IGNITION SOURCE ISOLATION

Guidance and priorities for isolating ignition sources can be found in industrial fire loss statistics.

One excellent source is the NFPA review of large loss fires (property damage of at least $500,000) in the period 1975 through 1984 (Redding and O’Brien, 1985). Table 2.5 summarizes the reported distribution of ignition sources responsible for large loss fires in mercantile, manufacturing, and storage properties.

The first two ignition source categories in Table 2.5, electrical origin and incendiary/suspicious ignitions, account for over half the large loss industrial fires with known ignition sources (34%

have unknown ignition sources). Thus high priorities in ignition source isolation should be (1) remote locations of open fired equipment, (2) maintenance of tight security to prevent access of arsonists, and (3) elimination of ignition-prone electrical equipment from areas with high concentrations of combustible and easily ignitable materials.

Open fired equipment includes furnaces, heaters, dryers, and flares. It would be prudent in most cases to locate this equipment far from sources of flammable liquids and vapors. In the case of the aerosol manufacturing plant, cans are passed through dryers upon emerging from water baths to check for can leaks. There is a need to provide adequate separation between the water bath (source of flammable vapor) and the dryer (potential ignition source even if it isn’t open fired equipment). In many plants, heaters are often used to shrink wrap the finished product. These are frequent ignition sources if not adequately isolated and maintained.

Security precautions to prevent entry of potential arsonists include supervision of exits and entrances. However, exit location supervision should not interfere with emergency evacuation and rescue capacity as described in Section 2.2.7. This may entail the use of alarmed exits and entrances.

The National Electrical Code (NFPA 70, 1989) is the most prominent and generally applicable Standard concerning ignition-prone electrical equipment. One particular aspect of the National Electrical Code that is especially relevant to ignition source isolation is the section on electrical equipment in hazardous locations (Articles 500 through 517). The intent is to eliminate electrical

Table 2.5. Large loss industrial fire ignition sources (based on data reported by Redding and O’Brien, 1985)^a

Cutting or Welding Torch – 8 6

Fuel-fired Heating or Other Equipment

9 10 7

Spontaneous Ignition – 6 5

Cigarette 4 3 4

aBased on 1227 fires with known ignition sources

ignition sources from areas possibly containing flammable gas-air mixtures, or combustible dust or fiber suspensions or accumulations. Classifications for flammable gas, combustible dust, and combustible fiber hazardous locations are designated Class I, II, and III locations, respectively.

An important and relevant subclassification for NEC hazardous locations pertains to the like-lihood of flammable gas or dust concentrations being present under normal and accidental conditions. The subclassification scheme is as follows:

• Division 1 locations are those in which flammable concentrations exist continuously, intermit-tently, or periodically under normal operating or maintenance conditions; or where equipment failure may simultaneously produce such concentrations and electrical ignition sources.

• Division 2 locations are those containing normally confined flammable gases, vapors, or dusts;

or those locations immediately adjacent to Division 1 locations.

Electrical equipment are rated for specific Class/Division classifications. In Class I Division 1 locations, electrical equipment are required to be either ‘explosion proof’, ‘intrinsically safe,’ or

‘purged.’ An explosion proof rating implies the equipment will contain an internal explosion (of a specific vapor-air mixture) and will have a surface temperature below the ignition temperature of the mixture. An intrinsically safe rating implies the equipment will not release sufficient electrical or thermal energy under normal conditions and under electrical faults during abnormal conditions to ignite the most readily ignitable concentration of vapor for that classification. In Class I Division 2 locations, equipment are required to be ‘nonincendive,’ which means it will not be an ignition source during normal operation (less restrictive than the ‘intrinsically safe’ rating).

Tests and listings of electrical equipment rated for use in hazardous locations are provided by Underwriters Laboratories and Factory Mutual Research Corporation in the US and by their counter parts, such as UL-Canada and T.U.V., in other countries.

The British counterpart to the NFPA/NEC hazard classifications is British Standard 5345:

Part 1, which defines the following three categories of hazardous areas:

• Zone 0 in which a flammable gas-air mixture is continuously present, or present for long periods.

• Zone 1 in which a flammable gas-air mixture is likely to occur sometime during normal operation.

