Best way to add a second vent?

Chief RA · Dec 13, 2014

I assume your talking fuel tand venting. I would check for blockage in the existing vent system first. Chief

thinwater · Dec 13, 2014

Since this is a sanitation forum, and 2 vents or larger vents only make sense in this context, I'm guessing he he adding a vent to the holding tank.

http://www.aussieglobe.com/uniseal1.htm

There are threads on this forum.
http://forums.s2.sailboatowners.com/printthread.php?t=130140

RichH · Dec 13, 2014

If you have clear access to your holding tank and its 'inlet' (from the head) consider to install a TEE into that 'inlet holding tank' line and as close as possible to the 'inlet to the holding tank', run new 'hosing straight up', keep the diameter of the new hose you place on that tee as close to the 1.25" to 1.5" diameter as possible to its new overboard vent.

Dont do this on the 'outlet' hose from the holding tank as this hosing is always run to the very bottom of the holding tank and therefore you wont get much air transfer with the end of that hose 'submerged'. (check to see if your 'existing' vent has been erroneously included in the 'tank outlet' hose - a quite common error).

If the tank is fiberglass, just put a new 'tank boss' onto the top of the tank and run the new line as 'straight up' as possible to the new overboard vent.

The 'alternate' (probably the best) is to install a powered 'air purge aerator' and simply use the existing vent. VERY Small (piston type) 12vdc air compressor that delivers its air into a porous 'stone' thats placed into the bottom of the holding tank (and its contents) and which 'creates small air bubbles' which constantly 'turns over' the tank contents as it supplies the air. You could tie in the pump to your engines ignition panel so that any time the engine is on, so is the small air pump; a small bypass switch so that you can run the pump when youre at the dock and charging your batteries while connected to the dock power.
You'd need to apply a new 'tank boss' on the top of the old tank to run the hose and allow for the large diameter of the 'porous stone' to do this. This will cause the 'fermentation' inside the tank to become aerobic and with 'significantly' less STINK output. Its also a functional KISS method.
This method of tank venting/purging has become very popular in the EU.

Here's a discussion from the Trawler Forum: http://www.trawlerforum.com/forums/s6/waste-holding-tank-aeration-17823.html

Ralph Johnstone · Dec 13, 2014

RichH said:
If you have clear access to your holding tank and its 'inlet' (from the head) consider to install a TEE into that 'inlet holding tank' line and as close as possible to the 'inlet to the holding tank', run new 'hosing straight up', keep the diameter of the new hose you place on that tee as close to the 1.25" to 1.5" diameter as possible to its new overboard vent.

I like Rich's idea of using the "T" off the inlet hose for the second vent. Very straightforward. Suffereing from OCD, I was compelled to add a second vent off the front of the tank to ensure "optomal" cross ventilation from front to back. Monumental love of labour in removing all connecting hoses and tank, washing out the tank interior as best as possible, having a new threaded fitting friction welded in place, replacing tank and all hoses.

Use a "T" in the inlet hose

............. much easier.

The only thing I would add is to place the second thru hull for the new vent hose as far forward or aft as possible, away from the original vent thru hull. Ensure the new vent hose maintains a slight grade upwards from tank up to the vent to ensure no water "traps" are formed.

In this arrangement, as air flows over the hull, the pressure at the two vents will often be (at least in theory) slightly different due to the different air velocities. This encourages some positive ventilation.

Can't say I ever put much faith in the idea that a single short vent hose from the holding tank to the thru hull could ever contribute anything to the ventilation other than just pressure relief. Considering that H2S has a specific gravity of 1.2 (relative to air), it's not going up the vent and neither is any CO2 at 1.5. If you ever get into aerobic digestion, maybe any newly created methane (SG = 0.6) might assist in transporting some of the H2S up and out. But if you already have aerobic digestion, you have little H2S, and you don't have a problem.

I'm certainly open to any theories as to how a single vent hose can contribute any ventilation to a holding tank

.

centerline · Dec 13, 2014

what material is your sanitation tank made of?.... if it is a poly tank, you can have ports thermally welded to it.... and if its fiberglass, you can epoxy either an FRP coupling or a nipple to it... either way, if you have the room to work on the tank, it wouldnt need to be removed from its bed...

centerline · Dec 13, 2014

Ralph Johnstone said:
I like Rich's idea of using the "T" off the inlet hose for the second vent. Very straightforward.

I'm certainly open to any theories as to how a single vent hose can ventilate a holding tank .

dont allow yourself to be confused with the differences in the words air vent and air circulation

the basic purpose and principal of the vent is not so much to allow the breeze to circulate and whistle thru the tank, but to allow for the displacement of the air when waste is pumped into it... and again when its sucked out.
without the vent, as ineffective as it is for allowing air circulation, the system cant work

.

Peggie Hall HeadMistress · Dec 13, 2014

If there's a way to over engineer it, y'all are sure to find it.

