Flexible Water Tanks - Thoughts?

[Jeremy]

I am at the stage where I need to order some water tanks:- I need three tanks (large fresh and grey tanks, which will be the same size, and a much smaller black tank). Because my bus is a full-chassis'd and mid-engineed I've only got small bays, but have lots of empty spaces in the middle of the bus, around and above the chassis rails. The water tanks will be going into these spaces, but since the spaces are completely open and irregularly-shaped, getting the tanks in to the spaces, and then fastened-down, will be a little tricky.

So I've been doing some research into tanks, and have looked at various options. I'm not going to attempt to make them from scratch myself, and can probably avoid having bespoke tanks custom-made - there are plenty of off-the-shelf plastic tanks available which will fit into the spaces I have - but I still have the problem of physically getting them in there.

During my research I have come across flexible water tanks made for fitting into boats. Here's a description from one supplier:

Vetus flexible waste water tanks can be installed quickly and will assume the shape of their surroundings. Features odour tight material with fully welded the seams, plus additional welded reinforcing strip. This makes the VETUS flexible tank resistant against much higher pressures, especially if the contents are moving when the boat is rolling or pitching.

The fabric of VETUS flexible waste water tanks consists of three layers:

    a wear resistant layer
    a reinforcement layer
    an odour tight layer




Is it me, or do these things sound ideal for my situation? For each tank I could build a sturdy outer compartment of a size and shape which made best use of the space available, and then simply place the flexible tank inside it - and I'd end up with a more storage capacity than I could achieve with a rigid tank, due to the difficulty of maneuvering a rigid tank into place. The tanks are available in sizes up to 220 litres (60 US gallons), and obviously you could daisy-chain them together to give more capacity - although that would start to get expensive.

I'm not clear on whether these flexible tanks could be used as black tanks though - and in some ways it's the black tank that's giving me the most problems, as it's the space I've allocated to the black tank which is the most awkward shape. I have two matching large rectangular spaces which will suit my fresh and grey tanks, but the problem there is that to get rigid tanks into those spaces I will need to jack the bus 2 or 3 feet in the air.

Any thoughts on the flexible tank idea?

Jeremy

[scanzel]

I can see the flexible for fresh water but on the black and gray how would you vent them. Gases building up in them could cause a problem with flushing the toilet and draining sinks and showers. Why not have someone build you some customed sized tanks. Make mockup's of the tanks with cardboard and then have someone duplicate them in stainless or plastic.

[Oonrahnjay]

     Two thoughts.  Here in the US, it's common to find a shop with a "pit" -- these are service depressions in the shop floor that are slightly narrower than the width of the inside wheel tread of a vehicle (some are deep enough for the service person to stand in, others are "lie down pits").  Is there any way to find one available to you?  You could do your fabrication for your support structures (I'd suggest a full lower flat pan with folded up sides of about 1 1/2 to 2" and suitable hanger brackets), drive to the pit, put your tanks in, position the bus over the pit, position the tanks and bolt up the supports.  Then you could drive back to your usual work area and complete the pipe installation.  You should be able to complete your work in minutes and minimize the amount you'd pay for the rent of the pit.

     Second, a bus is going to have much heavier vibration and front-rear-side-vertical G-forces than a boat that the flexible bags were made for.  I'd be very leery of a flexible bag; I know they're listed as "anti-wear layer" etc., but I just don't see how they'd last in a bus.  Also, buses get road dirt, salt, road water splashed everywhere.  That makes an abrasive soup that will cause wear to components that can move -- as a bag of water (or "black tank") would.  My opinion ... "maybe - just *maybe*" and you'll sure be unhappy if you have a black tank failure down the road.

     Just my opinions.   BH  NC USA

[Jeremy]

I can see the flexible for fresh water but on the black and gray how would you vent them. Gases building up in them could cause a problem with flushing the toilet and draining sinks and showers. Why not have someone build you some customed sized tanks. Make mockup's of the tanks with cardboard and then have someone duplicate them in stainless or plastic.

As I explained, I don't need custom tanks - just a way of getting a tank into a space that's difficult to access.

I think the problem with using the flexible tanks for the black tank is that they don't appear to be available with connections bigger than 38mm - and yet they sell 38mm diameter 'sanitation hose' to use with these tanks, and a 'WC extraction' fitting for the end of that hose. That suggests black tank to me, but I assume only in conjunction with a macerator.

