Environmental Engineering Reference
In-Depth Information
Scaling also often occurs in solution rotameters, making flow read-
ings impossible and freezing the flow indicator in place. Various valves
can freeze up, and pressure-sustaining valves can freeze and become
plugged. Various small diffuser holes fill with scale. To slow the rate of
scaling, many facilities purchase water from local suppliers to dilute
hypochlorite for the return activated sludge and miscellaneous uses.
Some facilities have experimented with the system by not adding lime
to it. When they did this, manganese dioxide (black deposits) developed
on the rotameter glass, making viewing the float impossible. In many
instances, moving the point of hypochlorite addition downstream of the
rotameter seems to solve the problem.
If remedial steps are not taken, scaling from hypochlorite solutions
can cause problems; for example, scale buildup can reduce the inside
diameter of a pipe so much that the actual supply of hypochlorite solution
required to properly disinfect water or wastewater is reduced. As a result,
the water sent to the customer or outfalled to the receiving body may
not be properly disinfected. Because of the scale buildup, the treatment
system itself will not function as designed and could result in a hazard-
ous situation in which the reduced pipe size increases the pressure level
to the point of catastrophic failure. Scaling, corrosion, or other clogging
problems in certain piping systems are far from an ideal situation.
Example 4.1
The scale problem can be illustrated by the use of an example.
Assume that we have a piping system that is designed to provide chemi-
cal feed to a critical plant unit process. The motive force for conveying
the chemical is provided by a positive-displacement pump at a given vol-
ume of solution at 70 psi through clean
pipe. After clogging takes place, the
pump continues trying to force the
same volume of chemical through the
system at 70 psi, but the pressure drops
to 25 psi because of friction. The reduc-
tion of the inside diameter of the pipe
increases the friction between the
chemical solution and the inside wall of
the pipe.
Key Point:
A basic principle in fluid mechanics
states that fluid flowing through a pipe is affected
by friction—the greater the friction, the greater the
loss of pressure. Another principle or rule states
that the amount of friction increases as the square
of the velocity. (Note that speed and velocity are
not the same, but common practice refers to the
“velocity” of a fluid.) In short, if the velocity of the
fluid doubles, the friction is increased four times
what it was before. If the velocity is multiplied by
5, the friction is multiplied by 25, and so on.
In Example 4.1, the pressure dropped from 70 psi to 25 psi because
the solution had to run faster to move through the pipe. Because the
velocity of the solution pushed by the pump had to increase to levels
above those when the pipe was clean, the friction increased at a higher
rate than before. The friction loss was the reason why a pressure of 25
psi arrived at the far end of the piping system. The equipment designed
to operate at a pressure of 70 psi could not work on the 25 psi of pres-
sure being supplied.
What is the solution to our pressure loss problem in Example 4.1?
Actually, we can solve this problem in two possible ways—we can either
replace the piping or clean it. Replacing the piping or cleaning sounds
