A centrifugal pump is a continuously acting
pump that moves liquid by accelerating it radially outward in a rotating member
(called an impeller) to a surrounding case. The impeller is essentially a
rotating disk with vanes attached to it. Arrows indicate the direction of
rotation and the direction of flow. The vanes on the impeller are curved
backward, since this shape provides the most stable flow characteristics. This
type of pump is by far the most common in use in buildings because of its
simple construction and relatively low cost.
This paper describes the different types of
centrifugal pumps, how they are constructed, and their performance and
efficiency characteristics, applications in buildings, installation, and maintenance.
Pump
Types and Nomenclature
The types of centrifugal pumps used in
buildings are often confusing because such pumps are identified in a number of
different ways, according to (a) the internal design, (b) single-suction versus
double-suction configuration, (c) the shape of the impeller and its operating
characteristics, (d) the casing design, (e) the type of connection between the
motor and pump, (f) the position of the pump in relation to the water being
pumped, and (g) the number of stages of the pump.
Internal
design: The casing of a pump is the
housing that encloses the impeller and collects the liquid being pumped. The
liquid enters at the eye, located at the center of the impeller. It is the
impeller that imparts energy to the liquid. After being rotated by the vanes on
the impeller, the liquid is discharged with a greatly increased velocity at the
periphery, where it is guided to the discharge nozzle through a spiral-shaped
passage called a volute. This shape is designed to result in an equal flow
velocity at all points around the circumference.
Single-suction
versus double-suction configuration: The
single-suction pump has a spiral-shaped casing and is most commonly used. The
water enters the impeller from only one side. In the double-suction pump, the
water enters both sides of the double-suction impeller so that hydraulic unbalance
is practically eliminated. Since only half the flow enters each side of the
impeller, problems with inlet design of higher-flow pumps are somewhat
relieved. The impeller is usually mounted between two bearings, and the casing is
split axially to permit convenient servicing of the pump.
Shape
of the impeller: Impellers are curved
to minimize the shock losses of flow in the liquid as it moves from the eye to
the shrouds, which are disks that enclose the impeller vanes. If an impeller
has no shrouds it is called an open impeller. This type usually is used where
the water being pumped contains suspended solids. If an impeller has two
shrouds, it is called a closed impeller; it requires little maintenance and
usually retains its operating efficiency longer than open impellers. If the impeller
has one shroud, it is called a semi open impeller.
Casing
design: Casing is typed as radially
split or axially split. The axially split casing is one that is split parallel
to the shaft axis so that the pump maybe opened without disturbing the system
piping, which makes it convenient to service. Radially split casings are split
perpendicular to the shaft axis, resulting in a simpler joint design.
Type
of connection between motor and pump:
A separately coupled pump is one in which the electric motor drive is connected
to the pump by means of a flexible coupling. Both pump and motor are mounted on
a structural baseplate to provide support and maintain shaft alignment. A close
coupled pump is one in which the same shaft is used for both the motor and
pump. This construction results in low initial cost and installation cost and
avoids alignment problems. It may also result in motor noise being transmitted
to the pump and piping. A motor-face-mounted pump is one in which the pump is
separately coupled with a face-mounted motor. This arrangement substitutes a
structural connection between the pump and motor. It eliminates the need for a
structural baseplate and minimizes coupling alignment problems.
Support
of the pump: Horizontal dry-pit support
is one where the pump is located with the shaft in a horizontal position in a
dry location such as a basement floor or even a special pit constructed for the
pump. The pump assembly is supported by the floor, and the structural baseplate
is usually grouted to the floor. This is the most common support arrangement. In-line
pumps are supported directly by the system piping; i.e., the piping carries the
weight of the pump. The pump-motor assembly is usually mounted vertically in order
to save floor space and center the weight over the piping. Some smaller pumps
may hang horizontally from the piping, and some larger vertically mounted pumps
may also rest on the floor. Wet-pit pumps are those which are immersed in the
liquid to be pumped. This is most common with sump pumps where the pumping end
is immersed in the liquid in the sump. The pump may be supported on the floor
of the sump, or it may be suspended from a structural floor above the sump.
Bearing
support: Shaft support is usually
provided by ball bearings which are lubricated by grease or oil. Some types of
pumps, such as submersible pumps (described below), depend on the liquid being
pumped to lubricate the bearings. In such pumps, sleeve or journal bearings are
used. A between-bearing pump is a centrifugal pump whose impeller is supported
by bearings on each side. This design is usually built with a double-suction
impeller and with the casing split in the axial direction so that the top can
be lifted off and the rotating element removed. An overhung impeller pump is a
centrifugal pump that has the impeller mounted on the end of a shaft that
over-hangs its bearings. In-line circulating pumps are of this type.
