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Steamdrivenandy

Flow Puzzle

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This isn't a caravan issue as it's on our campervan but the depth of knowledge on CT might assist in sorting the issue.

 

We have a 54 litre fresh water tank under the van. The water take-off is at the very bottom of the tank, but on a side wall. I fill the tank to the brim when on site and the water is pumped into the van by a Fiamma Aqua 8 pump. The cold flow splits and runs either to the kitchen tap,  Thetford toilet  or the Propex boiler, from where it's piped to the hot side of the tap.

 

When the tank is full all is fine, the cold and hot flow takes the usual spurting to settle down but if I keep topping up the tank once a day there's no problem.

 

However if I let the tank content run down a bit both hot and cold start spurting and coughing as if the pump is struggling to scavenge water from the tank. By my reckoning the point at which this occurs is after we've used maybe 20 or 25 litres, at which point there should be between 29 and 34 litres left in the tank and the tank outlet should be well covered and there should be plenty for the pump to use.

 

Any thoughts and suggestions welcome?

 

Whilst writing this, one thought has occurred to me. Before I purchased the van it had a large wheelchair lift removed. This was in the form of a sort of large metal cassette that stored under the middle of the van. I'm wondering if, in order to accommodate the lift they had to reduce the depth of the fresh water tank and it doesn't actually take the standard 54 litres specified for this model van?

 

I might call the builders to check.

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Water pick up pipe(s) do not reach the bottom of your water tank?

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The water pipe exit is right at the bottom of the tank, although on a side wall rather than the actual bottom of the tank. I can't believe that having fitted the pipe exit there they'd then run the pipe on into the tank so it could only access half of the content. I guess it's possible but poor build if that's what's been done.

 

I'm not sure what a normal watering can holds but  from empty it only took about four or five to brim the tank. Which is  10 to 14 litres per can to reach 54 litres.

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When we had a wheelchair adapted car the compromise was to cut the fuel tank down by about two thirds.

The lift has to go somewhere.

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A standard size watering can is usually 10 litres.

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What is the pump height relative to the surface of the water in the tank?

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The pump is in a rear seat box above the tank which is suspended under the floor of the van.  I guess it's about  3 inches from the bottom of the pump to the top surface when the tank is full.  The tank itself I'd estimate to be about 8 or 9 inches deep.

 

When I get home, as well as phoning the converter I'll drain the tank and measure how much is needed to refill. I'll then run the pump 'til it 'scavenges' and drain the tank whilst measuring what comes out.

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As you say the draw off is in the tank wall, it is quite possible than as the water level gets down near to that air gets pulled from the surface.  [Usually it is via a vortex being formed]. If it is easier to draw air in than water that is what an inlet will do.

This can be cured  by techniques like fixing a plate of sorts immediately above the inlet so the vortex can't readily reach to the surface. That needs to be a tight fit to the tank side so there is not a short cut for the vortex.*

Or plugging a pipe into the inlet routed to some point in the tank that serves as a "well", though this inevitably smaller bore pipe will restrict the flow somewhat.

 

Edit: * thinking here of bonding, say a bit of alloy angle with alloy plate 4" square to the tank wall. Sikaflex PU bonding products will live with the wet as long as the tank itself is of a bondable type plastic.

Edited by JTQ

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I'm not sure where the tank is relative to the axle line of the caravan, or what ground clearance there is below it but would it be possible to add a new take-off point in the base of the tank and blank of the existing one? Your Fiamma Aqua 8 pump looks similar to the Shurflo 8000 pump so should be self priming and easily cope with the few inches it has to draw water up into the caravan pipework. It sounds to me like the flow rate is sufficiently high to cause the vortex others have referred to so could the flow rate realistically be reduced slightly to minimise this effect? Any chance of a piccie?

Water take off from onboard tank.jpg

This is how close to the bottom the takeoff point was on the tank I fitted to our caravans and we never experienced any problems when scavenging the last few drops of water from the tank.

Gordon.

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20180824_161316.jpg

 

I laid down on wet grass between cloudbursts to get this shot, so I didn't hang about and I now realise it could have been clearer if it had shown the end panel of the tank.

 

Anyway, what it shows is the exterior of the rear end of the tank, with the take-off point (blue hose, rusty jubilee clip) almost at the bottom  of the tank. Above it and slightly inboard is the black corrugated pipe that's the fresh feed, which enters at the top side of the tank.  The other blue pipes further inboard are a feed from after the pump, across the van to the toilet. Even further inboard is the nice new exhaust pipe, running across the bottom of the pic.

