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Thermal radiation hazards and separation distances for industrial cellulose nitrate
Thermal radiation hazards and separation distances for industrial cellulose nitrate R. Merrifield and T. A. Roberts* Health and Safety Executive, Technology, Health Sciences Division, Bootle, Merseyside, L203QZ, UK *Health and Safety Executive, Research and Laboratory Services Division, Explosion and Flame Laboratory, Buxton, Derbyshire, SKI79JN, UK
Many forms of industrial cellulose nitrate burn fiercely with the emission of large quantities of thermal radiation. Models are considered for predicting the variation with distance of the irradiance from burning cellulose nitrate. These models are evaluated using data from HSE and German trials. An approach to setting quantity safety distances for cellulose nitrate is given. (Keywords:
cellulose nitrate, thermal radiation hazard, modelling)
The Health and Safety Executive (HSE) Explosion and Flame Laboratory (EFL) has performed several series of trials’-” on industrial cellulose nitrate in order to determine the critical parameters for assessing thermal radiation hazards. Other countries such as Germany’ have set safety distances based on their own cellulose nitrate burning trials. This paper considers methods of predicting the irradiation from burning cellulose nitrate and suggests an approach to setting quantity safety distances.
Thermal
radiation
models
Two basic approaches6 are generally used to predict the variation of irradiance with distance. These are the point source and solid flame models.
Point source model In this model, the energy radiated from a flame is specified as a fraction of the heat of combustion and the irradiance is assumed to vary inversely with the square of the distance from the source. Hence the irradiance is given by: Z = F - M’ - H/(4aY2)
(1)
The main problems with this model are estimating the mass burning rate at a particular time and estimation of the fraction of heat of combustion radiated. Whilst the average burning rate can be easily measured, it is very difficult to estimate the burning rate at a specific time for industrial cellulose nitrate in steel drums where there may be burning material within the drums, burning columns of material ejected into the air and burning material scattered on the
Received 30 July 1992 0950-4230/92/05031149 @ 1992 Butterworth-Heinemann
ground. In addition, the point source model overestimates the radiation received by a target close (c.a. less than five flame diameters) to the fire.
BAM modified point source model In the Bundesanstalt fiir Materialforschung und -prl.ifung (BAM) version7 of this model, mass burning rates for the packaged material are estimated using single stack package trials. A measure of how steadily the packaged cellulose nitrate bums is given by the ratio of the maximum irradiance to the mean irradiance. Using the BAM model, the maximum irradiance is given by: I,
= S. I, = S - F - H exp (0.6667 In [M/M,] + In [M6]}/(497Y*)
(2)
The extrapolated mass burning rate for 10 000 kg is normalized to give a corrected mass burning rate as: ML = M:( H/335OO)(F/O.25)(S/2.78)
(3)
where the standard heat of combustion is taken as 33 500 kJ kg-l, the standard fraction of the heat radiated as 0.25, and the standard ratio of maximum irradiance to mean irradiance as 2.78. This corrected mass burning rate criterion is then used to allocate the different types of cellulose nitrate, and other vigorously burning materials, to storage groups as shown in
Table I. The German quantity safety distances’ for occupied buildings/plant and other stores containing p ‘ otentially explosive’ materials are summarized in Table 2. There are reductions in the safety distances depending on the construction of the store, the type of protection available and whether there are quantity limits set for the store. Tncreases in distance are required if the mass per unit store area exceeds that used in the test trials.
Ltd
J. Loss Prev. Process Ind., 1992, Vol5, No 5
311
Industrial cellulose nitrate-radiation Table1 criteria
German corrected
mess
burning
hazards and separation
rate storage
group
Corrected mass burning rate
Storage group
25 kgsm’ < 5 and z 2.333 kg s-l c 2.333 and * 1 kg s-’ < 1 kg s-’
IA
I6
distances: R. Merrifieid
and T. A. Roberts
targets close to the fire and it can take effects such as blooming and obscuration into account. In the EFL trials, the flames were approximately cylindrical in shape. The horizontal and vertical geometric view factors for a vertical cylinder are given9 by: 7iVH = Ib - (l/s)1
II III
x {tan-’
[(b + 1) * (s - l)/(b - 1)
x (s + 1)}]0-5}l[bZ Modified BAM model Germany uses the BAM approach for industrial cellulose nitrate and other energetic substances such as organic peroxides and sponge blowing agents. For packaged organic peroxides, the authors have proposedS a modified form of the German point source model which utilizes the mass burning rate per unit area: Z = S . F - M”. A . H/(4nY2)
- [a - (I/s)1 X {tan-l
Cylindrical solid flame model In a simplified version of this model, a flame is assumed to radiate uniformly over its entire surface (more sophisticated models with a segmented approach are available) and the irradiance is given by: Z= E0V.T
(5)
To apply this model, measurements of the surface emissive power and flame dimensions are required. The advantage of this model is that it can be used for
[(a + 1) . (s - l)/{(a
- 1)
x (s + 1)}]0.5}/[a2 - 1]0.5
+ {[h/s] * {[a/(a’
- 1)“.5]
X tan-’ [(a + 1) * (s - l)/{(a - 1) * (s + 1)}]“.5 - tan-’ [(s - l)/(s + l)]“.‘}
v = (V-C + V&)O.S
(8)
where a = (h2 + s2 + 1)/(2s) and b = (1 + s2)/(2s)
Test data EFL has performed burning trialsla2 on unconfined and packaged (steel and fibreboard drums) isopropanol (IPA)-damp and dibutyl phthalate(DBP)-plasticized industrial cellulose nitrate. BAM have also performedlo trials on pigmented cellulose nitrate. The EFL data is in a form which can be used for both point source and solid flame models. EFL data for modified point source models The data required for the modified point source models are the heat of combustion, mass burning rate, shape factor and the fraction of the heat of combustion radiated. The results from trials’ where 40 mm deep layers of IPA-damp cellulose nitrate and DBP-plasticized cellulose nitrate (3.29 by 3.33 m area) were ignited are summarized in Table 3.
and unoccupied
stores
Quantity (kg)
Minimum
IA IA
Occupied Unoccupied
>lOO >I00
25 IO
0.092M”fl 0.115Mfi’~.
IB IB IB
Occupied Occupied Unoccupied
>200 s 10000 >10000 >200
25 10
5.5 M’h l&M@ 1.6 M@
II II
Occupied Unoccupied
>200 >200
25 10
1 1 API3 1:l M’p
III Ill
Occupied Unoccupied
,200 >200
10 10
_
312
mass burning rates are in kg min-’
J. Loss Prev. Process Ind., 1992, Vol5, No 5
distance(m)
Distance (m)*
Building type
*Corrected
(7)
and the maximum view factor is given by
Table 2 German quantity safety distances for occupied industrial buildings/plant Storage group
(6)
ZTV” = {[1,/s] *tan-’ [h/(s* - 1)“.5]}
(4)
The authors adopted a mass burning rate per unit area approach as most organic peroxides are liquids or would be stored with liquids and if such a store were bunded then the burning area would be fairly well defined. However, industrial cellulose nitrate is normally commercially available in solid form and would not be held in a bunded area. As indicated earlier, it would be very difficult to estimate the mass burning rate corresponding to the maximum flame size for cellulose nitrate in steel drums although reasonable results could be obtained for alcohol-damp cellulose nitrate in fibreboard drums where no violent behaviour is observed.
