H2Tech - Q1 2021 - 35

SAFETY AND SUSTAINABILITY

Experimental record of H2 ignitions.
Recent papers and presentations by the
UK Health and Safety Executive or affiliated agencies9 have explored possible H2
ignition mechanisms experimentally and
through literature review. In some cases,
the conclusions are not clear. Hooker et al.5
evaluated ignitions in a cavity downstream
of a bursting rupture disk; their data is
summarized in FIG. 3. There is no obvious
trend in this data, suggesting that factors
other than source pressure are involved.
Swain et al.10 experimentally investigated the ignition of H2 releases. The results
from this study suggested that the effects of
increased pressure on increasing ignition
probability (as relates to static discharge
potential or increased overall flowrate)
may not be as great as one might expect:
* Releases at Mach 0.1 velocity
ignited farther away from the
source than releases of the same

nominal flowrate (using a smallerdiameter orifice) at Mach 0.2
* The concentration of H2 at the
farther distance where ignition took
place was approximately 7 vol% at
Mach 0.1, but 10 vol% at Mach 0.2.
Since it is unlikely that there will be
'strong' ignition sources, such as a fired
heater in an operating plant at the distances used in the study (approximately
5 ft), this may suggest that the dominant
mode of H2 jet ignitions near the source
may be static. It may also indicate that the
probability of delayed ignition (and not
just the probability of explosion) may be
related to the degree of congestion/confinement in the surrounding plant.
Dryer et al.4 performed a series of experiments with H2 in which the presence
of obstructions or confinement at the
point of release was found to influence
the odds of ignition. The ignition phenomenon was observed only for release

pressures greater than 200 psig. The requisite confinement was effective down
to a discharge piping length of approximately 1.5 in., below which ignition did
not occur. Ignition also disappeared at
lengths greater than approximately 40 in.,
which was attributed to combustion heat
removal by the piping.
Also of interest were releases into open
atmospheres that did not produce ignition at release pressures as high as 800
psig. However, the implications for release of high-pressure streams with low
ignition temperatures are significant-
that ignition may occur in releases taking
place in confined spaces (e.g., relief device
discharge piping or perhaps flange leaks),
but not into open spaces (e.g., a pinhole
leak in a process vessel).
Grune/Kuznetsov et al.11 provided experimental evidence supporting a shock
wave ignition mechanism, using a rupture
disk release apparatus. Their test data is

1-Heat from fuel-fired equipment
2-Electrical spark
3-Smoking material
4-Open flame
5-Hot object
6-Explosives/fireworks
7-Heat from natural source
8-Heat from other fire
9-Electrostatic discharge
10-High-temperature, spontaneous
11-Catalyst
12-Welding
13-Lightning
14-Runaway chemical reaction
15-Unknown

Frequency, %

20
15
10
5
0
0

7 8 9 10 11 12 13 14 15
Ignition source

FIG. 2. Hydrogen incident statistics, 1961-1977, distribution by ignition source.

Cavity pressure, bar

Factory Mutual6 analyzed reported
H2 fire and explosion events from 1961-
1977 for the U.S. government. TABLE 2
shows the findings.
The extent of any data bias is unknown.
One might expect that unignited events
are underreported relative to fire events,
and perhaps fire events are underreported
compared to explosions. The recorded ignition sources are identified in FIG. 2. The
" unknown " category is large. This could
simply be indicative of an incomplete reporting or lack of knowledge by the reporter; however, it could also reflect the presence of unfamiliar ignition mechanisms.
While most of this article describes
releases of gaseous H2, there is one record
of tests with liquid H2 by Witcofski.7 The
focus of Witcofski was on dispersions, not
ignitions; so ignition was not desired, and
one can assume that no obvious ignition
sources were allowed in the area. The author notes that no ignitions occurred during the tests of seven liquid H2 releases,
each of 5.7-m3 volume.7
Thomas et al.8 provided a review of
unconfined H2 vapor cloud explosions
(VCEs) that included incidents beyond
those discussed here. More recent incidents included those at the Silver Eagle
refinery and Muskingum River power
plant. H2 VCEs reported following the
publication of Thomas et al.8 include
incidents at an H2 fuel station in Norway (2019) and at the OneH2 facility in
North Carolina (2020).

100
90
80
70
60
50
40
30
20
10
0

Ignitions
Non-ignitions

100

150

200

250

350
300
Burst pressure, bar

400

450

500

550

600

650

FIG. 3. Plot of cavity pressure against burst pressure for cases of ignition and non-ignition.
H2Tech | Q1 2021 35

H2Tech - Q1 2021

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