Explanation of Anti-Siphon Valve?

[cgrooves]

I asked this question in another thread, but rather than thread-jack, I figured I should start my own.

"If the anti-siphon valve is a 1-way valve, I assume it will allow fuel to flow in the direction 'from the tank to the fuel pump'. In the event of a fuel line failure between the anti-siphon valve (at the tank outlet, as I understand it) and the fuel pump, how would it keep fuel from emptying into the bilge since it is installed to allow fuel go in that direction?

Am I just wrong in my description that it is a 1-way valve? All anti-siphon valves that I am familiar with (i.e. plumbing related) are 1-way valves, but it sounds like the anti-siphon valve at the fuel tank will allow fuel to flow if there is a strong vacuum created (i.e. fuel pump) but not allow fuel to flow if there is just the pull of gravity present (i.e. fuel line failure causing an open system)."

Can someone explain this for me? I need educating

 

[HT32BSX115]

Re: Explanation of Anti-Siphon Valve?

Howdy,​

It's just simple ball-type check valve. It's a little different from a "regular" check valve in that it also has a spring on the ball so fuel cannot just run out via a siphon (if the hose routes below the tank on it's way to the fuel pump) Any fuel pump has enough suction to overcome the spring though......​

It is a one-way valve too. Fuel cannot run back into the tank but that function is not really needed.

By the way, anti-siphon (check)valves are only required on boats that have fuel tanks that fuel hoses that route below the tank on their way to the filter/pump/engine.

If the fuel tank is lower than the engine/pump/filter/and associated fuel delivery hoses, the anti-siphon valves is not required (since fuel couldn't "siphon" in the event of a fuel line leak. They're still not a bad idea though.​

Cheers,​

Rick​

I asked this question in another thread, but rather than thread-jack, I figured I should start my own.

"If the anti-siphon valve is a 1-way valve, I assume it will allow fuel to flow in the direction 'from the tank to the fuel pump'. In the event of a fuel line failure between the anti-siphon valve (at the tank outlet, as I understand it) and the fuel pump, how would it keep fuel from emptying into the bilge since it is installed to allow fuel go in that direction?

Am I just wrong in my description that it is a 1-way valve? All anti-siphon valves that I am familiar with (i.e. plumbing related) are 1-way valves, but it sounds like the anti-siphon valve at the fuel tank will allow fuel to flow if there is a strong vacuum created (i.e. fuel pump) but not allow fuel to flow if there is just the pull of gravity present (i.e. fuel line failure causing an open system)."

Can someone explain this for me? I need educating

 

[cgrooves]

Re: Explanation of Anti-Siphon Valve?

Thanks, Rick!
That makes perfect sense.

I'm now wondering if my boat needs one. I recently emptied my fuel tank by disconnecting the hose at the fuel/water seperator and siphoning at that point. Fuel didn't start coming out of the hose until I put suction on it, so that may indicate that there is one installed (or that one is not needed since it didn't siphon on its own). Sounds like a good idea to have one, however, whether you 'technically' need it or not.

 

[jerryjerry05]

Re: Explanation of Anti-Siphon Valve?

Depends on if you have an I/O or outboard. All I/O have a fuel pump that will overpower the spring and ball in the valve.Most outboards are a low pressure and might not have enough suck to make it work.Jerry

 

[rodbolt]

Re: Explanation of Anti-Siphon Valve?

google the USCG regulations on the anti-siphon valve.
you will find they are made in all sorts of head pressure ratings.
the same valve that will work ok for a tank that is 1ft above the lowest part of the fuel system wont be legal for use in a hull where the drop is say 4 or 5 feet.
luckily the USCG site has a formula to calculate the headpressure break point and allows those of us technicians that acually read the regulations to select the correct valve for the application.
if NO portion of fuel system drops below the top of the tank then it may not be applicable.
and trust me, it wont be the first time I have seen a water seperator mounted 4 ft below the tank siphon off 200+ gallons of gas into the bilge due to a cannister rust out or a line that got broken when the batterys or something was messed with.
it happens.
it should be required on diesel as well, had 320 gallons of diesel pumped into the creek one night over a broken line at the water seperator, every time the bilge filled the auto pump pumped as designed, luckily the wind contained it in the basin and the cleanup cost wasnt but about 350K.

 

[cooter2506]

Re: Explanation of Anti-Siphon Valve?

So if your tank is 100% below all lines and fuel system you dont "legally" need one?

 

[Bondo]

Re: Explanation of Anti-Siphon Valve?

So if your tank is 100% below all lines and fuel system you dont "legally" need one?
1988 Chaparral 198 XL 5.0L

Ayuh,....

There ain't Nowhere in a 20' boat that's below the motor,...
Just before it breaks over onto plane, under hard acceleration....

 

[JustJason]

Re: Explanation of Anti-Siphon Valve?

correct.
Fuel can't siphon uphill.