3.1 Creating Water Pressure

Pressure is what forces water out through the sprinklers or trickle emitters. Water pressure is created with the weight of the water itself or by mechanical means with a pump. The pressure at any point in a static column of water is simply the weight of the water above that point. For example, the column of water could be a water tower (discussed later) or tank of water on a hill. They create elevation above the point where water is used and in effect create a column of water that has weight and thus creates pressure. How this occurs will be covered next.

Pressure is a weight applied to an area.

Pressure can be illustrated by using the common weight and volume relationships of water:

One cubic foot of water weighs 62.4 pounds

One gallon weighs 8.3 pounds (from 62.4 lbs. divided by 7.48 gal per cu. ft.)

One cubic inch weighs 0.036 pounds (from 62.4 lbs. divided by 1,728 cu. in. per cu. ft.)

For example, using these relationships, stack twelve 1-inch cubes of water one on top of the other. These weigh 12 x 0.036 pounds or 0.433 pounds. The area under the column is one inch square (1 in. x 1 in.). The column exerts 0.433 pounds per square inch of pressure.

One pound per square inch (psi) of pressure can be created using a 1-in. square column of water nearly 28 inches or 2.31 feet high.

This is calculated from the two relationships just developed:

1 psi                 0.036 psi                      0.433 psi               1 psi
x inches       =   1 inch column                 1 foot column   =     x feet

0.36x  =  1                                                  0.433x  =  1

x  =  27.7 inches                                          x  =  2.31 feet

Because pressure changes with differences in elevation, two useful relationships to remember are:

1 foot of water = 0.433 pounds per square inch pressure

2.31 feet of water = 1 pound per square inch pressure

A PSI to Feet Scale illustrates a scale for converting units of pressure and head.

Pressure at a tee in a pipeline is equal in all directions when the flow divides. The pressure in a column of non-flowing (static) water is equal in all directions at any given point and is independent of the pipe surface area. Pressure can be expressed in different units, but they will all be equal. The total force depends on the area upon which the weight is applied.

Water Tower Example.

A municipal water system provides an illustration on maintaining water pressure in a large water system. If the top of a full water tank is 100 feet above the ground, the 100 ft. of water causes 100 ft. x 0.433 psi per ft. of column or 43.3 psi pressure at ground level. This is also expressed as 100 "feet of head." To fill the water tank, a pump must develop 43.3 psi or 100 ft. of head to lift water to the top of the water tank. Similarly, a water tower 150 ft. high would create 150 x 0.433 or 65 psi pressure at ground level. Look at Water Tower Pressure for a visual illustration of weight creating pressure.

Only vertical height affects pressure. The size of the tank does not affect the pressure. The water demand influences the size of the water tank; a large tank will empty slower for a large flow demand.

If the water level drops slowly with demand, the height of the water column remains fairly constant to maintain the pressure. Also, a 500 gpm pump can be used to fill the tank and the large reservoir can service intermittent demands of 1 to 2,000 gpm.

Pump Example.

A pump continually pushes water into one end of a pipe.

The pump creates a mechanical force against the water called pressure.

The small openings in sprinklers or emitters provide a release point for the water as more is pushed in. The release openings must be matched to the pumps capacity to push water into the pipe. Pressure is maintained while the restriction (sprinkler nozzle opening) allows the same amount of water to escape as is being pushed in at the beginning. If the pipe has a large opening, such as a break, water would flow through as fast as the pump pushed it into the pipe, but the pump would not create much pressure because there was no restriction to the flow.

Pump design and capacity vary widely. A sump pump is designed to lift a large volume of water a few feet out of a basement. A high-pressure pump is designed with close tolerances so it can push a small volume of water to great heights (high pressure). The high-pressure pump is made to close tolerances so water cannot leak back internally. More power is needed to operate the high-pressure pump.