 

20180824_161452.jpg

 

This picture shows the Fiamma pump in the seatbox with the fresh water feed pipe passing through from the fill point on the side of the van, through the floor and out under the van and into the tank. Usually when I fill the water overflows out of the fill point, which suggests the tank and the fill pipe are full. In theory making a tad more than 54 litres. The big white blob is actually a pale grey Propex 6 litre electric storage water heater.  In theory therefore to fill the tank, boiler and associated pipework requires 60 plus litres and a bit in the length of the feedpipe between the top of the tank and the fill point. so at a rough estimate, say 63 litres all told.

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The reason I asked about pump height is IF the pump is having to suck the water up then it's entirely possible to have air being pulled in through valves and connectors etc.   As the water level falls the pump has to suck more and hence why the air is pulled in part way down a tank. Just a theory, it may well be the vortex, that could easily be disproved or proved by running the tap slowly.

 

Can you fit a clear section of hose on the inlet to the pump? you could then see the air as it arrived at the pump. If that shows air transfer it onto the outlet of the tank and again run it, if you see air at that point you know it's vortex etc.

Edited by AJGalaxy2012

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Thing is, when the tank is first filled then the flow is fine, after initial splutters. It's only after a day, or so's, use that the spluttering starts and if you persist in running the pump, the splutters quickly turn to no flow at all.  When it starts spluttering the pump won't turn off automatically because it can't build up the pressure required to operate it's internal pressure switch. At that point you have to switch it off at the Zig box. When the tank is full the pump switches off automatically with no problem.

 

So it seems to me that the pump is working as it should but I don't seem to be getting 54 litres of water use. Either there's not 54 litres available or the pumps not accessing all the water in the tank.

 

I can drain the water out, once it's stopped pumping, to see if that's the problem and once it's empty I can measure the water I put in to fill it right up again. That should give me answers to both queries. 

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49 minutes ago, Steamdrivenandy said:

Thing is, when the tank is first filled then the flow is fine, after initial splutters. It's only after a day, or so's, use that the spluttering starts and if you persist in running the pump, the splutters quickly turn to no flow at all.   When it starts spluttering the pump won't turn off automatically because it can't build up the pressure required to operate it's internal pressure switch. At that point you have to switch it off at the Zig box. When the tank is full the pump switches off automatically with no problem.

 

So it seems to me that the pump is working as it should but I don't seem to be getting 54 litres of water use. Either there's not 54 litres available or the pumps not accessing all the water in the tank.

 

I can drain the water out, once it's stopped pumping, to see if that's the problem and once it's empty I can measure the water I put in to fill it right up again.  That should give me answers to both queries.  

Vent for the tank blocked? pump pulling a vacuum and cavitating?

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2 hours ago, AJGalaxy2012 said:

Vent for the tank blocked? pump pulling a vacuum and cavitating?

 

Would it not do that even when the tank was full. Though I have to say that on my previous model of the same van the tank overflowed somewhere under the van when overfilled. This one just sends back any 'surplus through the filler spout. So it doesn't seem as if it has any venting except the filler.

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7 hours ago, Steamdrivenandy said:

 

Would it not do that even when the tank was full. Though I have to say that on my previous model of the same van the tank overflowed somewhere under the van when overfilled. This one just sends back any 'surplus through the filler spout. So it doesn't seem as if it has any venting except the filler.

If there isnt a vent then a partial vacuum will form until such point the pump cant extract any more water. When it's spluttering take off the filler cap and see if normal operation is resumed.

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That's a good thought, I'll check that out. I've just had a look at the CAK Tanks website and identified the tank fitted to the van. They say it has a breather but I've no idea what one looks like or where it's supposed to be. Presumably it should be on the highest point of the tank, which is not visible unless you drop the tank from the van. I'm wondering if it has a breather that's somehow been blocked.

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23 minutes ago, Steamdrivenandy said:

That's a good thought, I'll check that out. I've just had a look at the CAK Tanks website and identified the tank fitted to the van. They say it has a breather but I've no idea what one looks like or where it's supposed to be. Presumably it should be on the highest point of the tank, which is not visible unless you drop the tank from the van. I'm wondering if it has a breather that's somehow been blocked.

 

There should be a breather on top of the hot water tank, they can get stuck with crud and limescale. it could be red or white.

