My black tank ruptured today.

Rene T said:

I still don't see how the tank would get pressurized. The nozzle has about 8 small holes in it which are probably 1/16" in diameter. Not much water going into the tank and then there's the vent. It still should have gone up the vent.

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This is the part that baffles me too! Been racking my brain trying to come up with a scenario that might explain the leakage without the tank actually being pressurized.
So far, a blank.

I am wondering if the vent has slid down into the tank some so that it gets covered before the tank is full.  I know you think it should come out the roof vent, however the pressure required to get the liquid to that height is likely more than the tank is designed to hold.  2' of water equals 1 psi.  Most tanks of that sort are only designed to between 3 to 5 psi(6 to 10'), so it is possible that the liquid is rising up the vent but you are exceeding the design pressure before liquid comes out the vent on the roof.

boatbuilder said:

I am wondering if the vent has slid down into the tank some so that it gets covered before the tank is full.

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Good point. But wouldn't there be signs of water leakage before the bursting point of the tank if the stack seal at the tank failed and the stack slid down into the tank?
Anyone know what actually secures the black water vent stack in place?
This is a learning session for me.

boatbuilder said:

I am wondering if the vent has slid down into the tank some so that it gets covered before the tank is full.  I know you think it should come out the roof vent, however the pressure required to get the liquid to that height is likely more than the tank is designed to hold.  2' of water equals 1 psi.  Most tanks of that sort are only designed to between 3 to 5 psi(6 to 10'), so it is possible that the liquid is rising up the vent but you are exceeding the design pressure before liquid comes out the vent on the roof.

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Good thought and point.

When I dump the Black using the tank flush..

1. Open the valve, let all the contents drain to a dribble..

2. Close the valve about 3/4*, turn on the flusher for about a minute. Open the valve, drain to a dribble.

3. Do step #2, 3 or 4 times, then close the valve to put some clear water in the tank. Then dump the Greys to flush the sewer hose.


*There's a  Warning Label above the Tank Flush fitting that sez: Do Not flush tank with drain valve closed.

(Frist Rig we've had that has that Warning Label ?)

On Edit. Hope this might help others in the future, also ?



 

I believe boatbuilder hit the nail right on the head. I just called the tank manufacturer. They said they test their tanks to 2 PSI.  I know one thing for sure, I'm going to change my flushing procedure TODAY.
Hope this information helps someone else in the future. 

Rene T said:

I believe boatbuilder hit the nail right on the head. I just called the tank manufacturer. They said they test their tanks to 2 PSI.  I know one thing for sure, I'm going to change my flushing procedure TODAY.
Hope this information helps someone else in the future.

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But I still don't understand how the tank would even get to 2psi pressure with an open vent into the same tank!
Doesn't make sense to me. Assuming the stack didn't slide down into the tank.
Again, what secures the vent stack?
Am I crazy?

No you are not crazy, well maybe a little, you have an RV.
Normally the vent stack is secured to the RV structure and is just sealed in the top of the tank with a Uniseal or similar sealing collar.
A column of water has weight that presses down on what is under it.  If you do the math, a column of water 2 feet tall will exert 1 pound per square inch of pressure.  This occurs even if the top of the column is open.  This is why water towers are built the height they are.  The height of the water level keeps the pressure in the system so that pumps just keep the tanks at the proper level and don't need pressure regulating devices.  The open top will keep the pressure from building up to the system pressure but will still exert enough pressure to cause damage to a tank that is not designed for the pressure.

My vents and drains (with the exception of the toilet) are attach with  pipe fittings.  The female is glued to the tank port and has a shoulder/flange.  The pipe has the male end.  No way it's falling into the tank.  I just don't see any way the tank is being pressurized.  The warning to keep the drain partially open IMO is to avoid over filling and possible flooding.

The most obvious answer is usually the correct answer.  And in my mind it's either a faulty tank design/installation, or regrettably an improper repair/replacement from the original incident.

At any rate all will be revealed when the tank is dropped, and personally I would do this myself to try and determine the cause.

Good luck with it!

The tank may not have burst. It could be leaking around where the toilet flange fits into the top of the tank. I am not sure but I believe  that on some tanks there is a rubber bushing lining the hole and when you push the toilet flange through the bushing it make a seal. Probably a really good seal for sloshing and splashing liquids but not much of a seal for any pressure. I will be curious to see what he finds when he drops the tank.

Charlie, I understand your explanation of the weight of the water in the vent stack. I agree. What I don't understand is how the vent stack would slide down into the black tank. It's not held in place by the tank seal alone, is it?

Some vent stacks are attached to a threaded fitting that is spin welded into the tank.  These are susceptible to vibration.  Some are just pushed through a sealing collar and attached to the structure of the RV.  Sometimes the straps will loosen and let the vent pipe slip down.

I learned something today. Thank you, Charlie. I just never heard of this kind of problem before.

BIG JOE said:

*There's a  Warning Label above the Tank Flush fitting that sez: Do Not flush tank with drain valve closed.

On Edit. Hope this might help others in the future, also ?

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Ours has the same warning

The way my vent pipe was installed is they drilled a hole what looks to be around 1 1/2". Then there's a fitting with maybe a 2" flange which drops into the hole after you load it up with a large amount of some type of black sealer. Once it cures, they then drop in the section of ABS piping and glue it in just like you would with PVC. There is a lip inside the adapter which stops the pipe from dropping into the tank. All the fittings going into the tank are tight. The water is coming from somewhere near the top and I believe it's probably the tank top which may have separated from the tank flange.

Good news. I called the dealer which installed the tank last July and he's going to make good on fixing it.  At the time, he said his work was guaranteed for a year. I have a appointment for May 2nd after we get home. I will ask to see the tank once they drop it.

I did dump my tank today after using it for 2 days and I used the flusher. I didn't see any evidence of any leakage, so I'll keep dumping every 2 days. After the tank was done draining, I shut the drain valve, turned on the flusher for 5 minutes, then I drained it again. I did this 3 times and crap was still coming out.  That's going to be my procedure from now on. You can take that to the bank. 

Good deal X2! You found out the tank will be fixed at little expense, if any, and your protocol of dumping works.
Sounds as if you will be a happy camper again in short order. That's all that matters. Sounds like maybe the sealant used on the prior repair failed. Good news the dealer will stand behind their work!

Hang in there. May will be here in no time and you can set your mind at ease again.