• Zone 2 in which a flammable gas-air mixture is not likely to occur during normal operation.

Thus, the BS Zone 0 and Zone 1 correspond to two subcategories of the NFPA/NEC Division 1 category, while the Zone 2 category is loosely equivalent to the NFPA/NEC Division 2 category.

Since the presence and extent of the NFPA and BS hazardous location areas depend on the potential formation of flammable vapor-air mixtures, it is important to consider both the size and likelihood of a flammable liquid/vapor release, and the effectiveness of ventilation in rapidly diluting the released vapor. It is sometimes necessary to make site-specific measurements with flammable vapor detectors or dust concentration probes. In most cases, generic estimates of these hazardous areas can be found in the standards. Examples of NFPA/NEC Division 1 and Division 2 locations for flammable vapors and gases are given in NFPA 497A (1986) for representative process equipment and potential release sites. A few example diagrams are reproduced here in Figures 2.8 and 2.9. The example in Figure 2.8 is for a ground level leakage source of flammable liquid in a building. If the building is adequately ventilated, it is unlikely that flammable mixtures will form anywhere except in a below grade sump or trench in which the leak will be contained.

Therefore, the sump/trench is classified as Division 1, while a 0.91 m (3 ft) high area surrounding the leakage sight and a semicircular area of 1.5 m (5 ft) radius is classified as Division 2. The bulk of the building is considered to be a nonhazardous nonclassified region. On the other hand,

Grade

Below grade location such as a sump either in division 1 or division 2 portion of building

Figure 2.8. Division 1 and Division 2 areas in flammable liquid buildings. Reprinted with permission from NFPA 497_, Classification of Flammable Liquids. Copyright_1997 National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject which is represented only by the standard in its entirety

z Spill

Figure 2.9. Air velocities and vapor concentrations in a well ventilated enclosure with a combustible liquid spill

if the building is not adequately ventilated, the lower 0.91 m is considered to be Division 1 and the bulk of the building is deemed to be Division 2.

How much ventilation is adequate to prevent flammable vapor-air mixture formation throughout a large section of the building? According to NFPA 497A, the mechanical ventilation should be equivalent to natural ventilation in an open area enclosed with at most one wall and a roof.

Since this is too ambiguous to quantify, the approach suggested here is to evaluate the ventilation required to maintain the volume-average concentration below one fourth of the vapor lower flammable limit. This in turn requires an estimate of the vapor generation rate, which could either be obtained empirically or via the following theoretical analysis.

Consider a spill or leak of flammable liquid onto the floor such that the area of the spill is A.

The actual value of A could be estimated from either the size of the sump/trench or, if there is no nearby sump, from the volume spilled and the equilibrium spill layer thickness as described in Chapter 7. The mass generation rate of vapor, Ev, is given by

Ev= MvkAPsat/(RTL) [2.2.1]

where Ev is the vaporization rate (kg/min), Mv is the vapor molecular weight (kg/kg-mole), k is the mass transfer coefficient (m/min), Psat is the liquid saturation vapor pressure (Pa) at TL, R is the ideal gas constant (8314 J/(kmol-^◦K), and T_L is the liquid temperature (^◦K).

Several different empirical correlations for the mass transfer coefficient are available. The simplest correlation, which is intended for outdoor releases, is (EPA-OSWER-88-001, p. G-3)

k= 0.25u^0.78

where u is the wind speed (m/s), which is usually measured at an elevation of 10 m.

A more comprehensive correlation, which is based on laminar flow over a flat plate representing the spill surface (AIChE CCPS, 1996), is

k= 0.664

Dv

Lsp



Sc^1/3Re^1/2 [2.2.3]

where Dvis the vapor molecular diffusivity (m²/min), Lspis the liquid pool dimension in direction of u (m), Sc is the vapor Schmidt number (kinematic viscosity/vapor diffusivity), and Re is the Reynolds number (velocity× Lsp/kinematic viscosity).