20150929_135624.jpg

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Isn't that a pressure relief valve Paul, rather than a breather?

 

Another thought though, if a vacuum is created in the tank you'd think there'd be a rush of air in when you remove the cap. Indeed it might even make the cap difficult  to remove. Neither of which seems to happen.

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They are side exit for the outlet protection under the MH for ground clearance and road debris and the tank can be mounted flat on a floor if required . The only issue I have found in my experience like those with tanks under the MH is freezing and the tank is insulated but the pipe exit freezes and causes fractures .

 

My new MH the tank is inside and it has a double floor for piping and a large access hole to clean it out .

 

Is there any joints between the tank and pump for inline filter as it sounds like it is pulling air possibly .

 

 

Dave

Edited by CommanderDave

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The inadequacy of a vent is a “red herring”. 
Whilst it can cause cavitation and inhibit the flow it can't generate "air" only pull out of solution any dissolved air, which in no way will be in the quantities being described. The cavitation does release gas but also water vapour, the former being of small orders in quantity the latter reforming instantly, sonically  and is the process that makes the cavitation noise
Air is being drawn in, through either a leak on the inlet system of a vortex from the surface, the former being far more the likely.   As well as checking all joints don't overlook any pump casing joint, or the inlet strainer's joint.
 

Edited by JTQ

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As the system works perfectly when the tank is full I don't think there are any  air leaks anywhere. It only misbehave when the water level drops.

 

I'm sceptical about the vortex because it would only form once the pump had started sucking and initially there'd be a 'surge' of water sucked from the still water, before the vortex formed. This doesn't happen, when at it's lowest no water comes through the pipes at all. 

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From the pic it looks as though the distance from the base of the tank to the top of the blue pipe is quite a lot.   As soon as the level drops to the very top of that blue pipe air will be drawn into the system.

As you said previously the tank depth is not a lot and no matter how big the volume is as soon as the level drops you'll get air - be it 60 litres or 260 litres.   Just take longer to reach that critical point.

If the take-off could be moved to the bottom of the tank then you'd get a lot more water than at present before spluttering I think.   (vehicle level of course!) 

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According to CAK's product diagram the tank is only 4 inches high at the rear end shown in the pics. It does go up in height further forward where there are spaces in the floor level that allow it to do so. At it's deepest it's 7 inches deep.

 

But as JB45 says suction will fall off as soon as the top of the blue pipe entry is exposed to air, which as the pipe is about an inch in diameter means that 20 to 25% of the contents, or thereabouts cannot be accessed. That's about 12 litres or so. Which means a top up to bring it up to full should be around 42 litres, or 4 watering cans full. Currently topping up takes about 2 watering cans. So something doesn't add up.

 

Maybe I'll have to pitch nose up to keep the tank exit covered with water for longer. Mind that's not a comfortable way to sleep.

 

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3 hours ago, Steamdrivenandy said:

Isn't that a pressure relief valve Paul, rather than a breather?

 

Another thought though, if a vacuum is created in the tank you'd think there'd be a rush of air in when you remove the cap. Indeed it might even make the cap difficult  to remove. Neither of which seems to happen.

 

It would act as one as excess pressure would push the rubber valve off its housing, however its primary purpose is to allow air into the hot water tank.

Explained better by Alde in this topic

 

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43 minutes ago, Steamdrivenandy said:

According to CAK's product diagram the tank is only 4 inches high at the rear end shown in the pics. It does go up in height further forward where there are spaces in the floor level that allow it to do so. At it's deepest it's 7 inches deep.

 

But as JB45 says suction will fall off as soon as the top of the blue pipe entry is exposed to air, which as the pipe is about an inch in diameter means that 20 to 25% of the contents, or thereabouts cannot be accessed. That's about 12 litres or so. Which means a top up to bring it up to full should be around 42 litres, or 4 watering cans full. Currently topping up takes about 2 watering cans. So something doesn't add up.

 

Maybe I'll have to pitch nose up to keep the tank exit covered with water for longer. Mind that's not a comfortable way to sleep.

 

If the tank is 4 inches deep where you've photographed it and the upper part is a "wedge shape" from 4 inches to 7 inches surely that part will hold far less water than the lower part and require less to fill? 

Without knowing the exact dimensions it does follow that a wedge shaped tank (the upper section) will hold half the amount of a rectangular tank of same base dimensions! 

Would that explain the dilemma 2 watering cans and not 4?  Lol  

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