NOVEMBER 1998 -- Six workers were killed in an explosion and fire involving a delayed coker unit at a petrochemical refinery. Trying to restart the unit after a weather-related power failure, the workers opened the bottom of a large pressurized drum expecting to find a "gooey tar ball" inside. Instead, a pocket of heated liquid fuel was waiting to be exposed to oxygen.
MAY 1999 -- Fire swept across the switching deck of a delayed coker unit. A coker operator was hospitalized in critical condition with burns on his face, neck and arms. The same refinery has been the scene of at least three other coker unit fires since 1993. One such fire in 1998 was blamed for a 10 cents a share drop in the company's quarterly earnings.
It is almost certain that fires and explosions involving delayed coker units will continue into the new millennium with disturbing frequency. In the last seven years 15 refinery workers have died in coker unit fires. In 1998 alone, eight refinery workers were killed in five separate coker fires. Beyond the tragedy of lost lives, these disasters represent more than $1 billion in property loss last year alone, not counting lost production.
"That tells me that we need to start readdressing the requirements for egress and fire protection on these coker units," said Jerry Craft, a consultant with Williams Fire & Hazard Control and a former refinery fire chief.
Craft addressed a workshop on the increasing incidents of coker fires at the 14th annual Industrial Fire World Conference & Exposition in April. He used his own experience in dealing with coker fires to illustrate the fire protection demands peculiar to this process.
"As a consultant I have been asked to confer with oil companies and access their emergency needs, in some cases after such incidents and sometimes before they can occur," Craft said. "The sharing of information about how these incidents happen has led to important changes."
Thermal cracking is only one of a series of processes used in refining crude oil into finished petroleum products. It breaks down large, high-boiling hydrocarbon molecules into smaller molecules through the use of high temperature for an extended time. Delayed coking utilizes thermal cracking to further process that part of crude oil left after every bit that can be used for gasoline, kerosine, jet fuel, diesel or feedstocks is extracted by other processes. This petroleum residuum is known in the trade as "bottom of the barrel."
"Coker units are designed to use the heavier part of the crude oil, the heavy bottoms if you will, and take it out of the practical distillation process," Craft said.
These "bottoms" are baked under high temperature. Liquid and gas product streams created are diverted to other uses. However, most of the material processed becomes a dense carbon known as petroleum coke. Coke with relatively low metal and sulfur content is used as a feedstock in the manufacture of anodes for aluminum and steel production. Coke with higher metal and sulfur concentration is ordinarily used as a fuel.
The delayed coker is the only main process in a modern petroleum refinery that is batch continuous. A fired heater with horizontal tubes reaches thermal cracking temperatures of 905 to 941 degrees F. A continuous flow of residuum passes through this furnace. Because it spends a relatively short time exposed to the heat, coking is thereby "delayed" until the residuum reaches large coking drums. A minimum of two drums are required, allowing one to be filled while the other is emptied and prepared for the next batch.
"De-coking" the drum is an arduous task in itself. The contents solidify as one enormous cake of coke. In the pioneering days of delayed cokers, "de-coking" required backbreaking manual labor done with picks and shovels. Today, a drilling rig similar to one found in the oil fields sits atop the drum. First a pilot hole is cut through the coke using high pressure water. Then the coke is cut apart and removed using a drilling bit equipped with horizontal water nozzles.
Tracing their modern origins back to the 1950s, delayed cokers operating in the U.S. numbered 49 in 1998. (This does not include fluid cokers or "flexicokers.") Most of these delayed cokers have been built since the 1970s. And, with their increasing numbers, coker units have become an important revenue source for their owners, Craft said. Hence, pressure to realize the maximum potential of these units.
"When they started out with coker units it was a 24-hour cycle," Craft said. "Then they went to 19-hour cycles. Now there is even talk of 10- to 12-hour cycles."
Adding further to the risk exposure is that delayed coking is a labor-intensive operation. A number of people work on these units in around-the-clock shifts.
"We have to think in terms of safety, protection and egress of those people," Craft said.
In August 1993, Craft helped extinguish and later investigate a major coker fire. The fire was in a unit that was part of a 10-drum complex, at that time the largest such complex in the world. A single drum in this complex is 100 feet tall and 23 feet in diameter. The walls of these drums are 11/4-inch thick carbon steel.
