Sketch description (layout):

At the top of the machinery space a foam generator is mounted on each of the two levels of the space, connected to a header pipe from a foam-making compound (foam concentrate) tank. Elements of the diagram: expansion/foam-generating devices (fans with a gauze/mesh and spray nozzles) fed by a pump; a dedicated foam concentrate storage tank (a tank of continuous volume or other approved supply); separate supply piping for foam solution or for a mixture of foam concentrate and seawater; control valves and non-return valves; a pressure gauge; and remote control valves at the control room. Typical pipework: water line from the fire main (or a dedicated seawater pump) goes to a proportioner/blender where foam concentrate is metered into the water stream to make the foam solution; the solution then flows to the foam generator which blows air through (a fan) so the solution is beaten into high-expansion foam (expansion ratio typically 300 to 1000:1 rather than the low-expansion which is ~10:1). The foam exits into the machinery space through a discharge opening/screen. Controls: a remote starting pushbutton and a local manual start, with the discharge flood valve operated from the control station. An alarm and the indication/light arrange at the control room.

On a fire being detected in the machinery space (e.g. by smoke/heat detectors or a manual alarm), the fire door is secured and ventilation closed. From the control room, the operator (or an automatic arrangement) starts the foam system: the foam solution pump starts, drawing water (seawater) from the sea through a sea suction; the pump discharges through a proportioner which injects foam concentrate in the correct ratio. The mixture (foam solution) passes to the foam generators where the fan rotates, drawing air in and creating a foam of very high expansion ratio (300:1 to 1000:1) by beating the solution with the air through a metal gauze or net. The foam is discharged into the space above the machinery (e.g. through openings near the top of the space) and builds up, flowing downward, filling the space. Because the foam has a high expansion ratio it rapidly fills the void (including under gratings and behind equipment) displacing air and cutting off the oxygen supply to the fire, smothering it, and also providing some cooling. The system carries on until the space is fully flooded with foam. Personnel must evacuate; the foam will make escape difficult and may restrict visibility, so the system is only operated when all personnel have left. The operating instructions are displayed, and the system is serviced/test-discharge regularly.

The foam concentrate (a liquid, generally protein, fluoroprotein, AFFF or synthetic) is mixed with seawater in a proportioner (inductor or ratio controller) in the correct ratio (e.g. 1-10% or 3/6 %) to form a foam solution. The foam solution is then forced at pressure through the generator where air is entrained by a fan and the solution is aerated/beaten as it passes through a fine mesh/gauze. This mechanical aeration - air bubbles mechanically entrapped in the low-surface-tension solution - produces a high-expansion, air-rich foam. The expansion ratio and the foam's stability depend on the type and concentration of the foaming agent, the quality of the water, and the design of the generator (the expansion device). Unlike chemical foam (foam made by the reaction of an acid and an alkali, generating CO2 bubbles), mechanical foam relies on the physical mixing of air and solution.

• Daily/weekly: check the foam concentrate stock level and specific gravity; ensure the tank is full and the sample is within specification (concentrate can deteriorate).
• Check the operation of the generator fan, the starter, the remote controls and the alarm.
• Inspect valves, filters and non-return valves; ensure no blockage of the discharge opening and air intake screen.
• Monthly/periodic: run the pump on water at test discharge to confirm correct pressure and flow (operational test); check the proportioner is correctly set and the ratio.
• 6-monthly/annually: sample the foam concentrate and have it tested by the manufacturer (it may degrade); replace the concentrate if out of specification; carry out a functional discharge/expansion test of a generator on the test line; drain check screens; and carry out cylinder/proportioner servicing.
• After any use or contamination, flush the system with fresh water to prevent blockage.
• All tests are recorded in the log and the system is kept in line with the manufacturer's and SOLAS/FSS requirements.