Medium speed engines. Air receivers and air bottles
The term ‘bottle’ is often used to cover both types of structure. Both have to be manufactured to specific requirements. On board maintenance amounts to regular surveys of the inside and mountings. The latter will include, at least, relief valve(s), pressure gauge connection, inlet and outlet valves, manhole, or inspection doors and drains. The drain should be situated at the lowest point of the bottle in such a way that accumulations of oil, water and solids can be blown out.
The water forms as the hot, compressed air cools down to below the relevant dew point. The ambient conditions of humidity and temperature will affect the amount of condensate so formed. In any case, the area around the drain should be inspected whenever possible to check on any corrosion that is taking place in that area. Sonic testing can be used to determine the residual thickness if corrosion has occurred. Cleaning and revarnishing (non-spirit based) of the affected areas is essential if further corrosion is to be avoided. The usual inspection of the other mountings should also be carried out during the survey period. Remember to treat the receivers as an enclosed space whenever entry is required and take the relevant precautions, particularly if cleansing fluids are being used or painting is being carried out.
Fusible plugs are fitted if the receiver is isolable from a relief valve. There will always be a relief valve on the high pressure side of the compressor so that when the compressor is being used the bottle is protected. However, this means that the receiver is only protected when the compressor is running. A fusible plug therefore offers protection against pressure development in the event of an engine room fire. The plug (lead, bismuth and antimony) softens as its temperature rises and extrudes from its fir tree type socket. The release of such large quantities of air would impair the effectiveness of any CO2 fire fighting gas unless extra gas is provided in compensation or the air is piped out of the engine room, say to the fidley.