Force, Pressure, Head, and Velocity Calculations - Basic Mathematics for Water and Wastewater Operators - page 176

62.4 lb
of
water

1 foot

1 foot

1 foot

FIGURE 12.1 One cubic foot of water weighs 62.4 pounds.

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. Figure 12.2 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 one foot would have 0.433 psi of pressure acting inward on all sides.
Note: Water and wastewater unit system pressures are measured in psi but cen-
trifugal pumps are rated in feet of total dynamic head (TDH).

Water and wastewater operators must be able to convert from one pressure unit to
the other. Thus, using the preceding information, we can develop Equations 12.1 and
12.2 for calculating pressure and head:

Pressure (psi) = 0.433 × Head (ft) (12.1)

Head (ft) = 2.31 × Pressure (psi) (12.2)

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 12.2 The relationship between pressure and head.