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New Mexico Wastewater Systems Operator Certification Study Manual - Chapter 8, Mechanical Systems 8-1
C
HAPTER
8: M
ECHANICAL
S
YSTEMS
Pumps serve many purposes in wastewater collection
systems and treatment plants. They are classified by the
character of the material handled; raw wastewater, grit,
effluent, activated sludge, raw sludge, or digested sludge.
Or, they may relate to the conditions of pumping: high lift,
low lift, recirculation, or high capacity. They may be further
classified by principle of operation, such as centrifugal,
propeller, reciprocating and turbine. The operation and
maintenance of these pumps are some of the most important
duties for many wastewater utility operators. The two most
common type of pump are the centrifugal pump and the
positive displacement pump.
Pumps are rated by the flow they produce and the pressure
they must work against. Centrifugal pumps are used for
high flow and low head pressure applications. Booster
pumps or primary service pumps are required to move high
volumes of water and usually operated at low head pressures
(200-300 feet of head for water and as little as 50 feet of
head for wastewater applications). Centrifugal pumps are
ideally suited to these types of applications and are much
more efficient than positive displacement pumps of
comparable size. Positive displacement pumps are used for
low flow and high-pressure applications. High pressure
water jet systems like those used for well screen or sewer
line cleaning use positive displacement pumps since
pressure in excess of 2500 feet of head are needed and the
flows seldom exceed 100 gpm. Sludge pumps and chemical
feed pumps are also likely to be positive displacement
pumps. Piston pumps, diaphragm pumps, and progressive
cavity screw pumps are the most common types of positive
displacement pumps.
Another difference between centrifugal and positive
displacement pumps has to do with how they react to
changes in discharge pressure. When the pressure that a
centrifugal pump has to work against changes, the flow
from the pump changes. As the pressure increases, the flow
from the pump will decrease, and when the pressure drops
the flow will increase. Positive displacement pumps do not
react this way. The flow does not change when the discharge
pressure changes. This is the main reason that positive
displacement pumps are used for chemical feeding and
sludge pumping. The operator knows that every time the
pump strokes, it is pumping the same amount of fluid. This
is important if accurate records are to be kept of chemical
dosages and pounds of solids that are moving through the
system.
C
ENTRIFUGAL
P
UMPS
A centrifugal pump moves water by the use of centrifugal
force. Any time an object moves in a circular motion there
is a force exerted against the object in the direction opposite
the center of the circle. This would be easier to explain if
we use an example consisting of a person with a bucket
full of water. If the person swings the bucket in a circle
fast enough, the water will stay in the bucket even when it
is upside down. The force that holds the water in the bucket
is called centrifugal force. If a hole is made in the bottom
of the bucket, and it is swung in a circular motion, the
centrifugal force will push the water out of the bucket
through the hole. The same principle applies when water
is moved through a centrifugal pump.
An impeller spins inside a centrifugal pump. It is the heart
of the pump. Water enters the center, or suction eye, of the
impeller. As the impeller rotates, the veins pick up the water
and sling it out into the pump body under pressure. It is the
pressure exerted by the vanes that moves the water out of
the pump and into the system. The suction created as the
water leaves the impeller draws more water into the
impeller through the suction eye.
I
MPELLER
R
OTATION

AND
C
ENTRIFUGAL
F
ORCE
The number of vanes and the sweep of the veins determine
the performance characteristics of the impeller. As vanes
are added, the impeller will produce higher discharge
pressures and lower flows. The same situation applies to
increasing the length or sweep of the vanes. Reducing the
number of vanes or the sweep of the vanes will increase
the flow and reduce the pressure.
TYPE OF PUMP
PRESSURE/FLOW RATING
CHARACTERISTICS
Centrifugal
Low Pressure/High Flow
Flow changes when pressure changes
Positive-Displacement
High Pressure/Low Flow
Flow doesnt change when pressurechanges
Table 8.1 - Pump Characteristics
Figure 8.1 - Centrifugal Pump Crossection 8-2
Figure 8.2 - Horizontal Nonclog Wastewater Pump with Open Impeller
Figure 8.3
Vertical Ball Bearing Type Wastewater Pump
C
ENTRIFUGAL
P
UMPS
Centrifugal pumps designed for pumping wastewater
usually have smooth channels and impellers with large
openings to prevent clogging.