- 1]“.5
AN AM
Industrial cellulose nitrate - radiation hazards and separation
Table3 Unconfined cellulose nitrate
trials
on
DBP-plasticized
Diluent concentration (%) Nitrogen content (%) Heat of combustion (kJ kg-‘) Mass (kg) Burning area (mZ) Mass burning rate per unit area (kg m-* s-l) Total burning time(s) Total dosage (kJ m-2) Mean irradiance (kW m-*1 Distance dosage measured (m) Period of fastest burning (s) Maximum average irradiance (kW m-2) Distance irradiance measured (rn) Shape factor Fraction of heat radiated
and
IPA-damp
DBP/NC
IPA/NC
19.7 11.8 14350 300 10.96 0.274 100 157.5 1.58 22.7 54 2.51 22.7 1.59 0.24
22.0 12.2 14910 286 10.96 0.065 400 275.8 0.69 22.7 150 1.21 22.7 1.75 0.42
Two types of four drum trials were performed in the first test series’ which give an indication of the maximum and minimum burning rates. One type involved removing the lid from the upwind drum and igniting the cellulose nitrate in this drum and allowing the flames to spread to the other three drums with their lids on. This gave an estimate of the minimum overall burning rate. The other type of trial involved heating all four drums, with their lids on, by a wooden crib fire. As all drums were engulfed by fire at the same time, Table4
Summary
of EFL package trial data for modified
Trial description
Linear configuration 4 x 143 kg steel drumsb with (N2 11.8%) 4 x 125 kg steel drumsb with (N, 11.5%) 4 x 63 kg fibreboard drumsc (N, 11.8%) 3 x 72 kg fibreboard drumsc
26.3% IPA/‘73.7% 20.2% DBP/79.8%
NC NC
with 26.3% IPA/73.7% with 20.9% DBP/79.1%
(N, 11.3%) and 1 x 72 kg with 20.2% DBP/79.8% Pallers adjacent 8 x 143 kg steel drums with 31.5% IPA/68.5% (N2 10.8%) 8 x 100 kg steel drums with 17.9-22.8% DBP/82.1%-77.2% NC (N2 11.5%-12.2%) Pallets stacked 8 x 143 ka steel drumsd with 31.5% IPA/68.5% (N,
NC
NC
NC NC
R. Merrifield
and T. A. Roberts
this gave an indication of the maximum overall burning rate. These tests were performed on two types of cellulose nitrate in steel and fibreboard drums. Only steel drums were used in the second series’ of trials as these are the drums used by a major United Kingdom cellulose nitrate manufacturer. These consisted of side-by-side and stacked pallet load trials using eight drums and with external heating by loose burning cellulose nitrate. The nitrogen content of the IPAdamp cellulose nitrate used in this second series of trials was lower than in the first series and the quantity of plasticized cellulose nitrate per drum was lower (100 kg instead of 120 kg) and hence the results, in particular from the IPA-damp cellulose nitrate, may not be fully representative of the industrial situation. The results are summarized in Table 4. Heats of combustion were measured using a bomb calorimeter. The mass burning rate was derived from the quantity of material present and the duration of burning. Shape factors were estimated as the ratio of the maximum average dosage to the overall dosage. A rough estimate of the fraction of the heat of combustion radiated was obtained from the observed irradiance at a specific distance using the unmodified point source model. BAM
data
The test results available3 and the storage categories assigned are summarized in Table 5.
point source model H (MJ kg-‘)
Square configuration 4 x 143 kg steel drumsb with 26.3% lPA/73.7% NC (NZ 11.8%) 4 x 125 kg steel drum+‘with 20.2% DBPp9.8% NC (N, 11.5%) 4 x 63 kg fibreboard drums” with 26.3% IPAj73.7% NC (N, 11.8%) 4 x 72 kg fibreboard drums’with 20.2% DBP/79.8% NC (N, 11.5%)
distances:
M (kg)
Aa (mZ)
M’ (kg s-’ )
S
F
16.34
572
1.03
2.5
3.08
0.19
14.35
500
1.03
7.0
1.71
0.29
16.34
252
0.68
0.5
1.10
0.19
14.35
288
0.68
2.1
3.57
0.29
16.34
572
1.03
7.0
3.6
0.12
14.35
500
1.03
11.9
3.1
0.15
16.34
252
0.68
1.1
1.3
0.14
13.71
288
0.68
2.0
1.5
0.19
16.42
1144
2.30
~2.3
1.48
-Co.11
13.86
800
2.30
1.67
0.24
16.42
1144
1.15
13.86
800
1.15
NC
3.6
Cl.3
1.23
<0.06
iO.i%)
8 x 100 kg steel drums’with DBP/82.1%-77.2% NC(N*
17.9-22.8% 11.5%-12.2%)
‘Number of unobstructed drums b0.937 m high, 0.572 m diameter “0.668 m high, 0.485 m diameter base held on by steel band 40.865 m high, 0.605 m diameter
5.0
3.33
0.25
x cross-sectional area with internal volume 0.241 m3 with lid designed to vent at 296 kPa with internal volume 0.113 m3 with 0.7 mm thick steel lid fastened with steel clamp and fibreboard with internal volume 0.249 m3 with lid designed to vent at 296 kPa
J. Loss Prev. Process Ind., 1992, Vol5, No 5
313
industrial
Table5
cellulose
nitrate
- radiation
BAM trials on packaged cellulose
hazards
and separation
distances:
R. Merrifield
and
T. A. Roberts
nitrate
Trial description (;J
ML (kg s-’
kg-l)
S
F
W (knmin-l)
Storage group
I
4 metal drums with venting device and plastic liners with 18% dioctyl phthalate/82% NC (N, 11.8-12.3%)
14.714
200
5.05
1.55
0.35
1416
IA
4fibreboard drums and plastic liners with 35% ethanol/65% NC(NZ 11.8-12.3%)
17.004
200
0.68
4.17
0.11
185
IB
dfibreboard cases each with 3 x IO kg paper bags with 16% plasticizer/40% carbon black/44% NC (N, 10.4-12.3%)
23.058
240
1.84
2.59
0.09
319
IA
4 metal drums with venting device and plastic liners with 35% IPA/65% NC (N, 11.8-12.3%)
18.357
246
0.95
3.70
0.16
306
IA
EFL data for solid flame models Under contract from the HSE Research and Laboratory Services Division @LSD), British Gas made accurate measurements of the surface emissive power and flame dimensions during both the EFL unconfined burning trials3 and the steel drum trials4. In the former, the surface emissive power was measured using an infra-red spectrometer and was also calculated from the radiometer measurements, flame dimensions and geometric view factor. The results are summarized in Table 6. In the two pallet load steel drum trials, British Gas
Table6 trials
British
Gas
measurements
from
Spectrometer (surface emissive power(kWm-*I) Radiometer (surface emissive power (kWm-*)I Distance(m) lrradiance (kW m-2) Transmittance View factor Flame width (ml Flame height(m)
Table 7 British Gas measurements
unconfined
burning
IPA/NC
DBP/NC
171
230
179
244
19.9 1.23 0.878 0.00783 4.0 5.0
22.7 2.56 0.862 0.01217 5.0 7.5
Process
Packaging Cellulose nitrate is transported in a variety of packagings. Most UK produced material is transported in 143 kg steel drums fitted with a lid designed to relieve at a pressure of 296 kPa. Imported cellulose nitrate tends to be transported in 50 kg fibreboard drums. Amendent 25 of the International Maritime Dangerous Goods Code allows the types of packaging shown in Table 9. In a fire, steel drums can vent their contents violently giving short duration fireball effects. These are in addition to the flames from material burning within the drum and on the ground. If the drum vents when on its side, then the body of the drum may rocket away. The violence of ejection will depend on the ullage space within the drum (cf. trials with 100 kg and 120 kg of DBP/NC in steel drums). Burning material
Adjacent DBP/NC
Stacked DBP/NC
Adjacent IPA/NC
Stacked IPA/NC
270 46.0 3.61 0.778 18.3 1.88 13.8 9.4 4.6 42 99
362 46.0 2.90 0.779 25.5 1.21 5.1 5.4 4.3 31 54
146 46.0 0.17 0.776 4.2 2.26 4.2 2.2 2.7 46 49
147 46.0 0.65 0.779 8.1 1.21 7.7 6.0 4.4 12 -1
“At half flame height bAt half drum height CFrom vertical dRelative to direction of measurement
J. Loss Prev.
Factors affecting the thermal radiation hazards
from steel drum trials
Surface emissive power (kW m-2j Distance (m) lrradiance (kW m-*1 Transmittance Flame height(m) Apparent stack width (m) Flame width= (m) Flame widthb (m) Wind speed (ms-‘j Tilt angle” (“) Wind directiona (“)
314
did not use a spectrometer and surface emissive power was estimated from the irradiance, flame dimensions and geometric view factor and an estimate of the transmittance. The results are summarized in Table 7. The flame dimensions from the first series of trials’ are given in Table 8.
