Anti-siphon in fresh water. Will this work?

[whipper_snapper]

I have an unusual arrangement in which the fresh water tank is above the galley sink - the only thing it supplies. To prevent water siphoning out there is a simple in-line tap. But that is a pain as it has to be turned on and off every time water is taken. It is often forgotten and precious water goes to waste.

Would it work I put a loop with a siphon break such as those used in engine water systems - the ones with a pipe to air rather than a valve ? Or would I find that the foot pump just pulls air rather than water from the tank, especially when the level is low. See sketch below.

Any better ideas?

Thanks

 

[Gwil]

Should work fine- same principle as a Lavac. It's a small pump though so the syphon break may only need a very small hole. Just a pipe loop with a pinhole at the top should do it.

 

[TonyS]

IMHO it will not work - you will just pump air. What you need is to take the line, after the pump, higher than the outlet from the tank and put the anti-syphon valve there and from the other side of the valve take the line back to the sink. You could use cheap flexible hose.

 

[boatmike]

Yes that would work OK but when you turn the tap off the water level in the piping will fall back to the same level as the water in the tank. This will mean that when you pump again you will get air down to the footpump which might cause it to lose its prime. When the tank is near full this might not be an issue but might be significant when the level in the tank drops as you indicated yourself. What you really want is a pressure sensitive valve that would stop water under gravity pressure but be overcome by the pump when in use. Gawd knows where you buy one though! Failing that Tony's idea might well work too. You would still pump air but the pump would not lose its prime.

 

[michael_w]

Why have a pump if the supply is above the sink? An ordinary domestic tap would do the job nicely as the water is supplied under pressure.

 

[boatmike]

Good question! (Doh!) Might take a little longer to fill the sink but strictly speaking you don't need to pump water downhill. (I'm embarrassed now!) /forums/images/graemlins/blush.gif

 

[whipper_snapper]

Because the head is very small - perhaps 6" between the bottom of the tank and the tap. The take-off has to be from the top because it is actually a series of 'lift-outable' 30L cans and when water does siphon it is only a trickle. And the siphon would loose prime when the tank is empty - then you would have to suck to restart it!


Re-reading my own question I can see it sounds bizarre! I seem to be saying I have a tap which controls water flow but I want to get rid of it in order to control water flow! But in reality the siphon is not always established and when it is it is just an annoying dribble, wastes a lot of water over several hours but is of no practical use.

 

[Phoenix of Hamble]

In that caes, i'd fit a small pump with a pressure switch in it, and a tap

 

[jmh2929]

Why not put a tap at the sink. Tap on - pump water - tap off. No syphon, dribbles or air locks.

 

[VicS]

The syphon break idea will only work if it is a pinhole. if it is too large you will just suck air in. Even with just a pinhole you will be getting a mixture of air and water, which may be acceptable.

If there is not enough head to use just a gravity feed to a tap use a combination of a pump and a tap. Depending upon the design of the pump the tap may be unnecessary or you may for many uses get a gentle flow without having to operate the pump.

For a fancy set up an electric pump and a tap with a built in switch to operate it is the answer as a variation on the pressure controlled pump suggested by Morgana. The pressure controlled pump will be a bit of a pain when the can runs empty.

 

[boatmike]

The syphon break idea will only work if it is a pinhole. if it is too large you will just suck air in. Even with just a pinhole you will be getting a mixture of air and water, which may be acceptable.

Er... That's why they put valves in them Vic.

 

[boatmike]

Ok then. If you don't want to go the expense of electrics, then do what Tony suggested. Leave your existing foot pump where it is with the existing feed, but route the pipe up again to a Vetus siphon valve above the tank and then back down to the sink. If you want to be clever you could avoid a valve and just use a small dia anti-syphon tube routed back into the top of the tank. /forums/images/graemlins/wink.gif

 

[whipper_snapper]

Yes you are right, that would work, I didn't quite grasp it when Tony 1st mentioned it. But as you say, it means that the 1-way valve in the anti-siphon valve is doing what it was designed to do rather than in my scheme where it was only seeing a 'suck'. And I like the idea of a small bore pipe going back into the tank.

Brilliant. Thanks all!

 

[holly1]

This does sound extreemly complicated for a tank above your sink i must have missed something...

 

[whipper_snapper]

I think what you have missed is that the tank's outlet is in the top and so needs a suck, you CANNOT just use a gravity flow. I don't want to change that because it is a beautifully simple, robust and flexible system in all other respects. It just needs a mile of plumbing to pump water out!!!!

edit -that typo probably didn't help!

 

[VicS]

All gone

 

[LittleSister]

You only need a valve if the line is under pressure (as per engine cooling water), to allow air in under suction, but prevent liquid from escaping under pressure. Complete waste of money for your purposes. In fact the valve may not work if your line is under suction (i.e. before the pump), because it will never close (it's never under pressure), and may well let in too much air for your (presumably small) pump to deal with.

Vic is right [in his first post]. All you need is a tiny hole (maybe 0.5mm - 1mm?) in the pipe at the top of its run from the tank to the pump. This will allow enough air in to break the syphon, but nowhere near enough to stop the pump drawing water. This is the normal arrangement on the water inlet to a Lavac head, and works a treat. (The air comes through the hole so slowly it takes about half a minute for the vacuum to subside enough to open the toilet lid (which is between pump and inlet) in that set up. You've probably got a smaller pump and volume under suction, and will get a quicker return to normal pressure (at which point water stops running in your set up) for the same size hole.)


Lavac will sell you a couple (choice of hole size) of little inserts to put in your pipe, but they're just a bit of plastic like a biro top with a hole in it. For your purposes a pinhole should do fine - just try it, you can always seal the hole up with sealant if you don't like it. If under test with the tap open the water continues to run for more than a few seconds after you've stopped pumping, just make the hole a little larger (using a bigger pin). Once you got the size of hole sorted right you can forget using the tap, just leave it open.

Edit: Vic got in while I was writing this. He was right first time, IMHO, but I don't think his amended diagram in the later post is right. Pinhole siphon-break goes where shown in the original diagram in this thread.

 

[whipper_snapper]

Wooaa - real confusion now.

VicS's scheme is exactly the same as I thought TonyS and Boatmike were saying.

And works because the valve (or fine bore tune) closes against pressure but allows suck so that the siphon collapses.

 

[boatmike]

Vic there is no need to be rude. Some of us, namely Tony first and then me already "engaged our brains" as you put it and realised that what you have drawn would work and the siphon valve would then operate as intended. You were at cross purposes with us. All you have done is engage your own brain and drawn a picture. I am however glad we all now agree though...

 

[boatmike]

Yes it does! Don't be diverted from the logic. What TonyS suggested and I supported, and Vic has now drawn a picture of will work. All else is frankly based on confused thinking. Go for it!