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calculating psi
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Posted by: sait2010

02/16/2010, 14:05:58

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If a building is 40 feet high and the psi at the top is 15 psi how do I calculate the psi needed at the base? I know it is probably a variation on a formula I have. Adversly, if the psi at the base is given, how do I calculate the psi at the top?







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: calculating psi Cool Job
: calculating psi -- sait2010 Post Reply Top of thread Engineering Forum
Posted by: Marky

02/16/2010, 15:14:20

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Hi and welcome to the forum.....Homework?







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: : calculating psi -- Marky Post Reply Top of thread Engineering Forum
Posted by: sait2010

02/16/2010, 15:40:44

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not homework-husband is plumber and forgets how to calculate in this application.







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: : : calculating psi -- sait2010 Post Reply Top of thread Engineering Forum
Posted by: Kelly Bramble

02/16/2010, 17:19:00

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Your wanting to calculate the pressure due to the weight of the water column.

Assuming a cylindrical pipe..

volume in.^3 = 3.14157 x r(in.)^2 x height(in.)

in. = inches
r = radius

one cu inch water weights 0.036127 lbs

Therefore;

Pressure at bottom (psi) = [(Calculated volume lbs/in^3) x 0.0.036127 lbs] + 15 psi

Somebody check my math...







Modified by Kelly Bramble at Tue, Feb 16, 2010, 17:19:42


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Posted by: zekeman

02/16/2010, 20:20:21

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First of all I doubt the plumber simply wants the water pressure of an open system.

I would make a small bet that he wants to size a circulating pump for a building 40 feet high, and if so, should ask the basic question.

The answer to the OP question:
pressure of a 40 foot column of water is
40/34*14.7=17.3 psi since it is well known that 34 feet of water is one atmosphere.And the pressure at the bottom is 17.3+14.7=32.0 psi. Another way to do the column is to take a column of water 1 square inch and 40*12 inches high Its weight would be the pressure due to the column. Taking the density of water. 0.0361 this comes to 40*12*.0361=17.33 psi, the same as before.
What is the application ?

Assuming I won the bet, a circulating pump in a closed system does not need to produce that value of pressure. Plumbers often make that mistake.

It would behoove the plumber to state the problem he has for a proper answer.








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