Water Hydraulics - Wastewater Operators - page 55

surface, the more pressure is created. This illustrates that the weight ,
not the volume, of water contained in a vessel determines the pressure
at the bottom of the vessel.
Nathanson (1997, pp. 21-22) listed some very important principles
that always apply for hydrostatic pressure:
1. The pressure depends only on the depth of water above the point in
question (not on the water surface area).
2. The pressure increases in direct proportion to the depth.
3. The pressure in a continuous volume of water is the same at all
points that are at the same depth.
4. The pressure at any point in the water acts in all directions at the
same depth.
2.4.2 effects of Water under Pressure *
Water under pressure and in motion can exert tremendous forces
inside a pipeline. One of these forces, called hydraulic shock or water
hammer , is the momentary increase in pressure that occurs when there
is a sudden change of direction or velocity of the water.
When a rapidly closing valve suddenly stops water flowing in a pipe-
line, pressure energy is transferred to the valve and pipe wall. Shock
waves are set up within the system. Waves of pressure move in hori-
zontal yo-yo fashion—back and forth—against any solid obstacles in
the system. Neither the water nor the pipe will compress to absorb the
shock, which may result in damage to pipes or valves and the shaking
of loose fittings.
Another effect of water under pressure is called thrust , which is
the force that water exerts on a pipeline as it rounds a bend. As shown
in Figure 2.3, thrust usually acts perpendicular (at 90°) to the inside
surface it pushes against. As stated, it affects bends but also reducers,
dead ends, and tees. Uncontrolled, the thrust can cause movement in
the fitting or pipeline which will lead to separation of the pipe coupling
away from both sections of pipeline or at some other nearby coupling
upstream or downstream of the fitting.
Two types of devices are commonly used to control thrust in larger
pipelines: thrust blocks and thrust anchors. A thrust block is a mass
of concrete cast in place onto the pipe and around the outside bend of
the turn. An example is shown in Figure 2.4. These are used for pipes
with tees or elbows that turn left or right or slant upward. The thrust is
transferred to the soil through the larger bearing surface of the block.
A thrust anchor is a massive block of concrete, often a cube, cast in
* This section is adapted from information contained in Hauser, B.A., hydraulics for
operators , Lewis Publishers, Boca Raton, FL, 1993, pp.16-18; AWWA, Basic Science
Concepts and applications: Principles and Practices of Water Supply operations , 2nd
ed., American Water Works Association, Denver, CO, 1995, pp. 351-353.