Effect of storage height, flue space, and aisle width

How does the fire severity and corresponding fire protection requirements vary with storage height? Data obtained by You (1989) for rack storage of the ‘standard plastic commodity’

(described in Section 5.4) indicate that the heat release rate early in the fire is directly proportional to the number of storage tiers. This data is shown here in Figure 5.6 and can be represented by the third power curve fit

Qcon= 0.0448N(t − t0)³ t− t0<26 s, 1 < N < 6 [5.3.1]

where Qcon is the convective heat release rate in kW at time t after ignition, t0is incubation time between ignition and self-sustained burning, and N is the number of tiers of storage.

Equation [5.3.1] is only valid early in the fire when t− t0 is less than about 26 seconds.

Convective heat release rates beyond this time increase less rapidly. Measurements of mass burning rates obtained by Kung et al. (1984) and shown in Figures 5.7 and 5.8 indicate that the mass burning rates for both the standard Class II commodity and the standard plastic commodity (the commodities are defined in Section 5.4) are directly proportional to the number of rack storage tiers for t− t0less than or equal to at least 180 sec in the case of the Class II commodity

1000

Convective heat release rate per tier (kW)

40 50 60

Figure 5.6. Heat release rate data for 2, 3, 4, 5-tier storage of Group A plastic commodity._2002 Factory Mutual Insurance Company, with permission

1.0

100 110 120 130 140 150 160 170 180

Burning rate per tier (KG/S) m/N⋅

Test 2 (t₀ = 0 sec)

Figure 5.7. Mass burning rates for 2-, 3-, and 4-tier storage of Class II commodity._2002 Factory Mutual Insurance Company, with permission

and 150 sec in the case of the plastic commodity. Thus it is tempting, albeit still premature, to generalize the conclusion that the mass burning rate and possibly the convective heat release rate is proportional to the storage array height for many commodities. (The convective heat release rate can be obtained by multiplying the burning rate by the effective convective heat for a specific commodity; values of effective convective heat for various commodities are discussed in Section 5.4.)

Mass burning rates for wood pallet piles are also proportional to pile height according to data obtained by Krasner and reviewed by Babrauskas (1995). The peak heat release rate for a fully involved stack of pallets 1.22× 1.22 m (4 × 4 ft) was represented by the correlation

Qmax= 1450(1 + 2.14hs)(1− 0.027Mw) [5.3.2]

where Qmax is the theoretical peak heat release rate in kW based on mass burning rate and a heat of combustion of 12 kJ/g, h_s is the height of pallet stack (m), h_s>0.5 m, and M_w is the pallet moisture content (wt %).

Babrauskas and others (for example, data in Table A.4) suggest that equation [5.3.2] can be generalized to other pallet sizes by normalizing on the basis of pallet floor area. The time required to reach the peak heat release rate given by equation [5.3.2] can be estimated from the character-istic 1 MW growth times listed for pallet stacks in Table 4.2. These charactercharacter-istic growth times

1.0

100 110 120 130 140 150

Burning rate per tier (KG/S) m/N⋅

Test 10 (t₀= 0 sec)

Figure 5.8. Mass burning rates for 2-, 3-, and 4-tier storage of plastic commodity._2002 Factory Mutual Insurance Company, with permission

generally decrease with stack height, implying that a given heat release rate is reached sooner in tall stacks than in short stacks.

In the case of a single wall, as represented for example by solid pile or palletized storage with minimal flue spaces burning on the aisle face, the peak heat release rate per unit wall width seems to vary as the wall height to the 1.25 power according to the data listed in Table A.3.

Thus, peak heat release for storage stacks seem to vary as stack height to a power between 1.0 and 1.25 depending on the stack configuration and flue space.

A pair of comparison tests reported by Dean (1980) demonstrates how the placement of an additional tier of storage on the top of a four-tier high rack storage array can overcome the design basis sprinkler protection. The baseline test with four-tier high storage of Class II Metal Lined Double Tri-wall Cartons opened 31 ceiling sprinklers and had a maximum ceiling steel temperature of 521^◦C (970^◦F). When an extra tier of storage was inserted on the top tier of the rack, 36 sprinklers opened and the maximum steel temperature increased to 655^◦C (1211^◦F).

The data are summarized in Table 5.5.

