Holding tank vent plugging alarm idea.

[Roger Long]

The whole take replacement project that started my Uniseal thread began with an overpressure event that found a weak spot in a hose joint and made removal of everything for access to the underside of the tank necessary. Now that I've purchased one of Dennis' level gauges

http://www.ferriellosales.com/

that shouldn't happen again but, as they say, sh!t happens.

I haven't been able to find a suitable low pressure alarm switch that I could install in a Tee in the vent line. (If anyone knows of one, please post link.) Like many systems however, mine has a diaphragm pump for emptying the tank offshore. Any significant pressure will leak right through the flapper valve and inflate the diaphragm. Fasten a magnet to the mechanism and mount a reed switch in the appropriate place and you have a pressure alarm. If being inspected for compliance with discharge regulations, you can say, "Look, I even have an alarm to let me know if someone is pumping when they shouldn't be."

There should, of course, be a suitable cut of switch for when you need to pump. The reed switch could be wired into the same alarm used for bilge level.

 

[Peggie Hall HeadMistress]

Wouldn't it be simpler to just maintain the vent?

There shouldn't be any joints in tank vent line...a tee in it would actually cause more problems than it solves.

You're WAAAY over-engineering a system that's actually very simple and only requires a little bit of an attention span and preventive maintenance to insure that it remains trouble free.

But then, that's what engineers do, isn't it?

 

[Roger Long]

There shouldn't be any joints in tank vent line..

True. Any fittings that inhibit the travel back to the tank of any debris that get into the vent line through overfilling will make it less likely to clear when pumping out.

I was actually going to use a Tee instead of an elbow at the tank top with one leg going to the vent and the other to the pressure transducer. There then wouldn't be any more obstruction than an elbow would have. Why pay for something expensive that would probably get eaten up by sewage anyway? A few bucks of Radio Shack burglar alarm parts on the pump will do the same job without compromising anything in the system.

Probably unnecessary with a tank gauge but, like you said, I'm an engineer and I like to tinker.

 

[RichH]

Roger -
Do websearch for "liquid level indicator" + "hall effect" etc.
If you have non-metal containment components ... the above will be suitable for at least indicating 'set points' of various stages of 'fill'. These are used throughout the chemical and 'fermentation' industries where such fluids are 'more reactive' than common sewage, etc. The Chemical process industry has zillions of variations on 'remote sensing' ... can be kind of expensive.

 

[Peggie Hall HeadMistress]

There should never be any liquid in a tank vent line

So what would be the purpose of a liquid level indicator in it? That wouldn't tell you when the vent has become totally blocked, only that you've overfilled the tank...again. Do THAT enough times without backflushing the line to rinse it out, and you have a blocked tank vent.

And before anyone brings up tank contents spilling into the vent line when the boat heels...that only happens when the tank vent fitting is in the wrong place (which it usually is). It belongs on, or at, the top of the tank at the edge that's closest to the CENTERLINE of the boat...the side that's closest to the hull. (That's also where the inlet fitting belongs too). That prevents contents from spilling into the vent line (or head discharge line) when the boat heels, because the tank contents flow to the other side of the tank when the boat heels to one side...or the vent line and head discharge line are running uphill when the boat heels to the other direction.

If your tank isn't plumbed this way, it's easy fix with a hole saw, a UniSeal and a threaded plug (available from any hardware store) for the existing vent . At the very least, replace the typical vent thru-hull with an open bulkhead fitting that can be backflushed with a hose every time you wash the boat and/or pump out to prevent the vent from becoming blocked.

 

[Roger Long]

Do websearch for "liquid level indicator" + "hall effect" etc.

Already have one in a box in the basement (see link in OP).

I was thinking about a blockage where stuff gets up into the the vent because of heeling when full (overfilling will never happen again on MY boat) and gets trapped in the hose by the barbs. Enough air passes to let the tank be emptied but the stuff packs up against the through hull hose barb and the vent plugs. This actually happened to me. Pumping was stiff and I found the tank blown up like a balloon but I had recently emptied it. This wasn't the first use after pump out and knowing the liquid level wouldn't have helped.

My boat had the ridiculously small vent of most production boats and the new one will be larger so a pressure alarm probably is lily gilding. The stuff that blocked the small vent probably couldn't span across the larger distance of a proper sized hose without blowing out.[FONT=&quot] [/FONT]

 

[RichH]

Same answer, 'hall effect' instrumentation. The difficulty is that the relative pressure differences are going to be quite small hence the need for 'expensive' to measure the defections on the transducer faces, etc.

IMHO - measuring pressure in a restricted vent on a holding tank is overkill and unnecessary; system design and piping orientation is usually the easiest solution.

A LARGE vent aperture would be the first requirement. Perhaps you would be better served by selecting vee-clamp (Tri-clover, etc.) 'sanitary' type connectors on the hosing and simply and easily remove entirely to a remote location to maintain the vent on a semi-routine basis - as thats the way its done in large volume fermentation vessels when there is a 'foam-out' --- its the simplest (engineering) way. More rugged and cost effective than 'cam locks'.

examples of Tri-clover or Tri-clamp fittings: http://www.cardinalfittings.com/Tri-Clover/Products/Fittings.htm Tri-clovers are also made in 'plastic'; reinforced poylprop being the most common plastic.

 

[Roger Long]

'expensive'

Doesn't work for me. I agree about a pressure alarm not being very necessary which is why my budget for this project is about 10 bucks.

I thought "Hall Effect" was a reference to remote liquid level gauging. I'll have to continue my Google education.

 

[Roger Long]

It (the vent) belongs on, or at, the top of the tank at the edge that's closest to the CENTERLINE of the boat...

