Calibration error Calibration error in the OCM is
one of the most common sources of error. Inability to get a zero reading
with clean water renders all subsequent readings suspect especially
since this is a single-point calibration. Inability to zero is usually
due to rouge or other material (biological film) coating the scatter
and transmit sensors and other parts of the internals. A regular inspection,
cleaning and maintenance programme of the OCM should be performed
according to manufacturers instructions.
Air bubbles The presence of air bubbles
can cause an OCM to yield non-zero readings when zeroed. Air bubbles
interfere with the transmission and detection of the light source(s)
and are perceived as turbidity by the OCM. Also, control of the effluent
entering temperature is important to the health of the OCM. Too high
an inlet temperature can damage the photo cells and render it unable
to zero in the presence of fresh water. (Rule of thumb: over heating
too high a sample temperature can damage the photo cells.
Temperatures over 60°C at the cells require a sample cooler be
installed.)
Detection of non-oily substances
Although it is not supposed to occur, it is well known that materials
which are not oil are sometimes detected by the OCM. The most common
are:
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▸ Fine particulate matter Usually soot as
a result of contamination from cleaning operations or iron and iron
compounds as a result of biological contamination. Biological contamination
is usually accompanied by a foul odour of the bilge water. If outgassing
occurs after the addition of citric acid to the bilge sample, this
is confirmation of the presence of iron. If no outgassing occurs,
add two drops of sulphuric acid. If no visible sheen develops and
black material is still suspended in the water, this is a confirmation
of soot. See Appendix II for test procedures. See annex 4 to test
for soot.
▸ Non-oil organic compound Soaps and solvents
together or alone will form droplets in water (emulsified droplets
approximately 0.1-1.0 millimetre in diameter). These droplets will
scatter light just as emulsified oils do, and will be detected by
the OCM. See Appendix II for test procedures. See annex 2 to test
for detergents and solvents; also refer to annex 3, Emulsions.
Detection of non-oily substances can result in occasional
false positives (high readings) or an inability to get reproducible
readings. This is not to say that the 60(33) or 107(49) meters are somehow defective.
A 60(33) is unable to detect clear
emulsions of any type and therefore may yield false negatives (low
readings). In that sense, a 60(33) is
imperfect by design in that clear emulsified oily wastes pass through
the OCM undetected. All instruments are able to detect certain materials,
and are unable to detect and/or will falsely detect others. Technology
commonly used in shipboard oil content meters is the best current
solution when one takes into account cost and potential problems with
other detection instruments. The 107(49) light-scattering
OCM is more sensitive by design than the older 60(33) units. It detects both turbid and clear oily emulsions
while the 60(33) units do not detect
clear emulsions, allowing these emulsions to pass. Newer 107(49) light-scattering unit OCM also
distinguish some types of particulates (e.g., iron oxide compounds).
If one understands how the OCM operates and performs regular maintenance
and calibration, it can be a reliable instrument. The instrument alone
cannot diagnose all the additions to the bilge which may have occurred
on a ship. It is important for proper OCM operation to prevent exposure
of the OCM to these confounding factors. Refer to annexes 2 to 6 to
diagnose and troubleshoot these problems.
There are many other sources of OCM malfunction or problems
that may occur, none of which are related to contaminants or the OCM.
These include, but are not limited to, the following items. Annexes
5 and 6 provide troubleshooting guidance for the same. The operator
should also refer to the manufacturers operating and maintenance
manual for diagnostic and troubleshooting guidance.
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▸ Oil has unusually high specific gravity and/or viscosity,
and/or the purge cycle of OWS is inoperable:
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Faulty/disabled capacitance probes.
Inoperable solenoid valves (purge).
Inoperable solenoid valves (pressurize for purge cycle).
Leaking isolation valves to allow pressurization.
Clogged purge piping viscous oil.
Ineffective heating of upper chamber.
Disabled circuitry or probes.
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▸ Too much air in system:
-
Leaking or holed piping on vacuum systems.
Inadequate air purging from the system.
Inadequate check valves in suction piping.
Air may be from the OWS unit itself due to agitation
of the effluent in conjunction with a loss of suction (i.e. pressure
drop).
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▸Sludge build-up in OWS:
-
Inadequate maintenance of unit.
System overwhelmed with too much bilge water.
Too much oil admitted to machine.
System operated beyond design capacity.
Failure to remove adequately surface oil prior to entering
machine.
Ingress of high specific gravity oil into the bilge.
Excess sludge in bilge holding tanks.
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▸ Corrosion or clogging of separator plates:
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Too much sludge accumulation (non-buoyant material).
Clogged sludge drain valves.
Too much solid debris in bilge.
Too much solid debris in rose boxes.
Internal corrosion of piping.
Failed strainers (excessive porosity) in bilge well and
inlet piping strainers.
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▸ Bypass of OWS:
-
Internal bypassing of unit.
Corroded
components.
Failed internal gasket(s).
Improper reassembly of unit.
Short circuiting of fluid flow.
-
▸ Flow rate too high:
-
▸ Pseudo-stable emulsion formed by shearing of pump
and transfer operations. See annex 3:
In many cases the causes above may be remedied by better management
of the bilge, including removal of foreign material and sludge, and/or
repair, reconfiguration and/or replacement of drains, strainers, piping
and pumps as indicated. The above may also indicate that either or
both the OCM and the OWS require cleaning, inspection and servicing
back to manufacturers specifications. More residence time may
be needed or the OWS should possibly be recirculated at a lower flow
rate. Refer to annexes 5 and 6 for specifics.