Bulge in full pot H2O tank

[PJHoffnet]

I recently installed tank level probes on my pot H2O and black water tanks. I had never 'fully' exposed the pot H2O tank, and I found when full, the tank bulged - a little more than an inch or so on each of the long sides (~5% of the three foot width ... tank is about four feet long and one foot high, 90 gallons).

I've lowered the tank to about three quarters full now, and there isn't much of a noticeable bulge. While I have the bed frame that surrounds the tank taken apart, I can easily put in plywood 'stiffeners' on each side of the tank to minimize the bulge.

Should the bulge concern me? Is it worth while to re-enforce the tank? Assuming the tank is OEM, it would be about twenty two years old. TIA

[rslifkin]

What's the tank made of? If it's a plastic tank, it's not uncommon to see a little bulging. As long as it's well supported from below, it shouldn't be an issue.

[peghall]

That depends on the wall thickness vs. size of the tank. Water and waste weigh 8.333 lbs/gal. "Off the shelf" tanks available from marine retailers typically have .25" walls, which is enough to support up to 18 gallons (150 lbs) without bulging...more than that, the tank should be supported on all sides as well as the bottom 'cuz bulging also means flexing as the weight of the contents varies with volume, and flexing will result in a cracked tank sooner or later.

In addition to supporting the tank walls, it's also vitally important to prevent vent blockages 'cuz all tank vents have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing.

Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.

When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing. In fact, the first clue that the tank vent may be blocked is a manual toilet that's becoming hard to pump or an electric toilet that "burps" and spits up flushes.

Do NOT use the toilet again or try to pump out or dump the tank until you've cleared the vent. Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.

All of this was the long winded way to say that if the walls of a tank are bulging even slightly, the walls need to be supported on all sides.

--Peggie

[PJHoffnet]

Originally Posted by rslifkin

What's the tank made of? If it's a plastic tank, it's not uncommon to see a little bulging. As long as it's well supported from below, it shouldn't be an issue.

Plastic tank ... fully supported on the bottom.

Originally Posted by peghall

That depends on the wall thickness vs. size of the tank. ... "Off the shelf" tanks available from marine retailers typically have .25" walls, which is enough to support up to 18 gallons (150 lbs) without bulging...more than that, the tank should be supported on all sides as well as the bottom 'cuz bulging also means flexing as the weight of the contents varies with volume, and flexing will result in a cracked tank sooner or later.

...
--Peggie

Concur with 0.25 inch thickness of tank. Surprised it would support so little (150 lbs ... shouldn't that be a pressure weight/area type of figure?).

- - - - -
Since things are apart now, no reason not to do the re-enforcing. Thanks all for the info.

[peghall]

Concur with 0.25 inch thickness of tank. Surprised it would support so little (150 lbs ... shouldn't that be a pressure weight/area type of figure?).


You've said that it only bulges when it's full. You haven't said how much the tank holds. I only used 18 gallons/150 lbs to illustrate how much water and waste weigh.

It's unlikely that your water tank is anywhere near that small. If you know its capacity, gals x 8.333 will give you the weight that's pushing against the tank walls. If you don't know its capacity, multiply L x W x H in inches and divide by 231 (cu in/gal. Because you're using outside dimensions and because not every last cu. in. in a tank is useable space, reduce that number by 5-8% to get very close to the tank's capacity. Fwiw, 40 gal--which is small for water tank on a 40'+ boat weighs 333 lbs.


-Peggie

[PJHoffnet]

Originally Posted by peghall

Concur with 0.25 inch thickness of tank. Surprised it would support so little (150 lbs ... shouldn't that be a pressure weight/area type of figure?).


You've said that it only bulges when it's full. You haven't said how much the tank holds. I only used 18 gallons/150 lbs to illustrate how much water and waste weigh.

...

-Peggie

I stated 90 gal in original post. So about 750 lbs. The inside of the tank (excluding top since it's not exposed to the weight of the water, has about 2,000 square inches ... so a SWAG would be about .4 lbs/in^2 (I hope, public math is hard)

Originally Posted by PJHoffnet

I recently installed tank level probes on my pot H2O and black water tanks. I had never 'fully' exposed the pot H2O tank, and I found when full, the tank bulged - a little more than an inch or so on each of the long sides (~5% of the three foot width ... tank is about four feet long and one foot high, 90 gallons).

[Cadence]

Originally Posted by PJHoffnet

I recently installed tank level probes on my pot H2O and black water tanks. I had never 'fully' exposed the pot H2O tank, and I found when full, the tank bulged - a little more than an inch or so on each of the long sides (~5% of the three foot width ... tank is about four feet long and one foot high, 90 gallons).

I've lowered the tank to about three quarters full now, and there isn't much of a noticeable bulge. While I have the bed frame that surrounds the tank taken apart, I can easily put in plywood 'stiffeners' on each side of the tank to minimize the bulge.

Should the bulge concern me? Is it worth while to re-enforce the tank? Assuming the tank is OEM, it would be about twenty two years old. TIA

If I understand you, make sure anything you place against the tank to stiffen it is bonded to it in order to preclude chaffing. A flexible bond like RTV might work.

[MikeHoncho]

It’s been doing it for 20 years so.......

[jamhass]

Ain't "just" the weight of the water, but the water pressure. If I overfill my tanks, I add a couple feet of head to the tanks, about 1 psi. This is enough to oilcan the SS tank, making a popping sound that lets me know the tank is full + a bit. That doesn't add appreciable weight, but does add pressure.

[PJHoffnet]

Drained tank (overtime, normal usage) down to about 1/4 full - easy to 'push in' bulged area. Put in simple plywood (with wool anti-chaffing/rubbing protection) pieces screwed into frame. Refilled tank. No bulge. Project complete. Bed frame restored. Admiral happy.

[David B]

Originally Posted by PJHoffnet

I recently installed tank level probes on my pot H2O and black water tanks. I had never 'fully' exposed the pot H2O tank, and I found when full, the tank bulged - a little more than an inch or so on each of the long sides (~5% of the three foot width ... tank is about four feet long and one foot high, 90 gallons).

I've lowered the tank to about three quarters full now, and there isn't much of a noticeable bulge. While I have the bed frame that surrounds the tank taken apart, I can easily put in plywood 'stiffeners' on each side of the tank to minimize the bulge.

Should the bulge concern me? Is it worth while to re-enforce the tank? Assuming the tank is OEM, it would be about twenty two years old. TIA

If the tank is bulging when full, imagine the flexing of those sides when you are at sea, and the tank is only partially full. I suggest that un-supported sides - especially if there is a molding seam there, are, splits waiting to happen.

I would support with decent thickness plywood, and a then layer of felt or similar between the tank and the ply.