At the top of the drum, 151 feet above grade, is the cutting deck. Derricks that stand 130 feet tall themselves sit at this level, waiting for the final phase of the coking operation when the solidified material is drilled and broken apart. At the 40 foot level of the unit is the switching deck which gives workers access to the switch valves on the bottom of the drum. It is here that the drilled coke is emptied out of the drum and transported away.
The fire began with the failure of an 8-inch pipe elbow at grade level. Reduced crude heated to 750 degrees F being pumped to the refractionator discharged at 250 pounds pressure for nearly six minutes before igniting.
"Why it didn't flash off immediately, no one knows," Craft said. "The operator told me 'Chief, when it flashed off it was as big as it ever got.' The fireball went from grade level all the way up to the 151 foot cutting deck. Within the next six minutes, the area engulfed was 250 feet by 50 feet and 160 feet high. That is how quick it expanded."
For firefighters, the coker unit fire presented a nightmare in hydraulics. The worst fire was at the top of the unit where a six-inch quench oil line carrying either naptha or diesel stock ruptured. (Quench oil is injected into product to lower the temperature and stop the cracking process.)
"We were using 2,000 gallon a minute streams from a Williams' Hired Gun flowing from grade at 130 pounds of this discharge," Craft said. "This is the only thing that would reach up there to the (cutting) deck."
At the peak of the fire responders were throwing 10,000 to 12,000 gallons a minute at it. Soon the fire was consuming water beyond the ability of the plant water system to supply at sufficient pressure. Portable pumps and pumper trucks were brought in to enhance and build up the pressure to get water to the necessary elevations.
The intense flames fed by the ruptured quench line contributed to the collapse of two of the derricks within the first 10 minutes of the fire.
"It wasn't instant," Craft said. "It was a gradual thing. The steel was bending and creaking. You could hear it but you couldn't see it because it was masked by the smoke. I'm just glad it didn't break free."
Had the derricks broken loose and fallen through the structure, it would have been extremely hazardous for the firefighters working at grade level. The debris would have fallen through piping charged with 20,000 to 30,000 parts per million of hydrogen sulfide.
"It wouldn't take much to tear out piping or drums and now you've got a massive H2S problem beside fighting the fire," Craft said.
After five hours, firefighters brought the blaze under control.
AFTER THE FIRE
Regulatory agencies monitoring the situation had one question they wanted answered immediately: Is there any radiation detected? To determine the level of liquid material inside the heavily insulated drums, radioactive sources are used. Nine radioactive capsules encased in lead and stainless steel are mounted on the walls of each drum. And although the radiation contained in each capsule is minimal, the regulators still wanted it accounted for.
"They wanted to know if any of these stainless steel containers had been breached," Craft said. "All I could say is 'I don't know.' We had to take our rescue squad, build boson's chair and slings to strap our hazmat people into and give them Geiger counters to check out these capsules."
Further complicating the issue, the radioactive sources were now regarded by officials as radioactive waste. Disposal would require permits and licenses from the Nuclear Regulatory Commission. The estimated cost of disposing of the 24 capsules used throughout the coker unit complex was estimated at $33 million alone.
"Our hazmat and rescue team confirmed that none of these instruments had been breached," Craft said. "We cut them lose from the drums, brought them down and secured them. Fortunately, the maker of these instruments came to our aid. They were licensed to get rid of radioactive waste."
It had been decided in pre-planning done years earlier that the worst fire possible in this coker complex would be in the common systems shared by the units. In fact, that was what had happened. The units on either side of the fire were down for more than three weeks. The unit where the fire occurred was down for more than a year due to necessary reconstruction.
It was time to come up with some solutions that would prevent a disaster of this magnitude striking the newly rebuilt unit.
The decision was made to augment the refinery's fire water system by giving the rebuilt unit its own 2,500 gallon per minute high pressure discharge pump. This pump would provide water to three separate sprinkler systems -- one at ground level on the pump side of the structure, one in the pipe run connecting the unit to neighboring units and one protecting the cutting deck.
Rather than tax the refinery's existing fire water system, this new pump pulls it supply from a nearby river. Furthermore, the pump, which is tested once a month, can be used to support the already existing fire water system with fires in other parts of the refinery.
