H2Tech - Q1 2021 - 34

SAFETY AND SUSTAINABILITY
flange-the corona discharge was
visible, which increased when the
pipe was tapped to stir up dust.
An ignition followed after the
tapping. Further work showed that
when sharpened copper wires were
used to promote corona discharges,
ignition occurred when the point
was bent away from the gas direction,
whereas no ignition occurred
when the wire was pointing in the
direction of flow. Consequently,
it is apparent that a corona
discharge was likely to have been
the source of ignition in this case.3

In addition, the following specific passages from Gummer and Hawksworth3
suggest that a static discharge mechanism
may be involved in some ignitions:
The 1922 incident investigation
After several spontaneous
ignitions of hydrogen at 2.1 MPa
being discharged to atmosphere had
been reported, work was undertaken
to determine the cause. Various
experiments were undertaken on
discharging hydrogen to atmosphere,
but no ignitions occurred despite
discharging through many different
types of nozzle made from differing
materials. However, cylinders had
been noted for having quantities
of iron oxide (rust) in them [...]
Therefore, mixtures of hydrogen
and oxygen were stored at an initial
pressure of 1.1 MPa at various
temperatures in the presence of iron
oxide to determine whether the
oxide catalysed the reaction [...] at
temperatures above ambient, the
pressure slowly fell, indicating that
the oxidation reaction was occurring.
The times were about 24 hours at
100°C, nine hours at 200°C, and
one hour at 380°C. There was no
explosion at any time.
Subsequent experiments on
discharging hydrogen into an open
funnel fitted with a long pipe showed
no ignitions except when the funnel
was obstructed by an iron cap [...]
when the hydrogen leaked out of a

The 1926 and 1930 incidents
and experiments
The second explosion occurred
when the isolation valve between
a pressurised pipeline and a
chromium plated vessel was opened
to depressurise the line from about
4.9 MPa. The explosion occurred
immediately [...] It was noticed that
there was ample evidence of fine
dust, presumably metal oxide, being
present in the pipe-work during the
examination after the explosion.
This led Fenning and Cotton to
surmise that the explosion had been
initiated by an electrostatic discharge,
presumed to have been generated
by the fine dust being blown along
the pipe by the high-velocity
hydrogen. However, despite
many attempts, no ignition was
achieved in their experiments.3

TABLE 1. Responses on likelihood of ignition
Ignition
Respondent Respondent Respondent Respondent Respondent Respondent
probability
1
2
3
4
5
6
Immediate
Ignition

0

0.1

0.1

0

0.1

0.1

Delayed
Ignition

1

0.75

0.01

1

0.9

0.01

TABLE 2. Distribution and type of hydrogen incident
Type of incident

Number of incidents

Fraction of total

Fire

74

0.264

Explosion

165

0.589

Pressure rupture

12

0.043

Unignited release

20

0.071

Fire and explosion

3

0.011

Other

6

0.021

Total

280

34 Q1 2021 | H2-Tech.com

Incidents reported by Bond
Bond reports two incidents,
sourced from a private conversation,
where hydrogen ignited. In the first
incident, hydrogen at a pressure of
11.1 MPa was leaking from a gasket
between two flanges. The hydrogen
had not ignited at the time when the
fitters arrived to tighten the bolts.
It was reported that on the second
strike of the hammer wrench that was
being used to tighten the bolts, there
was an ignition. It is not apparent
whether the ignition source was an
impact spark from a hammer wrench
being used to tighten the bolts on the
joint, or attributed to the mechanism
of a diffusion ignition. The second
incident refers to a cylinder of
hydrogen being connected to a
piece of laboratory apparatus. The
laboratory technician cracked the
valve open to clear any dirt out of
the connection, and when he did so,
the escaping gas ignited immediately.
Bond attributes this ignition to the
phenomenon of diffusion ignition.3
Jackass Flats Incident, 1964
This incident [...] involved the
deliberate release of a large quantity
of hydrogen to determine the sound
pressure levels. The hydrogen was
released from storage at an initial
pressure of 23.6 MPa and an initial
rate of 54.4 kg s-1, for a period of 10
seconds [...] In the run where the
gas was not deliberately ignited,
after 10 seconds, the 150 mm
diameter valve was being closed, and
three seconds after starting to close
the valve, ignition occurred.3
Gummer and Hawksworth3 summarized the work through 2007 by saying
that the proposed mechanisms do not
account for all the reported ignitions (or
non-ignitions) of H2 ; however, particulates and/or static appear to be a factor
in several of the reported cases. They
cite specific incidents in which ignitions
occurred in an obstructed discharge but
not in unobstructed discharges. This is
similar to what was observed by Dryer et
al.4 and Hooker et al.,5 and was attributed
to the relative presence or absence of the
formation of turbulence near the point of
release and may be symptomatic of the
adiabatic compression mechanism.
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