Science of Water : Concepts and Applications

52 The Science of Water: Concepts and Applications
Flow
containers, the higher pressure at the bottom of this container would cause some water to fl ow into
the container having the lower liquid level. In addition, the pressure of the water at any level (such as
Line T) is the same in each of the containers. Pressure increases because of the weight of the water.
The farther down from the surface, the greater the pressure 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) points out 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.
EFFECTS OF WATER UNDER PRESSURE *
Thrust
FIGURE 3.3 Shows direction of thrust in a pipe in a trench (viewed from above).
Flow
According to Hauser (1993), 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 fl owing in a pipeline, pressure energy is
transferred to the valve and pipe wall. Shock waves are set up within the system. Waves of pressure
move in horizontal 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,
valves, and shaking of loose fi ttings.
Another effect of water under pressure is called thrust. Thrust is the force that water exerts on a
pipeline as it rounds a bend. As shown in Figure 3.3, 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 fi tting 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 fi tting.
* This section is adapted from Hauser, B.A. (1993), Hydraulics for Operators, Lewis Publishers, Boca Raton, FL,
pp. 16–18, and American Water Works Association (1995), Basic Science Concepts and Applications: Principles and
Practices of Water Supply Operations, 2nd ed., American Water Works Association, Denver, pp. 351–353.
90°