Monitoring Oil In Water

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

Requirements for a monitor.

The oil pollution regulations put limitations on the quantity of oil discharged into the sea.

There can be a requirement to monitor the overboard discharge for oil from:‑

            A tanker ballast line as it discharges directly overboard

            A tanker ballast discharge after an oil/water separator 

            The bilge discharge from the machinery space of any ship.

General Specification Requirements.

Must be suitable for the marine environment  (accurate laboratory methods are well tried and tested but not all can be used on board ship).

Suitable for reading both high and low levels of contamination and to respond fast to sudden changes in those levels.

Effect due to the presence of sand, rust and other debris, should be minimal

Must operate irrespective of the type of oil (or at least be capable of calibration to allow for this factor.)

Must be easy to operate and maintain

Should be unaffected by considerable periods of idleness

Accurate to about ± 10%

Principles Of Measurement.

            Conductivity

            Capacitance

            Acoustic Attenuation

            Sonic Impedance

                                                           

            Infra Red Absorption

            Ultra Violet Absorption                     

            Visible Light Absorption

            Visible Light Scattering

            Ultra Violet Fluorescence

The first four are all poor with respect to sensitivity and would usually be used only to detect oil‑water interfaces (in an oil/water separator)

Infra Red absorption is potentially the most promising method as most oils absorb in the 3.4 μm wavelength. The variation in absorption rates between heavy oils through to the light diesels is only approximately 10%.

However water also has a strong absorption at the same wavelength and this makes detectors using this technique complex.

The latest view is, that it would be useful if the oil was extracted from the water with a suitable solvent, the solvent having no absorption of the infra red wavelength. However this would not allow a speedy response.

Ultra Violet absorption does not encounter the water absorption problem as it uses a wavelength of 0.254 μm but the requirement, for the opto‑electronics to detect small changes in a high light level, limits the low range capability.

Absorption devices using any wavelength, on a system where the oil is present in the form of particles, suffers from the effects of sand, rust etc. distorting the accuracy. This is significant and adds greatly to the problem of inaccuracies.

Devices using visible light are usually cheaper, simpler and are non‑specific with respect to oil types. However, they also detect, without distinguishing between, oil and non‑oil particles of similar diameter.

Of the two visible light techniques scattering is the most sensitive.

Ultra Violet fluorescence suffers from a wide variation in response to different types of oil.

Ballast Monitor.

A representative sample must be extracted. This is achieved by a strengthened intrusion pipe in the ballast line and the sample is then conveyed to the monitor by a pump.

After the measurement the sample can be discharged overboard or be put back into the ballast line.

To ensure a representative sample is obtained and to encourage good mixing, the sample point is usually in the middle of the ballast pipe, near to the discharge pump. Care must be taken to ensure the ballast line is always full so that no settling out occurs.

If the response time of the monitoring system is lengthy then considerable pollution can occur before the large discharge valves can be closed.

Additionally it is important that the operation of the valves should not be initiated by a spurious result caused by a small spike of oil exceeding the alarm level.

Generally the response of the monitor is instantaneous and most of the system response delay is in the sampling pipework.

To reduce the delay, short lengths of sample pipe with a minimum number of bends, utilizing a fast sample velocity are adopted.

This pipework often becomes clogged during periods of inactivity and, when restarted, erroneous readings are obtained as oil, deposited during periods of idleness, strips off the pipework.

Development work on the original design indicated the means for the practical arrangement of an oil content meter which are still applicable to most instruments. As an oil content meter must necessarily contain hydrocarbons, which may appear in such concentrations that a hazard exists, and furthermore, as the measuring technique is likely to involve using electrical equipment in one form or another, there is always an intricate problem in isolating the electrical side from the hazardous area. The solution, adopted by Bailey in the original instrument, entailed mounting the analyzing equipment on the bulkhead between the pump­room and the safe spaces in such a way that the optical sensors can work through lenses in the gas-tight barrier, is common to all instruments.

They generally also require a pump for the sample water, which is normally mounted to the pump-room bulkhead and driven by an electric motor on the engine-room side.

The Classification Societies' rules concerning the safety of such installations may eventually become part of the IMO guidelines. Subject to certain conditions, these installation rules also permit the analyzing equipment to be placed in a gas-tight casing mounted in a safe area such as the cargo control room. The casing must be purge-ventilated to the outside air, and certain other safety precautions must be observed.

Most monitors depend on an optical technique and this leads to problems with the sealing and cleaning of the optical windows. A fast sample flow rate helps in keeping the windows clean.

Bilge Monitor

The installation and operational problems with a bilge monitor are less than those for the ballast monitor.

The bilge monitor must provide a simple alarm after the separator/coalescer at 15 p.p.m.

With the bilge system the type of oil can vary from fuel oils to lubricating oils hence the monitor should not be specific to oil type.

Turbidity meters

(Scattered Light Detector)

If an oil/water mixture with a low oil content is heavily agitated so that the oil droplets become very small, the water will turn 'milky' to varying degrees, depending on the amount of oil present; the actual colour of the oil droplets is of no importance.

This method can be used for indicating the oil content, provided the conditions for homoge­nizing the sample are well controlled. If a light beam is projected through a test cell containing sample water with well-homogenized droplets, part of the light passing through the cell will be scattered. The intensity of light picked up by a photocell at the end of a straight path through the cell will be reduced, whereas the intensity of scattered light sensed by a photocell mounted at an angle to the original path will increase.

Laser light may be used to obtain a well defined light beam and possibly a selective light-scattering effect.

This principle is used in one oil content meter currently available, in which the light beam and the signals picked up by the photocells are transmitted via optical fibres to the electronic part in the engine-room, thereby making the bulkhead penetrations quite small.

A similar instrument, operating with infra-red light, is also available.

A third instrument, based on the turbidity principle, operates only on the direct trans­mitted light through the test cell in which the heavily agitated sample water circulates.

Instruments of this type can measure oils ranging from heavy crude oils to gasoline, but some changes in the calibration are required to cover the extremes of the range. As the instruments measure the number of particles in the water, they are rather sensitive to other contaminants such as rust or air bubbles.