Anti-siphon valves for fuel tanks: how they work and when you need one
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An anti-siphon valve is a normally closed valve in the outlet piping of an aboveground tank. It keeps fuel from flowing or siphoning out of the tank if a line breaks, and opens only when the pump runs (PEI). You generally need one whenever the fuel level in the tank sits higher than any part of the piping or the dispenser.
The fire code language many local fire departments use reads: "Approved antisiphon devices shall be installed in each external pipe connected to the tank when the pipe extends below the level of the top of the tank" (Central Arizona Fire and Medical Authority checklist, International Fire Code).
How a tank siphons, and how the valve stops it
If the fuel level in the tank is higher than the pipe or dispenser, gravity pushes fuel toward the low point. When a line breaks, it keeps coming until the tank drains down to the level of the break. PEI puts it simply: fuel in an elevated tank will try to flow out, so you need a positive shutoff except when you mean to pump (PEI).
This happens even when the piping leaves through the top of the tank. Once the line is full of fuel, it works like a siphon hose. A tank on a stand, a tank on higher ground than the dispenser, and piping that runs down a hill or to a dock are all common setups that do this.
The valve holds a spring-loaded poppet closed against that gravity pressure. When the pump starts, it overcomes the spring and fuel flows. When the pump stops, the spring closes the valve again (OPW). Most also have a built-in relief for pressure from fuel expanding in a sun-warmed line (Franklin Fueling Systems; Morrison Bros.).
When codes require one
Fire codes, NFPA 30A, and several state tank rules all require an anti-siphon valve where an aboveground tank creates gravity head on the piping. Your local fire marshal decides which code edition applies.
| Rule | What it says |
|---|---|
| International Fire Code (as used on local permit checklists) | Antisiphon devices are required in each external pipe connected to the tank when the pipe extends below the top of the tank (Central Arizona Fire; Freeport, TX Fire Marshal) |
| NFPA 30A (2021), Section 11.2 | Referenced by Florida as the standard for installing and maintaining anti-siphon valves (Florida DEP) |
| PEI RP200 (2019), Section 7 | Industry practice for installing aboveground tank systems, also referenced by Florida (Florida DEP) |
| PEI RP1000, Section 4.4 (marinas) | Where product piping is below the tank's liquid level, use a normally closed solenoid valve that opens and closes with the pump (NC DEQ) |
| New Mexico 20.5.109.902 | Tanks that create gravity head on dispensers or piping need an anti-siphon or solenoid valve, adjusted so fuel can't flow or siphon out if the piping fails (NMAC) |
Valve order matters. Florida's rule spells out the sequence: tank, then block (shutoff) valve, then anti-siphon valve (Florida DEP). PEI describes the same placement: downstream of the block valve, outside the tank shell (PEI).
Farm tanks. Nebraska's farm tank guidance, based on NFPA 395, says a pump discharge must have an effective anti-siphoning device or a self-closing nozzle on the hose, and siphons are prohibited (Nebraska State Fire Marshal). Check with your state: farm rules vary a lot.
Types of anti-siphon valves
There are three main types: spring-loaded mechanical valves, normally closed solenoid valves, and siphon breakers (PEI).
| Type | How it works | Where it fits |
|---|---|---|
| Spring-loaded (mechanical) | A spring holds the valve shut; pump suction or pressure opens it | The most common type on farm, fleet, and generator tanks |
| Normally closed solenoid | Wired to the pump so it opens and closes with the pump motor | Required for marinas under PEI RP1000; PEI says mechanical valves aren't recommended there (PEI; NC DEQ) |
| Siphon breaker | A normally open solenoid at the high point of the pipe. It closes while the pump runs and opens to let air in and break the siphon when the pump stops | Piping that leaves through the top of the tank (PEI) |
Picking the head setting
Spring-loaded valves are rated in feet of head. Pick the range that covers the height from the tank's highest fuel level down to the lowest point in the piping. One maker's guide calculates it as the height of the maximum liquid level minus the height of the lowest point of the delivery pipework (Elaflex).
Common ranges are 0 to 5, 5 to 10, 10 to 15, and 15 to 20 feet of water column (Morrison Bros.). OPW's 199ASV comes in four factory-set ranges, and OPW says not to field-adjust it (OPW). Some others are adjustable and lock once set (Franklin Fueling Systems).
Example: A tank on a stand with a full fuel level 9 feet above the ground, feeding a dispenser whose lowest piping is at ground level, has about 9 feet of head. That calls for the 5 to 10 foot range.
Field note: Morrison and OPW are the two brands I install. Both are widely stocked by fuel equipment distributors. Shop: Morrison anti-siphon valves on Amazon ยท OPW anti-siphon valves on Amazon
Installing it
- Put it after the shutoff valve: tank, block valve, then anti-siphon valve (Florida DEP).
- Top-mount valves go on the tank top, with the suction tube on the inlet and the line to the pump on the outlet (OPW).
- Check orientation. Some are vertical-only (Franklin Fueling Systems); others work horizontal or vertical (Morrison Bros.).
- With a line leak detector, put the detector right downstream of the anti-siphon valve (NC DEQ).
Troubleshooting
The pump has to overcome the valve's spring every time it runs (OPW). On a suction pump, that's extra lift on top of everything else, so a valve set much higher than needed can cause trouble. For pump basics, see how to choose a 12V fuel transfer pump.
| Symptom | Things to check |
|---|---|
| Pump won't prime or loses prime | Head setting too high for the pump; air leaks in suction fittings |
| Low flow | Wrong valve setting or size; clogged strainer or filter |
| Fuel still drains out with the pump off | Debris on the seat or a worn seal; replace or service the valve |
| Line pressure building with the pump off | Thermal expansion relief not working |
FAQ
Do I need an anti-siphon valve if my pump is on top of the tank?
If the hose or piping runs below the top of the tank, fire code checklists still call for one (Central Arizona Fire). Farm rules can differ: Nebraska allows a self-closing nozzle instead (Nebraska State Fire Marshal).
Is an anti-siphon valve the same as a check valve?
A spring-loaded anti-siphon valve is a type of check valve, but it's set to hold back a specific head of fuel and usually includes thermal relief (PEI; Morrison Bros.).
Does it need to be tested?
Florida's rules have no annual test requirement for anti-siphon valves, but manufacturers may have their own (Florida DEP).
Sources
- PEI: Antisiphon valve (2014)
- Florida DEP: Block valves and anti-siphon valves (rev. July 2023)
- NC DEQ: Anti-siphon valves at marinas (2022)
- New Mexico Administrative Code 20.5.109.902
- Central Arizona Fire and Medical Authority: Above-ground fuel tank checklist
- Freeport, TX Fire Marshal: Aboveground storage tanks
- Nebraska State Fire Marshal: Installation and use of farm tanks (2003)
- OPW: 199ASV anti-siphon valve
- Morrison Bros.: Fig. 910 anti-siphon valve
- Franklin Fueling Systems: Anti-syphon valves
- Elaflex: Anti-siphon valve information
This guide is general information. Your fire marshal or tank regulator decides what's required at your site.