Basic Math Operations - Mathematics and Statistics for the Environment - page 68

1 lb of water

2.31 ft

0.433 lb of water

1 ft

1 in. 2 area

1 ft water = 0.433 psi

1 in. 2 area

1 psi = 2.31 ft water

FIGURE 2.11 Relationship between pressure and head.

Therefore, the pressure in pounds per square inch (psi) is

2 2

22

62.4 lb/ft
1ft

62.4 lb/ft
in./ft

2

= = 0.433 lb/in. (psi)

2

144

If we use the bottom of the container as our reference point, the head would be 1 foot. From this, we
can see that 1 foot of head is equal to 0.433 psi—an important parameter to remember. Figure 2.11
illustrates some other important relationships between pressure and head.

Note: Force acts in a particular direction. Water in a tank exerts force down on the bottom and out
of the sides. Pressure, however, acts in all directions. A marble at a water depth of 1 foot
would have 0.433 psi of pressure acting inward on all sides.

Using the preceding information, we can develop Equations 2.12 and 2.13 for calculating pressure
and head:

Pressure (psi) = 0.433 × Head (ft) (2.12)

Head (ft) = 2.31 × Pressure (psi) (2.13)

2.12.2 h ead
Head is the vertical distance the water must be lifted from the supply tank or unit process to the
discharge. The total head includes the vertical distance the liquid must be lifted ( static head ), the
loss to friction ( friction head ), and the energy required to maintain the desired velocity ( velocity
head ):

Total head = Static head + Friction head + Velocity head (2.14)