Wastewater Conveyance - Wastewater Operators - page 220

Several temperature scales have been developed to provide a stan-
dard for indicating the temperatures of substances. The most commonly
used scales include the Fahrenheit, Celsius, Kelvin, and Rankine tem-
perature scales (see Figure 4.44). The Celsius scale is also called the
centigrade scale . The Fahrenheit (°F) and Celsius (°C) scales are based
on the freezing point and boiling point of water. The freezing point of a
substance is the temperature at which it changes its physical state from
a liquid to a solid. The boiling point is the temperature at which a sub-
stance changes from a liquid state to a gaseous state.
Note: Thermometers are classified as mechanical temperature-sens-
ing devices because they produce some type of mechanical action or
movement in response to temperature changes. The many types of ther-
mometers include liquid-, gas-, and vapor-filled systems and bimetallic
thermometers.
Figure 4.44 shows an industrial-type thermometer that is com-
monly used for measuring the temperature of fluids in industrial piping
systems. This type of measuring instrument is nothing more than a
rugged version of the familiar mercury thermometer. The bulb and capil-
lary tube are contained inside a protective metal tube called a well . The
thermometer is attached to the piping system (vat, tank, or other compo-
nent) by a union fitting. Another common type of temperature gauge is
the bimetallic gauge shown in Figure 4.45. If two materials with different
linear coefficients of expansion (i.e., how much a material expands with
heat) are bonded together, their rates of expansion will be different as
the temperature changes. This will cause the entire assembly to bend
in an arc. When the temperature is raised, an arc is formed around the
material with the smaller expansion coefficient. The amount of arc is
reflected in the movement of the pointer on the gauge. Because two dis-
similar materials form the assembly, it is known as a bimetallic element ,
which is also commonly used in thermostats.
4.15.4 vacuum breakers
A common ancillary device found in pipelines is a vacuum breaker
(see Figure 4.46). A vacuum breaker is a mechanical device that allows
air into the piping system, thereby preventing backflow that could oth-
erwise be caused by the siphoning action created by a partial vacuum.
In other words, a vacuum breaker is designed to admit air into the line
whenever a vacuum develops. A vacuum, obviously, is the absence of air.
Vacuum in a pipeline can be a serious problem; for example, it can cause
fluids to run in the wrong direction, possibly mixing contaminants with
purer solutions. In water systems, backsiphonage can occur when a par-
tial vacuum pulls nonpotable liquids back into the supply lines (AWWA,
1996). In addition, it can cause the collapse of tubing or equipment.
As illustrated in Figure 4.46, this particular type of vacuum breaker
uses a ball that usually is held against a seat by a spring. The ball is
contained in a retainer tube mounted inside the piping system or inside
the component being protected. If a vacuum develops, the ball is forced
(sucked) down into the retainer tube, where it works against the spring.