Bench and Large-scale assessment of smoke toxicity

Abstract

The overall goal of the project was to provide evidence to support the regulation of smoke toxicity in order to reduce death and injury in unwanted fires. This entailed the development of a robust methodology for assessing smoke toxicity on a laboratory bench-scale using the steady state tube furnace (SSTF), ISO/TS 19700, and relating it to the toxicity of large-scale fire tests. A review of the literature relating to bench- and large-scale fire toxicity assessment has been undertaken and is reported.
Research was conducted on a bench-scale to optimise the methodologies developed and assess the current techniques used in smoke toxicity research. In addition, the formation of the main asphyxiants, carbon monoxide (CO) and hydrogen cyanide (HCN), was investigated under different fire conditions. In most cases, where nitrogen was present in the fuel, the formation of HCN mirrored the formation of CO. HCN and CO formation were found to be steady and relatively consistent starting approximately 5 to 7 min after sample ignition. This research was used to test the assumptions related to steady state burning and sampling times stated in ISO/TS 19700.

For smoke toxicity to be regulated as a part of the Construction Product Regulations (CPR), a robust methodology for assessing smoke toxicity for large-scale fires is required as a “reference scenario”. As the current large-scale methods for construction products assess flammability, a revised methodology needed to be developed. In addition, the instrumentation and methodologies for assessing smoke toxicity on a large-scale required development and construction.

To measure the smoke toxicity on a large-scale, gas analysers suitable for operating at large-scale test facilities were required. As no such analysers are commercially available, portable analysers were designed and built. The analysers continuously monitor CO, CO2 and O2, with specific sampling of HCN and irritant gases produced during a fire test. The specific sampling was controlled by a mass-flow meter to ensure that equal masses of fire effluent were collected in each sample, and used program-controlled switches for sample collection. To validate the analyser, it was tested alongside the standard SSTF analysers, and used when conducting the research into HCN formation described above.

To identify the fire condition of the test in terms of the equivalence ratio, a phi meter was designed and built for this research, based on modifications to the original design. It was smaller and simpler than the original design, increasing portability and performance. The final apparatus was tested and calibrated using the SSTF where the equivalence ratio is controllable and well-defined. The phi meter was used to investigate the effect of sampling location within the SSTF by studying the equivalence ratio at specific locations inside the apparatus. No significant variation of the equivalence ratio with sampling location was found. The phi meter was successfully used to identify the equivalence ratio during the large-scale fire tests.

The ISO 9705 room corner test was modified to assess smoke toxicity. The novel methodology used either 1 or 2 L-shaped Single Burning Item (SBI) (EN 13823) test rigs placed on a load cell in the centre of the ISO test room. The measurements specified in the ISO 9705 standard from the exhaust duct were recorded throughout the tests. Fire effluent composition was also monitored using the portable gas analysers in the exhaust duct and the doorway of the test room. To enable future gas yield calculations to be made, McCaffrey probes were used with sensitive pressure transducers to estimate the gas flows in and out of the room. The tests aimed to represent a range of fire conditions, from well-ventilated to under-ventilated flaming. Two methods were investigated to replicate different fire conditions: limiting the ventilation; and increasing fuel loading.

Four products, which included non-homogenous and predominantly non-combustible components (plasterboard, OSB, flexible polyurethane foam and electric cables), were burned in the large-scale tests. Under-ventilated flaming occurred in tests with combustible products conducted using two SBI rigs, where well-ventilated flaming had predominated with single SBI rigs. Under-ventilated flaming was not achieved when restricting the ventilation by partially blocking the doorway. These experiments showed that restriction of the ventilation reduced the rate of burning rather than forcing the fire to transition into under-ventilated flaming. This is clearly dependant on the ratio of the heat release from the fuel to the size and heat capacity of the test enclosure.

The fire behaviour of the materials was predicted before testing using ConeTools fire modelling software, using test data from cone calorimetry. As ConeTools had not been written for the novel test layout used, the data was used to create heat release predictions for an SBI test and an ISO room test conducted with the product as a standard wall-lining. ConeTools overestimated the heat release predictions compared to previously reported SBI test data. When used to predict the heat release from products in this study, they were underestimated.

This research has provided key information and methodologies to support the regulation of smoke toxicity within the CPR. It has provided the revised methodologies which would be necessary for ISO/TS 19700 to become a full standard and provided robust research to reinforce existing methodologies. The methodology of testing smoke toxicity on a large-scale has also been enhanced, including details of specific equipment required to assess specific parameters during a large-scale test.