Max Water Pressure from a gravity fed water tank

Gringo

Gringo

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I have a water tank on the roof to gravity-feed the house. Will I get more pressure coming out of the tank with a 2" line mounted on the side wall of the tank (at the bottom), or a 2" line mounted on the bottom surface of the tank in the middle?

Thanks
Paul
 
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Don't confuse volume with pressure. The larger the pipe, the more volume you can get, but it will have no impact on pressure. If you try to get more volume than a pipe can supply, all outlets will slow down. This is where larger pipes or higher pressure make a difference.

Taking a shower with a gravity fed supply can be less than thrilling...there, you need a pump or height to increase the pressure.
 
If your water level is 15 feet above grade, you will have 6.5 psi. This will work for showering if you have a lot of volume. It will not work well using one of today's regulated shower heads.
 
Ja, but the showerhead is not likely to be at grade. If the showerhead is 7.5 feet above grade the pressure will be 3.25 PSI.
 
LLigetfa said:
Ja, but the showerhead is not likely to be at grade. If the showerhead is 7.5 feet above grade the pressure will be 3.25 PSI.
Click to expand...

Very true.
This may very well be much of the reason people did not commonly have showers in their homes 100 years ago. :p
 
Another Question Regarding Gravity Flow System

Lets say the tank (on roof) has a capacity of 1200 liters. If I come out of the tank with a 1" line, run this line across the roof (horizontaly) and then reduce to a 3/4" line vertically downwards through the walls to the fixtures, what size of air inlet tube do I need at the top of the tank to maximize pressure throughout the system?

Thank you
Paul
 
abc said:
Lets say the tank (on roof) has a capacity of 1200 liters. If I come out of the tank with a 1" line, run this line across the roof (horizontaly) and then reduce to a 3/4" line vertically downwards through the walls to the fixtures, what size of air inlet tube do I need at the top of the tank to maximize pressure throughout the system?

Thank you
Paul
Click to expand...
Just add a pump !
 
The answer is "it depends" on the temperature of the air and the water and the length of the tube. My guess is 1/4" would do it.

According to Wikipedia, it is just simple math.
http://en.wikipedia.org/wiki/Viscosity#Viscosity_of_air
Viscosity of air
Pressure dependence of the dynamic viscosity of dry air at the temperatures of 300, 400 and 500 K
The viscosity of air depends mostly on the temperature. At 15.0 °C, the viscosity of air is 1.81×10−5 kg/(m·s), 18.1 μPa.s or 1.81×10−5 Pa.s. One can get the viscosity of air as a function of temperature from the Gas Viscosity Calculator
Viscosity of water


Dynamic Viscosity of Water
The dynamic viscosity of water is 8.90 × 10−4 Pa·s or 8.90 × 10−3 dyn·s/cm2 or 0.890 cP at about 25 °C.
Water has a viscosity of 0.0091 poise at 25 °C, or 1 centipoise at 20 °C.
As a function of temperature T (K): (Pa·s) = A × 10B/(T−C)
where A=2.414 × 10−5 Pa·s ; B = 247.8 K ; and C = 140 K.
Click to expand...
 
How is the tank filled? When I lived in the Middle East, they pumped water into the roof-top tank(s) from a tanker truck in one place, but in another, there was municipal water, but it only came on for an hour or so during any 24-hour period. That one had a float valve to shut off the city water once the tank got full. The showers were pretty anemic! In one place, we had a 3-story apartment, and the showers on the lower floors were definately better (but not great) than those on the top floor.

You could install a vacuum breaker on the tank, and it would allow air in as the tank emptied. You'd need another valve to let air out if you pumped water into the tank through a fitting, rather than a door.

To maximize house pressure, you could install a pump similar to what is used on a well along with a bladder storage tank. You'd need an automatic safety shutoff (float switch) to disable the pump if the tank got too low since running the pump dry tends to ruin them quickly.
 
Gravity Fed System

If the tank on the roof has sufficient venting at it's top, are additional air vent tubes necessary at various points in the sytem?

Paul
 
No need for additional vents in the supply system...not to be confused with vents for the drainage system which is totally separate.
 
You probably have an "open tank" system, so there is no need for vents of any kind. The pressure delivered to the toilet, shower, or sink depends on how high the "top of the water" is from the fixture. The lower the faucet, the more pressure you will have. A shower head, because it is "high" above the floor will be closer to the "water level" and thus have less pressure. It has NOTHING to do with the pipe sizes.
 
I have a spring 70' up a hill from a house which should give me about 20psi, will that be enough for a shower without needing a pump? With 50 gallons an hour flow from the sping, how many gallons should I plan for a holding tank?
 
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David MJ said:
I have a spring 70' up a hill from a house which should give me about 20psi, will that be enough for a shower without needing a pump? With 50 gallons an hour flow from the sping, how many gallons should I plan for a holding tank?
Click to expand...
It depends on what you want to do! 50g/hr is less than one gallon/minute. In the USA, a typical shower head is limited to a maximum of 2.5gpm. A vanity faucet sometime like 2.1, but a washing machine is not flow restricted, nor is a tub filler and a large tub could use 80gallons or more!

What I'd do, is figure out what your worst case volume needs are per hour, and have a tank that's at least double that in case things slow down if there's a dry season...more is probably better. Washing your car, you could go through lots of water, and a top loader WM can use lots of water (the front loaders are more efficient in water use).

With that low pressure, you'll want to use large enough pipe to not drop your dynamic pressure any, but that also depends on the distance the water needs to flow along with the rate of flow...you want to preserve all of the pressure you have. Should you decide to add a pump, things get a little less critical, but more complicated. Your static pressure will be the same regardless of the pipe size, but the dynamic pressure drops based on the friction of the moving water and how far it has to travel, so the more water you try to pass, the more friction, the lower the pressure at the end point of use.
 
70' is 30 PSI. Install a wet well or catch basin that will hold 50 gallons or so at the spring. This will let you have the first 50 gallons as fast as needed. Install large pipe all the way to the shower and drill out the holes in the shower head and maybe you won't have to run around in the shower to get wet. :)
 
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Where to place the outlet pipe? I would place it a few inches above the bottom of the tank coming out the side. Also put a drain valve on the bottom of the tank to allow flushing out any debris and sand that may accumulate at the bottom.
 
Gringo, First answer, makes no difference where you make hole in tank middle or near side.
vent size for a gallon per minute? I'd probably just put 1 inch unless for some reason its important to know if it will function by going smaller
 
Sizing your tank depends a lot on what you want to do with it, and your safety level tolerance.

For example, is your flow the same all year long? What about in the middle of winter, or at the end of a dry spell in the summer?

Do you want to have any provisions for fire fighting? Having a few hundred extra gallons around might mean the difference between putting a smaller fire out, and having the home burn down. How far are you from the nearest fire station? How long would it take for them to get there, and where is the nearest place they could fill a pumper (if they even have one?!)? If you're using a spring, you may be out in the country, so there may not be a nearby fire hydrant, so any fire-fighting water would have to come from either your storage, or the pumper truck...neither one is a bottomless source.

You might get by through a weekend in a trailer with a 20-30g water tank, but it could easily be all used up by one person on one shower. What do you want at your place?
 
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