Ind.,
7992,
Vol5,
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Industrial
cellulose
Table 8 Flame dimensions
nitrate
- radiation
hazards
and separation
distances:
R. Merrifield
and
T. A. Roberts
(m) from four drum trials
Trial
Stack width
Flame width
Flame height
Square configuration Steel drums (IPA/NC) Steel drums (DBP/NC) Fibreboard drums (IPA/NC) Fibreboard drums (DBP/NC)
1.15 1.15 0.93 0.93
8 10 10 8
10 15 5 15
Linear configuration Steel drums (IPA/NC) Steel drums (DBP/NC) Fibreboard drums (IPA/NC) Fibreboard drums (DCHP/DBP/NC)
3.20 3.20 2.76 2.76
10 15
5 10 8 10
may be stacked two high. If such a stack were involved in a fire it is expected that:
Table 9 Packagings for industrial cellulose nitrate Type of packaging*
Gross mass (kg)
Cans (20 litre) packed in a wooden box Plastic bottles or bags in a wooden box Plastic bottles or bags in a fibreboard box Lined wooden box Plastic bag in fibreboard drum Water-resistancefibreboard drum Metal drum
125 125 40 125 225 225 225
*Packaging possible.
should
be so constructed
that
explosion
::
is not
only be in the air for approximately two seconds but will continue burning on the ground for a much longer period adding to the effective width of the flames. Fibreboard drums tend to burn relatively slowly with both the flame dimensions and duration of burning depending on the quantity and stack configuration. However, DBP-plasticized material, which can deflagrate rapidly, may be violently ejected if a deflagration is initiated whilst the drum still provides a degree of confinement, e.g. if heat is transmitted through a metal lid or base. Again the main effect of violent ejection will be to increase the effective flame width. The BAM shape factor compares the average maximum irradiance (over at least a IS second period) with the mean irradiance over the main burning period and does not directly indicate the irradiance during violent ejection although the shape factors given in Tables 4 and 5 do take this into account to some extent. After ejection, material is left burning on the ground which increases the effective flame width. As the burning columns of material are only in the air for a short time and the dosage is increased more from the material burning on the ground than from that burning in the air, it is considered more realistic to allow for ejected material by adjusting the effective flame width used in a solid flame mode1 rather than trying to estimate a shape factor and mass burning rate for a point source model.
will
Stack configuration Steel drums containing cellulose nitrate are normally stacked one high on a pallet and the pallets themselves
material would be ejected from the bottom layer of drums provided they vented by distortion of the lid (as observed in the stacked steel drum trials); with high nitrogen content material, material could be violently ejected from the top layer of drums; the flame height will be greater than from a single drum stack; the overall flame area may be less from stacked drums than from the same number of unstacked drums. little
Fibreboard drums containing plasticized material are likely to behave in a similar manner although it would be expected that the stack stability would be less and the burning time, for the same quantity, would be longer. Type of cellulose nitrate In general, plasticized cellulose nitrate burns much faster than alcohol-damp material and gives more thermal radiation per unit mass. It burns rapidly within a drum whereas the alcohol-damp material tends to burn slowly within a drum with the rate of burning controlled by the rate of vaporization of the alcohol. In fibreboard drums, with fibreboard lids, alcohol-damp material burns very gently with small smokeless flames. Plasticized cellulose nitrate burns with the creation of black smoke but this does not give much obscuration. The higher the nitrogen content then the more likely it is for the cellulose nitrate to burn vigorously, although the heat of combustion reduces with increasing nitrogen content. Addition of organic diluent generally reduces the burning rate, although the physical form can reverse the effect, and increases the heat of combustion.
Parameters models
required to use the point source
The parameters used in the modified point source models for prediction of irradiance are the shape factor, heat of combustion, fraction of the heat of combustion radiated and the mass burning rate. The shape factors measured are extremely variable and it is difficult to derive representative values for a point
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Industrial cellulose nitrate - radiation hazards and separation source model. In general, the fraction of the heat of combustion radiated is less from isopropanol-damp cellulose nitrate that from dibutyl phthalate-plasticized material. Estimation of a representative mass burning rate is difficult because of the variation in the amount of material ejected from the drums and the amount of material burning at any one time. The modified BAM model proposed by the authors for mainly liquid organic peroxides uses a mass burning rate per unit area. This model does not appear appropriate for solids which do not melt and run out of the packaging and where the stores are not bunded. All these factors suggest that it would be difficult to predict the level of thermal radiation from a particular cellulose nitrate storage situation using a point source model.
Parameters model
required to use the solid flame
The authors consider that a cylindrical solid flame model is appropriate for the prediction of irradiance from burning cellulose nitrate. The parameters involved and the assumptions made are considered below, Surface emissive power An estimated value for the surface emissive power is only valid for the particular model (flame area) used to derive it. However, the measurements obtained by British Gas from the unconfined burning trials by two independent methods are consistent and indicate that a value of 240 kW mm2 is reasonable for DBP-plasticized material and a value of 175 kW me2 is reasonable for IPA-damp material (higher values were obtained by calculation from the steel drum trials but these were affected by the ejection behaviour).
Flame dimensions In practice, because of packaging and stack configuration effects and wind effects, it is difficult to predict flame dimensions, in particular the flame length to diameter ratio, for a specific storage situation. In addition, because there will usually be burning material on the ground, the flame width will usually be considerably larger than the stack width. An expression, derived by the authors from the observed flame dimensions, which takes account of some of these factors is: D = 8W”.‘j
(I~W~lOO)
(9)
The above expression is reasonably representative of the dimensions measured in the field trials although it is not clear whether it would hold for large stacks. In the DBP/NC stack trials (see Tuble 7) monitored by British Gas, the flame height for an eightdrum single stack arrangement was 18.3 m and 25.5 m for a double drum stack. In the earlier trials (see Table 8), the flame height from four-drum single stacks varied from 5 to 15 m with burning material being ejected up to 30 m in height. The flame height is normally proportional to the flame width but to simplify the
316
J. Loss Prev. Process lnd., 1992, Vol5, No 5
distances: R. Merrifield
and T. A. Roberts
calculations it seems more appropriate to take a fixed flame height of 15 m for a single stack and 25 m for a double stack. IPA/‘NC in steel drums can be ejected violently but, as it burns with relatively small flames when unconfined, the overall flame height tends to be lower than from DBP/NC in steel drums. For single and double drum stacks, corresponding flame heights of 9 and 15 m appear reasonable. For IPA/‘NC in fibreboard drums, flame heights of 6 and 10 m may be appropriate for single and double drum stacks respectively. Geometric view factor In field trials, the flames were always approximately cylindrical and hence a view factor calculated using a right-cylinder model should be reasonable. The rightcylinder model can be easily adjusted to take obstacles, e.g. a store wall, into account. If a store is close to a nearby building etc. it may be necessary to use a tilted cylinder model to take wind effects into account. Atmospheric attenuation The importance of atmospheric attenuation effects will depend on the irradiance criteria chosen and the distances implied by them. Over relatively short distances, e.g. 10 to 20 m, the transmissivity is likely to be over 0.8 and hence attenuation effects could be neglected and the transmissivity taken as 0.95. If attenuation effects are likely to be significant, e.g. for large stores where the critical distance(s) may be large, then the method of evaluation suggested by Simpson’l could be used.
Calculation of quantity safety distances We propose that the variation of irradiance with distance from burning cellulose nitrate may be calculated using the assumptions summarized in Table 10. In Germany, an irradiance of 8.0 kW me2 is used to determine the manufacturing buildings’ separation distance and in the UK an irradiance of 12.6 kW rn-’ is used to set inter-building separations. In order to allow comparison of the distances predicted by the German method (based on their 8 kW mm2 criteria) with those from the current approach, the values for an irradiance of 8 kW m-’ from plasticized cellulose nitrate in metal drums (German storage group IA), from alcohol-damp cellulose nitrate in metal drums (German storage group IA) and from alcohol-damp cellulose nitrate in fibreboard drums (German storage group IB) are given in Tables 11, 12 and 13 respectively. The German approach does not directly take the stack width into account although the effect of stack height is reflected in the mass used. Only for the most severe storage category, IA, is the corrected mass burning rate used. However, of the four types of cellulose nitrate tested by BAM, all but the 35% ethanol-damp in fibreboard drums (IB) have been assigned to group IA. From Tables 11, 12 and 13, it can be seen that the
Industrial cellulose nitrate - radiation hazards and separation Table 10 Assumptions
made in calculating
irradiance
distances:
Alcohol-dampb
Metal orfibreboard
ae.g. dibutyl, dicyclohexyl be.g. ethanol, isopropanol
Table 11 Comoarison
Number
1.2 1.2 2.4 2.4 3.6 3.6 4.8 4.8 6.0 6.0
1X2X2 2X2X2 1X4X4 2~4x4 1X6X6 2x6x6 1X8X8 2x8x8 1 x10x10 2 x 10 x 10
Table 12 Comparison
M’*
of
1.2 1.2 2.4 2.4 3.6 3.6 4.8 4.8 6.0 6.0
1X2X2 2X2X2 1X4X4 2x4x4 1X6X6 2~6x6 1 X8X8 2x8x8 1 x10x 2x 10x = 0.092 M;‘/z.