At ground level the pump supplies the sprinkler systems with .50 gallons per square foot. On the cutting deck, the sprinkler system requires .30 gallons per square foot.
"It takes 2,500 gallons per minute minimum with the discharge at 200 psi to get adequate volume to that system," Craft said.
The sprinkler system can be operated by remote control from the refinery's computer control center or by an emergency switch at every egress level on the unit. It takes only seconds to have full flow from 56 sprinkler heads at the cutting deck level.
"If there is a fire that starts at the 151 foot level and those sprays are cut on, it gives people time to get off the structure," Craft said. "We sprinkled the stairwell so that if people can get down that route it would provide protection all the way down."
With that much water raining down, the sprinkler system would minimize the fire loading throughout the structure. Beyond bringing adequate water to bear, the system can also support the use of AFFF foam.
"The system has fire department manifolds," Craft said. "We can bring in those 2,000- and 3,000-gallon-per-minute foam pumpers and convert the water system to a foam system. If water acts to aid egress, foam is acting as an extinguishing measure."
Two years ago the system was tested in earnest when a fire broke out on the redesigned unit.
"They cut the sprinkler system on and everybody got off safely with no injuries," Craft said. "The responders hooked up the truck to convert water to foam and put out the fire. The unit was up and running in two days."
At the switching deck level with its extensive piping, something more than a sprinkler system was needed. Two 750-gallon-a-minute oscillating monitors were installed, one at each end of the 60-foot-wide, 200-foot-long deck. Once activated, the monitors make a 90-degree sweep at a 75-degree raised angle every 15 seconds.
"This is what saved the day in the more recent fire," Craft said. "The fire was on this deck."
The company has since installed similar sprinkler systems and monitors on the neighboring process units in the coker complex.
Egress for workers was another important issue addressed in rebuilding the unit. The goal is to make getting off the unit in an emergency as easy as possible.
"There was an elevator which obviously you wouldn't use due to the chance of being trapped," Craft said. "On one side of the unit you have a stairwell. But then, on the other side of the unit, you have a hand-over-hand caged ladder. That is not necessarily ease of egress when you have to climb down 150 feet to reach safety. People are obviously going to take the most convenient, quickest way and that is down a stairwell."
However, stairwells are typically placed only on one side of the unit. A worker caught on the opposite side would have to cross through a congested maze of piping and equipment to reach the stairs. Ideally, sprinkled stairwells should be available on each side of a coker unit, Craft said.
Another solution employed in rebuilding the damaged coker was adding two gantries at the cutting deck level linking the unit to its neighbors. Workers can now egress directly off the unit rather than work their way to grade level first.
"By nature this structure is very tall, very complex and congested with equipment," Craft said. "It is also a high occupancy unit with people working on it 24 hours a day. You need to look at and address ease of egress from a very high elevation under the most critical of circumstances."
These improvements may not eliminate coker unit emergencies but could substantially reduce the severity of these emergencies. However, coker operations will always be haunted by the same three factors that plague any industrial endeavor: human error, mechanical failure and acts of God.
"These fire protection measures provide some element of protection, but there is only so much we can do," Craft said.
The Importance of Metallurgy
Coker units process a crude oil residuum known in the trade as "bottom of the barrel." The residuum used in cokers today is closer to the bottom than ever before, fire protection consultant Jerry Craft said.
"I guess the biggest difference today from the early days of delayed coking in the 1960s and 1970s is the composition of the crude stock," Craft said. "Now there is more sulfur and metal content. There are things in the crude oil now that are detrimental -- either corrosive or erosive -- to piping systems and everything else."
Premium crude oil contains light materials that are relatively easy to distill to produce gasoline, kerosine and other products. However, modern refinery practices are capable of converting more of the heavier materials into feedstocks for making gasoline. To process the bottoms that are left requires an increase in the severity of the high temperature thermal cracking required for delayed coking.
"Metallurgy has to be upgraded in order to compensate for the bottoms being processed today," Craft said. "Standard carbon steel is not an acceptable metal for surfaces where you are feeding hot, heavy products that are heated at a very high temperature. The severity factor gets to be so critical that you've got a high potential for corrosion acceleration."
Every coker operation should have a positive material identification (PMI) program in place to test that the correct steel is being used in every step of the process, Craft said.