Environmental Engineering Reference
In-Depth Information
rise in the tube. Because atmospheric pressure is essentially universal,
we usually ignore the first 14.7 psi of actual pressure measurements
and measure only the difference between the water pressure and the
atmospheric pressure; we call this
gauge pressure
. Water in an open
reservoir is subjected to 14.7 psi of atmospheric pressure, but subtract-
ing this 14.7 psi leaves a gauge pressure of 0 psi, indicating that the
water would rise 0 feet 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 psi × 2.31 ft/psi = 277 ft (rounded)
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
(2.5)
2.5.1 static head
Static head is the actual vertical distance the liquid must be lifted:
Static Head = Discharge Elevation - Supply Elevation
(2.6)
Example 2.7
Problem:
The supply tank is located at elevation 118 ft. The discharge
point is at elevation 215 ft. What is the static head in feet?
Solution:
Static Head = 215 ft - 118 ft = 97 ft
2.5.2 friction head
Friction head
is the equivalent distance of the energy that must
be supplied to overcome friction. Engineering references include tables
providing the equivalent vertical distance for various sizes and types
of pipes, fittings, and valves. The total friction head is the sum of the
equivalent vertical distances for each component:
Friction Head = Energy Losses Due to Friction
(2.7)
2.5.3 velocity head
Velocity head is the equivalent distance of the energy consumed in
achieving and maintaining the desired velocity in the system.
Velocity Head = Energy Losses to Maintain Velocity
(2.8)
