Bulk carrier owners can choose from two compliant technologies to detect gas levels in the hold.
To establish the levels of gas present in a coal hold, crews on board bulk carriers commonly use one of two types of gas detector: catalytic bead detectors or infrared (IR) detectors.
Catalytic detectors are often chosen on dry cargo vessels because they are generally cheaper and simpler to use, while fully meeting mandatory carriage requirements. However, when carrying coal, catalytic detectors can give misleading readings if the oxygen level in the cargo hold falls too low.
During a voyage, crews carrying coal are required to monitor the atmosphere in the cargo spaces. The key gas readings usually recorded cover Oxygen (O2 – as a percentage), Carbon Monoxide (CO – as parts per million), and Methane (as a percentage of the LEL – lower explosive limit).
The accuracy of catalytic detectors can reduce significantly when the oxygen level in the hold falls below 10%, particularly when measuring methane as a percentage of the LEL.
Catalytic sensors rely on a chemical reaction, or oxidation, at the catalytic bead. The heat generated by that reaction is used to calculate the concentration of flammable gas. If there is not enough oxygen present, the reaction may be incomplete or may not occur at all, meaning the LEL reading may be unreliable.
In contrast, IR detectors do not rely on this chemical reaction and are generally therefore considered better suited to low-oxygen atmospheres.
The dilution solution
Where a catalytic detector is being used in a low-oxygen cargo hold, a dilution tube can be used to introduce additional oxygen into the sample before it reaches the sensor. In simple terms, the dilution tube draws in ambient air from outside the hold and mixes it with the hold sample, helping the catalytic sensor to operate correctly.
However, once a dilution tube is used, the detector is no longer measuring the exact atmosphere in the hold. The reading has been diluted, so it must be interpreted with care. Factors such as hold pressure, sampling arrangements and equipment condition can also affect the result.
To estimate the actual LEL, crews should calculate the dilution percentage using the oxygen readings. As a practical approach, they should:
- Take a measurement from within the hold without the dilution tube and record the oxygen reading.
- Take a second measurement from within the hold with the dilution tube fitted and record both the oxygen and methane LEL readings.
The following equation can then be used to derive an estimate of the actual LEL:
Actual LEL concentration = LEL reading with splitter x [(20.9 – oxygen reading without splitter) / (20.9 – oxygen reading with splitter)]
Precautionary practice
Before using this approach, crews should confirm that the gas detector is calibrated for methane. Other gases may require a different calculation or correction factor.
The safest option is to use a dilution tube supplied by the gas detector manufacturer. It should always be used in accordance with the manufacturer’s instructions.
If a manufacturer’s dilution tube is not available, a temporary arrangement may be made using plastic tubing and a Y-piece splitter. In this case, one hose should remain outside the hold sampling point so that ambient air can be drawn into the sample.
Crews should always follow the requirements of the IMSBC Code. Expert advice should be sought if the carbon monoxide level in any cargo space reaches 50 ppm or shows a steady rise over three consecutive days, or if methane levels give cause for concern.
If the use of a dilution tube is being considered, it is also recommended that expert advice is obtained at an early stage, particularly where readings are unusual, inconsistent or difficult to interpret.