Single-stage
versus multistage pumps: A
single-stage pump is one which has only one impeller. The total head is
developed by the pump in one stage. A multistage pump is one which has two or
more impellers. The total head is developed in multiple stages. Vertical turbine pumps are a unique type of multistage pump. They are
designed primarily to pump water from deep wells and are long and slender.
Centrifugal
Pump Construction
Materials: Centrifugal pumps used for most building services are
built with cast-iron casings, bronze impellers, and bronze small parts.
Stainless-steel impellers and stainless-steel small parts also are common.
Cast-iron impellers may be used, but the life of a cast-iron impeller is
shorter than that of a bronze or stainless-steel impeller.
Shafts,
seals, and bearings: The shaft used
to drive the impeller of the pump enters the casing through an opening that
must be sealed to prevent leakage around the shaft (i.e., the seal must prevent
liquid from leaving and air from entering). Two types of seals are used: soft
fiber packing and mechanical face seals. Where packing is used, the shaft enters
the opening through a stuffing box. Liquid is prevented from leaking out by
filling this opening with a soft fiber packing. The packing material, which is
relatively inexpensive, can usually be replaced without disassembling the pump.
However, the packing will leak about 60 drops per minute and requires periodic
adjustment. Mechanical seals are commonly used instead of packing because they
are reliable, have good life expectancy, are practically leak-free, and do not
require periodic adjustment.
Pump
Characteristics
Capacity: The capacity of a pump is the rate of flow of liquid
through the impeller expressed in gallons per minute (gpm) or cubic meters per hour (m3/h).
Total
head: Head h is the energy per unit
weight of a fluid due to (a) its pressure head hp, (b) its velocity head hv,
and (c) its elevation head Z above some datum. It is commonly expressed as the
height of a column of water in feet (or meters) which is necessary to develop a
specific pressure. The total head developed by a pump is equal to the discharge
head hd minus the suction head hs. The discharge head is the energy per unit
weight of fluid on the discharge side of the pump. The suction head is the
energy per unit weight on the suction side of the pump.
The static head Z is the static elevation
measured in feet (meters) at the same point where the pressure is measured. Note
that if a pressure gage is used, the center of the gage is the measurement
point for the static head. The centerline of the pump impeller is usually used
as the reference point for such measurements. The symbols and units used in
this section are the same as those used by the Hydraulic Institute.
Efficiency: The efficiency in percent with which the pump
operates is the ratio of the output power to the input power multiplied by 100.
Efficiency varies with capacity reaching a maximum value at one capacity where
the sum of all losses is a minimum.
Net
positive suction head: Net positive
suction head (NPSH) is the total suction head in feet (meters) of liquid in
absolute pressure terms determined at the pump impeller, minus the vapor
pressure of the liquid in feet (meters). The net positive suction head required
(NPSHR) by the pump is determined by test and is the NPSH value at which the
pump total head has decreased by 3% because of low suction head and resulting
cavitation within the pump. In multistage pumps, the 3% head reduction refers to
the first stage head and the NPSHR increases with capacity.
Speed: Usually a centrifugal pump is driven by a constant-speed
electric motor. However, it is more efficient to control a pump by a
variable-speed drive. The extra cost of variable-speed drives can be justified
by the resultant savings in electric power.
System head curve: In order to move liquid through any system of pipes,
the pump must produce a total head equal to or greater than the total head
required by the system. The system head usually increases with flow rate, and if
plotted versus capacity, it is called the system head curve. The shape of the
system head curve is an important consideration in the proper selection of a
pump in building services. The total head required to pump liquid through a system
is the sum of the static head and the head due to friction loss in the system.
For example, to pump water to the top of a 50-ft (15-m) building, the total
head required is 50 ft (15 m) plus some friction loss. If the friction loss at the
required flow is equivalent to a head of 10 ft (3 m), the total head required
is 60 ft (18 m). When the flow is zero, there is no friction loss so the total
head required is only 50 ft (15 m). The pump will operate where the pump curve
intersects with the system head curve; at this point the full flow required
will be pumped.
Because the pump is subject to wear, the
total head output is reduced. As a result, there is a reduction in flow. However,
note that the reduction is greater when there is a high static head than when
the head is due only to friction losses. Hence, it is important that the system
head curve and pump characteristic curve be compared at the time of pump
selection to ensure that a 10% reduction in pump output, due to wear, does not
result in a significant reduction in flow rate.
Pump
efficiency: Centrifugal pumps are
more efficient at high flow rates and moderate heads than at low flow rates and
high heads.
Source: https://dticorp.com/improving-lives-with-water-pumps-and-equipment/
