Hydraulic calculation for Storage tank protection with foam pourerThis hydraulic model represents a large storage tank with a fixed cone roof and is protected by three foam chambers. The foam chambers are located above the liquid level of the tank and the deflector is located inside the tank to distribute the foam solution over the surface.
The number of foam chamber is determined by the tank diameter for a fixed cone or open top tank and the flow rate can be calculated by multiplying the area by the required density.

The tank in this example has a diameter of 30m and therefore a surface area of 707m2. If we base the design density on 4.1 mm/min this will give us a minimum flow rate of 2899 lpm and will require a minimum of two foam chambers but to give a give faster foam distribution we have used three. Also the volume of foam from each pourer is also reduced which will allow for a smaller riser pipe to each of the foam chambers. For this design we have used 80mm Viking Model FC foam chambers and each chamber will protect 236m2 area and require a minimum of 699 lpm discharge. By using the manufactures design table we can determine that we will require a minimum pressure of 4.14 bar at the foam chamber inlet.

With the above information we can now proceed to start the hydraulic calculation for the systems but instead of using a sprinkle or head as the output devices we can specify in FHC the required flow rate and pressure which require for each foam chamber on our system in the optional items section in the Project Data. When we calculate the hydraulic model in FHC we find that we will require a source duty of 2122 lpm @ 5.525 bar.

You can find out more information about protection to storage tanks in NFPA 11: Standard for low, medium, and high-expansion foam.

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