Table 13 Comparison
(M;‘fi
M’j3
175 0.95 8 W’6 10
= 1.60934M’J3
1.15 1.15 2.3 2.3 3.45 3.45 4.6 4.6 5.75 5.75
1 x2x2 2X2X2 1X4X4 2x4x4 1X6X6 2~6x6 1X8X8 2x8x8 1 x 10 x 10 2x10x10
Distance(m) German*
34.6 42.9 44.2 54.9 50.7 63.2 55.9 69.7 60.3 75.2
27.1 34.1 43.0 54.2 56.4 70.8 68.3 86.0 79.2 99.8
of 8 kW m-* from isopropanol-damp
of 8 kW mm2 from alcohol-damp
s 10000 kg); distance = 1.6. M’13
cellulose
nitrate in metal drums Distance(m)
Authors’
German*
23.9 29.4 30.4 37.6 35.0 43.4 38.7 47.9 41.9 51.9
25.0 25.0 25.0 26.7 27.8 35.0 33.7 42.4 39.1 49.2
for 61.5 kg of 35% isopropanol-damp
of drums
NC in metal drums = 1416 kg min-‘)
(kg)
572 1144 2288 4576 5148 10296 9152 18304 14300 28600 (M, ‘lb
Authors’
kg di-octyl phthalate-plasticized
Quantity
of distances for an irradiance
(M
for50
10 10
Number
= 5.5 Ml&
175 0.95 8 W0.6 9 15
drum
from plasticized cellulose nitrate in metal drums
480 960 1920 3840 4320 8640 7680 15360 12000 24000
of drums
Stack width (m)
*Distance
Fibreboard
Quantity (kg)
of distances for an irradiance Number
of 8 kWm-2
drums
= 3.461941V’/~
Stack width (m)
*Distance
Metal drum
or di-octyl phthalate or butanol
of distances for an irradiance
= 0.092M;‘/2.
drum
240 0.95 8 W”.6 15 25
Stack width (m)
*Distance
and T. A. Roberts
for different types of cellulose nitrate Plasticized”
Surface emissive power (kW m-2) Transmittance Flame width (m) Flame length (I drum high cm)) Flame length (2 drums high fm))
R. Merrifield
NC in metal drums = 306 kg min’)
cellulose nitrate in fibreboard Distance(m)
Quantity (kg)
252 504 1008 2016 2268 4536 4032 8064 6300 12600 (M
> 10000
drums
Authors’
German*
19.8 24.6 25.3 31.3 29.1 36.1 32.3 39.9 34.9 43.7
25.0 25.0 25.0 25.2 25.8 29.6 28.9 35.2 31.6 37.2
kg) (for 50 kg of 35% ethanol-damp
NC in fibreboard
drums)
J. Loss Prev. Process Ind., 1992, Vol5, No 5
317
Industrial cellulose nitrate - radiation hazards and separation German distances are in broad agreement with those derived using the assumptions in Table 10, assuming that the material is stacked in the way suggested. However, if the material was in a long thin stack, the authors’ method would correctly predict larger separation distances for a target face-on than for a target end-on. The German method would predict the same distance in both cases. Buildings adjacent to a cellulose nitrate store fire could be at risk from either flame contact or radiant heat (via piloted ignition of wood). Whether or not the building does catch fire will depend upon a number of factors including design, type and materials of building construction, distance, wind speed and direction, and duration of the fire. The separation criterion of 12.6 kWmm2 implied in the 1985 Building Regulations” could be used as one basis for setting safety distances. The relationships between the separation distances for an irradiance of 12.6 kWm_’ and the stack width for different types and packaging of cellulose nitrate are approximated by the equations given in Table 14. The predicted distances for an irradiance of 12.6 kW me2 for large stores is consistent with the observed damage from a fire13 involving 3322 steel drums containing isopropanol-damp and 660 of plasticized cellulose nitrate.
Separation
criteria
In addition to the risk to property mentioned above there is also the safety of people to be considered, who may be nearby either inside a building or out-of-doors. To provide adequate safeguards from such fire dangers, it would be necessary to make a specific assessment covering all the relevant aspects of the operations carried out at each individual site. If, as part of such an assessment, other higher or lower irradiance levels are required, suitable predictions may be derived using the assumptions summarized in Table 10.
l
Acknowledgements The authors thank Dr P. Wandrey and Dr D. Lemke of BAM for their explanation of the German system and use of their results and Imperial Chemical Industries (ICI) plc for provision of the cellulose nitrate used in the HSE trials. We also thank the British Gas staff who performed thermal radiation measurements at some of the HSE trials.
References 1 Roberts,
2
3
4
different separation distances are required depending on both the type (plasticized or alcohol-damp) 5
Table 14 Quantity safety distance relationships of 12.6 kW m-2
for an irradiance
6 7
Type of NC and configuration
Distance (m)
8
of stack
Plasticized, double stack of metal orfibreboard drums Plasticized, single stack of metal or fibreboard drums
30.70 Lw3”3s 9 25.48 W”.3584
Alcohol-damp, Alcohol-damp,
double stack of metal drums single stack of metal drums
2 1.30 wo.3724 17.65 W0.3740
Alcohol-damp, drums Alcohol-damp, drums
double stack of fibreboard
18.32 W”.3737
singlestackof
15.08 WO.=”
10 11
fibreboard
12 13
318
J. Loss Prev. Process Ind., 7992, Vol5, No 5
and T. A. Roberts
and the packaging (metal or fibreboard drum) of the cellulose nitrate; the separation distance required increases in the order alcohol-damp/fibreboard drum < alcoholdamp/metal drum < plasticized/metal or fibreboard drum; irradiance predictions based on measured surface emissive powers, fixed flame heights for single and double drum stacks, flame diameters based on the relation D = S(stack width)0.6 and a right-cylindrical view factor model are reasonable; and for a square (as opposed to a rectangular) storage arrangement, this method is in good agreement with those predicted by the German method (which is independent of stack configuration) and is sufficiently flexible to allow prediction to any stack arrangement up to two drums high.
Conclusions The following conclusions are made:
distances: R. Merrifield
T. A. . Turner, B. C. and Freeder, B. G. ‘External and internal ignition trials on industrial cellulose nitrate contained in steel and fibreboard drums’, HSE Internal Report No. IR/L/HM/‘82/5,1982 Roberts, T. A. and Freeder, B. G. ‘Measurements of the thermal radiation from burning propan-2-01 damp and dibutyl phthalate plasticised industrial cellulose nitrate in trials with British Gas’, HSE Internal Report No. IK/‘L/HM/X7/5,1987 Pritchard, M. ‘Thermal radiation measurements on two nitrocellulose fires - a record of work conducted under contract (RPS 1294) to the Health and Safety Executive’, British Gas Report MRS 14270,1986 Murphy, D. J., Pritchard, M. J. and Shale, G. A. ‘Thermal radiation and geometry measurements on four cellulose nitrate fires - a record of work conducted under contract (RPS 1294) to the Health and Safety Executive on 7/S October 1986’. British Gas Report MRS 14436,1987 BAM ‘The assignment of dangerously explosive substances to storage groups according to their hazard’, OECD-IGUS Report No. 248,1981 Mudan, K. S., Prog. Ener Combusr. Sci., 1984,10,59 Roberts. T. A. and Merrifield, R., .I. Loss Pi-w. Process Id., lYYO,3. 244 German ‘Guidelines on the storage of potentially explosive materials’ (Spreng LR 300, 19/12/88). HSE Translation No. 13288,1989 Rai, P. K. and Kalelkar, A. S. ‘Assessment models in support of the hazard assessment handbook (CG-446-3)‘, Chapter 9, Technical Report for the US Coast Guard, NTIS Publication No. AD7766i7,1974 BAM ‘Examples for the assignment of dangerously explosive substances to storage groups according to their hazards’, 1983 Simpson, I. ‘Atmospheric transmissivity - the effects of atmospheric attenuation on thermal radiation’, UKAEA, SRD Report R304,1984 ‘Building and Buildings; The Building Regulations’ Sl No. 1065, lY85 Auld, I., Blair J er a[., ‘Fire in nitrocellulose drum storage
Industrial cellulose nitrate-radiation
hazards and separation
compound on Wednesday May 23 1973 - Report of Committee Enquiry’, ICI Report No. ST55,1973
Nomenclature I F M M’ H Y
MP tb 4
Irradiance (kW me2) Fraction of heat of combustion radiated Total mass of cellulose nitrate (kg) Mass burning rate (kg s-l) Heat of combustion (kJ kg-‘) Distance (m) Mass content of package (kg) Time between the first sign of burning of the packaged substance and when the irradiance has fallen to 5% of the maximum (s) Mass burning rate of packaged substance, Mp/tb
(km')
of
M; IIn Ia s ML
M" A E V
VH VV
T h s
D W
distances:
R. Merrifield
and T. A. Roberts
Estimated mass burning rate for 10 000 kg (kg s-t) Maximum irradiance (kW rne2) Mean irradiance (kW III-~) Shape factor, I,/I, Corrected mass burning rate (kgs-‘) Mass burning rate per unit area (kgs-t m-2) Pool burning area (m2) Surface emissive power (kW mm2) View factor corresponding to the relative geometry of the flame and target Horizontal view factor Vertical view factor Transmissivity Cylinder height/cylinder radius Distance from vertical axis/cylinder radius Flame width (m) Stack width (m)
@ 1992 Crown Copyright
J. Loss Prev. Process Ind., 7992, Vol5, No 5
319
Many forms of industrial cellulose nitrate burn fiercely with the emission of large quantities of thermal radiation. Models are considered for predicting the variation with distance of the irradiance from burning cellulose nitrate. These models are evaluated using data from HSE and German trials. An approach to setting quantity safety distances for cellulose nitrate is given. (Keywords:
cellulose nitrate, thermal radiation hazard, modelling)
The Health and Safety Executive (HSE) Explosion and Flame Laboratory (EFL) has performed several series of trials’-” on industrial cellulose nitrate in order to determine the critical parameters for assessing thermal radiation hazards. Other countries such as Germany’ have set safety distances based on their own cellulose nitrate burning trials. This paper considers methods of predicting the irradiation from burning cellulose nitrate and suggests an approach to setting quantity safety distances.