Just to play devil's advocate:

If your primary concern is to keep liquid out of the vent, along with any clog promoting stuff that may be floating on top, the proper place for the vent is in the center of the tank. That will give you the maximum liquid level on either tack before the level reaches the vent. If you do fill right up, you may have some go out the vent though.

My vent is in the location that you suggest but I've had vent dribbles because I'm forced to have a short run of horizontal hose before it runs up to the vent. The horizontal hose section fills on one tack when the tank is near full and doesn't fully drain when I come about. Motion and sloshing of the contents then pump the liquid up and out the vent.

I'm thinking of putting my new vent on the tank centerline for this reason. "Full", for purposes of installing and calibrating the level gauge, will be a distance from the top of the elbow equal to 1/4 of the width of the tank. The rest will be emergency reserve. This level would be where liquid could enter (neglecting sloshing) the horizontal section at 30 degrees heel.

A larger vent line which drains quicker may eliminate the problem by itself so I'm still thinking.

 

[RichH]

Just to play devil's advocate:

If your primary concern is to keep liquid out of the vent, along with any clog promoting stuff that may be floating on top, the proper place for the vent is in the center of the tank. That will give you the maximum liquid level on either tack before the level reaches the vent. If you do fill right up, you may have some go out the vent though.

My vent is in the location that you suggest but I've had vent dribbles because I'm forced to have a short run of horizontal hose before it runs up to the vent. The horizontal hose section fills on one tack when the tank is near full and doesn't fully drain when I come about. Motion and sloshing of the contents then pump the liquid up and out the vent.

I'm thinking of putting my new vent on the tank centerline for this reason. "Full", for purposes of installing and calibrating the level gauge, will be a distance from the top of the elbow equal to 1/4 of the width of the tank. The rest will be emergency reserve. This level would be where liquid could enter (neglecting sloshing) the horizontal section at 30 degrees heel.

A larger vent line which drains quicker may eliminate the problem by itself so I'm still thinking.

Agree ... eliminate any 'horizontal' .... minimum to the horizontal at ~45°. If your boat goes over more than 45° youll have other pressing duties besides thinking about poop in a pipe.

 

[Peggie Hall HeadMistress]

Just to play devil's advocate:
If your primary concern is to keep liquid out of the vent, along with any clog promoting stuff that may be floating on top, the proper place for the vent is in the center of the tank.

Not necessarily...it depends on the location of the tank and the shape of it. If the tank lies along the hull and is long and skinny, contents can still run into a vent that's in the center of the top.

A larger vent line which drains quicker may eliminate the problem by itself so I'm still thinking.

It prob'ly would in a 3" vent...but it only takes about twice as long to plug up a 1" vent line as it does to plug up a standard 5/8"...and dirt daubers can even plug up a 3" hole overnight.

And if you want to have a prayer of keeping the tank aerobic, the vent line has to be straight as an arrow, no more than about 5' long (<3 is optimal) and the vertical rise has to be <45 degrees.

It's impossible to engineer enough ways to save every fool from himself...and most of the gadgets that try end up creating more problems than they solve because they fail to take into account that everything on a boat is a component in a system...and anything you do to one component impacts the whole system. There really isn't any reliable way to avoid doing ANY preventive maintenance.

 

[Roger Long]

And if you want to have a prayer of keeping the tank aerobic, the vent line has to be straight as an arrow, no more than about 5' long (<3 is optimal) and the vertical rise has to be <45 degrees.

I'm going to either have to give up on aerobic or give up a berth. Tough choice. I'm going to have to think about that one

I wish you'd been around to talk to the designer of the boat. There isn't another place to put the tank that I can see.

The big ketch that was launched while I was working at Philip Rhodes had it's holding tank vent run all the way up to the top of the mizzen mast. Lack of planning or other error on the part of the engineer shortly after resulted in a Texas style gusher of you know what all over the boat during a cocktail party. That story was legend for years.

 

[Peggie Hall HeadMistress]

That legend is an URBAN legend that began when...

...Malcolm Forbes HIGHLANDER I was built (by Palmer Johnson, I THINK). Every boatyard on the east coast that's built a large sailboat since then has tried to lay claim to it as happening there.

According to legend, the building crew were using the heads once they were installed, even though it was strictly against the rules, and filled up the holding tank all the way to the top of vent line up the mast. The first flush of a guest during the launch party was allegedly the one that created the shower. Only one thing wrong with the story: that boat was built long before tanks were required. It had tanks...and the vent lines did go up the mast. But the toilets were plumbed to flush overboard.


I'm going to either have to give up on aerobic or give up a berth. Tough choice. I'm going to have to think about that one

You shouldn't have to do either. Give me a call...let's talk about it.

 

[Roger Long]

...Malcolm Forbes HIGHLANDER I was built (by Palmer Johnson, I THINK). Every boatyard on the east coast that's built a large sailboat since then has tried to lay claim to it as happening there.

I'm sure it's happened more than once. I know it happened to "Sea Star" because I was asked shortly after in a job interview just a few weeks after the event if I had worked on the piping systems. I was able to tell them the boat was completely designed before I got to Rhodes but I didn't get the job anyway.

 

[Scott T-Bird]

I have decided to add a 10 gallon freshwater fill tank to the bow, just forward of the 10 gallon holding tank. The sole purpose of the tank would be to provide water for the head rather than using nasty Hudson River water. The holding tank has about a 4' vent run to the small anchor locker and the run is pretty close to horizontal. I know it didn't have aerobic conditions in the boat for quite some time as the P.O. left a plugged line which couldn't be blown out at any pressure.

My plan was to tee the vent lines for both tanks for one common vent. That sounds like it may not be such a good idea ...