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A valve installed in an aboveground tank system to prevent liquid from accidentally flowing out of the tank. An anti siphon valve is often needed where the liquid level in the tank is higher than the elevation of the dispenser or any of the product piping.
To understand how such a valve works, visualize a [fuel] tank located above a fueling station.
The force of gravity causesfuel in the tank constantly to try to flow through the pipe connecting it to the dispensing pump at the marina below. Some positive shut off device needs to be present in the piping to prevent this from happening, except during those periods when the operator specifically desires that product flow through the pipe.
The device used for this purpose is referred to as an anti siphon valve. Such a valve is positioned in the tank’s discharge pipe, downstream from the block valve that is located just outside the tank shell.
An anti siphon valve is designed to remain in a closed position, thus preventing the flow of liquid except when some positive action, such as the turning on of the pump, is taken. An anti siphon valve may operate on the solenoid principle. Others are spring-loaded check valves. Both are designed to open only when the pump is operating.
An alternate anti-siphon scheme operates not as a flow preventing valve, but rather as a siphon breaker. On systems where the piping connections come out of the top of an AST it consists of a small normally open solendoi valve which is installed in the highest point in the pipe run. It is de-energized (open) when the pump is not running. Loss of prime back into the tank is prevented by installing a check valve in the suction riser pipe coming up out of the tank. When the pump is turned on, the solenoid is energized, and it closes. Normal pump suction draws fuel up out of the tank, and down to the pump. When the pump is shut off the solenoid (siphon breaker) opens again. If there is a leak, or a valve in the pump or downstream of the pump is accidentally left open, or of a pump seal fails, fuel will flow out of the system, but with the siphon breaker introducing air into the piping as fuael flows out, only the amount of fuel held in the pipe will spill out. None will be drawin out of the tank by siphon, because the siphon breaker has introduced air into the suction line and broken the continuaou siphon effect.
Another type of anti-siphon valve for suction line a p[plication is called a Tank Safety Valve, and consists of a diaphragm globe valve with a spring pushing it closed. , It is opened by a double sided power diaphragm having one side connectged by small piot line to a fitting near the suction inlet of the pump, and the other side connected similarly to the high pressure discharge side of the pump. When the pump is operating the pressure differential causes sufficient force on the power diaphragm to move the valve diaphragm open ane permits flow.
In marina applications, mechanical anti siphon valves are not recommended.
See also Solenoid valve.