Trailer shook with a boom

[Pitchbb9]

2010 3400RL
I had just drained my black tank and was backwashing the tank. I heard a boom / thud and the RV moved a little. I thought the worse, my tank had fallen and the connections have cracked. I immediately went out and pulled the drain and shut the flusher off. I observed water dripping from the underbelly.
I have pulled down the underbelly sheeting and found my tank has not fallen and appears to be intact. The connection for the stool looks good. I have not looked at the vent connection yet.

Any Ideas?

Could there have been trapped air in the tank and it was forced out of the vent connection?

Maybe there is a hairline crack that I haven’t found yet?

[DQDick]

Check and make sure you don't have a crack on the upper side of the tank.

[Pitchbb9]

I assume I need to fill the tank and see if there is a leak in the top of the tank.
I also failed to mention that there was dirty water in the stool.

[firestation12]

The black or gray tanks should never be flushed with their drain valves closed. Upon filing the black tank to check for a leak, you'll find out why. The dirty water in the stool is from the pressure in the black tank trying to find a way out. Even with a vent and or an overflow tube, hose pressure will quickly overcome them and cause the tank to fail.

[Pitchbb9]

So based on the fact I had water leaking from under the RV, I have cracked my tank?

[firestation12]

Originally Posted by Pitchbb9

So based on the fact I had water leaking from under the RV, I have cracked my tank?

I suppose there is a VERY small chance the vent pipe in the top of the tank (which is usually held in by a type of putty compound) was pushed out, which would have released products from your flushing process without damaging the tank. But, I'm betting the outcome of this will be a tank replacement. A typical black tank has a L X W measurement of 20" X 40" = 400 square inches. With just 6 psi of overpressure (because the drain is closed), this results in 2400 pounds of lifting pressure applied to the tank's top. Yes, there is a vent in the top of the tank that extends to the camper's roof, but it takes approximately 6 psi of pressure in the tanks to cause the water to rise to the roof (.434 psi per foot of rise). If the black tank's contents were to block the vent, just imagine what the garden hose supplying 50 psi to the tank's 400 square inches could exert to the tank's top (400 X 50 = 20,000 lifting pounds. After emptying my black tanks, I have on occasion closed the tank dump to partially fiil the tanks for a second flush, BUT, I always set my smart phone's timer for no more than 2 1/2 minutes so I don't get distracted and forget. It's risky to do this without a fail proof reminder!

[Pitchbb9]

Is the best removal method to cut the basement floor and lift the tank out?

Would a plastic weld work on a cracked tank?

[Daryles]

You said you were flushing, boom. Water dripping down from underbelly. You checked for obvious damage. Is your flushing system, hose, still intact? A hose under pressure coming off the flushing device and whipping into the side of the skirt could cause a boom and a shudder. And explain the water dripping down.
Hoping this is it. Easy fix.
Your year RV has a tank that has a tub and lid glued on top ( 2 piece). Worse case, the lid blew off. Hopefully not. Only one way to find out. Drop the coroplast and inspect. Get an endoscope (Amazon). Also while you have the bottom off, go ahead and give everything an upgrade. Valves, tank heaters, tank gauges, extra insulation etc....
Hoping for the best..
Let us know what happens

[PeteandJoan]

Originally Posted by Pitchbb9

2010 3400RL
I had just drained my black tank and was backwashing the tank. I heard a boom / thud and the RV moved a little. I thought the worse, my tank had fallen and the connections have cracked. I immediately went out and pulled the drain and shut the flusher off. I observed water dripping from the underbelly.
I have pulled down the underbelly sheeting and found my tank has not fallen and appears to be intact. The connection for the stool looks good. I have not looked at the vent connection yet.

Any Ideas?

Could there have been trapped air in the tank and it was forced out of the vent connection?

Maybe there is a hairline crack that I haven’t found yet?

This is exactly what happened to me when someone left the flusher running and went into rig....BOOM, thought someone had rammed us with a truck . Water was running down front DS jack, some idiot left the water running, took about 8-10 min. Split the tank at the seam where the top half meets the bottom half. I time it now . Learned the hard way.

[Montana Man]

Keystone sometimes uses two piece welded tanks. They do not mount those tanks according to the manufacturers recommendations. As such, merely filling the tanks can cause them to break and yes, cause the boom you heard.

