Bilge Alarm Readings

The test procedures for the 15ppm Bilge Alarm are described in MEPC 49 Test Specifications. Basically, the accuracy of the Bilge Alarm is determined by comparing its readings against a known flow of Test Fluid injected into a known flow of water. The grab samples are analyzed in a laboratory to Standard ISO9377-2:2000, using solvent extraction and gas chromatography for the determination of the hydrocarbon oil index in water.

The methodology is clear and the oil content for the test effluent is known. Therefore, in the approval process, the Bilge Alarm is demonstrated under controlled, predictable and repeatable conditions. To get the Bilge Alarm approved, the instrument needs to correctly display the optical effect created by the known oily water mixture, as the equivalent parts per million of oil content. In other words, the instrument is calibrated for this purpose before the approval test, then shown to read oil content correctly.

As we all know, bilge water aboard a ship, or more accurately the effluent from the Bilge Separator is not the same mixture of oil and water as the one in the approval process. In the approval process the IMO Test Fluid "C" is used, which is a 1kg mixture of water, containing 25g of heavy fuel RMG 35, plus 25g of light fuel DMA , plus 0.5g surfactant and 1.7g of iron oxides. By comparison,the effluent coming out of the Bilge Separator contains typically more than 2 types of oils as it contains heavy fuel, light fuel lube oils as well as other oils; it contains probably more than 0.5g of surfactants as cleaners and detergents are present, from the cleaning of surfaces and as part of modern lube oils as well; solids content of the effluent most likely is quite different too. Then of course there is the color of the bilge water, also different than the test liquid. All this to say that the Bilge Alarm does not compare apples with apples; it is tested with "apples" but then required to read "oranges" aboard a ship.

The Bilge Alarm is approved by IMO for on-line, continuous measuring, using an optical value as a reference for oil content. The measuring system is at best an approximation, rather than a scientific measurement of hydrocarbon content in water. Port State Control procedures confirm that compliant bilge water effluent is determined by a calibrated Bilge Alarm and not a laboratory analysis.

Because the Bilge Alarm is calibrated to a specific oily water mixture, there will be a difference between the actual hydrocarbon content of compliant real life effluent and the test liquid. How big the difference will be is dependent on the complexity of the bilge water the Bilge Separator has to treat.