Waste tank expansion (movement) on top side

[Larryect]

Well, I had a real brain fart this past weekend.......

Was setting up and hooked up the water thinking I was hooking up to the city water inlet.

Instead, I had hooked up the water to the tank flush and turned it on.... Was sitting out side when we heard a "POP" and got splashed, then water started running of the roof.....

Well, shut off the water and started investigating. Figured out I hooked up to the flush and overfilled the black tank. Drained the tank and started looking for damage.

No sewage inside. But, the vent pipe and toilet moved upward with enough force to rip the toilet mounting screws out of the floor.

I decided to refill the tank from the toilet and check for leaks. Found no leaks. But as the tank got very close to full, somehow it flexes suddenly and lifts the toilet and vent pipe. The vent pipe moves upward about 3 inches..... Toilet not as much.

Would have been no damage except for the tank flexing. It seems really odd to me that the top of the tank would move that way and with so much force.

I am think of cutting the drain pipes apart inside the storage area and reassembling with flexible couplings to allow some movement so future potential flexing doesn't rip the toilet off the floor and ruin the vent pipe seal on the roof.

Have any of you ever heard of such a thing?

[Larryect]

mikensallyt wrote:
Me, being such a newbie and I dread forgetting to turn off the water. Don't want or need poop all over the place. That's for my young pup to keep me on my toes. I know there's (for lack of the correct word) sensors that need cleaning just in case. But couldn't I just leave the drain open while running water through it?

My problem occurred because my black tank flush connection is right next to my fresh water hook-up and when hooking up, I connected to the wrong fitting, and I did not realize I was filling the black tank.....

Per the instructions you are supposed to leave the drain valve open when using the flush.

If you are asking about normal usage - It is not wise to leave the black tank valve open when using it. It is better to allow waste to accumulate and then drain it out after it has filled some. The flushing action helps carry solids out of the tank. Otherwise you can get a pyramid of solids building up under the toilet drain pipe.

The grey tank drain can be left open if you want while in use. Some people do that. I generally let it accumulate for a day or two before dumping, but once I hook the drain hose up, I may leave it open.

The sensors are notoriously not very accurate, but give a general idea of the status of the tank.

[mikensallyt]

Me, being such a newbie and I dread forgetting to turn off the water. Don't want or need poop all over the place. That's for my young pup to keep me on my toes. I know there's (for lack of the correct word) sensors that need cleaning just in case. But couldn't I just leave the drain open while running water through it?

[Larryect]

Okay,

I pulled the underbelly down to check things out. The tank is supported by supports on both sides (across the width of the trailer). The front brace is welded and the rear one bolted in place. Nothing was broken. The tank has lips molded in that sit on the braces.

I filled the tank again and found nothing looking wrong underneath. The toilet and vent pipe moved up again. But this time it seemed to be gradual. I think the time before, I thought it was sudden only because my foot may have slipped off the valve. The floor of the storage compartment also flexed upward significantly.

Taking measurements indicated that the supports underneath didn't move, but the tank did pull in a bit, away from the support.

Looking online it seems that 30 inches of water column creates about 1 psi of pressure and 120 inches is about 4 psi.

The tank seems to be made of a fairly thin plastic. Pushing on the bottom of the tank when empty moves it pretty easily.

My conclusion is that the build up in pressure (up to around 4 psi) caused the tank to expand like a balloon - top and bottom. Even pulling the sides in a bit.

There is some space between the top of the tank and the storage compartment floor. But the flange holding the drain pipe is tight against the floor so it pushes the floor up as soon as it starts to move.

I think originally, the resistance of the floor, the toilet flange bolts etc. held against the pressure until the flange bolts let go. They were only about 1/4" sheet metal screws into 1/2" plywood.

I ended up reinforcing the toilet mount by taking about a 12" square of 3/4" plywood, cutting a hole for the 3" drain pipe, cutting the drain apart, sliding the wood up underneath the bathroom floor and mounting it there with some screws. Then I went inside and drilled 1/4" holes through the flange mounting holes all the way through the floor and plywood. Then I put 1/4" x 4" bolts through and secured them underneath with fender washers and nuts. I reassembled the drain pipe with a couple of flex couplings.

I also put a clamp around the 1 1/2 " vent pipe to the side of the plywood support to give it some resistance to movement. Then I added a 1 x 2 furring strip to the edge of the storage compartment floor to stiffen it some. I screwed it into the front compartment vertical divider right at the bottom edge.

I don't know for sure if this is enough reinforcement to prevent all damage if the tank is overfilled in the future. But hopefully it will help.

I think the key thing is going to be to not overfill the tank again.............