The vapor diffusivity is usually evaluated based on the diffusivity of water vapor, D_w, and the molecular weights of water and the vapor in question, as

Dv= Dw

Combining equations [2.2.3] and [2.2.4] with the definitions of Schmidt and Reynolds number (and approximating the vapor kinematic viscosity with that of air, υa) yields

k= 0.664

The EPA Guidelines for Risk Management recommend that the spill area A in equation [2.2.1]

be taken as the area corresponding to the full liquid volume with a depth of 1 cm, unless this calculated area is larger than the room floor area. They further recommend for their submittals that the air velocity u be input as about 0.1 m/s for a room/building ventilation rate of 0.5 volume changes per hour and typical sized ventilation fans. Of course, the actual effective value of u

depends on the local air flow velocity over the spill which depends on ventilation duct location as well as ventilation rate. EPA does not offer guidance on how the value of u should be adjusted for different ventilation rates and duct locations, so the options are to conduct in-situ measurements or flow field calculations. In the context of hazardous location determination ventilation rates for flammable liquid spills, we can consider u to be the average velocity between floor level and the 3 ft elevation level.

Once Evis determined from equations [2.2.1] and [2.2.2] or [2.2.5], the volumetric ventilation rate needed to dilute the vapors to one-fourth of the LFL can be calculated as

V = 4Ev/(χlflρv) [2.2.6]

where V is the required ventilation rate (m³/min), χ_lfl is the lower flammable limit volume fraction, and ρ_v is the vapor mass density (kg/m³).

Using the ideal gas law for ρv and equation [2.2.1], we obtain V ≥ 4kAPsatTa

χlflPaTL

[2.2.7]

where Pa and Ta denote ambient pressure and temperature, respectively.

If the ventilation inflow is below elevation, h= 3 ft, sweeps across the floor and exits at the opposite wall as sketched in Figure 2.9, the ventilation rate may be written as V = uhw, where w is the room width. In this case, u appears on both sides of equation [2.2.7], so that an explicit solu-tion can be obtained for the minimum ventilasolu-tion velocity to achieve the desired vapor dilusolu-tion.

Similar approximations can be used to estimate vapor or gas leakage rates and required venti-lation rates in other scenarios such as from vented storage tanks or leaky pipe fittings, or more catastrophic failures. Even when adequate ventilation is provided, there will inevitably be local regions of flammable vapor concentrations in the immediate vicinity of the release site. As an example, Figure 2.10 shows the Division 1 and Division 2 boundaries given in NFPA 497A for a well ventilated building containing a flammable liquid tank or vessel with a vent line emerging on the roof and an emergency dump tank adjacent to the building. In view of the multiple leakage

3′ radius from vent

Figure 2.10. Division 1 and Division 2 classified areas in an adequately ventilated building containing a flammable liquid vessel or tank. Reprinted with permission from NFPA 497A_, Classification of Flammable Liquids. Copyright_1992 National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject which is represented only by the standard in its entirety

sites and Division 2 areas in the situation shown in Figure 2.10, it sometimes becomes more cost effective to classify the entire building as Division 2 rather than restrict unrated electrical equipment to the unclassified regions of the building.

In the case of combustible dusts processed in mixers, grinders, extruders, etc., guidance on classified Class II (dust) areas is offered in NFPA 497B (1991). Electrical equipment in Class II, Division 1 areas is required to be free of ignition sources, whereas equipment in Division 2 areas need only be dust tight. The relevant classification considerations are: (1) the potential for a large dust cloud; (2) accumulation of deep (>3 mm (1/8 in)) layers dust that are not readily discernible on equipment, floors, etc.; (3) the type of dust cloud or layer produced upon failure of dust collection equipment; and (4) the composition of the dust. In the case of unenclosed or only partially enclosed dust processing equipment producing a dust cloud, the Division 1 boundary either to a radius of 6.1 m (20 ft) or to the edge of the visible cloud. This often leads to an entire room being designated as Division 1, and a Division 2 area extending 3 m (10 ft) beyond it through a frequently opened door (NFPA 497B, 1991).

One other consideration in isolating ignition sources from combustible dusts and vapors is the potential for electrostatic discharge. This concern often leads to strict requirements for grounding equipment and for avoiding charge generations on operating personnel. In the case of at least one aerosol manufacturing facility, this entailed building the gas rooms with floors containing stainless steel grids extending up through the surface of the concrete.