Air pressure in lieu of water pump?

[jesusgatos]

Thanks for the suggestion. That sounds like a great idea. If you could put that soda keg after the main water tank (inline with the water supply with a one-way check-valve) somewhere low enough that the main tank could gravity-feed it, then the main tank wouldn't even need to be pressurized at all. Not sure that will work in my case (tanks will be mounted pretty low), but maybe I could do a low-pressure/high-pressure system. Just pressurize the main tank enough to fill the soda keg, and then pressurize that to whatever pressure I want at the water outlets.

 

[Phreak480]

dont forget some type of check valve then to keep the pressure in the smaller tank from trying to backfeed into the larger tank

 

[ntsqd]

Thanks for the suggestion. That sounds like a great idea. If you could put that soda keg after the main water tank (inline with the water supply with a one-way check-valve) somewhere low enough that the main tank could gravity-feed it, then the main tank wouldn't even need to be pressurized at all. Not sure that will work in my case (tanks will be mounted pretty low), but maybe I could do a low-pressure/high-pressure system. Just pressurize the main tank enough to fill the soda keg, and then pressurize that to whatever pressure I want at the water outlets.

Seems like a lot of complexity and effort to avoid a pump, be it manual or electric.

 

[jesusgatos]

Yeah, probably. And I'm not even trying to avoid using electric pumps. I've already got a bunch of really nice Shurflo pumps. But I've already got onboard air, and was thinking that I'd like to have a backup system in case the electrical system goes out. Had just been planning on installing a manual pump/faucet at the kitchen sink, but if I can use air-pressure and get full-pressure at all the outlets, that would be pretty cool. I was planning on plumbing air into the water lines anyway (so I can blow them out for easy winterizing), so I'm not sure how much complexity this would really add. I've enjoyed the research and reading all about this stuff though, and even if I don't use it on this project, it's good to have in the library.

 

[Tony LEE]

A common RV water tank can be easily pressurized to 40 PSI, which should provide a good flowrate. Big tanks are usually pretty thick to withstand the weight of shifting water as you cruise down the road.

Any engineering design calculations to back this up?

Most RV tanks are rectangular prisms and are not engineered as pressure vessels. Shifting water is not the same load as applying 40psi.

Another example...Have you seen those big blue plastic barrels used to haul things like corn syrup or raisin extract? I was playing around with one to see if I could make a temporary automatic irrigation system for a community garden that lacks both a dedicated water supply and electricity. I pressurized one of those bad boys to 100 PSI by accident with no ill effects.

The blue barrels are round, have dished top and bottoms and are designed to handle the pressures developed by certain hazardous materials. Yes, they might stand 100psi under some conditions, but add higher or lower temperatures, ageing embrittlement or scratches that weaken the wall and you would be creating what is effectively a bomb. If you want to see a similar effect, get a PET bottle, put a bit of water inside, add a small amount of dry-ice and screw the lid on. Best move back a way ASAP.

Failure of a pressure vessel pressurised with gas is way way way more dangerous than one filled with water.

Best not to "play around" without doing the engineering first.

 

[CJ2AinAZ]

Blitz Cans not a good idea

We used air pressure for our OBM (On board misting system) worked great other then 15 psi made the bottom of the blitzs cans balloon out

Only other issue was getting covered in dust, as it stuck to you more then normal.

Flow was never an issue.

On another note on the trailer we had a line from the bottom of the tank and could always use gravity feed if the pump was an issue.

 

[ntsqd]

Any time a tank has flat panels pressurizing it is suspect. No matter what it is made out of. Under pressure a tank will be trying to form a round bodied shape with rounded ends. Put enough pressure in it and assuming the material doesn't rupture first it will assume that shape. Witness the shape of truck air brake tanks, air compressor tanks, and propane tanks to name a few. They are like that for a reason, and enough people died in learning about pressure vessels in their early days that is it wise to do the research first.

That said, such a flat sided tank might be acceptable at a very low pressurization. The concept to grasp is "psi", Pounds per Square Inch. So a panel that is retaining 15 psi has 15 pounds of force pushing on each square inch. If that panel is 100 square inches (10" to a side) then there is 1500 pounds pushing on that panel. Just moving the panel size up to a square foot (12" x 12") increases the force to 2160 pounds. The force numbers get large pretty fast. This force is always perpendicular to the surface ("Normal") at any spot on the surface.

 

[rusty_tlc]

I may try to mod up the water tank in the Goony Bird to operate on air pressure this weekend just to test the idea.

That stupid hand pump is ridiculous it's only up side is you conserve water, because before you get enough out to wash your hands you give up and walk away.

 

[RHINO]

my father in law made one with a keg, it works great but is heavy bulky and takes up much space.