Impellers may be of the open or closed type. Submersible
pumps usually have open impellers and are frequently used
to pump wastewater from wet wells in lift stations.
P
ROPELLER
P
UMPS
There are two basic types of propeller pumps, axial-flow
and mixed-flow impellers. The axial-flow propeller pump
is one having a flow parallel to the axis of the impeller.
The mixed-flow propeller pump is one having a flow that
is both axial and radial to the impeller.
V
ERTICAL
W
ET
W
ELL
P
UMPS
A vertical wet well pump is a vertical shaft, diffuser type
centrifugal pump with the pumping element suspended
from the discharge piping. The needs of a given installation
determine the length of discharge column. The pumping
bowl assembly may connect directly to the discharge head
for shallow sumps, or may be suspended several hundred
feet for raising water from wells. Vertical turbine
centrifugal pump consists of multiple impellers that are
staged on a vertical shaft. The impellers are designed to
bring water in the bottom and discharge it out the top. This
results in axial flow as water is discharged up through the
column pipe. Staging the impellers in these pumps can
create very high discharge pressures, since the pressure
increases as the water moves through each stage. 8-3
Figure 8.4 - Submersible Wastewater Pump 8-4
Figure 8.7 - Propeller Pump
Figure 8.5 - Impellers
Figure 8.6
Propeller-type Impellers 8-5
P
OSITIVE
D
ISPLACEMENT
P
UMPS
R
ECIPROCATING

OR
P
ISTON
P
UMPS
The word reciprocating means moving back and forth,
so a reciprocating pump is one that moves water or sludge
by a piston that moves back and forth. A simple
reciprocating pump is shown below. If the piston is pulled
to the left, check valve A will be open and sludge will enter
the pump and fill the casing.
When the piston reaches the end of its travel to the left and
is pushed back to the right, Check Valve A will close, Check
Valve B will open, and wastewater will be forced out the
exit line.
A reciprocating or piston pump is a positive-displacement
pump. Never operate it against a closed discharge valve or
the pump, valve, and/or pipe could be damaged by excessive
pressures. Also, the suction valve should be open when
the pump is started. Otherwise an excessive suction or
vacuum could develop and cause problems
I
NCLINE
S
CREW
P
UMPS
Incline screw pumps consist of a screw operating at a
constant speed within a housing or trough. When the screw
rotates, it moves the wastewater up the trough to a discharge
point. Two bearings, one on top and one at the bottom,
support the screw.
Figure 8.9 - Incline Screw Pump
Figure 8.8 - Simple Reciprocating Pump
Reprinted, with permission, from Operation of Wastewater Treatment Plants, Vol. II, 5
th
ed.,
Office of Water Programs, California State University, Sacramento Foundation 8-6
P
ROGRESSIVE
C
AVITY
P
UMPS
Operation of a progressive cavity pump is similar to that
of a precision incline screw pump. The progressive cavity
pump consists of a screw-shaped rotor snugly enclosed in
a non-moving stator or housing. The threads of the screw-
like rotor make contact along the walls of the stator (usually
made of synthetic rubber). The gaps between the rotor
threads are called cavities. When wastewater is pumped
through an inlet valve, it enters the cavity. As the rotor
turns, the waste material is moved along until it leaves the
conveyor (rotor) at the discharge end of the pump. The
size of the cavities along the rotor determines the capacity
of the pump.
These pumps are recommended for materials that contain
higher concentrations of suspended solids. They are
commonly used to pump sludges. Progressive cavity
pumps should NEVER be operated dry (without liquid in
the cavities), nor should they be run against a closed
discharge valve.
Figure 8.10 - Progressive Cavity (screw-flow) Pump
Figure 8.11 - Pumping Principle of a
Progressive Cavity Pump 8-7
Figure 8.12 - Protecting the Shaf</b>t
C
ENTRIFUGAL
P
UMP
C
OMPONENTS
Before we can discuss operations and maintenance of a
centrifugal pump, it is important to understand how a pump
is put together and what the role is of each of the pump
components. A centrifugal pump is constructed from about
a dozen major components. Lets take a look at how these
pieces fit together to make a pump.
The impeller is attached to the pump shaft. The shaft must
be straight and true so that it will not cause vibration when
it rotates. The shaft should be protected from potential
damage caused by the failure of other pump parts. A shaft
sleeve is used to protect the shaft in the ar