A tee in a vent line won't allow any ambient air to ventilate the tank, air will only travel across the top of the tee between the two thru-hulls.

Adding/moving a tank fitting on any tank is actually pretty easy to do these days, thanks to a li'l doodad called the Uniseal UNISEAL Uniseal Instructions It will NOT leak! And is also VERY inexpensive.

All you need is a hole saw, a piece of pvc pipe that will become the new tank fitting, and a threaded plug and some teflon tape to plug and seal the old tank fitting--all of which are available from the plumbing section of any decent hardware store.

If the vent line can be short enough--<5', straight, and limited to rise that doesn't exceed 45 degrees, a single 1" vent should be enough to get the job done. "Vent" thru-hulls are only available in 5/8"...but a "vent" thru-hull defeats the purpose of a larger shorter, straighter vent line anyway. Use an open "bulkhead" thru-hull...that will allow you to backflush the line to make sure the vent line stays open.

Otoh, it the tank so low in the bilge that the vent line has to be long and vertical, a second vent line prob'ly won't help...in which case, aeration is the best solution. As an absolute LAST resort in a system that's so poorly designed that PREVENTING odor is impossible, a vent line filter to trap it may be the only answer.

Peggie Hall HeadMistress · Dec 13, 2014

centerline said:
dont allow yourself to be confused with the differences in the words air vent and air circulation the basic purpose and principal of the vent is not so much to allow the breeze to circulate and whistle thru the tank, but to allow for the displacement of the air when waste is pumped into it... and again when its sucked out. without the vent, as ineffective as it is for allowing air circulation, the system cant work.

You're correct about the BASIC function of a vent in any tank, but unlike water and fuel, sewage must also be managed to create the environment in the tank necessary for breakdown and odor elimination...and OXYGEN is the key to both--something that the marine industry and boat builders in particular failed to recognize when new laws made it necessary to install holding tanks (most still don't have a clue--or want one--how to design a system can be managed). So the vent line(s) on a black or gray water holding tank, unlike vent lines on water and fuel tanks, must serve a third purpose: to provide a means of keeping the tank aerobic by VENTILATING it as well as just venting it.

RichH · Dec 13, 2014

Centerline -

That may be true so that you dont over-pressurize and cause tank failure ... .

But, as pertains to 'fermentation' (the stink generator) if you dont have enough mass transfer of air into the tank, either by natural chemical equilibrium of the air(oxygen) 'wanting' to get in there through the vent or by pumping air, you'll wind up with an anaerobic fermentation which produces sulfides, "putrenes", etc. - the noxious STINK. The larger the total venting (area) the less anaerobic and more aerobic (less stinky) fermentation.

Pumping air additionally 'sweeps out' the gaseous contents and provides MORE air(oxygen) - LESS stink. So, if youre looking just to 'sweep' the gases out of the tank and provide a wee bit more of air(oxygen) into the tank ... simply put a small amperage draw small piston (gas) pump onto that 'new' tee and forget about running that new tee to an overboard vent. Just keep the pump 'high and away' so you dont backfill it when its not powered up and running. A few ml/cc (25-50ml/min) of air per minute should be sufficient. The outlet check valve on the head discharge will have to be 'tight' and probably regularly 'cleaned' - vinegar, etc. If the generated carbon dioxide from fermentation is not removed so it doesnt form a 'blanket' over the 'crud' the fermentation will become 'anaerobic' - STINKY ... more reason to pump the air in and the stink out.

centerline · Dec 13, 2014

Peggie Hall-HeadMistress said:
You're correct about the BASIC function of a vent in any tank, but unlike water and fuel, sewage must also be managed to create the environment in the tank necessary for breakdown and odor elimination...and OXYGEN is the key to both--something that the marine industry and boat builders in particular failed to recognize when new laws made it necessary to install holding tanks (most still don't have a clue--or want one--how to design a system can be managed). So the vent line(s) on a black or gray water holding tank, unlike vent lines on water and fuel tanks, must serve a third purpose: to provide a means of keeping the tank aerobic by VENTILATING it as well as just venting it.

I would never disagree with that

I fully understand the concept and desire to have some air circulation thru the tank, but as much as it is a desirable feature, most systems dont have it...
it is my opinion that there are many more boats that have a vent line that is at least partially restricted due to effluent particulate trapped in it, than there are with a healthy sanitation systems...

the vent is absolutely necessary and must be kept free of debris at all times...

Ralph Johnstone · Dec 13, 2014

I Just Knew It ........................

Peggie Hall-HeadMistress said:
If there's a way to over engineer it, y'all are sure to find it.

...................... my disparaging remarks about a SINGLE vent line would bring you out of semi-retirement. Good to hear from you again as always.

I guess my thoughts are that if you have a single vent line, it doesn't matter how it's run or what the diameter or the length for that matter. It won't ventilate ................ just equalize the pressure in the tank to atmospheric.

