An anti-syphoning valve is designed to prevent the fuel in an above ground storage tank being lost as a result of syphoning action in the event of the leak in the pump suction line or from the pump itself.
The anti-syphoning valve remains closed, preventing any fuel passing along the suction line, until the pump is activated creating a suction which, in turn, opens the valve. When the pumping stops the anti syphoning valve closes automatically.
The anti-syphoning valve can be fitted in any orientation but must be installed at the lowest point in the suction line as any leak occurring at a lower level than the valve could result in its opening due to a syphoning action.
The anti-syphoning valve will operate consistently for tank fuel levels ranging from 0 to 30 metres (100 feet) above the level of the pump with very little change in the amount of pumping effort required throughout this range. This means that it would be within the capability of a relatively small pump to draw fuel at a consistent flow rate from a large storage tank regardless of the fuel level.
A cheaper alternative to the anti-syphoning valve is to install a check valve, fitted with a spring, to the tank outlet. The pressure rating of the spring must be matched from ground level to the top of the tank. Example, a maximum height of 3.6 metres (12 feet) a 6psi (0.4 bar) spring is required to effectively hold back the fuel in the event of a leak from the pump or suction line. When the tank is at or near full capacity this anti syphon method works well because very little effort is required by the pump to overcome the spring loaded valve due to the “assisting” pressure created by the height of the fuel level.
The disadvantage of this anti-syphon method is that, as the tank fuel level drops, the pump has to work harder to draw fuel up through the suction line inside the tank & then through the spring loaded check valve. This can result in lower flow rates & more “wear & tear” on the pump itself leading to a shorter lifespan.