The Man in the Middle — Some Accidents Caused by Simple Mistakes
| Pages | 11-13 |
| Date | 01 March 1982 |
| DOI | https://doi.org/10.1108/eb057237 |
| Published date | 01 March 1982 |
| Author | T.A. Kletz |
| Subject Matter | Economics,Information & knowledge management,Management science & operations |
The Man in the Middle
—Some Accidents Caused by Simple Mistakes
by T.A. Kletz
ICI Ltd, Petrochemicals Division, Wilton, UK
Introduction
This article describes some incidents which occurred
because someone made a simple mistake, such as forget-
ting to open a valve or pressing the wrong button. The
mistakes were not due to lack of training or ability—in all
the cases described the men knew what they should do and
were capable of doing it, but had a moment of aberration
or forgetfulness. The article discusses the probability of
such mistakes and the action that should be taken to pre-
vent them.
Accidents Caused by Simple Mistakes
1.
Forgetting
to open a
valve
Vervalin[1] has described an operation—starting a
coker—which was carried out correctly 6,000 times until
on one occasion a vent valve was left in the closed posi-
tion. As a result a column of air in a distillation column
was compressed by the rising level of oil and an explo-
sion occurred—a diesel engine effect—killing one man.
The operators were well aware of the importance of
opening the valve but unaccountably failed to do so.
The rising pressure was observed but was attributed to
the viscosity of the oil as the day was very cold. The
operators were somewhat rushed as the plant had been
handed back late by the maintenance organisation and
they were trying to get as much done as possible before
the end of their shift.
2.
Opening
the
wrong valve
Figure 1 shows three waste heat boilers which share a
common steam drum. No 2 unit had been shut down
while Nos 1 and 3 continued on line. An operator was
asked to close valves C2 and D2 but closed C2 and D3
instead. The valves were arranged so that C2 was near
D3.
On the first occasion that this occurred the waste
heat boiler was severely damaged. High temperature
alarms were then fitted to the boilers. On the second oc-
casion, these prevented serious damage but some tubes
still had to be changed. Two errors occurred in a total of
1,000 operations.
3.
Opening a vessel
before
releasing
the pressure
On a number of occasions operators have opened
filters,
autoclaves or other vessels using quick release
couplings before blowing off the pressure inside. For ex-
ample, a pressure filter was opened when the pressure
inside was 30psig and the operator was killed; he was an
experienced man and knew very well that the pressure
had to be blown off first, but forgot to do so. The inci-
dent occurred a few hours before the start of his annual
holiday[2].
In the second incident, plastic pellets were blown out
of a road tanker with compressed air. The driver opened
the manhole to confirm that the tanker was empty. He
did so before blowing off the pressure inside; he was
blown off the top of the tanker and killed.
4.
Pressing
the
wrong
button
Two men were fixing new blades to No 2 unit in a pipe
coating plant. A third man wanted to start up another
unit. By mistake he pressed the wrong button. No 2 unit
moved and one of the men was killed[3].
A similar incident caused an explosion[4,5]. Beverage
vending machines are also a simple example of equip-
ment on which errors are caused by pressing the wrong
button[4].
The users are well-trained, capable of using
the machines and well-motivated but still make occa-
sional mistakes.
5.
Passing
railway
signals
at
danger
The literature on railway accidents[6, 7] contains many
accounts of accidents which have occurred because the
drivers of railway trains passed signals at danger. If an
accident results the driver is the person most at risk and
many drivers have been killed in this way. Nevertheless,
incidents still occur[8, 9]. The probability is about
1
in
10,000 signals at danger approached.
Past Attitudes Towards Errors
Attitudes towards errors such as those described have
swung between two extremes.
At times managers and designers have assumed that a
man would always do what he was expected to do, provid-
ed he was well-trained and what he was asked to do was
within his physical and mental capabilities. If he forgot to
close a valve, or closed the wrong one, he ought to be
MARCH/APRIL 1982 11
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