Promoted ignition testing : an investigation of sample geometry and data analysis techniques

Abstract

Metallic materials and oxygen can be a volatile combination when accompanied by

ignition mechanisms. Once ignited, metallic materials can readily burn in high pressure

oxygen atmospheres, releasing an enormous amount of energy and potentially

destroying equipment, space missions and resulting in the loss of life. The potential

losses associated with these fires led to research into the conditions under which metal

fires propagate. Several organisations, including the American Society for Testing

and Materials (ASTM) and the International Organisation for Standardisation (ISO),

have published recommended standard test practices with which to assess the relative

flammability of metallic materials. These promoted ignition tests, so called because

samples are ignited with an overwhelming source of energy, are typically used to examine

two important parameters as an indication of a metallic material's flammability:

Threshold Pressure (TP) and the Regression Rate of the Melting Interface (RRMI). A

material's TP is the minimum pressure at which it burns, therefore, TPs of different

materials can be compared to assess which materials are most suited for a range of

high pressure applications. The RRMI is a useful measure for ranking materials, particularly

if they have the same TP, but can be used as a ranking method irrespective of

TP. In addition, it is a crucial parameter to aid in understanding the complex burning

process and is one of the few experimental parameters that can be measured.

Promoted ignition test standards specify a standard sample geometry to use when

performing the test, typically a 3.2 mm diameter cylindrical rod. The recent addition

of a 3.2 × 3.2 mm square rod as an optional standard sample geometry raises

the issue of how the geometry of a sample affects its flammability. Promoted ignition

test results for standard geometries are often applied to assess the flammability

risk for the complex geometries of real components within oxygen systems, including

regulators, valves, piping etc. Literature shows that sample geometry has a significant

effect on material rankings when rankings are based on testing of standard geometries,

for example, cylindrical rods, compared to non-standard geometries, for example, sintered

filters and meshes. In addition, the RRMI has been shown to be dependent on a sample's cross-sectional area (XA). However, it remains unclear, from a simple heat

transfer analysis, why the RRMI is dependent on XA or how the shape of a sample

affects its melting rate. These questions are particularly relevant since understanding

how sample geometry affects burning contributes to two important research goals: to

be able to accurately model and predict the flammability risk of a metallic component

without the need for physical testing, and to understand the effects of different sample

geometries on their relative flammabilities within the standard tests used.

Promoted ignition tests were conducted on iron rods with cylindrical, rectangular

and triangular cross sections for a range of XAs. Their RRMIs were measured and

analysed using a statistical approach which allowed differences in RRMI to be quantitatively

assessed. Statistically significant differences in RRMI were measured for rods

with the same XA but of different shape. Furthermore, the magnitude of the difference

was dependent on XA. Triangular rods had the fastest RRMIs, followed by rectangular

rods and then cylindrical rods. Differences in RRMI based on rod shape are due to heat

transfer effects and the dynamic motion of the attached molten mass during the drop

cycle. The corners of the rectangular and triangular rods melt faster due to their locally

higher Surface Area to Volume ratio (SA/V). This dynamic effect increases the area of

contact between the molten mass and the solid rod (solid liquid interface (SLI)) which

facilitates increased heat transfer to the rod resulting in a faster RRMI. This finding

highlights the importance of the SLI in the heat transfer process. Although the SLI is

largely dependent on the XA, the shape of the rod causes subtle changes to the size of

the SLI and thus affects heat transfer, burning and observed RRMI.

The relationship between rod diameter, test pressure and Extent of Reaction (ER),

the proportion of metal that reacts (oxidises) whilst attached to the burning rod, was

investigated. During promoted ignition testing of iron rods of varying diameter the

detached drops were rapidly quenched by immersion in a water bath. Microanalysis

techniques were used to qualitatively assess the ER as a function of pressure and

rod diameter. It was found that the pressure dramatically affects ER. High pressure

tests resulted in a slag mass consisting of oxide, with no unreacted iron, whereas low

pressure tests resulted in a significant fraction of unreacted iron within the slag. This

indicates that the ER contributes directly to the observed increase in RRMI with increasing

test pressure. At high pressures the ER is not affected by rod diameter, since

all available liquid metal reacted, but at low pressures ER is a function of rod diameter,

ER decreases as XA increases.

This thesis also investigates the analysis of promoted ignition test data through suitable statistical methods. Logistic regression is identified as an appropriate method

for modelling binary burn/no-burn test data. The relationship between the reaction

probability, defined as the probability that a sample will undergo sustained burning,

and pressure, is evaluated for two different data sets. The fits of the logistic regression

models are assessed and found to model the available data well. The logistic regression

method is contrasted with the confidence levels associated with binary data based

on the Bernoulli distribution. It is concluded that a modelling approach is beneficial

in providing an overall understanding of the transition between pressures where no

burning occurs and pressures where burning is expected.