1 In an ideal world, the simple solution for preventing
oil mist fires is to ensure no leaks occur in the first instance,
but the harsh reality is that oil mist fires do occur and the problem
needs to be addressed. One practical answer is to install an oil mist
detection system that will detect an oil mist before it can reach
levels where it saturates the atmosphere to such an extent that there
is a risk of fire.
2 It is generally accepted that oil mist can be
formed in one of two ways:
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.1 minute leaks in oil lines which, under pressure,
produce a very fine atomised spray; or
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.2 oil, when allowed to come into contact with
a hot surface, will boil off producing a fine vapour.
3 In the first instance, the danger occurs when
the particle size formed is between 3 to 10 microns and is allowed
to form a mist in the atmosphere. When oil vapour reaches the range
of flammability, the condition can be classed as truly hazardous and,
if no action is taken, a fire may result. The ignition temperature
for this type of oil mist can be extremely low depending on the type
of oil being atomised.
4 Oil mists generated by being boiled off can
produce particles between 3 to 10 microns. This mist is visible and
is known as a blue smoke. Temperature and area of surface contact
affect the rate of oil mist generation. At this stage, a temperature
as low as 150°C could result in ignition.
5 Sources of oil mist include pump seals, leaking
injectors; loose or incorrectly fitted pipe fittings, weld fractures
and poor maintenance of machinery.
6 Possible heat sources causing the ignition include
heat exchangers, exhaust pipes, turbocharger, electrical contacts,
static electricity, faulty wiring and high- and low-pressure turbines.
7 Types of detection systems:
8 Location of detectors and sampling lines
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8.1 In each case, the number of detectors or sampling
points to be used is dependent on the size and layout of the particular
application. For a single point application, the unit may be mounted
next to the application or connected via a sampling line. Multiple
sampling systems are fitted in a suitable location away from the application.
Sample lines are fed to a common manifold with a suitable control
unit to allow alternative samples to be taken from continuously flowing
sample streams, thus one unit can be utilised to monitor several points.
The oil mist may be drawn into the unit by its own built in fan or
by an independent blower.
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8.2 To determine suitable positions for mounting
detectors or for fitting sampling lines, a smoke test would be required
to verify air movements in relation to application. In general, air
will move towards ventilation extractors and turbo chargers, so any
detector or sampling line should be positioned as close as possible
to the machinery. Likewise, detectors or sampling lines should not
be sighted next to ventilation blowers as these will prevent mist
formation from being drawn into the unit. On installation, a smoke
test should be carried out with all engines, ventilation and machinery
fully operational to ensure that detectors/sampling lines are correctly
positioned.
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8.3 If detector units are to be located close
to the source of application, care should be taken to avoid locating
the unit in places where: vibration is excessive; extremes of temperature
may be experienced; it would be difficult for maintenance personnel
to gain access; high levels of humidity and water may occur; and there
is a risk of electromagnetic interference.
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8.4 Locating of any detector in an explosive atmosphere
should not be undertaken unless the unit is certified intrinsically
safe for the hazard area.
9 Setting alarm levels
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9.1 This may be determined by the requirements
of the end user. In general, there are likely to be two stages: first
an early warning that something is wrong; and then a secondary alarm
indicating a full alarm. These should be advisory and the monitor
should be able to define the areas where oil mist has been detected.
In certain circumstances, alarms may be used to shut down individual
sections of the plant if deemed necessary, but there should be an
overriding control.
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9.2 The alarm level set initially should take
into account the atmospheric condition when there is no problem, for
example there is always a small amount of mist generated within an
oil purifier room. The initial level set will change with each application,
but should not exceed concentrations of greater than 2 ppm atmospheric
oil content. Alarm levels will be indicated as the level rises to
certain preset percentages of this set value.
10 Test procedure
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10.1 As with most electronic equipment, units
should be fixed by the components used in the design stage and have
built-in calibration routines to correct slight deviations.
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10.2 The manufacturer should calibrate the detector
against a known oil mist measurement. No adjustment to the calibration
should be possible by the user. It is not satisfactory to set up a
system against an electronic procedure or a piece of filter glass.
If deemed necessary, a calibration certificate should be issued.
There is very little maintenance which can be carried out
on the units in relation to the electronic components. The main areas
which will require attention are in line filters either in the sample
line or within the unit itself. These will require either cleaning
or replacement depending on the type, and recommendations of the detector
manufacturer. Sensor faces will also require cleaning periodically
with approved cleaner to clean any oil film build up. Some detectors
may have built-in fault diagnostic circuitry which will give an indication
that cleaning is required or that filters need attention due to fall
off in flow.