The semi-dry deck water seal has a separate holding chamber which is connected by pipe to the inert gas main via an eductor. Instead of passing through a water trap, the inert gas flowing through the venturi creates a suction effect in the holding tank which draws the sealing water from the loop seal into the holding tank. This mitigates the undesirable ‘carry over’ of moisture to the cargo tanks.
When the gas flow ceases, the venturi effect is lost and the water from the holding tank rapidly flows back into the inert gas main loop seal thus creating a water seal which serves to prevent the backflow of hydrocarbon vapour and/or inert gas from the hazardous to non-hazardous area.
The following monitoring equipment options should be considered as having the potential to enhance the safety of operation of the deck seal:
- Deck Main High Pressure Shutdown
- Differential Pressure Alarm Shut Down
- Atmospheric Vent Valve,
- Power Operated Deck Main Isolating Valve.
- Deck Main Non-Return Valve With Disc Position Indicator.
- Deck Seal Water Supply Low Pressure (or Flow) Alarm
- Ultrasonic Type Level Sensors
Proposed measures to reduce the risk of backflow:
The risk of hydrocarbon vapour and/or inert gas backflow can be sIGnificantly reduced by ensuring that a proper and effective maintenance regime is adopted and followed on board the ship. Such a regime should allow for:
- Frequent physical and visual inspection of those pipelines which have potential for blockage or corrosion to ensure that water may freely flow through them and their integrity is not impaired due to corrosion. Whilst it is appreciated that this recommendation may involve removing sections of pipeline, it is suggested that this inconvenience is insIGnificant when compared to the potential for a serious incident should the deck seal fail to operate.
- Checking the diameter of the pipeline connecting the venturi to the holding tank with a view to increasing it to ensure the opportunity for blockage is reduced. Similarly the diameter of the pipeline from the holding tank to the seal loop in the inert gas main should be checked to ensure it is adequate to allow rapid transfer of water to the loop in the event of a cessation of inert gasflow.
- It is strongly recommended that any alterations to the diameter of these pipelines is made only after a proper technical investIGation or consultation with the manufacturer of the seal.
- The selection of materials used in critical pipelines to be such as to avoid blockage due to internal corrosion. Critical pipelines should also be installed in such a way as to facilitate removal for inspection. The installation of a trap at the connection between the seal loop and the pipeline to the holding tank should reduce the possibility of debris from the inert gas pipeline blocking the smaller diameter pipe.