'Anti-siphon valve' - Please explain

[achris]

I am reading a lot of fuel related threads and see reference to an 'anti-siphon valve'. Could someone (I know I'll get many) please explain :

1. What they are.
2. How they work.
3. Why they are required.

I don't have anything in the line between the fuel end of the pickup in my tank and the engine mounted primary water separating fuel filter....

Thanks again,

Chris............

 

[Lone Duck]

Re: 'Anti-siphon valve' - Please explain

It is a valve in your fuel line that stops fuel from siphoning back to your fuel tank. when you pump your bulb the suction pulls the ball bearing open and allows fuel to pass to carbs. When you shut down ball seals again this prevents you from having to pump bulb every time you start up. Your primer bulb also has one in it. They should be checked every now and again if they stick they can limit the amount of fuel to carbs and lower your RPM at wot.

 

[Don S]

Re: 'Anti-siphon valve' - Please explain

The anti siphon valve has one purpose. It prevents fuel from siphoning out of the gas tank incase the hose is cut or burned off at some point and hangs below the level of fuel in the tank. Which would cause the gasoline to siphon out of the tank. Hence the name anti siphon valve.
It's normal location in US boats is the hose barb fitting that is screwed into the fuel pickup tube coming out of the fuel tank.
They are either aluminum or brass, and look just like a hose barb fitting, except the have a check ball and spring in them.
Many people mistakenly think they are there to prevent fuel from flowing back into the fuel tank when the engine is not running. That is done by the fuel pump, not the anti siphon valve.

http://a257.g.akamaitech.net/7/257/....gpo.gov/cfr_2003/julqtr/pdf/33cfr183.568.pdf

http://www.iboats.com/Tempo_Aluminu...d.238200--list_time.1200547527--view_id.20087

 

[achris]

Re: 'Anti-siphon valve' - Please explain

The anti siphon valve has one purpose. It prevents fuel from siphoning out of the gas tank incase the hose is cut or burned off at some point and hangs below the level of fuel in the tank. Which would cause the gasoline to siphon out of the tank. Hence the name anti siphon valve.

Thanks Don, but how does the valve know the difference between when fuel flow is demanded by the engine and a ruptured hose? Flow is flow...

Chris...........

 

[Don S]

Re: 'Anti-siphon valve' - Please explain

It takes about 1.5 inches of vacuum to pull the check valve off seat and allow flow. This is above what a siphon can cause, but well below what a fuel pump will pull.

 

[Lone Duck]

Re: 'Anti-siphon valve' - Please explain

Thanks Don

 

[HT32BSX115]

Re: 'Anti-siphon valve' - Please explain

The operative word here is "Siphon"

The fuel can siphon if any part of the fuel line from the tank runs below the (fuel) level of the tank. If a break occurs in the line anywhere below the level of the tank fuel will siphon out of the tank at the break

The slight vacuum produced by a siphon would not un-seat the check-ball in the anti-siphon valve.....Of course, any fuel pump can easily produce enough vacuum to unseat the ball. [Actually a siphon COULD unseat an anti siphon ball but it would require a siphon hose to be probably 10+ feet below the tank to produce enough vacuum to overcome the spring against the ball....depending on the hose diameter.......ain't gonna happen in most boats!]


If the fuel line is never physically below the level of the tank fuel can never siphon from a break in the line.

183.568 Anti-siphon protection.
Each fuel line from the fuel tank to
the fuel inlet connection on the carburetor
must:
(a) Be above the level of the tank top;
or
(b) Have an anti-siphon device or an
electrically operated fuel stop valve:

Technically. If your fuel tank is an in-floor tank and the fuel line never runs below level of the tank, an anti-siphon valve would not be required. It's still not a bad idea to have one though.





[Now that Don provided the 1.5" (of Mercury) number a geek engineer like me could calculate exactly how far a siphon hose could run below a tank using gasoline before the ball was unseated......but if I did I would bore everyone more than I have already.....I'll save that one for another time....

EDIT: Don, you always type faster than me! (I never learned to type.....)

 

[starsnstripers]

Re: 'Anti-siphon valve' - Please explain

Hey Chris how many feet below the fuel tank must the line be to siphone a 1.5 (of mercury) to open the anti-siphon valve? Give him something to do. haha, This is a good thread i have a below the floor tank and i'm going to get one. I like it!

 

[HT32BSX115]

Re: 'Anti-siphon valve' - Please explain

Hey, if we work it right we can either turn it into an oil thread or a Captains Call bashing thread!

Hey Chris how many feet below the fuel tank must the line be to siphone a 1.5 (of mercury) to open the anti-siphon valve? Give him something to do. haha, This is a good thread i have a below the floor tank and i'm going to get one. I like it!

 

[achris]

Re: 'Anti-siphon valve' - Please explain

Hey Chris how many feet below the fuel tank must the line be to siphone a 1.5 (of mercury) to open the anti-siphon valve? Give him something to do. haha, This is a good thread i have a below the floor tank and i'm going to get one. I like it!

What do you think I am?!?!?!? Some sort of walking computer!!!! I refuse to play your childish games!!!!

It's 2ft 4in....

Chris..............

----------------------------------------------------------------
Workings for those who think I'm wrong... I might be

SG of mercury = 13.57
SG of petrol ('gas' for my US friends) = 0.737

To pull 1.5" of Hg would require (13.57/0.737*1.5)" of 'gas'
= 27.659" (2 ft 3.659in)

 

[MikDee]

Re: 'Anti-siphon valve' - Please explain

I read that the main purpose of an antisiphon valve, is to prevent fuel from leaking out of the tank in case of an accident, so as not to contribute to a fire or explosion.
Whatever, these are pesky little buggers, any little bit of dirt, or debri, will clog them up, creating havoc. I learned about this valve (the hard way) yrs. ago, on my first I/O with a built in fuel tank. My second (newer) boat of the same make, & design, gave me the same issues, but I knew where to look, & what to do, then.