Either way, I don't think having a vent for a grey tank would be a problem. The manufacturer apparently factory-fits one connection fitting themselves, and provides a second fitting loose to be fitted by the customer. And additional fittings can be bought and installed if required - but none of them bigger than 38mm.

Jeremy

[junkman42]

Flexible tanks have been used in airplane wings since the 1940's.  I would think that modern materials would hold black water safely.  In aircraft wings there are snaps that hold the tanks from collapse when they are empty.  Vents do not seem to be a problem for the flexible tanks used in boats.  The forces on tanks in boats far exceeds the forces in the hold of a bus i would think.  You can fold flexible tanks up and insert them in places no rigid tank will ever fit.  My take for what its worth.  Regards John L

[Jeremy]

     Two thoughts.  Here in the US, it's common to find a shop with a "pit"...

That's a brilliant idea, and yet so obvious I don't know why I didn't think of it 

When I first bought the bus I took it to a local place for a check-over and to have a tow hitch fabricated and installed - and they had a full size pit. It was run by a friendly and helpful guy, and I could definitely take it back there to get the tanks in place. Assuming it's not closed down since then, which I have a sneaking suspicion that it might have done. But even if they have I dare say I could find somewhere else with a pit - it's a good idea, thanks.

I'm not sure about whether the stresses on a tank in a bus are greater than a tank in a boat - apart from in an accident, I don't think a bus would ever get close to generating forces equivalent to a heavy boat falling off a wave in a big sea for instance. Certainly using a flexible tank on the underside of a bus would require it to be fully protected from beneath, hence my 'sturdy outer compartment' idea. But I'm sure I would still be happier with conventional rigid tanks, if I could get them in there.

Jeremy

[Oonrahnjay]

Flexible tanks have been used in airplane wings since the 1940's.  (snip)

    True, but we're talking about a different environment under a bus.  I hope someone will try this out but I don't want it to be a friend of mine.  And I like Jeremy.  

[Jeremy]

I do know that all race cars use flexible tanks for the fuel, for safety reasons - as the fuel is used up the tank collapses, thus avoiding the large chamber of highly explosive vapour which an almost empty rigid fuel tank represents. So I don't doubt that the tanks are tough enough in principle to hold dangerous and volatile liquids, and resist big forces.

But as Oonrahnray points out - it's a nice theory until you're the one having to deal with a black tank which has split and deposited it's contents everywhere.

Jeremy

[Seangie]

Jeremy,

Keith (PCC) posted that he used bags for his freshwater in his conversion.  He posted in an earlier conversation when I was asking about tanks.  You might be able to PM him directly for more details and pictures.  His post/reply to me is here:
http://www.busconversions.com/bbs/index.php?topic=22752.msg249878#msg249878

-Sean

[Iceni John]

Getting whatever tanks you decide on up into position and secured should be the least of your concerns at this time!   While a pit makes such lifting easy(ish), there's nothing wrong with jacking up the bus to get tanks in place.   For each of my four tanks I jack up one side of my 27,000 lb bus with a 12-ton bottle jack under the front axle and a 20-ton under the rear, blocking under the tyres with 6 x 8" lumber, then slide the tanks in under the bodyside, lower the bus down to terra firma, manhandle them up the first 9" so I can then jack them up into position and secure them.   With their support frames, and with full plywood sheathing for the fresh water tanks, each tank is up to 100 lbs, but they're nothing that leverage and sweat can't move!   Just make sure that everything will fit before The Big Lift  -  I test-fit and triple-check everything beforehand to prevent surprises later.   This Saturday I'll be installing my grey tank, and my poo tank the following weekend.   Oh frabjous joy!

You may run into a similar situation as my drain pipe routing dilemma that I recently posted about here.   Us full-frame folk have to think about things that other bus owners take for granted!   Venting for the fresh tanks is easy, but the grey and poo tanks' vents need careful consideration.

FYI, all my tanks are roto-moulded 3/8" polyethylene, with umpteen fittings spin-welded in.   So far, so good.

Good luck, John

[TomC]

You'll always get more capacity by using square tanks that fit the space closely, then the flexible tanks.  Venting can be a problem, and sloshing around. 
I had Ronco plastic tanks on my truck.  They were mounted under the drive shaft on the fuel tank supports.  They were bouncing around (on 3/4" plywood supports) for over 800,000 miles without problem.  Needless to say my tanks are once again made by Ronco.  Good Luck, TomC

[Jeremy]

You'll always get more capacity by using square tanks that fit the space closely...