You've got to look at two vent hoses if you intend to ventilate the tank.

Ralph Johnstone · Dec 13, 2014

holdech said:
Just curious what the best course of action is for my particular installation.

Chris

I'd still go with Rich's idea of the vent teed off the inlet hose to the tank. It's the simplest and would give you almost the same results as a new vent take-off from the other end of the tank.

I fully undersrtand that you intend to run this new teed vent to its own dedicated vent line with its own thru hull.

RichH · Dec 13, 2014

That 1/2"Ø vent line is most probably "THE" problem.

Id be more apt to change that 1/2" to a 1.25 or 1.5"Ø as a first attempt in 'improvement'. A 1.5"Ø hose is going to have well over 6 times the 'volumetric flow' over a 0.5"Ø hose. Suggest you do a websearch for direct contact a poly tank mfg. to find out where you can get 'semi-soft' etc. connectors that can be 'pushed' into the correct sized bore hole and still 'perfect seal' the connection to the tank. Poly tanks simply cant have connectors 'glued' to them, reliably that is.
If that change to larger vent hose diameter doesnt give 'some relief' then progress to more/other remedies, as in any system remedy you really need to change out that 1/2" vent.

But again, active air pumping is still probably your ultimate best solution, and usually doesnt need large vent hosing, etc.

thinwater · Dec 13, 2014

a. A single large vent WILL provide sufficient ventilation if it is no more than 3-4 feet long and on an angle (works better than vertical to encourage 2-way flow--this is common practice in chemical processes). I did considerable side-by-side testing of this and variations, to investigate Peggy's suggestions and those of others. Yes, a single vent can be quite effective, and generally a bubbler is not going to be needed, though it certainly can work, as can vent filters. But the simplest solution, if the problem is moderate and the tank not too deep in the bilge, is a single large vent. I have to agree with Peggy that they can be over engineered.

b. We like the centerline, since it reduces plugging due to slosh and heel. Do fore or aft vent locations increase plugging? Seems to me they could be problematical if the boat is sailed very vigorously, though perhaps very few sail that vigorously. I would tend to avoid the extreme ends.

c. I've used Uniseals successfully on PE tanks. Perfectly safe for a center line vent application. I don't know about FRP; should be easy to glass on a fitting.

Ralph Johnstone · Dec 13, 2014

Care to Explain ..................

thinwater said:
a. A single large vent WILL provide sufficient ventilation if it is no more than 3-4 feet long and on an angle (works better than vertical to encourage 2-way flow--this is common practice in chemical processes).

.................. the mechanism of just HOW it works ?

Agreed, if the holding tank gasses are lighter than air but H2S and CO2 ? ? ?

RichH · Dec 13, 2014

thinwater said:
a. A single large vent WILL provide sufficient ventilation if it is no more than 3-4 feet long and on an angle (works better than vertical to encourage 2-way flow--this is common practice in chemical processes).

Sorry, stink reduction, is a fermentation process', and thats what is occurring inside a holding tank, and needs LOTS of mass transfer of air to remain aerobic. Without a forced or free flowing air/gas transfer via multiple vents, the gas densities of H2S, CO, CO2 will all 'blanket' the contents and retard diffusion and thus oxidation and the anaerobic reduction reaction will predominate. Just do simple convection and two-way diffusion calcs. for CO2 and H2S vs. Air(Oxygen) through a 1/2" or 1.5"Ø pipe and see what happens to the transfer - minimum to nil. Thats why one needs an air-pak SCBA when going down into ANY container w/ CO2 or H2S present, even if 'vented' .... density which causes stratification of the varying gas compositions and which do not readily diffuse into one another.
Take a walk down to the waste water treatment in just about any Chem Plant and witness the air 'sparging' going on, then ask them what happens when the compressors go off line.

Thread one of these into that newly installed and easily removed Tee with pierced and sealed cap and all the problems vanish (find one rated for continuous duty and has sufficient force more than the weight from the 'height' of tank contents - Static head pressure of the 'fluid'): http://www.amazon.com/dp/B000EYWNVQ...TF8&colid=2DQNN1I4V9TVS&coliid=I31CETR76ZDVBV

Stu Jackson · Dec 13, 2014

Ralph Johnstone said:
.................. the mechanism of just HOW it works ?

Agreed, if the holding tank gasses are lighter than air but H2S and CO2 ? ? ?

Buy Peggie Hall's book. Right here on this website. A very good read.

Ralph Johnstone · Dec 13, 2014

Stu Jackson said:
Buy Peggie Hall's book. Right here on this website. A very good read.

I don't doubt it in the least .............. except for this one small, single detail.

The numbers are all wrong for a single vent line of any diameter travelling at any angle to permit ventilation in a holding tank. The gasseous products of aneorobic digestion are heavier than air. The gasses will not rise out of the tank and gaseous diffusion is so small it's not worth mentioning.

Best way to add a second vent?

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