[Dayle1]

Had a friend do the same thing a few yrs ago, but his tank definitely flexed down due to the weight and expansion. His underbelly was rigid plastic and the expansion started popping screws holding the plastic. Also some water leaked around the toilet wax seal. If an oversized tank was installed that was taller than the I-beam, I can see it expanding when filled to capacity, but I can't imagine any tank being so well secured underneath that it would expand upwards. Curious to learn what you ultimately find out.

[Larryect]

Dannyabear1 wrote:
I don't think there is a fitting on top of the tank, just a tight hole cut in the top of tank and black pipe pushed into it.

There is definitely some sort of fitting. Looks like a flange glued to the tank. Right now I am not sure if it could be the soft type in the diagram above. But, I think from what I saw (and remember) it is a glued flange with pipe glued into it.

[Larryect]

That is an interesting connector.

I can see a bit of the tank top through the floor of the storage compartment, where the pipes are connected to the top of the tank. Sure looks like it is glued. But, the flange in your picture may be hard to tell the difference from the little I can see. I will have to look at it again. They may use a soft connector on the grey and solid connections on the black.....

I will drop the plastic covering under the trailer to see if I find anything broken underneath when I have time. Maybe this weekend if my schedule allows.

[Old-Biscuit]

Uniseal for waste tank vent line



Soft seal for waste tank line



Then again some RV mfg. hard connect waste tanks lines

Until tank is inspected one doesn't know for sure......and sometimes that requires dropping tank

[Dannyabear1]

I don't think there is a fitting on top of the tank, just a tight hole cut in the top of tank and black pipe pushed into it.

[Larryect]

rrbowman wrote:

Larryect wrote:
I am thinking there is some odd wrinkle in the tank that changes due to the weight of the water.

Possibly I should open the bottom of the trailer so I can see better what is going on.......

I'm thinking that what has happened is that a brace or strap that holds the tank in place has broken and the tank when full is shifting or sliding because of the broken strap or bracket. I recommend that you remove the heavy plastic covering from beneath the trailer and look to see what is actually happening. If not the fixes you are thinking of trying will lead to a complete failure. I believe the tank is actually tipping over due to the bracket or strap being broken which would result in the toilet lifting and the vent pipe being forced upwards.

That would make sense. I will have to pull the cover off and look before doing anything else........

Thanks

[rrbowman]

Larryect wrote:
I am thinking there is some odd wrinkle in the tank that changes due to the weight of the water.

Possibly I should open the bottom of the trailer so I can see better what is going on.......

I'm thinking that what has happened is that a brace or strap that holds the tank in place has broken and the tank when full is shifting or sliding because of the broken strap or bracket. I recommend that you remove the heavy plastic covering from beneath the trailer and look to see what is actually happening. If not the fixes you are thinking of trying will lead to a complete failure. I believe the tank is actually tipping over due to the bracket or strap being broken which would result in the toilet lifting and the vent pipe being forced upwards.

[Larryect]

There would have been some pressure the first time - whatever it takes to force the water up the vent pipe at least....

[Larryect]

I am thinking there is some odd wrinkle in the tank that changes due to the weight of the water.

Possibly I should open the bottom of the trailer so I can see better what is going on.......

[Larryect]

Don't know the GPM. But, the second time we were filling through the toilet water valve, only, with the toilet flush valve open - allowing no pressure build up, when the tank again suddenly moved again knocking my foot off the valve. It was like something flexed sudden like. Not a slow pressure build up. But with the 3 inch flush valve open watching water flow into the drain there is no way there could be pressure.

[Old-Biscuit]

Larryect wrote:
Thanks for the thought.

I am planning on bracing and re-securing the toilet by using wood underneath the floor and running bolts all the way through the flange with nuts and washers. But, I was thinking the flex couplings would help prevent transferring the tank movement force to the flange in the future. The piping is not a straight through affair. There are 2 45 degree fittings creating on off-set drop which is where I should be able to allow a little flex.

I don't think the tank had any more pressure exerted on it other than the weight of the water.

How can a 5/8" hose going to probably a 3/8" inlet line to the tank flush let water in the tank faster then a 1-1/2" vent line let air out?

The 2nd time the tank moved, the toilet valve was wide open. It seems to me when it gets a certain amount of weight, it suddenly flexes in a manner that causes the top of the tank to move.

Another thing I found odd was, in the past I called the manufacturer about the grey tank leaking when overfilled and I was told they use a soft connection at the top of the tank to allow movement and flexing to happen without breaking things. But now I find nothing but hard pipe glued solidly between the toilet and black tank.

Tank movement is upward.that is not from weight of water

Water going into tank is under pressure...at how many gallons a minute???

Soft connection on tank is by using a grommet at top of tank where line connects to tank. Line slips thru grommet into tank. Grommet seals tank opening and allows for minor movement..........not over-pressurizing of tank as witnessed by toilet ripping away from floor and blowing out top of vent line creating a poop shower.