Thermal
radiation
models
Two basic approaches6 are generally used to predict the variation of irradiance with distance. These are the point source and solid flame models.
Point source model In this model, the energy radiated from a flame is specified as a fraction of the heat of combustion and the irradiance is assumed to vary inversely with the square of the distance from the source. Hence the irradiance is given by: Z = F - M’ - H/(4aY2)
(1)
The main problems with this model are estimating the mass burning rate at a particular time and estimation of the fraction of heat of combustion radiated. Whilst the average burning rate can be easily measured, it is very difficult to estimate the burning rate at a specific time for industrial cellulose nitrate in steel drums where there may be burning material within the drums, burning columns of material ejected into the air and burning material scattered on the
Received 30 July 1992 0950-4230/92/05031149 @ 1992 Butterworth-Heinemann
ground. In addition, the point source model overestimates the radiation received by a target close (c.a. less than five flame diameters) to the fire.
BAM modified point source model In the Bundesanstalt fiir Materialforschung und -prl.ifung (BAM) version7 of this model, mass burning rates for the packaged material are estimated using single stack package trials. A measure of how steadily the packaged cellulose nitrate bums is given by the ratio of the maximum irradiance to the mean irradiance. Using the BAM model, the maximum irradiance is given by: I,
= S. I, = S - F - H exp (0.6667 In [M/M,] + In [M6]}/(497Y*)
(2)
The extrapolated mass burning rate for 10 000 kg is normalized to give a corrected mass burning rate as: ML = M:( H/335OO)(F/O.25)(S/2.78)
(3)
where the standard heat of combustion is taken as 33 500 kJ kg-l, the standard fraction of the heat radiated as 0.25, and the standard ratio of maximum irradiance to mean irradiance as 2.78. This corrected mass burning rate criterion is then used to allocate the different types of cellulose nitrate, and other vigorously burning materials, to storage groups as shown in
Table I. The German quantity safety distances’ for occupied buildings/plant and other stores containing p ‘ otentially explosive’ materials are summarized in Table 2. There are reductions in the safety distances depending on the construction of the store, the type of protection available and whether there are quantity limits set for the store. Tncreases in distance are required if the mass per unit store area exceeds that used in the test trials.
Ltd
J. Loss Prev. Process Ind., 1992, Vol5, No 5
311
Industrial cellulose nitrate-radiation Table1 criteria
German corrected
mess
burning
hazards and separation
rate storage
group
Corrected mass burning rate
Storage group
25 kgsm’ < 5 and z 2.333 kg s-l c 2.333 and * 1 kg s-’ < 1 kg s-’
IA
I6
distances: R. Merrifieid
and T. A. Roberts
targets close to the fire and it can take effects such as blooming and obscuration into account. In the EFL trials, the flames were approximately cylindrical in shape. The horizontal and vertical geometric view factors for a vertical cylinder are given9 by: 7iVH = Ib - (l/s)1
II III
x {tan-’
[(b + 1) * (s - l)/(b - 1)
x (s + 1)}]0-5}l[bZ Modified BAM model Germany uses the BAM approach for industrial cellulose nitrate and other energetic substances such as organic peroxides and sponge blowing agents. For packaged organic peroxides, the authors have proposedS a modified form of the German point source model which utilizes the mass burning rate per unit area: Z = S . F - M”. A . H/(4nY2)
- [a - (I/s)1 X {tan-l
Cylindrical solid flame model In a simplified version of this model, a flame is assumed to radiate uniformly over its entire surface (more sophisticated models with a segmented approach are available) and the irradiance is given by: Z= E0V.T
(5)
To apply this model, measurements of the surface emissive power and flame dimensions are required. The advantage of this model is that it can be used for
[(a + 1) . (s - l)/{(a
- 1)
x (s + 1)}]0.5}/[a2 - 1]0.5
+ {[h/s] * {[a/(a’
- 1)“.5]
X tan-’ [(a + 1) * (s - l)/{(a - 1) * (s + 1)}]“.5 - tan-’ [(s - l)/(s + l)]“.‘}
v = (V-C + V&)O.S
(8)
where a = (h2 + s2 + 1)/(2s) and b = (1 + s2)/(2s)
Test data EFL has performed burning trialsla2 on unconfined and packaged (steel and fibreboard drums) isopropanol (IPA)-damp and dibutyl phthalate(DBP)-plasticized industrial cellulose nitrate. BAM have also performedlo trials on pigmented cellulose nitrate. The EFL data is in a form which can be used for both point source and solid flame models. EFL data for modified point source models The data required for the modified point source models are the heat of combustion, mass burning rate, shape factor and the fraction of the heat of combustion radiated. The results from trials’ where 40 mm deep layers of IPA-damp cellulose nitrate and DBP-plasticized cellulose nitrate (3.29 by 3.33 m area) were ignited are summarized in Table 3.
and unoccupied
stores
Quantity (kg)
Minimum
IA IA
Occupied Unoccupied
>lOO >I00
25 IO
0.092M”fl 0.115Mfi’~.
IB IB IB
Occupied Occupied Unoccupied
>200 s 10000 >10000 >200
25 10
5.5 M’h l&M@ 1.6 M@
II II
Occupied Unoccupied
>200 >200
25 10
1 1 API3 1:l M’p
III Ill
Occupied Unoccupied
,200 >200
10 10
_
312
mass burning rates are in kg min-’
J. Loss Prev. Process Ind., 1992, Vol5, No 5
distance(m)
Distance (m)*
Building type
*Corrected
(7)
and the maximum view factor is given by
Table 2 German quantity safety distances for occupied industrial buildings/plant Storage group
(6)
ZTV” = {[1,/s] *tan-’ [h/(s* - 1)“.5]}
(4)
The authors adopted a mass burning rate per unit area approach as most organic peroxides are liquids or would be stored with liquids and if such a store were bunded then the burning area would be fairly well defined. However, industrial cellulose nitrate is normally commercially available in solid form and would not be held in a bunded area. As indicated earlier, it would be very difficult to estimate the mass burning rate corresponding to the maximum flame size for cellulose nitrate in steel drums although reasonable results could be obtained for alcohol-damp cellulose nitrate in fibreboard drums where no violent behaviour is observed.
