Educate me on Vented loops please

[Nautiville]

I'm in the middle of my winter project, and I think I'm going to make it last the whole winter. I'm adding a Purasan hold n treat system to my existing holding tank. I have learned much. I have replaced many feet of old hose, and learned who your real friends are! Moved a Y valve, changed a macerator pump, relocated an AC pump, (fixed leaks I created) and on and on.

The question is. On the exit from the Purasan to the underwater discharge, I am fitting a vented loop. The previous discharge line had one with a hose on the loop to a through hole on the side of the boat. I will do this on the new one also. I've had to use a new one, since I cut the old one upon removal, and really really don't want leaks in this system! So, a vented loop without a hose barb has a little duckbill valve in it, but ones with a hose barb seem to not have one. Seems like they both should have one. I would think that otherwise "stuff" would push it's way up the external vent line, instead of the line simply providing air to operate the air/siphon break.

What am I missing here?

Thanks.

Bob!

 

[jtybt15]

I'm not quite seeing a vent line without a hose fitting to the vent fitting. I've never seen a thru-hull fitting without a barb fitting.

The only way 'stuff' can go up the vent line is if the discharge line is completely clogged. Then I see the purpose of a reversed duck bill at the thru-hull. Just put a sign saying 'don't flush tampons'. Ask any plummer.

...and I really don't see the purpose of a loop in this application.

 

[Nautiville]

Plain vented loop with little duckbill valve.

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**From Forspar **
The vent caps (#903002 MF 841-replacements available) have a one-way duck-bill style valve inside. These caps can be replaced with an optional ¼” hose barb (#903004 MF-846 Vent Barb
Assembly) if desired. Note that if the hose barb option is used, the duck bill one-way valve is eliminated

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As I understand the operation, as fluid is pushed (not pulled) through the loop, the duckbill stays closed and fluid passes on. When the pushing stops, the duckbill can open, allowing air to enter and the siphon is broken, preventing a reversal if something goes bad.

Since this should only vent air "in" I wonder why there is a need to plumb it to an above water thru hull. I suppose as a precaution if there was a problem with the duckbill valve.

If there is no valve in the vent line, Why will the fluids continue merrily upon their way to the underwater through hull, when it would be easier (no water to push out of the way) to simply shoot out the loop vent. Even without clogs or problems with the underwater through hull, a vent out the side is easier to get out than an underwater one.

I do feel the vented loop is necessary since the output of the Purasan is below water level and I"m not a fan of reversals in case something goes wrong.

I'm just curious as to why there is a duckbill in one type of loop and not in the other.

 

[jtybt15]

The pump being below the water line turns the pump into a critical component that can possibly flood the boat. Above waterline, not so much.

 

[Nautiville]

The pump draws from the holding tank and feeds the purasan as it requests it. Holding tank is below the water line, Pump is above and angled so that it will drain back into the tank after cycle. Purasan discharge is below water line level. Thru hull is below waterline. The vented loop is between discharge of purasan and underwater thru hull.

Fresh water vacuflush draws water from fresh water tank, flushes into holding tank. Purasan system monitors tank and begins processing when needed. Waste cycles through purasan and exists overboard under water. The last leg of the system, both components (exit of purasan and underwater thru hull)are below water line.

 

[Radioactive]

It is desirable to have the purasan discharge up, to well above the waterline, to a vented loop ( aka: vacuum breaker ) then flow downward to the through-hull below waterline.

One good reason for the vacuum breaker is so that "treated, but contaminated" water does not remain in the discharge line from above the waterline to the through-hull. When the purasan stops pumping, the flow will continue to exit and the vacuum breaker will allow air into the line from the high point to ( at least ) the waterline.

The duckbill allows "high" pressure in the line while the pump is active, but allows air into the line when a vacuum is present. Note that it will also prevent any suction, for any reason, from drawing water up the discharge into the system and/or bilge. ie: it is a safety issue.

System will function without this, but it will not fail-safe.