By the Book

If a facility—be it a plant, a laboratory or a farm—plans to store flammable and combustible liquids, there's a lot more involved than merely putting the volatile materials in an out-of-the-way place. Consideration must be given to many different aspects of fire-protection and life safety, as the inherent physical properties of flammable and combustible liquids—flash point, va...

08/01/2002


If a facility—be it a plant, a laboratory or a farm—plans to store flammable and combustible liquids, there's a lot more involved than merely putting the volatile materials in an out-of-the-way place. Consideration must be given to many different aspects of fire-protection and life safety, as the inherent physical properties of flammable and combustible liquids—flash point, vapor pressure and vapor density—can result in hazards to both building occupants and property.

Such stringent detail is necessary because of the relatively low flash point of flammable liquids, combined with their relatively high vapor pressure. Thus there is a natural concern about the presence of flammable vapors in concentrations within the flammable limits. Plus, the relatively high spread rate of flammable liquids can result in large spill areas and extension of the hazard well beyond the original storage area. Additionally, flammable vapors are typically heavier than air and settle to floor level, seeking out ignition sources (see Table 1, opposite page).

In preparing a protection plan, the first step involves identifying the type of storage room that must be protected.

Flammable liquid storage rooms can vary, from an open warehouse situation to rooms along exterior walls to internal areas with no exterior walls.

This is an important factor, as the major codes which govern such storage—the 2000 International Building Code (IBC), the 2000 International Fire Code (IFC) and NFPA 30, Flammable and Combustible Liquids Code (2000 Edition)—dictate different measures for different room types. Additionally, these codes often call out seven specific fire-protection criteria that must be taken into account when designing such rooms (see "Seven Code Keys," to the left).

A comprehensive fire-protection scheme is critical considering all these criteria. Before plunging ahead, wise designers will take the time to break down the codes as they apply to the aforementioned categories.

Sizing/locating storage rooms

As noted, there are limitations to the location of storage rooms and floor areas themselves. This is something designers must be keenly aware of, as the codes sometimes differ in how they address these criteria. For example, one important difference between the IBC and the Building Officials and Code Administrators' National Building Code—still applicable in many jurisdictions—is the permitted location of flammable storage rooms in a separated mixed-use building. Under BOCA, flammable storage rooms are typically classified as Use Group H-2, High Hazard.

But under the IBC, two use group classifications are used:

  • Use Group H-2 - Flammable Liquids in Open Systems or Closed Systems >15 psig

  • Use Group H-3 - Flammable Liquids in Closed Containers or Closed Systems &15 psig

This H-3 classification allows the location of storage rooms on higher floor locations.

On the other hand, IBC also requires that 25% of all flammable liquid storage rooms greater than 1,000 sq. ft. be located along an exterior wall. This allows for incorporation of deflagration venting—if required—and provides access for firefighting operations.

In contrast, NFPA 30 limits the floor area of flammable liquid storage rooms based on the type of room and its fire separation from other building areas. An "inside" room—defined as a room totally enclosed within a building, with no exterior walls—is limited to a maximum of 500 sq. ft. with two-hour fire separation. Size limitation for "cut-off" rooms—structures made up of at least one exterior wall—is based on the area limits for Use Group H-3 (separated mixed-use) in the IBC. NFPA 30 does not limit the size of cut-off rooms.

Furthermore, the IBC requires that there be two exits from Use Group H-2 rooms greater than 300 sq. ft. Additionally, such-sized rooms must have a common path of travel greater than 25 ft.

Electrical classifications

The second major code regulation associated with flammable liquid storage rooms involves electrical classifications. In order to minimize the potential for ignition, all electrical equipment, including lighting and switches, should have the appropriate hazard rating. NFPA 30 and NFPA 70, National Electrical Code (1999 Edition), provide the requirements for electrical classification, based on flammable liquid type and the nature of the activities in the storage room.

NFPA 70 classifies all flammable liquid storage areas as Class I. The distinction between Division 1 and 2 is dictated by the storage vs. dispensing activities in the storage room. If containers are being filled from full drums, for example, electrical equipment within 3 ft. of the vent and fill openings in any direction should carry a Class-I, Division-1 rating. Electrical equipment in the remainder of the room should be rated for Class I, Division 2.

NFPA 70 further distinguishes between types of Class-I flammable vapors by classifying materials into groups. Three of the groups are listed according to their ignition properties, specifically the Maximum Experimental Safe Gap (MESG) and the Minimum Igniting Current Ratio (MIC) (see Table 2 above). Lower MESG and MIC values translate to a higher performance requirement for electrical equipment.

Suppression vs. control

The third step in meeting code involves fire protection design itself. Consideration must be given to whether the overall goal is fire suppression or control.

In many cases, simply controlling a fire is adequate; therefore, a conventional water-sprinkler system is appropriate. It's true that water will not suppress a flammable liquid fire, but it will cool adjacent containers, limiting a fire's spread. The IFC specifies that automatic sprinkler protection for flammable liquid storage rooms be designed for Ordinary Hazard Group 2 over a minimum design area of 3,000 sq. ft. Since most flammable liquid storage rooms are less than 1,500 sq. ft., this corresponds to a minimum design density of 0.20 gpm per sq. ft., as specified in NFPA 13, Standard for the Installation of Sprinkler Systems , 1999 Edition.

However, if suppression is the goal—at times required by insurance carriers—alternative agents, such as foam/water, should be considered.

Agent selection is particularly a challenge when storing liquids with flashpoints greater than 200°F. Since the temperature of a burning high flashpoint liquid is significantly higher than the boiling point of water, application of sprinkler water will cause spattering. In such cases, alternative fire-protection agents, such as aqueous film-forming foam/water or high expansion foam—should be used.