- 1]“.5
AN AM
Industrial cellulose nitrate - radiation hazards and separation
Table3 Unconfined cellulose nitrate
trials
on
DBP-plasticized
Diluent concentration (%) Nitrogen content (%) Heat of combustion (kJ kg-‘) Mass (kg) Burning area (mZ) Mass burning rate per unit area (kg m-* s-l) Total burning time(s) Total dosage (kJ m-2) Mean irradiance (kW m-*1 Distance dosage measured (m) Period of fastest burning (s) Maximum average irradiance (kW m-2) Distance irradiance measured (rn) Shape factor Fraction of heat radiated
and
IPA-damp
DBP/NC
IPA/NC
19.7 11.8 14350 300 10.96 0.274 100 157.5 1.58 22.7 54 2.51 22.7 1.59 0.24
22.0 12.2 14910 286 10.96 0.065 400 275.8 0.69 22.7 150 1.21 22.7 1.75 0.42
Two types of four drum trials were performed in the first test series’ which give an indication of the maximum and minimum burning rates. One type involved removing the lid from the upwind drum and igniting the cellulose nitrate in this drum and allowing the flames to spread to the other three drums with their lids on. This gave an estimate of the minimum overall burning rate. The other type of trial involved heating all four drums, with their lids on, by a wooden crib fire. As all drums were engulfed by fire at the same time, Table4
Summary
of EFL package trial data for modified
Trial description
Linear configuration 4 x 143 kg steel drumsb with (N2 11.8%) 4 x 125 kg steel drumsb with (N, 11.5%) 4 x 63 kg fibreboard drumsc (N, 11.8%) 3 x 72 kg fibreboard drumsc
26.3% IPA/‘73.7% 20.2% DBP/79.8%
NC NC
with 26.3% IPA/73.7% with 20.9% DBP/79.1%
(N, 11.3%) and 1 x 72 kg with 20.2% DBP/79.8% Pallers adjacent 8 x 143 kg steel drums with 31.5% IPA/68.5% (N2 10.8%) 8 x 100 kg steel drums with 17.9-22.8% DBP/82.1%-77.2% NC (N2 11.5%-12.2%) Pallets stacked 8 x 143 ka steel drumsd with 31.5% IPA/68.5% (N,
NC
NC
NC NC
R. Merrifield
and T. A. Roberts
this gave an indication of the maximum overall burning rate. These tests were performed on two types of cellulose nitrate in steel and fibreboard drums. Only steel drums were used in the second series’ of trials as these are the drums used by a major United Kingdom cellulose nitrate manufacturer. These consisted of side-by-side and stacked pallet load trials using eight drums and with external heating by loose burning cellulose nitrate. The nitrogen content of the IPAdamp cellulose nitrate used in this second series of trials was lower than in the first series and the quantity of plasticized cellulose nitrate per drum was lower (100 kg instead of 120 kg) and hence the results, in particular from the IPA-damp cellulose nitrate, may not be fully representative of the industrial situation. The results are summarized in Table 4. Heats of combustion were measured using a bomb calorimeter. The mass burning rate was derived from the quantity of material present and the duration of burning. Shape factors were estimated as the ratio of the maximum average dosage to the overall dosage. A rough estimate of the fraction of the heat of combustion radiated was obtained from the observed irradiance at a specific distance using the unmodified point source model. BAM
data
The test results available3 and the storage categories assigned are summarized in Table 5.
point source model H (MJ kg-‘)
Square configuration 4 x 143 kg steel drumsb with 26.3% lPA/73.7% NC (NZ 11.8%) 4 x 125 kg steel drum+‘with 20.2% DBPp9.8% NC (N, 11.5%) 4 x 63 kg fibreboard drums” with 26.3% IPAj73.7% NC (N, 11.8%) 4 x 72 kg fibreboard drums’with 20.2% DBP/79.8% NC (N, 11.5%)
distances:
M (kg)
Aa (mZ)
M’ (kg s-’ )
S
F
16.34
572
1.03
2.5
3.08
0.19
14.35
500
1.03
7.0
1.71
0.29
16.34
252
0.68
0.5
1.10
0.19
14.35
288
0.68
2.1
3.57
0.29
16.34
572
1.03
7.0
3.6
0.12
14.35
500
1.03
11.9
3.1
0.15
16.34
252
0.68
1.1
1.3
0.14
13.71
288
0.68
2.0
1.5
0.19
16.42
1144
2.30
~2.3
1.48
-Co.11
13.86
800
2.30
1.67
0.24
16.42
1144
1.15
13.86
800
1.15
NC
3.6
Cl.3
1.23
<0.06
iO.i%)
8 x 100 kg steel drums’with DBP/82.1%-77.2% NC(N*
17.9-22.8% 11.5%-12.2%)
‘Number of unobstructed drums b0.937 m high, 0.572 m diameter “0.668 m high, 0.485 m diameter base held on by steel band 40.865 m high, 0.605 m diameter
5.0
3.33
0.25
x cross-sectional area with internal volume 0.241 m3 with lid designed to vent at 296 kPa with internal volume 0.113 m3 with 0.7 mm thick steel lid fastened with steel clamp and fibreboard with internal volume 0.249 m3 with lid designed to vent at 296 kPa
J. Loss Prev. Process Ind., 1992, Vol5, No 5
313
industrial
Table5
cellulose
nitrate
- radiation
BAM trials on packaged cellulose
hazards
and separation
distances:
R. Merrifield
and
T. A. Roberts
nitrate
Trial description (;J
ML (kg s-’
kg-l)
S
F
W (knmin-l)
Storage group
I
4 metal drums with venting device and plastic liners with 18% dioctyl phthalate/82% NC (N, 11.8-12.3%)
14.714
200
5.05
1.55
0.35
1416
IA
4fibreboard drums and plastic liners with 35% ethanol/65% NC(NZ 11.8-12.3%)
17.004
200
0.68
4.17
0.11
185
IB
dfibreboard cases each with 3 x IO kg paper bags with 16% plasticizer/40% carbon black/44% NC (N, 10.4-12.3%)
23.058
240
1.84
2.59
0.09
319
IA
4 metal drums with venting device and plastic liners with 35% IPA/65% NC (N, 11.8-12.3%)
18.357
246
0.95
3.70
0.16
306
IA
EFL data for solid flame models Under contract from the HSE Research and Laboratory Services Division @LSD), British Gas made accurate measurements of the surface emissive power and flame dimensions during both the EFL unconfined burning trials3 and the steel drum trials4. In the former, the surface emissive power was measured using an infra-red spectrometer and was also calculated from the radiometer measurements, flame dimensions and geometric view factor. The results are summarized in Table 6. In the two pallet load steel drum trials, British Gas
Table6 trials
British
Gas
measurements
from
Spectrometer (surface emissive power(kWm-*I) Radiometer (surface emissive power (kWm-*)I Distance(m) lrradiance (kW m-2) Transmittance View factor Flame width (ml Flame height(m)
Table 7 British Gas measurements
unconfined
burning
IPA/NC
DBP/NC
171
230
179
244
19.9 1.23 0.878 0.00783 4.0 5.0
22.7 2.56 0.862 0.01217 5.0 7.5
Process
Packaging Cellulose nitrate is transported in a variety of packagings. Most UK produced material is transported in 143 kg steel drums fitted with a lid designed to relieve at a pressure of 296 kPa. Imported cellulose nitrate tends to be transported in 50 kg fibreboard drums. Amendent 25 of the International Maritime Dangerous Goods Code allows the types of packaging shown in Table 9. In a fire, steel drums can vent their contents violently giving short duration fireball effects. These are in addition to the flames from material burning within the drum and on the ground. If the drum vents when on its side, then the body of the drum may rocket away. The violence of ejection will depend on the ullage space within the drum (cf. trials with 100 kg and 120 kg of DBP/NC in steel drums). Burning material
Adjacent DBP/NC
Stacked DBP/NC
Adjacent IPA/NC
Stacked IPA/NC
270 46.0 3.61 0.778 18.3 1.88 13.8 9.4 4.6 42 99
362 46.0 2.90 0.779 25.5 1.21 5.1 5.4 4.3 31 54
146 46.0 0.17 0.776 4.2 2.26 4.2 2.2 2.7 46 49
147 46.0 0.65 0.779 8.1 1.21 7.7 6.0 4.4 12 -1
“At half flame height bAt half drum height CFrom vertical dRelative to direction of measurement
J. Loss Prev.
Factors affecting the thermal radiation hazards
from steel drum trials
Surface emissive power (kW m-2j Distance (m) lrradiance (kW m-*1 Transmittance Flame height(m) Apparent stack width (m) Flame width= (m) Flame widthb (m) Wind speed (ms-‘j Tilt angle” (“) Wind directiona (“)
314
did not use a spectrometer and surface emissive power was estimated from the irradiance, flame dimensions and geometric view factor and an estimate of the transmittance. The results are summarized in Table 7. The flame dimensions from the first series of trials’ are given in Table 8.
Ind.,
7992,
Vol5,
No 5
Industrial
cellulose
Table 8 Flame dimensions
nitrate
- radiation
hazards
and separation
distances:
R. Merrifield
and
T. A. Roberts
(m) from four drum trials
Trial
Stack width
Flame width
Flame height
Square configuration Steel drums (IPA/NC) Steel drums (DBP/NC) Fibreboard drums (IPA/NC) Fibreboard drums (DBP/NC)
1.15 1.15 0.93 0.93
8 10 10 8
10 15 5 15
Linear configuration Steel drums (IPA/NC) Steel drums (DBP/NC) Fibreboard drums (IPA/NC) Fibreboard drums (DCHP/DBP/NC)
3.20 3.20 2.76 2.76
10 15
5 10 8 10
may be stacked two high. If such a stack were involved in a fire it is expected that:
Table 9 Packagings for industrial cellulose nitrate Type of packaging*
Gross mass (kg)
Cans (20 litre) packed in a wooden box Plastic bottles or bags in a wooden box Plastic bottles or bags in a fibreboard box Lined wooden box Plastic bag in fibreboard drum Water-resistancefibreboard drum Metal drum
125 125 40 125 225 225 225
*Packaging possible.
