Anti-syphon loops and all that.

[sequena1]

I read (several times) with interest the contributions regarding anti-syphon loops. I wonder if anyone has any thoughts on this setup. I'm installing a Lavac which will be connected through a manual pump directly to a large Tek-tanks holding tank below the water-line. There will be no option to bypass it. The latter will be emptied, again with a manual pump and an above the water-line loop to the underwater discharge skin-fitting. I know I need a vent in the water inlet loop but do I need vents in either the pipe exiting the loo or the discharge pipe. Doesn't the large (1 1/2 inch) vent pipe from the holding tank emerging way above the waterline act as an anti-syphon for both sides of the holding tank?

 

[HeadMistress]

No anti-siphon device is needed in a line that goes only to a tank. You will need one in the line to dump the tank at sea, however...the tank vent above the surface of the tank contents can't introduce an air break into line coming from the bottom of the tank--regardless of whether a pickup tube or a fitting at the bottom--until/unless the tank is empty.

 

[boatmike]

Peggie,
I have learned to respect your posts but if the tank is vented at the top as mine is with a vent pipe to atmosphere (it's a cat so it goes overboard on the bridgedeck looking directly downward with a big loop on top) why do I need a vent in the line I pump out to sea with that comes from the bottom of the tank? Seems to work OK without and if I had one I would surely be pumping poo up it wouldn't I?
Mike

 

[boatmike]

Humble apologies Headmistress!!

I misread the original post in haste and you are correct. My holding tank is above WL. His is below. Therefore any siphon could fill his poo tank with seawater.... While the pump valves should be an ipediment to this happening and the seacock only needs to be open when actually pumping out making syphons unlikely, it is not impossible so I guess you are right.
I would add however that the simplest syphon break in this case is via an anti syphon valve from the highest point in the pipework back to the top of the tank. This keeps all smells etc contained and avoids more holes in the hull. Do you agree?

 

[HeadMistress]

Re: Humble apologies Headmistress!!

No apology needed. /forums/images/graemlins/smile.gif

The simplest siphon break is a vented loop with an air valve in it that only allows air IN, not out...no need for any line on it going to any other line. In fact, putting a vent line on it instead of an air valve is a bad idea IMO because the diameter is so small that waste squirting out would quickly clog it up, turning it into nothing more than a loop that can no longer provide an air break.

 

[sequena1]

Many thanks to you for your help; much appreciated! One small extra point; the A-S loop on the discharge pipe would be situated between the tank and the pump and not on the pressure side between pump and skin-fitting?

 

[HeadMistress]

Nope...loop in the tank overboard discharge line should go between the pump and the thru-hull--in the line through which the waste is being PUSHED by the pump. If you put it between the tank and the pump--in the line in which the pump is PULLING waste out of the tank, it will also pull air into the line that can interfere with the pump's ability to pull waste out of the tank...but it can't interfere with anything being pushed through.

So when installing vented loops, always think, "will liquid be pulled through this line, or pushed through it," and try to put it where liquid will be PUSHED over it, not in a line through which liquid (waste, water) will be pulled. I realize this not always possible, but should be the "rule of thumb" to start from.

 

[HeadMistress]

Btw...

I'm leaving tomorrow morning for Florida on a job...staying on the boat while I'm there, so will have very limited--if any--internet access for the next week (and I'm already beginning to feel withdrawal symptoms). So if you have questions for me, I'm not ignoring you...PM me and I'll answer when I get home if I can't get on while I'm away.

 

[sequena1]

Thanks again. Invaluable to have such expertise to tap into!