 

[Robj]

Re: 'Anti-siphon valve' - Please explain

The anti-siphon valve is on the top of my fuel tank, not actually in it. It also has another purpose which is to prevent fuel from flowing back into the tank when the engine is turned off.

Have a great day,

Rob.

 

[rodbolt]

Re: 'Anti-siphon valve' - Please explain

the anti-siphon valve isnt desined to stop flow into the tank, like DonS says its desighned as a siphon break in case of a line leak below the liquid level in the tank.
flow aint flow
most folks, even most techs dont know nor understand these valves are built with specific head presure ratings,
the USCG and the ABYC sites have the mathematical fourmulas to determine maximum expected head presures due to maximum line drop from the maximum expected liquid level. with a 50Ft drop on a 3/8ths line you would most likly need something around 5"Hg opening pressure.
so not all anti siphon valves are the same.
I would tell ya how to modify the head presure rating but then Id have to ask for a beer.

 

[achris]

Re: 'Anti-siphon valve' - Please explain

... with a 50Ft drop on a 3/8ths line you would most likley need something around 5"Hg opening pressure.

I believe the maximum you can lift gas is 41ft. So 41ft will pull 30" of Hg

so not all anti siphon valves are the same.
I would tell ya how to modify the head pressure rating but then Id have to ask for a beer.

If it's a good quality check valve, then just turn the screw adjustment inside. If it's a cheapie (and we don't use cheapie rubbish on our boats, do we ) you would need to change the spring tension.

Chris...............

 

[rodbolt]

Re: 'Anti-siphon valve' - Please explain

chris
a 50ft drop would not require that much, now if I tried to lift it 50 ft it would most likly boil and give me vapors. the antisiphon valve doesnt care about lift,just how low the leak is below the liquid level in the tank.
but the formula is there in the USCG regulations.
and 50ft wasnt my idea anyway
most boats we see have the cheap aluminium antisiphon valve of unknown origins.
depending on the hull I try to shoot for a spring tension of 1/2 to 1"Hg over the stated formula for line drop below the maximum liquid level above the lowest part of the fuel system.

 

[Don S]

Re: 'Anti-siphon valve' - Please explain

Someone want to tell me the name of a boat that is powered with gas engines that has over a 6' rise or drop from the gas tank to the engine?
Or is all this 50' garbage done on that stupid looking boat from Hypothetical Marine?

 

[starsnstripers]

Re: 'Anti-siphon valve' - Please explain

HAHAHA! Man you guys get all carried away with your math stuff and figures. I'm getting a brain cramp here. All in all it seems like a safe good idea to my simple mind. Here's a thought on the clogging thing. What if you like angled the antisiphon valve at an upward angle from the tank so if any debris that got to it would drain back to the tank. Might work to a point unless it jams the valve. It would be almost self cleaning,HUH? OR, A screen/filter close to tank before the valve? Is there such thing as a fittinf screen? You know just like the antisiphon valve but with a fine screen? Like in some gm carbs? Oh thanks for those figures Chris, you enjoyed it.

 

[achris]

Re: 'Anti-siphon valve' - Please explain

chris
a 50ft drop would not require that much, now if I tried to lift it 50 ft it would most likly boil and give me vapors. the antisiphon valve doesnt care about lift,just how low the leak is below the liquid level in the tank.
but the formula is there in the USCG regulations.
and 50ft wasnt my idea anyway
most boats we see have the cheap aluminium antisiphon valve of unknown origins.
depending on the hull I try to shoot for a spring tension of 1/2 to 1"Hg over the stated formula for line drop below the maximum liquid level above the lowest part of the fuel system.

I'm not trying to argue with you Rod, just running the maths....

SG of mercury is 13.57, SG of gas is 0.737. That makes Hg 18.41 times heavy than gas. So to draw the same vacuum as 1.5" of Hg, the gas column would have to be 18.41 times longer. 1.5" times 18.41 is 27.615 inches....

Running the same with a full vacuum, the gas column would have to be 18.41 times 30"... that's 46 ft... for a full 30" vacuum.

Running it for water, a full vacuum (the longest you can draw water, ask anyone who has set a bore) is 34 feet.

It doesn't matter if you are siphoning or lifting the liquid. It's all to do with the weight of the liquid column at the head.... and the atmospheric pressure...

Maybe someone could do a test for us... I'm not anywhere I can do this, but I'll try it when I get home... See how far a hose has to be lowered to crack open an anti-siphon valve....

If my maths is wrong can someone please point out exactly where....

Chris.............
(Line diameter will have absolutley no effect on the head height, merely the volume transferred)

 

[starsnstripers]

Re: 'Anti-siphon valve' - Please explain

HAHA! Your having to much fun Chris! Let me know what it is And make sure we know which valve and the intended spring pressure. Have fun I'm just gonna go fight with my anode bolt

 

[rodbolt]

Re: 'Anti-siphon valve' - Please explain

chris, its simple, find a 50 ft drop, place a 5 gallon bucket on top, run a 50 ft hose to the bottom, you can now either T in a vacum Guage at the bucket or use a presure guage and do the math at the bottom.
the 5 gallon bucket will be the "gas" tank and the 50 ft drop will be some rig a commercial fisherman figgured out for reasons only he knows.
you will find it will siphon just fine.
remember a siphon can start anytime the liquid is below the head level.
its also why I cringe watching the commercial guy with a bowcap mounted tank and 25ft of 3/8ths hose running to the 60 HP on the back.