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Fire Protection for Flammable and Combustible Liquids

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1. Introduction: The hazard of flammable and combustible liquids comes from the vapours evaporated from the liquid rather than from the liquid. When a flammable or combustible liquid is heated to a temperature above the flash point, vapours evaporated from the liquid can be ignited and burn. Since, by definition, most flammable liquids are stored and handled above their flash point temperature, vapours are continuously released by the liquid resulting in an ignitable mixture of vapours and air. Ignitable mixtures occur when concentrations of vapours in air are within a definite percentage range, commonly referred to as the flammable (explosive) range. The lower limit of the range is known as the lower flammable limit (LFL) or lower explosive limit (LEL). The upper limit of the range is known as the upper flammable limit (UFL) or upper explosive limit (UEL). Therefore, storing flammable and combustible liquids in properly closed containers and minimizing the exposure of the liquid to air while in use are fundamental safety measures.

Accumulations of flammable vapour–air mixtures can occur in a confined space, such as containers, tanks, rooms, or buildings. The violence of a flammable vapor explosion is dependent on the nature of the vapours, the enclosure containing the mixture, the quantity of the mixture, and the concentration of vapour in the mixture with air.

2. Classification of Flammable and Combustible liquids: For fire protection purposes, an arbitrary division between liquids and gases has been established. Based on the definition of a flammable liquid found in NFPA 30, Flammable and Combustible Liquids Code, the liquids are defined as those fluids with a vapour pressure not exceeding 2068 mm Hg (40 psia), which is approximately 25 psi gauge pressure at 37.8°C (100°F). The definition and classifications of flammable and combustible liquids are provided below.

Flammable Liquids:

  1. Flammable liquids have flashpoints below 37.8°C (100°F) and vapour pressures not exceeding 2,068 mm Hg at 37.8°C (40 psia at 100°F).
  2. Class I liquids include those with flashpoints below (37.8°C (100°F) and may be subdivided as follows:

(a) Class IA liquids include those with flashpoints below 22.8°C (73°F) and with boiling points below 37.8°C (100°F).

(b) Class IB liquids include those with flashpoints below 22.8°C (73°F) and with boiling points at or above 37.8°C (100°F).

(c) Class IC liquids include those with flashpoints at or above 22.8°C (73°F) and below 37.8°C 100°F). Combustible Liquids

Combustible Liquids:

Liquids with flashpoints at or above 37.8°C (100°F) are referred to as combustible liquids and may be subdivided as follows:

  1. Class II liquids have flashpoints at or above 37.8°C (100°F) and below 60°C (140°F).
  2. Class IIIA liquids have flashpoints at or above 60°C (140°F) and below 93°C (200°F).
  3. Class IIIB liquids have flashpoints at or above 93°C (200°F

3. General Requirements for Fire Protection of Flammable and Combustible Liquids: Flammable and combustible liquids fires can be controlled and extinguished by using agents like foam, dry chemical agents, carbon dioxide, water, while using Foam Extinguishing Agents and Systems, Dry Chemical Agents and Application Systems, Carbon Dioxide and Application Systems, and Water Spray Protection System or the appropriate extinguishers. Below are some of the commonly used National Fire Protection Association (NFPA) codes and standards used for fire protection requirements of flammable and combustible liquids.

NFPA 30, Flammable and Combustible Liquids Code

NFPA 30A, Code for Motor Fuel Dispensing Facilities and Repair Garages

NFPA 32, Dry cleaning Facilities

NFPA 33, Standard for Spray Application Using Flammable or Combustible Materials

NFPA 34, Standard for Dipping, Coating, and Printing Processes Using Flammable or Combustible Liquids

The fire protection requirements for facilities handling, processing, and storing Flammable and combustible liquids in buildings can be found in building codes like International Building Code, National Building Code of Canada, and Ontario Building Code, and fire codes like International Fire Code, National Fire Code of Canada, Ontario Fire Code, while for the facilities handling, processing, and storing flammable and combustible liquids in process industries, the fire protection requirements are given in respective industry standards.

4. Fire Protection for Flammable and Combustible Liquids within Buildings: There are numerous requirements about fire protection of flammable and combustible liquids within buildings. Only few are discussed below.

(a) As per NFPA 30, storage of flammable and combustible liquids in drums or other containers that do not exceed 450 L (119 gal) individual capacity, Portable tanks that do not exceed 2500 L (660 gal) individual capacity, and Intermediate bulk containers that do not exceed 3000 L (793 gal)  should have following fire protection requirements.

Portable fire extinguishers must be provided in accordance with NFPA 10 and NFPA 30. Portable fire extinguishers shall meet the following requirements:

(a) At least one portable fire extinguisher having a rating of not less than 40:B shall be located outside of, but not more than 3 m (10 ft) from, the door opening into a liquid storage area.

(b) At least one portable fire extinguisher having a rating of not less than 40:B shall be located within 9 m (30 ft) of any Class I or Class II liquids located outside of a liquid storage area.

(b) Automatic fire protection systems for inside storage of flammable and combustible liquids in containers, intermediate bulk containers, and table tanks are specified in NFPA 30. Included in NFPA 30 are requirements for automatic sprinkler systems or low-expansion foam-water sprinkler systems to protect storage of liquids.

5. Fire Protection Requirements for Flammable and Combustible Liquids in Industry: Some examples of fire protection requirements of flammable and combustible liquids within industry are given below.

(a) In the process industry handling flammable and combustible liquids the process areas are defined in the three risk categories below:

High-Risk Process Area: A “high-risk process area” is an area where equipment is processing any of the following substances at autoignition temperature: flammable liquid, combustible liquid, or combustible gas.  The high-risk process area is normally limited to 1860 m² (20,020 ft 2).

Medium-Risk Process Area: An area shall be designated a “medium-risk process area” when a hydrocarbon product is processed or handled above its flash point.

Low-Risk Process Area: An area in which combustible liquids are processed or handled at ambient temperatures is designated as a “low-risk process area”.

The details of Fire Water Demand for each risk area are given below.

For high-risk process areas: The fire water demand for a high-risk area is computed by either calculating flow rate using the water density [minimum 0.21 L/m²s (0.30 gpm/ft²)] for the entire designated area within the risk area boundaries or summing the design flow rates of each monitor and spray system, whichever is greater.

For medium-risk process areas, the demand requirement is calculated in the same manner to that of high-risk process areas. The flow rate provided to a medium-risk process area is at least 0.14 L/m²s (0.2 gpm/ft²).

For low-risk process areas: The minimum water application density for exposed surface area in low-risk process is at least 0.10 L/m²s (0.15 gpm/ft²).

(b) Fire Protection for Aboveground Storage Tanks:   A fire-extinguishing system in accordance with an applicable NFPA standard should be considered for vertical atmospheric fixed-roof storage tanks larger than 50,000 gal (190 m3) capacity, storing Class I liquids, if located in a congested area where there is an unusual exposure hazard to the tank from adjacent property, or to adjacent property from the tank.

Protection against fire or explosion required for large flammable liquid storage tanks should consider the use of a fixed, semi-fixed (see fig. 1), or portable protection system, designed in conformance with good engineering practice such as those described in NFPA 11, Standard for Low-, Medium-, and High Expansion Foam, or NFPA 15, Standard for Water Spray Fixed Systems.

Please contact us at PLC Fire Safety Solutions, if you have any questions or concerns regarding Fire Protection for Flammable and Combustible Liquids.

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