Verify conformance with a safe entry permit
system to ensure that appropriate measures are
taken prior to and during entry into any enclosed
area or vessel where coke may be present.
- Establish personnel protective measures to
protect against inhalation or personal contact
with coke dust or potentially contaminated mists
from water used for cutting, quench, or coke
conveyance (see section on Toxic Exposures,
Dust Irritants, and Burn Trauma, below).
Emergency Evacuation - Preparations
and Procedures
Despite best efforts to prevent incidents, DCU
operators should anticipate the need for emergency
evacuation and other response measures, operate in a
manner that will minimize the severity of an incident,
and prepare for and implement emergency
procedures to protect worker safety.
The following specific actions are recommended:
- Review and address weaknesses associated with
the location and suitability of emergency escape
routes. Protected stairways, preferably detached
from the coke drum structure, are the most
effective conventional means of emergency
escape route (egress) from tall structures, such as
those serving the coke drums. Consider installing
horizontal walkways to adjacent structures.
Some refineries are exploring the use of
commercially available escape chutes. Also, slip
resistant walking surfaces will help prevent
falling during an emergency evacuation.
- Establish or verify the operability of an
evacuation signal (Scram Alarm) to expedite
personnel clearing the structure in the event of an
emergency. Alarm signal actuation (triggering)
stations should be deployed at work areas and
along the escape routes.
- Install water sprays to protect work stations and
emergency escape routes. Include activation
stations at work stations and along the escape
route.
- Provide heat shields to protect work stations and
escape routes. Ensure that the shield will not
interfere with evacuation and will not entrap
fugitive vapors.
- Conduct regular emergency exercises to test the
plan as well as to ensure familiarity with
emergency signals, evacuation routes, and
procedures.
Toxic Exposures, Dust Irritants, and
Burn Trauma
The following actions could help control exposures to
toxic substances, irritants, physical stress, and
hazardous conditions associated with DCU
operations, in general:
- Configure coke drum inlets and outlets with
doubleblock valve and steam seal isolation to
reduce the likelihood of unanticipated leakage.
- Establish burn trauma response procedures,
including procedures for interacting with
emergency medical service providers and the
burn trauma center that would be used in the
event of a burn incident.
- Conduct burn trauma simulation exercises to
ensure appropriate use of the emergency
response procedures and the training level of
relevant personnel.
- Evaluate health exposure potential and establish
appropriate protective measures based on an
industrial hygiene survey plan that anticipates
variations in the range of DCU feed stocks and
operating conditions.
- Shovel, sweep, vacuum, and provide proper
ventilation to keep exposures to dust around a
DCU to within acceptable limits.
Information Resources
Internet resources - The search entry, “Delayed
Coker Unit,” yields many sources of information that
are believed to be useful. However, neither EPA nor
OSHA control this information and cannot guarantee
the accuracy, relevance, timeliness or completeness
of all facets of the information.
Further, the citation to these resources is not intended
to endorse any views expressed, or services offered
by the author of the reference or the organization
operating the service identified by the reference. The
following are examples of informative additional
reading.
- http://www.coking.com - focuses on coking
best practices, safety, reliability, and
communications within the DCU industry.
- http://www.fireworld.com/magazine/coker.
html – describes a May 1999 coking unit fire
and offers recommendations on fire protection.
For More Information:
To report an emergency, file a complaint, or seek OSHA advice, assistance, or products, call:
1-800-321-OSHA (6742)
TTY 1-877-889-5627
24-hours
Visit the OSHA Home Page: http://www.osha.gov
Contact EPA’s RCRA Superfund & EPCRA Call Center
(800) 424-9346 or (703) 412-9810
TDD (800) 553-7672
Monday-Friday, 9 AM to 5 PM, Eastern Time
Visit the OEPPR Home Page:
http://www.epa.gov/ceppo
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