Ventilation guidelines

As intimated in IBC's room location requirements, ventilation is the fourth major area to address when designing around flammable liquid storage rooms. In general, the goal of ventilation for such storage areas is to maintain the concentration of flammable vapors in air at less than 25% of the Lower Flammable Limit. In order to accomplish this, NFPA 30 specifies a minimum exhaust rate of 1 cfm per sq. ft. of floor area—but not less than 150 cfm—for flammable liquid storage rooms.

Since flammable vapors are typically heavier than air, exhaust ducts should be located within 6 in. of the floor. In addition, an airflow switch is required to be interlocked with an audible alarm for notifying facility personnel in the event of an exhaust system failure. The IFC also requires a manual "break-glass" switch outside the storage room so that emergency responders can shut down the ventilation system, if desired.

An independent exhaust duct should serve each flammable liquid storage room, with no fire dampers. Exhaust ducts should be enclosed in rated shafts; the shafts should not contain any general ventilation ductwork. Where exhaust ducts penetrate rated fire separation assemblies, the horizontal runs should be enclosed in appropriately rated construction. Supply air ducts should be equipped with fire dampers wherever the ducts penetrate rated fire-separation assemblies.

Explosion venting

Along a similar vein, the fifth major code requirement deals with explosion venting. Explosion or deflagration venting is required for flammable liquid storage rooms in which Class-IA liquids are dispensed or stored in containers greater than a 1-gallon capacity.

Vents should be sized according to the requirements of NFPA 68, Guide for Deflagration Venting (1998 Edition) for low-strength enclosures—those able to withstand a maximum explosion pressure of 1.5 psi (see equation below).

A V = Explosion Vent Area = C*A s /(P red )1/2

In this case, A s = internal surface area; C = fuel characteristic construction = 0.17 psi1/2(for flammable liquid vapors); and P red = 1.5 psi—the maximum value that can be used for for this variable in a low strength enclosure.

The IFC contains specific requirements for clearances from deflagration vents, depending on the type of vent panel used. If releasing-style vent panels are used, a 50-ft. separation is required between the exterior wall of the building and the property line. If vent panels on tethers are used, a 10-ft. vertical separation is required between the discharge opening and any windows or exits; a 20-ft. horizontal separation is required from the discharge opening to exits, windows and exits in adjacent buildings and the property line.

In certain instances, explosion control is acceptable as an option to deflagration venting. Explosion control methods are provided in NFPA 69, Standard on Explosion Prevention Systems, 1997 Edition.

Spill control and secondary containment

The sixth element fire-protection designers need to be cognizant of, is spill control and secondary containment. The relatively high spread rate of combustible liquids in storage room of origin is the root of concern. If uncontained, burning liquids mixed with fire-protection water could flow out of the storage room and involve other combustible materials in the remainder of a building.

As a general guideline, for all flammable and combustible liquids—except Class IIIB liquids stored in containers greater than a 10-gallon capacity—a form of containment is required for the volume of the largest container, plus the expected volume of fire-protection water. In other words:

Volume of fire protection water = design density x design area or room area (smaller area) x 20 minutes.

For example, a 300-sq.-ft. liquid storage room with a sprinkler design density of 0.20 gpm per sq. ft. would require 1,200 gallons of containment, plus the volume of the largest stored container.

The final code element to be aware of deals with fire separation requirements . Note the requirements contained in the IBC and NFPA 30 for flammable liquid storage rooms are somewhat different. For example, the IBC requires a 2-hour fire separation assembly between Use Group H-3 rooms (flammable liquid storage above exempt amounts) and other areas of a building. NFPA 30 requires either a 1- or 2-hour separation, depending on room type—inside or cut-off—and size.

Seven rules to live by

Fire-protection engineers can save their clients and owners considerable construction headaches by keeping these seven major code factors at the forefront of a project's overall design. This, of course, means these items must be discussed and incorporated early on in the process. Considering the potential for disaster when working with flammable or combustible materials, it's a given that local building officials will certainly reference and call out these life-safety measures in reviewing any design. That being said, it's better they be a non-issue, than a construction issue.

Table 1 - Classification of Flammable and Combustible Liquids

Classification

Flash Point

Boiling Point

Examples

Source: NFPA 30: Flammable and Combustible Liquids Code 2000 Edition. 1.7.3.1 and 1.7.3.2

IA Flammable Diethyl Ether

&73°F

&100°F

Ethylamine

IB Flammable

&73°F

100°F

Acetone, Methanol

IC Flammable

73°F &100°F

Xylen, Turpentine

II - Combustible

100°F to &140°F

Diesel Fuel

IIIA - Combustible

140°F to &200°F

Phenol

IIIB - Combustible

200°F

Corn Oil


Table 2-Electrical Classifications for Flammable Liquid Storage

Class I Group

MESG (mm)

MIC

Examples

A

NA

NA

Acetylene (gas)

B

0.45

0.40

Hydrogen (gas), Propyl; Nitrate (liquid vapor)

C

>0.45 up to 0.75

>0.40 up to 0.80

Dimethylamine, Hydrazine, Tetrahydrofuran (liquid vapors)

D

>0.75

>0.80

Acetone



Seven Code Keys

Size and location

Electrical classification

Fire-protection design

Ventilation

Deflagration venting

Spill control and secondary containment

Fire separation

Which Code Applies to You?

The IBC and IFC are currently adopted in many states as the applicable building and fire codes, most notably in New York, North Carolina, Ohio and Michigan. The third code, NFPA 30, is a long-standing applicable code in most jurisdictions, and is usually adopted as part of most municipal fire codes.

The 2000 edition of NFPA 30 contains many clarifications and updates from the 1996 edition, which is still applicable in many locations.

In general, where the applicable building code and NFPA 30 speak to the same issue, the more restrictive requirement is enforced.



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