Problems Concerning Pressure

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Basic Hydraulics
 
 

Hydraulics is the study of fluids.  For this text, hydraulics will concern only water.  The following brief outline on hydraulics represents a simple and logical discussion of that portion of hydraulics which the operators of water works systems are likely to practice.

In the operation of water works systems, pressures are dealt with continuously.  Water pressure is directly dependent upon the weight of water.

A  12"  x  12"  x  12" container, one cubic foot, of water weighs approximately 62.4 pounds.  The total pressure exerted against the bottom of the container is 62.4 lbs.  Thus each square inch of the bottom of the container has a pressure exerted against on it of 0.433 lbs. per sq. in. (62.4 lbs. divided by 144 sq. in.).  Water pressure is usually stated in pounds per square inch (psi).  So a column of water one foot high exerts a pressure of 0.433 psi, or 0.43 psi.

The most common U.S. unit of measure of water is the gallon.  A gallon of water contains 231 cubic inches.  The cubic foot container which we just used contains 1728 cubic inches (12"  x  12"  x  12").  By dividing the 1728 cubic inches by 231 cubic inches, we find there are 7.481 gallons of water in a cubic foot.  It is necessary to deal with gallons per cubic foot frequently and the approximation of 7.5 gallons per cubic foot is normally used.  One gallon of water weighs 8.34 lbs. (62.4 lbs. divided by 7.48 gal.).

Since water pressure is dependent on weight, it is also directly proportional to elevation.  If the container were ten feet high instead of one foot, the pressure on the bottom would be ten times more than a one foot column or 4.3 psi.  If the pressure in a water main resulted from water in a storage tank, with the top of the water 100 feet above the main, the pressure in the main would be 43 psi.  (100  x  0.43 psi).

The difference in elevation which determines pressure is referred to as head.  A 100 foot head has a pressure of 43 psi.  Head is generally measured in feet and pressure in pounds per square inch.  To convert from pressure in pounds per square inch to head in feet, multiply  pressure by 2.31 (43 psi  x  2.31  =  100 ft.)


Hydraulic Detention Time:


Detention Time in Hours Using Tank Volume in Cubic Feet:


Detention Time in Hours Using Tank Volume in Million Gallons:

 



Detention Time in Days Using Tank Volume in Cubic Feet:


Detention Time in Days Using Tank Volume in Million Gallons: