Fixed monitors, the second method of water application, permit the use of fixed hydrant mounted monitors or individual monitors connected to the fire main to apply water to the fire area. In this case, water application is accomplished by operators manually opening valves to allow the flow of water to the LPG sphere. This procedure exposes operators to high heat fluxes and places them dangerously close to vessels under fire conditions. It is important to carefully study the plant and vessel layout if this method is elected. Proper placement, location, and quantity of fixed monitors must be reviewed and field tested to ensure that proper application and even distribution of water to all parts of the vessel is accomplished. In some cases, remote activation and operation is suggested when proper spacing of monitors is not a possibility. Additionally, annual testing and preventative maintenance are necessary to ensure parameters have not changed and that coverage is still adequate.
The third method of application is the use of water spray systems. These are systems that are comprised of a piping network of spray nozzles that distribute water over the surface area of an LPG sphere. The spray nozzles are positioned to form a grid pattern which facilitates the complete coverage of the sphere's surface area. Larger orifices and piping should be considered to help reduce blockage due to scale and mussel build-up and other potential problems. It is also important to properly size the strainer to prevent blockage. Inspection of strainers should be part of the preventative maintenance program.
The last method available is the deployment of portable monitors and hoses. This method utilizes hand carried portable or trailer mounted monitors deployed by the fire department. Although not one of the three primary methods of water application, preparations and planning for this type of application should not be forgotten. Quantity of monitors, monitor flow calculations, and predetermined hose lays should be reviewed prior to an incident to ensure adequate capabilities are available. This method is considerably more dangerous that previously mentioned methods due to exposing personnel to the hazards and risks associated with LPG fire fighting.
When utilizing the four water application techniques discussed previously, a combination of techniques provides ample fire protection; such as the use of a deluge or water spray system and portable monitors. A combination of a water deluge/distributor with a fixed water spray system with portable monitor back up from the fire department provides excellent coverage.
A water application rate for these fixed fire protection systems depends on the type of fire situation. When a vessel is exposed to only radiant heat without direct flame contact, a density of 0.1 gpm per square foot of vessel surface area is the minimum. If direct flame contact, or impingement, occurs, a density larger than 0.1 gpm, up to 0.25 gpm per square foot of vessel surface area is the minimum.
When dealing with fixed or portable monitors, 250 to 500 gpm is the minimum flow that should be initially considered. However, field verification and flow testing is necessary to ensure adequate and proper coverage is provided. Placement of monitors must also be field verified against approved plans to ensure acceptable spacing and access.
Vapor, heat, or flame detectors mounted in the vicinity of a vessel can complete automatic activation of these systems. The use of vapor detection provides early detection and warning, but activation of water application systems must be confirmed through flame detection. Flame detection provides quick activation, but use caution when positioning these detectors to prevent false activation from sunlight. Consideration need also be given to the installation of UV/IR combination detectors to reduce the false indication rate. These devices require testing and preventative maintenance programs. An evaluation of the facility is necessary in order to determine the correct type and location of devices.
Even with the proper installation of fixed fire protection system, the importance of emergency response to LPG fires cannot be disregarded. LPG fires can escalate quickly, and a lack of manual activities by the fire department can lead to vessel failure. As part of this response, an up-to-date and complete emergency response plan is an integral part. The plan should include:
- Hydrant layouts
- Hose lays and lengths
- Multiple response approaches (wind dependant)
- Vessel inventories
- Fixed protection information
- Scenarios for both un-ignited and ignited leaks