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Hypoxia
One of the most common causes of fatal accidents due to gas is when
the oxygen concentration drops.
It may, for instance occur when another gas
displaces the oxygen.
Man's own warning signals for lack of oxygen
are not reliable.
The symptoms are almost unnoticeable and the
victim does not react until it's too late.
Gagging symptoms include shortness of breath,
increased breathing rate, numbed responsiveness and impaired muscle
coordination.
Oxygen concentration in the air is normally
20.9%.
The table below shows what happens to the
oxygen concentration when another gas dilutes the air:
|
Concentration of
diluted gas
(%) |
Oxygen concentration
(%) |
|
0.5 |
20.8 |
| 1 |
20.7 |
| 5 |
19.9 |
| 10 |
19
(flashlight) |
| 15 |
18.2 |
| 20 |
17.4 (gas alarm sign) |
| 25 |
16.7 |
Symptoms of hypoxia are already shown when
the concentration falls below 17% O2.
If the O2 concentration drops another 2-3% this indicates mortal
danger.
Alarm limits for decreasing concentration of gas is set to
18-19%.
Other reasons why oxygen levels can fall are
among others:
-
Combustion of somthing which consumes
oxygen
-
Corrosion of something that reduces the
oxygen concentration
-
Oxygen consumption due to breathing in
dense areas.
Problems usually arise in confined spaces
such as tanks, tunnels, etc.
Oil- and gas industry
The normally closed spaces like tanks, containers, pontoons, etc..
corrosion may have contributed to making oxygen levels drop to low
values.
A worker entering such an area may die immediately.
Therefore, it is important to first check the oxygen concentration
in the compartment.
In some cases, other gases may be present,
both toxic and explosive. Therefore it may be necessaru to use a
portable instrument that measures several gases at the same time.
Before stepping into, for example, a tank
you should check the oxygen concentration in the tank with a
portable gas detector.
Underground
In tunnels, such as sewers and other underground facilities the
oxygen concentration may drop for for various reasons.
In addition, other, both explosive and toxic
gases may have been formed as a result of biological reactions.
For example, CH4 (methane), H2S (hydrogen
sulfide), CO (carbon monoxide).
An combustion engine underground may consume the oxygen and leave
carbon monoxide behind.
Leakage of different gases that are lighter or heavier than air, may
lead to accumulation in different cavities.
When someone enters into such a cavity
without first checking gas concentrations with an instrument it may
lead to immediate death.
Sensor placement for monitoring oxygen
In cases where you want to monitor oxygen concentration in a
room, it is important to bear in mind which gas that could displace
the oxygen.
Examples of gases that can displace oxygen,
thereby causing suffocation are the inert gases, such as Nitrogen
(N), argon, helium, carbon dioxide, used in various industrial
processes, hospitals, etc.
Important to mention here is that the fact
that the carbon dioxide in addition to displacing the oxygen is also
toxic itself.
If there is leakage of, for example, krypton,
which has a higher density than air and will sink, the oxygen
detector should be placed low.
If a gas with lower density than air, such as helium, should
leak, the oxygen sensor should be placed high.
If the oxygen in a room risks being consumed
by any form of combustion, the oxygen concentration will be reduced
across the room which means that the sensor placement here is not as
critical as in the above examples.
High concentrations of oxygen
If the oxygen concentration in the air increases, the risk of fire
dramatically increases. All combustible products will be much more
flammable.
Alarm limit is set at 23% O2.
Inhalation of high oxygen concentrations over time can be harmful.
How many detectors are needed in a stationary system?
This must be decided from case to case
depending on the local area that you want to monitor, but as a rule
of thumb you can use the following:
One detector per 100m2,
with additional detectors at locations where leakage is likely occur.
References:
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