[Carl n Susan]

The tank will come out from under the rig. There is no need to remove the floor above it. As others have written, the top of the tank has separated from the tub and likely will need to be replaced.

[firestation12]

Originally Posted by Pitchbb9

Is the best removal method to cut the basement floor and lift the tank out?

Would a plastic weld work on a cracked tank?

As Carl & Susan replied, the tank must be removed from under the camper. The underbelly membrane (Coroplast...short for corrugated plastic) was the first step which you have done. To answer your question,"Would a plastic weld work on a cracked tank?" No, your tank will not be repairable, it will need to be replaced. Once the Coroplast is removed, the process of replacing the tank should apparent. If it isn't apparent, then don't attempt to do it yourself. Be nice to yourself, you are not the first one to do this.

[Ozarkwoods]

If the vent was blocked or plugged for some reason I can see the pressure from filling could burst the tank. But if the vent was fully functional and working correctly I can’t fathom pressure building. Even if you happen to over flow the tank it should go up the vent stack and flow out the 1 1/2” vent pipe. So I would seriously look at your vent being plugged or improperly installed.

[Rondo]

Yup, Carl beat me to it again. It sounds to me that you have a separated tank. The tank is a two piece tank and the top is plastic welded to the bottom bowl. This boom and water in the belly is a very common thing with units from before 2015 or so. You have to remove the tank from the underside of the unit. When you do so just be careful because there is still a lot of "stuff" in that black tank even if you empty it. If you do it yourself make sure you put either some extra bracing under the tank or strapping to hold the tank up. The only thing that is holding those tank up is a small lip on the edges that sit on the frame of the unit. I have had to replace all three of mine tanks and I've had a service tech do it because I'm not that brave under that unit when it comes to dropping a tank and especially the black tank. I flush my black tank from both directions. The regular tank flush and also a flush elbow that connects to the outbound sewer pipe. I do close my valve when using the regular tank flush BUT I do not leave the area of the tank valves or leave it run over 5-7 minutes then dump and if needed do it two or three times that way and then back flush from the outlet side several time also.

[richfaa]

If I recall correctly at the factory tour at the MOC rally at least 2 years ago we were told that the 2 piece tank is no longer used in the Montana line and we did observe the one piece tanks on the builds on the assembly line. Rondo said and he is absolutely correct "The only thing that is holding those tank up is a small lip on the edges that sit on the frame of the unit.

[firestation12]

Originally Posted by Ozarkwoods

If the vent was blocked or plugged for some reason I can see the pressure from filling could burst the tank. But if the vent was fully functional and working correctly I can’t fathom pressure building. Even if you happen to over flow the tank it should go up the vent stack and flow out the 1 1/2” vent pipe. So I would seriously look at your vent being plugged or improperly installed.

The sewer vent pipe is NOT a pressure relief device. It's purpose is to allow sewer gasses to escape. Water does not magically rise to the vent's top without consequences. For water to rise up to the top of the vent pipe, requires a pressure of approximately 5 psi at the base of the vent. That 5 psi BACK pressure pushing the water up to the top of the vent, is exerted on all directions inside the tank. The back pressure acts cumulatively at right angles to flat surfaces such as the top, bottom and sides. Since the top and bottoms of the tanks have the greatest number of square inches of surface area, the 5 psi times the total area, which in the example I gave of 20 X 40 inches, creates a total lifting force of 2400 lbs. Think of how an air mattress supports the body weight with only a fraction of a psi inside or a tire supports thousands of pounds with only a few pounds of air pressure. Propane gas pressure in your trailer is measured in the amount of pressure required to lift a column of water 11 inches or about .34 psi.

[prndl]

Here you go:
https://www.icondirect.com/holding-t...drain-ht543ed/

[jcurryii]

Originally Posted by Pitchbb9

So based on the fact I had water leaking from under the RV, I have cracked my tank?

These tanks are made in a mold and the top is a separate piece that is glued on. From what I have heard from repair guys is that the top where it’s glued is the weakest point. Repair can be made but most likely the tank will have to be removed. You might be better off to replace as there could be other places around the top that have been weakened.

[katdon]

Had an RV neighbour that cracked his tank doing the same thing. I am thinking that's what you did.

[Bnrc73]

Had the exact thing happen on my 2010 3400RL. My black tank was made with an open tank and the top was glued on. The glue let go on one side. Only leaked when it was full. Mine wasn’t fixable. Had to replace the tank.