should
be so constructed
that
explosion
::
is not
only be in the air for approximately two seconds but will continue burning on the ground for a much longer period adding to the effective width of the flames. Fibreboard drums tend to burn relatively slowly with both the flame dimensions and duration of burning depending on the quantity and stack configuration. However, DBP-plasticized material, which can deflagrate rapidly, may be violently ejected if a deflagration is initiated whilst the drum still provides a degree of confinement, e.g. if heat is transmitted through a metal lid or base. Again the main effect of violent ejection will be to increase the effective flame width. The BAM shape factor compares the average maximum irradiance (over at least a IS second period) with the mean irradiance over the main burning period and does not directly indicate the irradiance during violent ejection although the shape factors given in Tables 4 and 5 do take this into account to some extent. After ejection, material is left burning on the ground which increases the effective flame width. As the burning columns of material are only in the air for a short time and the dosage is increased more from the material burning on the ground than from that burning in the air, it is considered more realistic to allow for ejected material by adjusting the effective flame width used in a solid flame mode1 rather than trying to estimate a shape factor and mass burning rate for a point source model.
will
Stack configuration Steel drums containing cellulose nitrate are normally stacked one high on a pallet and the pallets themselves
material would be ejected from the bottom layer of drums provided they vented by distortion of the lid (as observed in the stacked steel drum trials); with high nitrogen content material, material could be violently ejected from the top layer of drums; the flame height will be greater than from a single drum stack; the overall flame area may be less from stacked drums than from the same number of unstacked drums. little
Fibreboard drums containing plasticized material are likely to behave in a similar manner although it would be expected that the stack stability would be less and the burning time, for the same quantity, would be longer. Type of cellulose nitrate In general, plasticized cellulose nitrate burns much faster than alcohol-damp material and gives more thermal radiation per unit mass. It burns rapidly within a drum whereas the alcohol-damp material tends to burn slowly within a drum with the rate of burning controlled by the rate of vaporization of the alcohol. In fibreboard drums, with fibreboard lids, alcohol-damp material burns very gently with small smokeless flames. Plasticized cellulose nitrate burns with the creation of black smoke but this does not give much obscuration. The higher the nitrogen content then the more likely it is for the cellulose nitrate to burn vigorously, although the heat of combustion reduces with increasing nitrogen content. Addition of organic diluent generally reduces the burning rate, although the physical form can reverse the effect, and increases the heat of combustion.
Parameters models
required to use the point source
The parameters used in the modified point source models for prediction of irradiance are the shape factor, heat of combustion, fraction of the heat of combustion radiated and the mass burning rate. The shape factors measured are extremely variable and it is difficult to derive representative values for a point
J. Loss Prev.
Process
Ind.,
7992,
Vol5,
No 5
315
Industrial cellulose nitrate - radiation hazards and separation source model. In general, the fraction of the heat of combustion radiated is less from isopropanol-damp cellulose nitrate that from dibutyl phthalate-plasticized material. Estimation of a representative mass burning rate is difficult because of the variation in the amount of material ejected from the drums and the amount of material burning at any one time. The modified BAM model proposed by the authors for mainly liquid organic peroxides uses a mass burning rate per unit area. This model does not appear appropriate for solids which do not melt and run out of the packaging and where the stores are not bunded. All these factors suggest that it would be difficult to predict the level of thermal radiation from a particular cellulose nitrate storage situation using a point source model.
Parameters model
required to use the solid flame
The authors consider that a cylindrical solid flame model is appropriate for the prediction of irradiance from burning cellulose nitrate. The parameters involved and the assumptions made are considered below, Surface emissive power An estimated value for the surface emissive power is only valid for the particular model (flame area) used to derive it. However, the measurements obtained by British Gas from the unconfined burning trials by two independent methods are consistent and indicate that a value of 240 kW mm2 is reasonable for DBP-plasticized material and a value of 175 kW me2 is reasonable for IPA-damp material (higher values were obtained by calculation from the steel drum trials but these were affected by the ejection behaviour).
Flame dimensions In practice, because of packaging and stack configuration effects and wind effects, it is difficult to predict flame dimensions, in particular the flame length to diameter ratio, for a specific storage situation. In addition, because there will usually be burning material on the ground, the flame width will usually be considerably larger than the stack width. An expression, derived by the authors from the observed flame dimensions, which takes account of some of these factors is: D = 8W”.‘j
(I~W~lOO)
(9)
The above expression is reasonably representative of the dimensions measured in the field trials although it is not clear whether it would hold for large stacks. In the DBP/NC stack trials (see Tuble 7) monitored by British Gas, the flame height for an eightdrum single stack arrangement was 18.3 m and 25.5 m for a double drum stack. In the earlier trials (see Table 8), the flame height from four-drum single stacks varied from 5 to 15 m with burning material being ejected up to 30 m in height. The flame height is normally proportional to the flame width but to simplify the
316
J. Loss Prev. Process lnd., 1992, Vol5, No 5
distances: R. Merrifield
and T. A. Roberts
calculations it seems more appropriate to take a fixed flame height of 15 m for a single stack and 25 m for a double stack. IPA/‘NC in steel drums can be ejected violently but, as it burns with relatively small flames when unconfined, the overall flame height tends to be lower than from DBP/NC in steel drums. For single and double drum stacks, corresponding flame heights of 9 and 15 m appear reasonable. For IPA/‘NC in fibreboard drums, flame heights of 6 and 10 m may be appropriate for single and double drum stacks respectively. Geometric view factor In field trials, the flames were always approximately cylindrical and hence a view factor calculated using a right-cylinder model should be reasonable. The rightcylinder model can be easily adjusted to take obstacles, e.g. a store wall, into account. If a store is close to a nearby building etc. it may be necessary to use a tilted cylinder model to take wind effects into account. Atmospheric attenuation The importance of atmospheric attenuation effects will depend on the irradiance criteria chosen and the distances implied by them. Over relatively short distances, e.g. 10 to 20 m, the transmissivity is likely to be over 0.8 and hence attenuation effects could be neglected and the transmissivity taken as 0.95. If attenuation effects are likely to be significant, e.g. for large stores where the critical distance(s) may be large, then the method of evaluation suggested by Simpson’l could be used.
Calculation of quantity safety distances We propose that the variation of irradiance with distance from burning cellulose nitrate may be calculated using the assumptions summarized in Table 10. In Germany, an irradiance of 8.0 kW me2 is used to determine the manufacturing buildings’ separation distance and in the UK an irradiance of 12.6 kW rn-’ is used to set inter-building separations. In order to allow comparison of the distances predicted by the German method (based on their 8 kW mm2 criteria) with those from the current approach, the values for an irradiance of 8 kW m-’ from plasticized cellulose nitrate in metal drums (German storage group IA), from alcohol-damp cellulose nitrate in metal drums (German storage group IA) and from alcohol-damp cellulose nitrate in fibreboard drums (German storage group IB) are given in Tables 11, 12 and 13 respectively. The German approach does not directly take the stack width into account although the effect of stack height is reflected in the mass used. Only for the most severe storage category, IA, is the corrected mass burning rate used. However, of the four types of cellulose nitrate tested by BAM, all but the 35% ethanol-damp in fibreboard drums (IB) have been assigned to group IA. From Tables 11, 12 and 13, it can be seen that the
Industrial cellulose nitrate - radiation hazards and separation Table 10 Assumptions
made in calculating
irradiance
distances:
Alcohol-dampb
Metal orfibreboard
ae.g. dibutyl, dicyclohexyl be.g. ethanol, isopropanol
Table 11 Comoarison
Number
1.2 1.2 2.4 2.4 3.6 3.6 4.8 4.8 6.0 6.0
1X2X2 2X2X2 1X4X4 2~4x4 1X6X6 2x6x6 1X8X8 2x8x8 1 x10x10 2 x 10 x 10
Table 12 Comparison
M’*
of
1.2 1.2 2.4 2.4 3.6 3.6 4.8 4.8 6.0 6.0
1X2X2 2X2X2 1X4X4 2x4x4 1X6X6 2~6x6 1 X8X8 2x8x8 1 x10x 2x 10x = 0.092 M;‘/z.
