IN THE REAL WORLD, oil-processing facilities are constructed in a variety of configurations, ranging from narrow and long to wide and tall, and the petrochemical hazards they present are rarely the size of those used in testing agencies’ test facilities. Consequently, it’s often necessary to think outside the box when designing a fire protection system for the petrochemical- and hydrocarbon-processing industries. Such was the case with the ConocoPhillips Alaska Alpine project. From start to finish, the project went beyond the normal caterpillar-to-butterfly transition; it more closely resembled the metamorphosis of a butterfly into a Boeing 747.
by Larry Owen, CFPS
When Arco Alaska, now called ConocoPhillips Alaska, Inc., of Anchorage considered fire protection options for its Alpine project, one of the factors was the life expectancy of the extinguishing agent. At that time, most of ConocoPhillips’ oil-processing facilities were protected by Halon 1301, and the company wasn’t convinced that the gaseous replacement agents then available would provide long-term fire protection and environmental solutions.
To survey the options available, ConocoPhillips engaged Dooley Tackaberry (DT), a fire protection company, of Deer Park, Texas. After numerous meetings and cost estimates, ConocoPhillips decided that water mist was the best long-term, environmentally safe solution, so DT set to work designing a water-mist system for the project, using hardware manufactured by Marioff, a company based in Finland.
Sometimes, the Hazards Grow
In Alaska’s Arctic environment, ConocoPhillips installs most of its oil-processing equipment in heated buildings or modules, and the initial layout of the Alpine site included modules small enough to comply with water-mist size limitations established by testing in Europe. As the design progressed, however, the modules grew until conventionally designed watermist systems became inadequate.
The only water-mist solution suitable for the Alpine project thus evolved into a performance-based design using total-flooding and local application coverage.
For years, carbon dioxide fire protection systems have been designed using an “either/or” approach. A combination approach is rarely used because there are established design methods for either total-flooding systems, such as those used to protect small spaces like enclosed turbines, or local-application systems for larger, unenclosed hazards, such as newspaper press lines. Unlike carbon dioxide systems, a combination approach to water-mist systems designed for hazards that don’t fit the “approved” template is a possible solution. As the Alpine project grew, DT decided to use a belt-and-suspenders approach, resulting in both total-flooding and local-application water-mist coverage.
Because this would be a performance based fire protection system, the authority having jurisdiction—the Alaska State Fire Marshal—was consulted and approved the basic design approach as long as the design was based on known test criteria. ConocoPhillips also approved the design approach.
All spaces at the facility would be protected by a total-flooding system, and each piece of process equipment that handles a hydrocarbon liquid in a space whose roof was more than 16 feet (5 meters) high would be protected by local-application nozzles. The total-flooding nozzles would cool the protected space, and the local-application nozzles would extinguish the fire.
On the surface, this combined approach might not seem like a big challenge until one explores the application rates and nozzle pressure requirements. One of the project objectives was to use as few different components as possible. To meet this objective, the same nozzles were used for both total flooding and local application, although the desired pressures and flow rates of each application are quite different. Total-flooding nozzles require a minimum pressure of 30 bar, or approximately 435 psi, while local-application nozzles require a minimum pressure of 70 bar, or approximately 1,015 psi. The total flooding nozzle flows approximately 2.75 gallons per minute (gpm) [10 liters per minute (lpm)], while the local-application nozzles flow between 4 and 5 gpm (15 and 19 lpm). How could a single system meet these different flow and pressure requirements?
The Alpine site’s potable water comes from a lake and is prepared for use by an on-site treatment facility. The water is also filtered twice and subjected to UV-water treatment to eliminate growths. It’s then stored in two elevated tanks with a combined capacity of approximately 55,000 gallons (208,194liters), from which the water-mist system’s central pumping system provides it to the suppression systems in 17 buildings. The system can protect the largest defined hazard for one hour.
In the Arctic, it’s a good design philosophy to circulate the fire protection water supply throughout the facility and monitor its flow. If flow isn’t present, there may be an ice plug or a break in the piping. Because the circulating water needn’t be at the water-mist discharge pressure, low pressure centrifugal pumps can be used to keep the water moving.
The Alpine project used a primary and secondary centrifugal pump, each flowing 200 gpm (757 lpm) at 50 psi, to move the water through the loop. The circulation-loop piping, which is heat-traced and insulated whenever it leaves a heated building, is made of stainless steel, as is all the piping in the water-mist system.
ConocoPhillips wanted to keep down project costs by using a single circulating loop, rather than individual high- and low-pressure loops, even though the pressure requirements of the local-application and total-flooding nozzles dictate different pump discharge pressures. To accommodate this imbalance, the designers used a single circulating loop, one side of which normally carries circulating water out into the plant and the other side of which returns it to the storage tanks.
