Science of Water : Concepts and Applications

54 The Science of Water: Concepts and Applications
14.7 psi leaves a gauge pressure of 0 psi. This shows that the water would rise 0 ft above the reservoir
surface. If the gauge pressure in a water main were 120 psi, the water would rise in a tube connected
to the main:
120 psi2.31 ft/psi 277
ft (rounded)
The total head includes the vertical distance through which 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 (3.5)
STATIC HEAD
Static head is the actual vertical distance the liquid must be lifted.
Static head = discharge elevation – supply elevation (3.6)
Example 3.7
Problem:
The supply tank is located at an elevation of 118 ft. The discharge point is at an elevation of 215 ft. What
is the static head in feet?
Solution:
Static head (ft) = 215 ft – 118 ft = 97 ft
FRICTION HEAD
Friction head is the equivalent distance of the energy that must be supplied to overcome friction.
Engineering references include tables showing the equivalent vertical distance for various sizes
and types of pipes, fi ttings, and valves. The total friction head is the sum of the equivalent vertical
distances for each component.
Friction head (ft) = energy loss due to friction (3.7)
VELOCITY HEAD
Velocity head is the equivalent distance of the energy consumed in achieving and maintaining the
desired velocity in the system.
Velocity head (ft) = energy loss to maintain velocity (3.8)
TOTAL DYNAMIC HEAD (TOTAL SYSTEM HEAD)
Total head = static head + friction head + velocity head (3.9)
PRESSURE/HEAD
The pressure exerted by water/wastewater is directly proportional to its depth or head in the pipe,
tank, or channel. If the pressure is known, the equivalent head can be calculated.
Head (ft) = pressure (psi) × 2.31 ft/psi (3.10)