Table 13 Comparison
(M;‘fi
M’j3
175 0.95 8 W’6 10
= 1.60934M’J3
1.15 1.15 2.3 2.3 3.45 3.45 4.6 4.6 5.75 5.75
1 x2x2 2X2X2 1X4X4 2x4x4 1X6X6 2~6x6 1X8X8 2x8x8 1 x 10 x 10 2x10x10
Distance(m) German*
34.6 42.9 44.2 54.9 50.7 63.2 55.9 69.7 60.3 75.2
27.1 34.1 43.0 54.2 56.4 70.8 68.3 86.0 79.2 99.8
of 8 kW m-* from isopropanol-damp
of 8 kW mm2 from alcohol-damp
s 10000 kg); distance = 1.6. M’13
cellulose
nitrate in metal drums Distance(m)
Authors’
German*
23.9 29.4 30.4 37.6 35.0 43.4 38.7 47.9 41.9 51.9
25.0 25.0 25.0 26.7 27.8 35.0 33.7 42.4 39.1 49.2
for 61.5 kg of 35% isopropanol-damp
of drums
NC in metal drums = 1416 kg min-‘)
(kg)
572 1144 2288 4576 5148 10296 9152 18304 14300 28600 (M, ‘lb
Authors’
kg di-octyl phthalate-plasticized
Quantity
of distances for an irradiance
(M
for50
10 10
Number
= 5.5 Ml&
175 0.95 8 W0.6 9 15
drum
from plasticized cellulose nitrate in metal drums
480 960 1920 3840 4320 8640 7680 15360 12000 24000
of drums
Stack width (m)
*Distance
Fibreboard
Quantity (kg)
of distances for an irradiance Number
of 8 kWm-2
drums
= 3.461941V’/~
Stack width (m)
*Distance
Metal drum
or di-octyl phthalate or butanol
of distances for an irradiance
= 0.092M;‘/2.
drum
240 0.95 8 W”.6 15 25
Stack width (m)
*Distance
and T. A. Roberts
for different types of cellulose nitrate Plasticized”
Surface emissive power (kW m-2) Transmittance Flame width (m) Flame length (I drum high cm)) Flame length (2 drums high fm))
R. Merrifield
NC in metal drums = 306 kg min’)
cellulose nitrate in fibreboard Distance(m)
Quantity (kg)
252 504 1008 2016 2268 4536 4032 8064 6300 12600 (M
> 10000
drums
Authors’
German*
19.8 24.6 25.3 31.3 29.1 36.1 32.3 39.9 34.9 43.7
25.0 25.0 25.0 25.2 25.8 29.6 28.9 35.2 31.6 37.2
kg) (for 50 kg of 35% ethanol-damp
NC in fibreboard
drums)
J. Loss Prev. Process Ind., 1992, Vol5, No 5
317
Industrial cellulose nitrate - radiation hazards and separation German distances are in broad agreement with those derived using the assumptions in Table 10, assuming that the material is stacked in the way suggested. However, if the material was in a long thin stack, the authors’ method would correctly predict larger separation distances for a target face-on than for a target end-on. The German method would predict the same distance in both cases. Buildings adjacent to a cellulose nitrate store fire could be at risk from either flame contact or radiant heat (via piloted ignition of wood). Whether or not the building does catch fire will depend upon a number of factors including design, type and materials of building construction, distance, wind speed and direction, and duration of the fire. The separation criterion of 12.6 kWmm2 implied in the 1985 Building Regulations” could be used as one basis for setting safety distances. The relationships between the separation distances for an irradiance of 12.6 kWm_’ and the stack width for different types and packaging of cellulose nitrate are approximated by the equations given in Table 14. The predicted distances for an irradiance of 12.6 kW me2 for large stores is consistent with the observed damage from a fire13 involving 3322 steel drums containing isopropanol-damp and 660 of plasticized cellulose nitrate.
Separation
criteria
In addition to the risk to property mentioned above there is also the safety of people to be considered, who may be nearby either inside a building or out-of-doors. To provide adequate safeguards from such fire dangers, it would be necessary to make a specific assessment covering all the relevant aspects of the operations carried out at each individual site. If, as part of such an assessment, other higher or lower irradiance levels are required, suitable predictions may be derived using the assumptions summarized in Table 10.
l
Acknowledgements The authors thank Dr P. Wandrey and Dr D. Lemke of BAM for their explanation of the German system and use of their results and Imperial Chemical Industries (ICI) plc for provision of the cellulose nitrate used in the HSE trials. We also thank the British Gas staff who performed thermal radiation measurements at some of the HSE trials.
References 1 Roberts,
2
3
4
different separation distances are required depending on both the type (plasticized or alcohol-damp) 5
Table 14 Quantity safety distance relationships of 12.6 kW m-2
for an irradiance
6 7
Type of NC and configuration
Distance (m)
8
of stack
Plasticized, double stack of metal orfibreboard drums Plasticized, single stack of metal or fibreboard drums
30.70 Lw3”3s 9 25.48 W”.3584
Alcohol-damp, Alcohol-damp,
double stack of metal drums single stack of metal drums
2 1.30 wo.3724 17.65 W0.3740
Alcohol-damp, drums Alcohol-damp, drums
double stack of fibreboard
18.32 W”.3737
singlestackof
15.08 WO.=”
10 11
fibreboard
12 13
318
J. Loss Prev. Process Ind., 7992, Vol5, No 5
and T. A. Roberts
and the packaging (metal or fibreboard drum) of the cellulose nitrate; the separation distance required increases in the order alcohol-damp/fibreboard drum < alcoholdamp/metal drum < plasticized/metal or fibreboard drum; irradiance predictions based on measured surface emissive powers, fixed flame heights for single and double drum stacks, flame diameters based on the relation D = S(stack width)0.6 and a right-cylindrical view factor model are reasonable; and for a square (as opposed to a rectangular) storage arrangement, this method is in good agreement with those predicted by the German method (which is independent of stack configuration) and is sufficiently flexible to allow prediction to any stack arrangement up to two drums high.
Conclusions The following conclusions are made:
distances: R. Merrifield
T. A. . Turner, B. C. and Freeder, B. G. ‘External and internal ignition trials on industrial cellulose nitrate contained in steel and fibreboard drums’, HSE Internal Report No. IR/L/HM/‘82/5,1982 Roberts, T. A. and Freeder, B. G. ‘Measurements of the thermal radiation from burning propan-2-01 damp and dibutyl phthalate plasticised industrial cellulose nitrate in trials with British Gas’, HSE Internal Report No. IK/‘L/HM/X7/5,1987 Pritchard, M. ‘Thermal radiation measurements on two nitrocellulose fires - a record of work conducted under contract (RPS 1294) to the Health and Safety Executive’, British Gas Report MRS 14270,1986 Murphy, D. J., Pritchard, M. J. and Shale, G. A. ‘Thermal radiation and geometry measurements on four cellulose nitrate fires - a record of work conducted under contract (RPS 1294) to the Health and Safety Executive on 7/S October 1986’. British Gas Report MRS 14436,1987 BAM ‘The assignment of dangerously explosive substances to storage groups according to their hazard’, OECD-IGUS Report No. 248,1981 Mudan, K. S., Prog. Ener Combusr. Sci., 1984,10,59 Roberts. T. A. and Merrifield, R., .I. Loss Pi-w. Process Id., lYYO,3. 244 German ‘Guidelines on the storage of potentially explosive materials’ (Spreng LR 300, 19/12/88). HSE Translation No. 13288,1989 Rai, P. K. and Kalelkar, A. S. ‘Assessment models in support of the hazard assessment handbook (CG-446-3)‘, Chapter 9, Technical Report for the US Coast Guard, NTIS Publication No. AD7766i7,1974 BAM ‘Examples for the assignment of dangerously explosive substances to storage groups according to their hazards’, 1983 Simpson, I. ‘Atmospheric transmissivity - the effects of atmospheric attenuation on thermal radiation’, UKAEA, SRD Report R304,1984 ‘Building and Buildings; The Building Regulations’ Sl No. 1065, lY85 Auld, I., Blair J er a[., ‘Fire in nitrocellulose drum storage
Industrial cellulose nitrate-radiation
hazards and separation
compound on Wednesday May 23 1973 - Report of Committee Enquiry’, ICI Report No. ST55,1973
Nomenclature I F M M’ H Y
MP tb 4
Irradiance (kW me2) Fraction of heat of combustion radiated Total mass of cellulose nitrate (kg) Mass burning rate (kg s-l) Heat of combustion (kJ kg-‘) Distance (m) Mass content of package (kg) Time between the first sign of burning of the packaged substance and when the irradiance has fallen to 5% of the maximum (s) Mass burning rate of packaged substance, Mp/tb
(km')
of
M; IIn Ia s ML
M" A E V
VH VV
T h s
D W
distances:
R. Merrifield
and T. A. Roberts
Estimated mass burning rate for 10 000 kg (kg s-t) Maximum irradiance (kW rne2) Mean irradiance (kW III-~) Shape factor, I,/I, Corrected mass burning rate (kgs-‘) Mass burning rate per unit area (kgs-t m-2) Pool burning area (m2) Surface emissive power (kW mm2) View factor corresponding to the relative geometry of the flame and target Horizontal view factor Vertical view factor Transmissivity Cylinder height/cylinder radius Distance from vertical axis/cylinder radius Flame width (m) Stack width (m)
@ 1992 Crown Copyright
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319