During a water-mist system discharge, the circulating pumps shut down and are isolated from the pipe network by a series of high-pressure ball and check valves. The circulating pumps’ discharge line then becomes the discharge line for the low-pressure water-mist pumps, while the return line becomes the discharge line for the high-pressure pumps. Nine motors drive the high-pressure pumps, and 12 drive the low-pressure pumps. Two of the pump sets are considered standby pumps. At the most remote point of the circulating loop, two check valves prevent the high-pressure water from entering the low-pressure discharge line. This whole process takes place in a matter of seconds.
Each pump has an unloader valve that is used to regulate its pressure. As the pressure requirements of the system are satisfied, the pumps with the lower unloader valve settings unload, and the water from the unloaded pumps goes into a recycle line, which returns the water to the storage tanks.
Compared to a typical centrifugal fire pump system, this unloader valve arrangement and the positive displacement pumps required a new way of thinking. Defined quantities of water are available at the pressures dictated by the unloader valve settings, and the typical centrifugal pump curve is replaced by a stair-step curve.
At the time the Alpine project was designed, Marioff had tested its total flooding system for spaces up to 16 feet (5 meters) high and provided spacing criteria and flow requirements for both the total-flooding and local application nozzles. DT adapted these test criteria to the Alpine hazards. Any time a roof exceeds 16 feet (5 meters), the local-application nozzles activate to complement the roof nozzles.
One feature of the Marioff system is high pressure. At 435 psi (30 bar), even its low-pressure discharge is much higher than most water-mist system discharge pressures. This high pressure results in both entrainment of the water particles in the fire plume and the penetration of the plume by the water particles. Expecting a nozzle more than 16 feet (5 meters) above a hazard and discharging at 30 bar to provide particles that penetrate a fire plume is probably fantasy. However, nozzles less than 16 feet (5 meters) high do an adequate job of delivering mist that can both penetrate the fire plume and become entrained in the plume.
Another challenge was the absence of software that used the Darcy-Weisbach formulas for hydraulic calculations, rather than the traditional Hazen-Williams formulas. Hydraulic calculations using Darcy-Weisbach formulas are necessary when designing water-mist systems because their flows exceed 25 feet per second (7.6 meters per second). After searching unsuccessfully for such a software package, DT adapted a nonsprinkler hydraulic program with Darcy-Weisbach formulas to calculate the Alpine systems.
Full-scale discharge tests proved the calculations accurate, even though, as custom calculations, they weren’t in a standard NFPA 13, Installation of Sprinkler Systems, format so reading them took a bit longer than reading a conventional NFPA 13 printout.
All the electrical components used on the Alpine project were listed or approved by a nationally recognized testing laboratory. For example, the fire alarm control panels carry Underwriters Laboratories’ UL 508 labels for process control panels and UL 864 labels, listed as Dooley Tackaberry Fire Alarm Signaling Control Panels Suitable for Releasing Service. ConocoPhillips wanted this double listing to eliminate any future problems related to the use of unapproved equipment. The watermist pumps, which are driven by UL-listed fire pump motors, were certified by DNV for their performance, and the solenoid-operated control valves, pressure switches, level switches, and flow switches all carry testing lab labels or approvals. The control system looks for a “fail-tostart” signal from the pump motor controllers. If the control system receives one, a standby motor and pump set is activated. If the control system receives a second fail-to-start signal, a second standby motor and pump set is activated.
High-pressure, electrically operated ball valves direct water from the standby pumps to the appropriate piping, and the site-critical control system records the water-mist system pressures on a realtime basis. Solenoid-operated control valves, located in each protected space, are opened with a signal from the fire alarm control panel.
Each valve is monitored for position, and control valves are monitored for leaks.
Once the water-mist system was installed and all the piping flushed and hydrotested, full-scale water-flow tests were conducted in all protected areas in the presence of the owner, the fire protection consultant, and representatives of DT.
The performance of both totalflooding and local-application nozzles was robust, and the resulting mist in the buildings was spectacular. Everyone involved was impressed with the system’s performance and the combined total-flooding and local-application approach. The positive report by the owner’s fire protection consultant successfully concluded the four-year project.
But is it really the end of the journey? Hardly!
In 2003, the Standards Council issued the new edition of NFPA 750, Water Mist Fire Protection Systems, which provides guidance for designing, installing, and maintaining water-mist systems. As new petrochemical industry projects requiring water-mist systems are built, fire protection engineers and contractors will find NFPA 750 helpful in applying water-mist hardware. And testing currently being performed around the world will contribute a wealth of information on water-mist systems to support their increased use.
On a smaller scale, the many lessons learned on the Alpine project about using both total-flooding and localapplication nozzles will serve all the parties involved for years to come. The job provided the project team with a working, full-scale, real-world laboratory and prepared team members for many future water-mist system challenges.
While water-mist systems don’t fit every project, they do provide an excellent tool for the fire protection professional serving the petrochemical market.