Drainage/Spill Control Systems
Large quantities of water may be used to fight fires in facilities handling chemicals. Since most flammable materials float on water, there is the potential for fire protection water to spread the fire. In addition, many chemicals have the potential to contaminate the soil and groundwater. Water used for fire fighting can disperse these chemicals spreading the pollution. As a result of all these factors, there is a need for drainage systems to control water runoff. An equally important point to make is that drainage systems can control flammable liquid spills.
Drainage facilities should be designed to simultaneously carry flammable liquids and fire protection water away from buildings, structures, storage tanks, pipe racks, and process equipment. Drainage systems should not expose adjacent plant facilities to burning or toxic materials during an incident. This may require diversionary curbs, trenches, collection sumps, skimmers, and holding ponds or basins.
In many cases, the water and chemicals collected during an incident will need to be “preheated” prior to disposal in a waste water treatment facility, or the rate that these materials are introduced to the treatment process controlled. Small holding ponds for specific process areas should be sized to hold 30-60 minutes of discharge as a minimum. Where there is the potential for fires of long duration, such as in petrochemical or oil refining facilities, special precautions will be necessary. It maybe possible to separate organics from fire water prior to disposal, thus reducing the size of the required holding area. As noted in the introduction to this section, the potential for soil and water contamination should not be used as a reason to avoid providing fire protection or drainage systems.
Drainage of liquids may not be desirable in some cases, since this may actually result in more serious concerns (such as large vapor clouds or contamination of drinking water), and in other cases, drainage and remote containment may not be practical due to space limitations or other reasons. In such cases, additional automatic fixed fire protection measures such as foam or waterspray may be necessary to ensure adequate protection is afforded.
Drainage/spill control systems are typically comprised of a combination of features to achieve the necessary spill control depending on the particular situation. These features include the following:
• Sloped surfaces (typically minimum 1%)
• Flumes or sluiceways
• Drains (with traps to prevent flashback)
• Curbs or dikes
• Sumps or pits (to contain small spills)
• Separators or skimmers (to separate contaminants from water)
• Remote containment basins or tanks
• Reactivity of chemicals with water or other chemicals in the drainage system
The design of drainage/spill control systems can be complex. Space does not permit a complete discussion of this subject, but some of the factors which should be considered in the design of drainage/spill control systems include the following:
• Volume of liquid which could be released as well as the rate and mode
• Properties of the liquid which could be released, including extinguishability, viscosity, water solubility, specific gravity, and volatility.
• Expected flow from water based fire extinguishing systems, such as sprinklers, foam systems, hoses, and monitor nozzles
• Surface type (earth, gravel, concrete, etc.)
• Spacing and location of facilities
• Rainfall (containment facilities should normally contemplate some rainfall in capacity design)
• Expected duration of fire (or time to implement contingency plans)
• Risk of environmental contamination (proximity to water supplies, geology, etc.)
• Local codes and regulations
• Need to separate organics from water to prevent drainage to rivers
NFPA 30 and the appendix of NFPA 15, “Water Spray Fixed Systems” should be consulted for details on the design of drainage/spill control systems. Additional guidance can be obtained from insurance company publications.
Drainage/spill control systems should be inspected on a regular basis to ensure they are in good condition. In particular, drains and trenches should be examined to ensure they do not contain any blockages. Rainwater should be drained or pumped out of containment facilities following each rainfall.