We frequently hear disconnects about what is expected from fire protection systems. It is important to understand what complying with a standard, such as a National Fire Protection Association (NFPA) or insurance company standard is actually “buying you” in terms of protection.
We started with sprinklers in manufacturing operations (please see Part 1) and continued with sprinklers in warehouse operations (please see Part 2). Part 3 addressed passive fire protection such as tank spacing, diking (bunding) and fire walls. This final article (Part 4) will address other water-based systems such as water spray, water mist, foam, and foam water.
Water Spray vs Water Mist
These terms are sometimes used interchangeably, but they are totally different types of systems that meet different fire protection goals. NFPA 15 covers water spray and NFPA 750 covers water mist.
Most readers of this article will see more water spray than water mist. Water spray is most commonly used for cooling to prevent damage from an external fire. Common uses are large transformers, pressurized and non-pressurized storage vessels, and chemical processing units. They are also used for conveyors and less commonly to protect an exposed wall. Different published standards will require different water application rates (in gpm/ft² or l/min/m²) This is based on assumptions about the allowable heat flux to the vessel. Sometimes they may be used for vapor dilution.
Water spray systems require high flows relative to the hazard they protect. Drainage is frequently needed to drain the cooling water away to prevent flammable liquids from floating on top and spreading throughout the area. The drainage requires as much design and maintenance as the system itself does.
Water spray systems might be set up as automatic, manual, or semi-fixed. Automatic spray systems are activated by a variety of detection methods, including gas detection. Manual systems are fully piped but rely on an operator opening a valve, frequently remotely from a control room. Semi-fixed systems have the piping in place but must be supplied by a fire department pumper. Semi-fixed systems are economical, but they must be supplied by a pumper quickly to be effective. Delayed alarms, or time lost looking for the connection can render them ineffective.
A drawback of water spray systems is that the piping can be destroyed in an explosion. In the case of transformers, the traditional bird cage piping is being replaced with goal post piping. Bird cage piping looks like a bird cage of small pipe all around the transformer with several small nozzles spraying water. Goal post piping looks like a football goal post with a few large pipes and high flow nozzles (perhaps 500 gpm (1900 l/min)). Sometimes these large nozzles discharge an additive such as an emulsifier. The emulsifier not only cools and protects the transformer; it also works to extinguish the fire. This can change the time scale of the operation from hours to minutes.
Water spray systems are intended to protect exposed property. In the case of a transformer, the transformer of origin is expected to be destroyed but the spray will protect the adjacent transformers and surrounding infrastructure.
In the photo, the bird cage piping protects the bottom of this sphere. Although it cannot be seen in the photo, a large pipe supplies a large distribution nozzle at the top which discharges water that flows down and protects the top half. This system is activated by flame detectors.
Water mist systems are not used for the same hazards as water spray systems. In industrial settings, they are primarily used in enclosed spaces. They could be thought of as an alternative to gaseous systems in these spaces. They discharge a very fine mist. There are several mechanisms of extinguishment such as steam generation, flame cooling, radiation shielding and all of these can work in combination.
Water mist systems are highly engineered and are not generally supported by the fire department, meaning that they typically do not have fire department connections. They frequently depend on steam generation in an enclosure such as a gas turbine house or engine room. Before entering, the fire department should be prepared to encounter steam and be able to extinguish any residual fire.
Unlike a gas system, they work better on larger fires, as might be present in an engine room, rather than on small fires, such as a computer server in a computer room. This is because steam generation is a major mechanism of extinguishment and larger fires will convert more of the mist to steam.
Foam vs Foam-Water System
Like water spray and water mist systems, the terms foam and foam-water systems are often used interchangeably but they are not. Both discharge a foam solution (foam and water mix) for a specified period. The difference is that once the foam runs out, a foam system, such as a foam chamber on a tank, provides no value with water alone and is probably detrimental. When the foam runs out in a foam-water system, the water discharge (from sprinklers or water spray nozzles) is still expected to have some value in terms of structure protection and cooling. They also have different standards, NFPA 11 for foam systems and NFPA 16 for foam-water systems.
Just this week there was a Facebook post on the need for a fire department connection on a “foam” system. The post was not clear as to whether this was a foam or foam-water system. With a foam system, an FDC would only be of value if foam-water solution could be pumped to it; as is the case with industrial foam pumpers. The only mention of an FDC in NFPA 11 is that if there is one, it must be shown on technical drawings.
Most municipal pumpers lack the capability to supply a foam system. For technical reasons, foam eductors are not suitable for this purpose. Around the pump proportioners will only work if the required flow is known. Semi-fixed foam systems, common at tank farms, are designed with the intent that an industrial foam pumper will supply the system through an FDC at a specific flow and pressure. The same is true of attempting to supply foam solution to a water only sprinkler or water spray system.
With a foam-water system, supplying water only would not alter the intent of the design. NFPA 16 does not require an FDC, but if one is installed, it must be connected on the supply side of the foam proportioner. The water pressure must be within the designed pressure range; however, a sign should be placed at the FDC with the maximum allowable inlet pressure. An industrial foam pumper has the capability to supply a foam solution through the FDC. This is especially valuable after the foam concentrate supply of the fixed system runs out. Before that; however, an industrial pumper’s foam solution could interfere with the foam that is already being supplied by the fixed system. This situation requires a case-by-case evaluation before the fire.
The most complicated arrangement I have seen combining the above systems was at a large chemical plant. One process structure had semi-fixed foam-water sprinklers at the top level and the need for manual foam concentrate injection at the intermediate levels. There was one fire department connection for foam-water solution and another for foam concentrate injection. The on-site foam pumper was capable of this; however, it was a complicated setup and one that could easily be forgotten in the middle of an incident. It would have been much better to design this as an automatic foam-water system for all levels. This situation does however, illustrate the continued need for active and recurring pre-planning.
Editor's note: An exception is low pressure water mist systems used as a substitute for sprinklers in museums and similar occupancies. These are not typical industrial occupancies. Cruise ships also have water mist systems and would have a way for the fire department to connect to them when the ship is in port.
Contact the author at [email protected] John Frank is the senior vice president of the AXA XL Risk Consulting's Loss Prevention Center of Excellence, where he is involved in loss prevention research and loss prevention training.