Measurement of fire effluents Krzysztof Lebek Fire Materials Laboratory Centre for Materials Research and Innovation University of Bolton
Introduction z Bench-scale measurement costs less, but
must relate to real fires. z All scale-up of fire is difficult. z Toxicity has often been ignored because of the difficulties in assessment.
How can smoke toxicity be measured? BS 7990, IEC 60695-7-50 (and ISO TS 19700) describe one of the physical fire models capable of making such measurements. The apparatus is known as the Purser furnace
The Purser Furnace Secondary oxidiser – silica wool at 900°C
exhaust gases
thermocouple
secondary air supply smoke sensor CO2
HCl analysis
Effluent dilution chamber
Furnace
primary air supply
The Purser Furnace (mixing chamber)
exhaust gases thermocouple secondary air supply smoke sensor
Effluent dilution chamber
Furnace
primary air supply
Measurements needed for simple fire toxicity assessment CO CO2 O2 Other significant gases e.g. HCl for PVC
Repeatability and Steady State O2 vs time
CO vs time
Oxygen %
21.5
CO ppm
5000
Series1 Series2
21 CO concentration / ppm
20.5 20 O2 / %
Series3
4000
19.5 19 Series1
18.5
Series2 Series3
18
Series4 Series5
17.5
Series4 Series5
3000
2000
1000
0 0
17 0
10
20
30
40
50
60
10
20
30
time / min
60
optical density vs time
CO2 %
Optical density
1.4 Series1 Series2
1.2
Series1 Series2
1.2
Series3
Series3
Series5
Series4
1
1
Series5
optical density
CO2 concentration / %
50
time / min
CO2 in mixing. chamber vs time 1.4
40
-1000
0.8 0.6 0.4
0.8 0.6 0.4 0.2
0.2
0
0 0
10
20
30
-0.2 time / min
40
50
60
0
10
20
30
-0.2 time /min
40
50
60
Equivalence ratio φ The toxic product yields depend mostly on the equivalence ratio φ Actual Fuel/Air Ratio φ= Stoichiometric Fuel/Air Ratio For “stoichiometric” combustion to CO2 and water, φ = 1. For well-ventilated fires, φ = 0.5 For fuel-rich (vitiated) combustion, φ = 2.
Importance of Equivalence Ratio z Different cables have different oxygen
requirements (e.g. EVA, ATH, CaCO3 etc.) z Different cables burn to different degrees (e.g. sheathing, bedding, insulation) z Different fire scenarios burn the cables in different ways
Equivalence ratio φ Problem
To determine the toxic product yields at different equivalence ratio, we must know the stoichiometric air requirement of the cable. Solution
To measure fuel content of gas phase by using O2 analyser on secondary oxidiser (the phi meter method)
Calculation of equivalence ratio φ = φ underventi lated + φ overventil ated ⎛ 20 .95 − [O 2 ]tube ⎞ ⎛ 50 × [O 2 ]box − [O 2 ]sec ondary φ =⎜ ⎟ + ⎜⎜ 20 .95 ⎝ ⎠ ⎝ 20 .95 × primary air flow O2 tube primary air supply
O2 secondary Furnace
O2 box
⎞ ⎟ ⎟ ⎠
Validation using Polypropylene Primary air flow /l min-1 4 15
φ under 0.87 0.82
φ over φ calculated 1.85 0.05
2.71 0.87
φ experimental 3.07 0.82
Determination of Equivalence ratio z For simple polymers, which burn
completely, the equivalence ratio comes from the chemical formula z For cables the equivalence ratio must come from the difference between oxygen supplied and oxygen consumed
Comparison of data from large-scale test and Purser Furnace for Polypropylene 0.2 Large Scale Steady State Tube Furnace
CO yield g/g
0.15
0.1
0.05
0 0
0.2
0.4
too much air
0.6
0.8
1
equivalence ratio φ
1.2
1.4
1.6
not enough air
Oxygen depletion 21
oxygen concentration / %
20,5
20
19,5
19 NYM NHMH
18,5
NHXMH RZ1-K
18 0,5
1,5
2,5
3,5 phi
4,5
CO2 yield 2,5 NYM NHMH
2
NHXMH
CO2 yield g/g
RZ1-K
1,5
1
0,5
0 0,5
1,5
2,5
3,5
phi
4,5
5,5
CO yield 0,25
CO yield / g/g
0,2
0,15
0,1 NYM NHMH NHXMH RZ1-K
0,05
0 0,5
1,5
2,5
3,5
phi
4,5
5,5
FED (Fractional Effective Dose) Fire toxicity is quantified by: FED - the fraction of a lethal dose (for 50% of the population) When FED = 1 then 50% of the population will die. contribution contribution FED = from oxygen + from CO depletion
contribution + from HCN
For example
[CO] LC50 (CO)
contribution + from other gases
FED vs phi 1,6 HC 1,4
HCl hypoxia
1,2
CO
FED
1 0,8 0,6 0,4 0,2 0 0,5 0,6 1,0 1,8 3,3 0,6 0,8 1,4 2,6 5,0 0,5 0,7 1,0 1,9 4,0 0,7 0,8 0,8 0,9 1,1 1,8 2,9 NYM
NHMH
NHXMH
RZ1-K
0
PVC data PVC Power FR Polyolefin FR Polyolefin Power data UTP Cat 5 NHXMH power RZ1-K RV-K NO7V-K
Under-ventilated 825°C
Well-ventilated 650°C
Oxidative Pyrolysis 350°C
Under-ventilated 825°C
Well-ventilated 650°C
Oxidative Pyrolysis 350°C
Under-ventilated 825°C
Well-ventilated 650°C
Oxidative Pyrolysis 350°C
Under-ventilated 825°C
Well-ventilated 650°C
Oxidative Pyrolysis 350°C
Under-ventilated 825°C
1,5
Well-ventilated 650°C
2
Oxidative Pyrolysis 350°C
Under-ventilated 825°C
Well-ventilated 650°C
Oxidative Pyrolysis 350°C
Under-ventilated 825°C
Well-ventilated 650°C
Oxidative Pyrolysis 350°C
Under-ventilated 825°C
Well-ventilated 650°C
Oxidative Pyrolysis 350°C
Under-ventilated 825°C
Well-ventilated 650°C
Oxidative Pyrolysis 350°C
Under-ventilated 825°C
Well-ventilated 650°C
Oxidative Pyrolysis 350°C
FED
FED for ten cables 2,5
HC HCl hypoxia CO
1
0,5
SSTP Cat 7
Conclusions – Bench Scale Toxicity data • The Purser furnace compares the combustion toxicity, independent of the flammability. • For the limited range of (PVC and Polyolefin) cables presented here polyolefin cables show lower toxic product yields than PVC during burning. • The higher CO yield for PVC results from interference of the flame reactions by HCl resulting in incomplete combustion.
Conclusions – comparison with large scale test z Correlation with large scale tests is difficult. z Cables must be burnt whole, to allow the FR
sheath to protect the more flammable insulation. z Large scale test data must be normalised by mass loss. z The steady state method is capable of reasonable correlation using data for a wellventilated fire scenario.