Square tanks will only closely fit a square space, and then only if you can get the square tank into the square space in the first place...


Iceni John is probably right in saying that I'm focusing too much on the wrong thing - and it's also possible that I'm worrying too much about trying to maximise the use of the space I've got.

So, different question:- for those people who have got dedicated (ie., toilet only) black tanks - how big are they? Would you rather have more capacity, or could you stand to have less?

I can work out how many flushes a tank would accommodate, but I don't really know how people decide when it's time to empty them (ie., whenever you get the chance, only when it's full, at the end of every trip, at the end of several trips etc)


Jeremy

[Iceni John]

I've gone to great lengths to fit in the absolute largest size tanks that will possibly fit!   With my two fresh tanks I have only an inch or so of space around them, and with my two waste tanks I'm down to small fractions of an inch space around them.   I'm literally calculating their spacing and fitting to eighths of an inch!   Even if I had Ronco custom-make grey and black tanks for me, they couldn't have been any larger at all.   As a result, I've shoehorned in a 65 gal poo tank and a 115 gal grey.   Sure, it would have been much easier to use smaller tanks, but I'm only doing this once so I'll do it as best as I can, and I would rather have too much capacity and not use all of it than wish I had more.   With a loo that's advertised as needing as little as one pint per flush (I think it will probably use slightly more in reality) positioned directly over the poo tank, and a thrifty use of water elsewhere, I should be able to comfortably manage at least one month off-grid, and for two of us at least a fortnight.   (Bear in mind these are emasculated colonial gallons, not the full-bodied Anglican versions!)

The generally-accepted wisdom is that poo tanks should not be emptied until they're at least half full (or half empty, if you're a pessimist).   This way there should be sufficient flow of efluvium to encourage the "solids" to proceed vigorously and without hesitation through the slinky, to prevent the dreaded constipation there.   I also have a small transfer pump to move excess grey water into the poo tank, partially for this reason and also to free up space in the grey tank.   A more vertical than flat configuration of poo tank will also promote better dumping flow.  

Fitting in the biggest possible tanks wil drive you crazy, and you'll be solving problems you didn't know could even exist, but at the end of it all you'll not have any regrets that you could have done anything better.   Think of it as a character-building exercise, sort of like being in the SAS but without being shot at.

Isambard Kingdom John

(I sometimes wonder if a psychoanalyst would worry about our obsession with the by-products of natural bodily functions.)

[Jeremy]

Interesting...my own instinct is also to extract the most you can get out of the space available, in order to avoid the dreaded "If only I had..." thoughts afterwards. Hence four slideouts, roof patio, huge awning, etc etc.

I was actually hoping that I might be told not to try too hard in this situation though, and that huge tanks weren't really a big benefit.

Any other views?

Jeremy

PS - Isambard Kingdom John - don't overdo it - remember that Brunel died of overwork aged only 53.

[Seangie]

Jeremy,

We have 4 kids - So we figured we would need a minimum of 100 fresh, 100 gray.  We are figuring on mostly being in campgrounds but in the occasion we are in between its better to have and not need than to need and not have.  If we are very thrifty with our water usage I figure maybe 2 possibly 3 days off the grid... factor in eating out and using public restrooms and we might get 4 days out of it .  The loo will only be 40 gallons so we will have to be conservative there.  We probably could have installed much larger tanks but storage space will be as important as fresh water and the potty with 4 kids.

Finding the right tanks was a bit of a challenge but with the Eagle the layout is very simple unlike what John is forced to be creative with.

Our bus is not huge...8 x 40 only 2 axles.  Roof is not raised so it will be tight for us.  Benefits are miles per gallon.  This is our first bus and our first adventure of anything of this nature so I want to keep it as simple as possible.  Once we figure all this stuff out and the kids are out of the house we will go big

I took Tom C's advice and ordered my tanks from Ronco today.  Linda from Ronco was awesome and worked with me on the pricing.  Best price I could find going from 1500.00 quoted from plastic-mart to 1000.00 quoted from Trionic to about 750.00 from Ronco. And if the tanks hold up for 800k mile I am golden

Lastly...to follow up on your original question...my preference is to use hard tanks.  If I did  use one of these bags it would only be for fresh drinking water and I think I'd use the space under the bed for the bag storage.

-Sean
www.herdofturtles.org