The amount of sprinkler water needed to suppress rack storage fires of different stack height increases with stack height in a manner investigated by Lee (1984). Lee measured the Required Delivered Density (RDD), i.e. the water spray flow rate per unit area at the top of the storage array; for the standard plastic commodity stacked three, four, and five tiers high. The RDD for

Table 5.5. Effect of an extra tier and of mixed commodity storage on sprinkler protection effectiveness for rack storage (data from Dean, 1980)

Test 69R2 Test C1 Test F1 Test F2

Commodity Class II Class II II+ Plastic II+ Plastic

Storage height 19 ft 23 ft 19 ft 19 ft

First sprinkler open (min:sec) 2:56 2:43 2:25 2:40

Total sprinklers open 31 36 51 88

Total water flow rate (gpm) 895 1085 1515 2700

Maximum Ceiling Steel Temp (C) 521 655 894 712

Notes: Ceiling Height was 30 ft and Sprinkler Discharge Density was 0.30 gpm/ft²from 286^◦F ¹₂-in orifice heads in all tests.

Plastic commodity was placed in the top tier for Test F1 and the bottom tier for Test F2

Table 5.6. RDD varies with stock height

Number of tiers Stack height (ft) RDD (gpm/ft²) Redevelopment fraction

3 14.5 0.30 1/6

4 19.5 0.40 1/3

5 24.5 0.50 2/4

each stack height was determined by applying water when the fire in a two pallet wide by two pallet deep by (3,4, or 5) tier high array reached a predetermined convective heat release rate in the range 350 – 1900 kW (331 – 1800 Btu/sec). The RDD corresponding to the boundary between fire suppression and fire redevelopment was independent of water application heat release rate in this range and varied with stack height, as shown in Table 5.6.

Thus, the RDD is roughly, but not exactly, proportional to stack height. The nominal RDD values specified for suppression are greater than those listed above by 0.05 gpm/ft²(2 L/min-m²) and 0.10 gpm/ft²(4 L/min-m²) for five tier high because the fire redeveloped in a certain fraction of the tests with the listed RDD, as indicated by the last column above.

Insurance loss data of the type described in Section 5.1 generally reflect the increased chal-lenge of higher storage heights. For example, the average loss for 5.5 m (18 ft) high rack storage is about $1.7 million, which is about three times the value for 3.05 m (10 ft) high rack storage. However, the data is too sparse to develop a quantitative correlation with stor-age height.

The Swedish National Testing and Research Institute (SP) has conducted tests to determine the effect of rack storage flue space on the rate of fire development and the maximum heat release rate. As the longitudinal and transverse flue spaces get larger, there is less re-radiation from carton to carton, and the convective heat transfer to adjacent cartons decrease. However, when the flue space becomes too small, air access to the burning surfaces becomes limited. Data reported by Ingason (2001) for the Standard Class II commodity tested at four different flue spaces in the range 7.5 – 30 cm, show that the optimum spacing from the standpoint of rapid fire growth rate, i.e.

time for the free burn heat release rate to reach 2 MW; is 15 cm (6 in). The tests with a 30 cm flue space required two minutes longer to reach 2 MW; however, the maximum heat release reached with the 30 cm flue space (greater than 10 MW) was higher than the maximum heat release rates with the narrower flue spaces. Furthermore, the time for the fire to grow from 2 MW to 7 MW was shorter with the 30 cm flue than with the narrower flues. Thus, the worst-case flue space depends on the time period and range of heat releases of interest. In most applications, the range of free burn heat release rates is limited to less than 2 MW because the ceiling sprinklers should actuate before that value is reached.

Aisle widths are important from the viewpoint of fire spread to adjacent storage rows. Since flame spread occurs via radiant heating of the exposed surface of the target row, the effect of aisle width show depend on how the radiant view factor decreases with distance from the flame to the exposed target. Sample calculations have been conducted to explore this effect using the methodology described in Section 5.7. In the case of 6.1 m (20 ft) high rack storage of the prototypical Group A plastic commodity, the calculated time to jump increased from 81 seconds to 94 seconds when the aisle width was increased from 1.83 m to 3.66 m (6 ft to 12 ft). These calculated times do not account for the effects of any sprinkler discharge, which is the primary defense against aisle jump. Warehouse sprinkler protection guidelines discussed in Section 5.6 give credit for larger aisle widths.

One other advantage of wide aisles is the improved access for locating and manually responding to a warehouse storage fire at an early stage of development. Similarly, fire cleanup operations can be conducted more expeditiously with the improved access.