Environmental Engineering Reference
In-Depth Information
Liquid
level
T
FIGURE 4.3
Hydrostatic pressure.
Hydrostatic Pressure
Figure 4.3 shows a number of differently shaped, connected, open containers of
water. Note that the water level is the same in each container, regardless of the shape
or size of the container. This occurs because pressure is developed, within water
(or any other liquid), by the weight of the water above. If the water level in any one
container is momentarily higher than that in any of the other containers, the higher
pressure at the bottom of this container would cause some water to flow into the con-
tainer 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 more pressure is cre-
ated. This illustrates that the
weight
, not the volume, of water contained in a vessel
determines the pressure at the bottom of the vessel. Some important principles that
always apply for hydrostatic pressure include the following (Nathanson, 1997):
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.
Head
Head is defined as the vertical distance the water must be lifted from the supply tank
to the discharge, or as the height a column of water would rise due to the pressure
at its base. A perfect vacuum plus atmospheric pressure of 14.7 psi would lift the
water 34 ft. If the top of the sealed tube is open to the atmosphere and the reservoir
is enclosed, the pressure in the reservoir is increased; the water will 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, for example, is subjected to the 14.7 psi of atmospheric pres-
sure, but subtracting this 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 is 120 psi, the water would rise in a tube connected to the main:
120 psi × 2.31 ft/psi = 277 ft (rounded)
