Water Pressure

[milmar]

How would I calculate water pressure in a vertical column? I had about a 17 foot PVC tube with a 4" diameter filled with water. What equations could I use?

[arildno]

Well, the water pressure at a given level must give rise to a force on the fluid above the level which balances the weight of the fluid above the level (otherwise, that part of the fluid would fall down due to gravity).

[milmar]

okay, so where do i go next?

[arildno]

Have you gone anywhere as yet?
Where's your equation derived on basis of what I told you?

[ceptimus]

What's the weight of water in the tube, and what is the area it is pressing on at the end of the tube?

[DeathKnight]

Cant we use the formula 'hpg' to calculate the pressure? Because what i've learnt so far (i've just started A'levels) pressure of a fluid depends on its depth not on the width/diameter of the container. I can be wrong. If it is the case please dont make fun of me and do correct me.

[ceptimus]

You're right that the pressure depends only on the depth (height) of the liquid, its density, and the acceleration due to gravity.

With a constant diameter tube of water though, it's easy to visualise why this is the case by calculating the total weight of water in the tube, and then considering what force is keeping the water where it is.

[Artman]

The formula for psi of static pressure is the height in feet times approximately .43 (or divided by approximately 2.33). Multiply the height of the water above any point by the area of the tube at that point and you can determine the total static pounds of pressure acting on that area.

Such as a point 10' down the tube would be about 4.3 psi x pi x 2"^2 = 54 lbs approximately.

[gerben]

You know that pressure is "force acting on a unit of area":
Pressure = \frac{F}{area}

A water column will apply a certain force on the water below due to the gravity acting on it. That force depends on the mass of the water column and on the acceleration due to gravity:
F = m*g

because the mass depends on the volume and on the density of the water you get:
F = volume*density*g

because the volume depends on the height and cross sectional area of the water column you get:
F = height*area*density*g

If you combine this expression of the force with the definition of pressure you get:
Pressure =\frac{height*area*density*g}{area} \Leftrightarrow Pressure =height*density*g

[sparky968]

Water weighs 62.4 pounds per cubic foot here on earth. There are 1728 cubic inches in a cubic foot. If you stack all 1728 cubes on top of each other the bottom cube would have 62.4 psi. 1728 divided by 12 inches per foot, then divided by 62.4 is approx. 2.3, so every foot of water increases the pressure by about 2.3.

[Loren Booda]

gerben,

You make barometrics bearable. Let milmar mind the millibars!

[spamlessjack]

Milmar -

I am a novice, here, but I think there is a standing formula for what you're asking.
Water pressure has a specific/general weight/foot-of-elevation (here on earth...) - there's a rule-of-thumb (I THINK!!!) for calculating this water pressure –
The rest of you PLEASE correct me if I'm wrong.

I am in the process of designing a water system for a remote location without a public or well supply (electricity/wind power limited or unavailable). I am going to collect rain water and pump it up to a water tower, so I can have a static pressure supply. I am trying to calculate the water pressure/foot of height. The constants are:

25’ tower
1000 gal tank – 64” radius x 80” height.

What I need to know is:

• When the tank is full (25’+6.6’=31.6’) how much static pressure will I have at the bottom?
• When the tank is pert near empty (25’) how much static pressure will I have at the bottom?
• Will it matter if I use a 2” pipe vs. a 4” pipe for the riser? (flow=yes, pressure=???)

Please help. . .

(How can a cubic foot of water weigh 62.4Lb??? I thought water only weighed 8.4Lb/Gal – Is there 7.4Gal of water in a CF? – that’s the biggest CF/H2O I ever saw. . . what did I miss?. . .)

[gmax137]

Yes, spamlessjack, a cubic foot is bigger than you think (or that gallon jug is smaller than it looks). There are 7.4805 gallons (US) per cubic foot. I don't know that one offhand for imperial gallons...

[Toymkr]

Spamlessjack-
Don't let your calculations be distracted by the diameter of your pipe or tank. Weight of the column is not the same as pressure on a gauge.Don't feel bad when it seems to you that volume should matter, lots of other bright folk make the same error. The ooonly thing that affects head pressure of water is elevation at .43 psi per foot of height. ( 10 foot head = 4.3 psi, 100 foot head = 43 psi) Do a search for "calculators online" and you'll find that there are more than you can count for a plethora of different things. They're set up so you just plug in the values you want to convert, hit "calculate" and you get an answer...as if by magic.
Be sure to design supports for your pipe and tank with consideration for total weight of contents plus pipe and tank plus a sizeable factor for wind pressure on the total structure.
Milmar & gmax137 are right on the weight of water and gallons per cubic foot.

As in all things......do the math. <grin>

Toymkr

[Zerber4]

I was trying to find out why someone had tried equating .43 psi/ft to kPA in an owner's manual I was proofreading, did some searching to find some kind of context that would help me understand what they were trying to say, and was able to use the posts here to understand what they were trying to do. THANKS very much.