Steelmaker incorporates fire-suppression system for its intricate needs
U.S. steelmaker gets out of a pickle.
Recently, a steelmaker in the United States was in the process of building a new cold mill linked to a pickle line and a new exposed hot-dip galvanizing line. The company encountered various obstacles during the design, one specifically revolving around fire-suppression needs.
This new facility incorporates a 72-in., five-stand tandem mill that produces 2.1 million tons of steel per year. The overall facility measures about 1/2-mile in length. This facility required a fire-suppression system that could meet all of its specialized needs, such as:
- Capability to suppress a fire inside the pickle line exhaust duct;
- Effectiveness and reliability in corrosive environments;
- Using a minimal amount of water to protect the sensitive motors and controls;
- Effectiveness in areas such as electrical rooms without hindering the room’s integrity;
- Minimal downtime after a system recharge; and
- Design flexibility to allow for ease of maintenance and future facility expansion.
These requirements led to a fire-suppression solution that is the first installation of its kind in the world. A special hazards fire system provider, a major Victaulic Vortex integrator and the project manager chose to install the Victaulic Vortex fire-suppression system for the new expansion.
The Victaulic Vortex system features hybrid technology, combining two separate forms of fire suppression — water mist and inert gas — to extinguish fires by both lowering the oxygen level and absorbing heat.
How does it work?
The system discharges a mixture of nitrogen gas and water droplets at high velocity (40 mph), with low pressure (15-25 psi at the emitter) eliminating the drag effect that often limits the effectiveness of traditional water-mist systems. The water droplets are less than 10 microns in size and represent a much greater surface area and provide an increased rate of heat absorption and total fire extinguishment.
Use of such fine water droplets results in minimal water discharge in protected areas. This means there is no water damage to the equipment. Because the system only discharges nitrogen and water (both nontoxic agents), personnel remain safe, even during activation. System design provides that oxygen levels do not drop below a safe breathing tolerance. Unlike other systems that require evacuation delays before discharging potentially toxic agents, there is no delay in system activation once a sensor detects smoke, heat or flame.
Pickle line challenges
The pickle line contains four polypropylene pickle tanks and one rinse tank, with each pickle tank holding hydrochloric acid at approximately 185° F. Polypropylene is an inherently combustible material, increasing the chance of a major fire on the pickle line.
To protect the tanks, the project manager considered relying on traditional water deluge systems, similar to water cannons. However, because the tanks are covered and the ducts are enclosed, water from the deluge system could only reach the exterior of the tanks and fume exhaust duct.
The Vortex solves the problem the deluge system could not by providing total flooding coverage to the pickle line. The coverage comes from specialized and patented PVDF (polyvinylidene fluoride) emitters strategically placed inside the tanks and fume exhaust ducts. They provide an enhanced ability to extinguish the fire.
The pickle line presents both total flooding and local application scenarios. PVDF is a unique plastic material most often used in applications which require resistance to strong acid and heat. For applications such as the pickle line, Victaulic offers emitters constructed with this material. Since the PVDF emitters allow discharge of the Vortex system inside the tanks and the fume exhaust ducts, the fire never has a chance to get out of control.
Because pickle line fume exhaust systems typically are fiberglass-reinforced polyester, they are highly combustible and pose significant fire protection challenges.
As fumes are drawn from the pickle tanks into the interior fume exhaust system, the inside of the ductwork becomes an extremely corrosive environment. This environment of acid fumes from the line can play host to sparks generated outside the ducts. The PVDF emitters, coupled with plastic piping, resist this corrosive attack and allow for the Vortex system discharge, providing fire suppression inside the fume exhaust ducts.
Along the tanks are steering rolls, immersion rolls, scrubber rolls and squeegee rolls that help keep the steel moving through the pickle line. Each of these rolls has associated bearings. The bearings and rolls are continually turning, and the lubricated bearings become another potential ignition source.
The system deploys at the bearing areas to protect the exterior of the tanks. Local application of the system is provided by emitters that point directly at the bearings, which extinguish the fire at the origin site and prevent the fire from spreading.
The storage tanks hold the acid from the pickle tanks when it is necessary to drain. Pumps are incorporated in order to put the acid back into the pickle tanks. These storage tanks also are made of polypropylene, making them combustible and vulnerable to other sources of fire. Utilizing corrosion-resistant emitters and piping, the system installs around the acid storage tanks in order to extinguish fires on the exterior surface of the tanks. Utilizing less than 25 psi of nitrogen, the Vortex system allows the use of plastic pipe and fittings, providing direct savings in installation and material costs.
The tandem mill exhaust duct system represents a particularly challenging fire suppression scenario. In the event of a fire in the mill, fire is pulled into the constantly running exhaust system and spreads quickly. Because these fires are three-dimensional, suppression is needed on all inner surfaces of the ductwork.
A total flooding system such as the Vortex is needed. This facility considered a dry-pipe sprinkler system for this application. However, these systems are really two-dimensional agents protecting, for example, only the flat bottom surface of the ducts. They often respond too slowly, with sprinklers activating after the fire has spread beyond their coverage areas.
The Vortex is designed to three-dimensionally disperse nitrogen and small water particles into total flooding spaces, making it very effective in protecting the inside ductwork.
Total flooding applications also are present in the pulpits and the electrical control rooms. The electrical rooms control the entire operational processes of the plant and represent the largest volume of overage area in the facility for the Vortex system.
Since the system utilizes naturally buoyant nitrogen and small water particles, room integrity testing is less of a concern and the facility opted for the Vortex system.
Author bio: Bob Ballard, P.E., is a senior project engineer technical leader with Victaulic and has 20 years of research-and-development experience in fire protection. Ballard is a technical committee member for both the NFPA 75 Standard for the Fire Protection of Information Technology Equipment and NFPA 318 standard for the protection of semiconductor fabrication facilities.