Drain hose on anti-siphon loop valve?

[HeadedToTexas]

I am in the process of replacing the tiny vent line on my holding tank which had become clogged with bad news. Part of the replacement is dealing with the wye fitting that accepted the occasional drip from one of the anti-siphon loop valves. The new holding tank vent line will be a straight shot of 1" hose from a UniSeal in the top of the tank to a new stainless mushroom type through hull fitting. I am currently at the disassembly stage of this project.

I (finally) understand the purpose of the anti-siphon valve including the consequences of the drain hose clogging thus preventing the siphon-breaking function. Should I replace the gray fitting on the left above with one like the one on the right? Servicing the duckbill valve inside the right valve is far easier than the check valve diaphragm in the left valve, but I could easily be overlooking something. What would that be?

 

[Juliet 15]

Just curious - what was the consequence of the drain hose clogging thus preventing the siphon-breaking function?

 

[Comodave]

Not sure I am understanding what you are asking. The vent hose doesn’t have anything to do with the anti-siphon valve. The vent should just run from the top of the tank to the through hull in the side of the hull. No valves at all.

 

[O C Diver]

Without seeing the rest of your plumbing and assuming the boat manufacturer used two different siphon breaks ( the black and the gray ones), I respectfully suggest that you don't understand the difference between the two loops. My guess is that one operates with a vacuum in the system while the other operates with pressure in the system.

Without understanding why they are different, I would be hesitant to change one to the other.

Btw, the dip in the vent line under the beam needs to be avoided. I made a habit of flushing some water through the vent hose after each pumpout to avoid build up in the hose.

Ted

 

[HeadedToTexas]

I respectfully suggest that you don't understand the difference between the two loops.

That is definitely the real issue here. The anti-siphon loop valve is an air check valve allows air into a section of hose to break a siphon. The siphon can be a problem because when the pump stops that filled the hose with liquid, the siphon can begin returning water or liquid from the tank or the sea (or lake) into the boat. Definitely a mess and far worse than that when the bilge pumps finally fail or exhaust the batteries.

While I have used siphon hoses for many things over the decades, this BoatUS article educated me about the two different types of anti-siphon valves common in marine applications. Near as I can tell, they work the same way. One has a diaphragm and the other has a duckbill valve.

Both can be used without a hose that directs gas and liquid that gets past the valve as it seats. No hose means venting that small "spurt" into the engine room. A hose such as the one in my photo above directs the "spurt" into the holding tank vent line and overboard, but once that tiny wye fitting clogs with bacteria and solids, the function of the anti-siphon loop valve is defeated introducing the risk of a siphon creating real problems.

Does ABYC address anti-siphon loop valve discharge?

 

[HeadedToTexas]

Btw, the dip in the vent line under the beam needs to be avoided. I made a habit of flushing some water through the vent hose after each pumpout to avoid build up in the hose.

That dip will be gone once the new 1" vent line is installed later this winter. And I doubt the manufacturer built the boat this way because the wye is copper and looks out of place compared to fittings throughout the rest of the boat. My guess is that at some point the original black anti-siphon (like the one on the right) failed or clogged and the owner at the time simply replaced it with that gray one and cut in the wye to direct the "spurt" overboard.

 

[O C Diver]

I wasn't clear.

A siphon break that operates under pressure (in the hose) is designed to allow air into the system ( preventing siphoning) by having a check valve that is closed when the system is under pressure and opens (allowing air in) when the system stops, pressure goes away, and the system goes to vacuum. Another form of this break is to run a line from the siphon break overboard (or possibly in this case to the holding tank). In this case, there wouldn't be a check valve in the siphon break.

A siphon break that operates under vacuum is designed to allow air into the system (preventing siphoning) by either having a spring loaded valve that opens the valve when vacuum is decreased or has a very small hole where air is always entering the system. The small hole principle requires enough vacuum (by volume) from the pump to overcome the steady small stream of air bubbles entering the liquid flow. This type of siphon break might include a hose overboard or routed back to a holding tank vent line to mitigate the smell of effluent coming out of the small hole.

By now, I've probably thoroughly confused you. So this is the important part to understand:

If the siphon break is operating before the overboard discharge pump (between holding tank and pump), that siphon break is operating in a vacuum system.

If the siphon break is operating after the overboard discharge pump (between pump and seacock), that siphon break is operating in a pressure system.

Obviously, determine pressure versus vacuum before making changes to the system.

Ted