Wider is better for fog cones used to control hydrofluoric acid. So is an automated system. Neither development happened by chance. Individuals committed to the safety of others made all the difference.
LIQUID HYDROGEN FLUORIDE is utilized as a catalytic agent in many modern alkylation units that are needed in the process of refining gasoline. Although it is a highly effective catalyst, hydrogen fluoride can also be highly corrosive and toxic. When exposed to water vapor present in the atmosphere, it forms a cloud of concentrated hydrofluoric acid (HF).
The most effective way to protect the surrounding area in case of an accidental release is to keep the vapor cloud from spreading and dilute the acid with large volumes of water. To do this, most refineries employ numerous sensors, cameras, and instruments to detect leaks, as well as a network of remote-control water monitors with fog nozzles to suppress and contain any vapor cloud that might form. The water monitors are positioned at critical locations and can form a protective water curtain around the entire unit.
Pre-Automation
James Roy Chidester was one individual who was instrumental in developing these automated systems. Chidester started his firefighting career in the Navy in 1967, where he worked on the flight deck of an aircraft carrier. After discharge from the service, he went to work for several chemical and refinery fire brigades and eventually became a fire protection instructor at Louisiana State University. In 1989, he left the university to take a job as fire chief for the Exxon refinery in Billings, Mont., where he helped develop one of the latest generations of automated HF mitigation systems.
Chief Chidester is retired now, but he remembers the difficulties associated with the older systems. “Before we installed the latest design in about 1999,” he recalls, “we would have had to send personnel out to operate each monitor manually. It was often a slow process because the personnel had to don special protective gear before they could get into position. The response took a lot of people and a lot of time, which could have allowed the vapor cloud to escape. It also potentially exposed personnel to severe weather and unnecessary hazards.”
To streamline the process, Chidester worked with various manufacturers to develop an automated system of monitors that could be operated from within an environmentally protected control room. Personnel in the control room could remotely shut down process valves to isolate a leak and then activate a system of high-volume water monitors to slow, dilute, and block the spread of any acid vapor that formed in the atmosphere. Ground drains within the affected area could be switched to divert the runoff water into holding dikes or containment vessels.
The Pattern Problem
“One of the problems we faced was getting the right stream shape and water droplet size from the nozzles on the containment monitors used around the perimeter,” says the veteran firefighter. “Some nozzles didn’t produce a wide enough fog cone with uniform coverage to allow interlocking patterns from adjacent nozzles. Without that, we knew we couldn’t effectively contain a vapor cloud. We contacted several nozzle manufacturers to help us with this problem and asked them for solutions.”
Akron Brass was one of the companies working on the problem, and Gene Dettra was an engineer assigned to the project team at Akron. “It was a challenging problem from many aspects,” he noted. “Getting a very wide fog stream was difficult. Nozzles that utilized spinning teeth produced good droplet size but had only a limited angle of coverage. In fact, most nozzles designed for firefighting were unsuitable because they produced such a narrow fog pattern.”
Eventually another engineer on the project team made some modifications to the internal geometry of the nozzle passage and added what is known as a Coanda ring, which draws the exiting water out in a very wide pattern. “It really produced an impressive fog bloom that covered a large diameter,” said Dettra.
Exercising Controls
The job of designing the monitor control system fell to Dettra, who also faced some interesting challenges. “First, we had to ensure the electrical controls could operate in a potentially explosive environment,” he said. “We used brushless servo motors with sealed housings to eliminate the possibility of an electrical arc. The servo motors also gave us excellent motion control, which was required to accurately position the streams.”
Akron made the controls programmable so that the monitors could automatically move to different positions based on shifting conditions. To ensure that the monitors could operate when exposed to salt air and other harsh conditions, the monitors, nozzles, and component housings were made of brass.
The end result was the Akron Brass Storm electric programmable monitor capable of flowing up to 2,000 gpm and producing fog patterns ranging from very narrow to very wide. The monitor was given an NEC Class 1, Division 2, Groups C&D rating. The programmable controls could adjust the monitor elevation, rotation, and stream pattern based on predetermined responses to different inputs.
To augment the containment monitors, Akron Brass also adapted the Storm monitors for two other applications in the HF mitigation system. The first was a group of “momentum breaker” monitors used to break up the vapor cloud and slow its movement with the wind. These monitors were usually mounted 6 to 8 feet (2 to 3 meters) above the ground and aimed slightly downward to deliver a more focused fog stream. They could be programmed to react automatically to the input of gas sensors or could be positioned by operators in the control room. The second was a series of “aim and shoot” monitors that could be remotely operated from within the control room to deliver a fog stream directly onto the area of the leak and wash away the liquid hydrogen fluoride before it could form a vapor cloud.
“We began development tests on the system in 1997 and continued through 1998,” remembers former Chief Chidester. “The wide fog angle produced by the Storm monitors on the perimeter provided an interlocking water curtain that could effectively contain a vapor cloud before it left the process area. The refinery management saw the value of the system and installed more pumps and water mains to support it. The installation was completed by 1999 and was one of the first fully automated HF mitigation systems in the United States.”