centrifugal pumps designs
All centrifugal pumps use an impeller and volute to create the partial vacuum and discharge pressure necessary to move water through the casing. The impeller and volute form the heart of the pump and help determine its flow, pressure and solid handling capability.
An impeller is a rotating disk with a set of vanes coupled to the engine/motor shaft that produces centrifugal force within the pump casing. A volute is the stationary housing (in which the impeller rotates) that collects, discharges and recirculates water entering the pump. A diffuser is used on high pressure pumps and is similar to a volute but more compact in design. Many types of material can be used in their manufactire but cast iron is most commonly used for construction applications.
In order for a centrifugal pump, or self priming, pump to attain its initial
prime the casing must first be manually primed or filled with water. Afterwards,
unless it is run dry or drained, a sufficient amount of water should remain in
the pump to ensure quick priming the next time it is needed.
As
the impeller churns the water (see figure above), it purges air from the casing
creating an area of low pressure, or partial vacuum, at the eye (center) of the
impeller. The weight of the atmosphere on the external body of water pushes
water rapidly through the hose and pump casing toward the eye of the
impeller.
Centrifugal force created by the rotating impeller pushes water
away from the eye, where pressure is lowest, to the vane tips where the pressure
is highest. The velocity of the rotating vanes pressurizes the water forced
through the volute and discharges it from the pump.
Water passing through the
pump brings with it solids and other abrasive material that will gradually wear
down the impeller or volute. This wear can increase the distance between the
impeller and the volute resulting in decreased flows, decreased heads and longer
priming times. Periodic inspection and maintenance is necessary to keep pumps
running like new.
Another key component of the pump is its mechanical seal. This
spring loaded component consists of two faces, one stationary and another
rotating, and is located on the engine shaft between the impeller and the rear
casing (see figure below). It is designed to prevent water from seeping into and
damaging the engine. Pumps designed for work in harsh environments require a
seal that is more abrasion resistant than pumps designed for regular household
use. 
Typically
seals are cooled by water as it passes through the pump. If the pump is dry or
has insufficient water for priming it could damage the mechanical seal.
Oil-lubricated an occasionally grease-lubricated seals are available on some
pumps that provide positive lubrication in the event that the pump is run
without water. The seal is a common wear part that should be periodically
inspected.
Regardless of whether the application calls for a standard, high pressure, or
trash every centrifugal pump lifts and discharges water in the same way. The
following section will point out design differences between these
pumps.
Standard Centrifugal Pumps
Standard
centrifugal pumps provide an economical choice for general purpose
dewatering. A number of different sizes are available but the most common model
offerings are in the 2 to 4 inch range with flows from 142 to 500 gallons per
minute (GPM) and heads in the range of 90 to 115 feet.
these pumps should only be used in clear water applications (agricultural,
industrial, residential) as they have a limited solid handling capability of
only 10% by volume.
The impellers typically use a three-vane design (see
figure below) and the volute is compact, preventing the passage of large solids.
The rule of thumb is the pump will only pass spherical solids 1/4 the diameter
of the suction inlet.
One advantage these pumps have over comparably sized
trash models is their low initial cost. There are several reasons for this
difference. Lower horsepower engines are utilized that are smaller in size and
more fuel efficient. The mechanical seals, since they are not subjected to harsh
working conditions, can be made of less costly material. Additionally, the
casings are smaller and have fewer machined parts that when combined with the
smaller engines make the pumps much lighter in weight.
High Pressure Centrifugal Pumps
High pressure centrifugal pumps
are designed for use in applications requireing high discharge pressures and
flows. Contractors may use them to wash down equipment on the job site as well
as install them on water trailers. Other uses include irrigation and as
emergency standby pumps in areas where there is a high risk of fire.
Typically these pumps will discharge around 100 GPM and produce heads in
excess of 240 feet. The pump may have a 2 inch suction port and up to three
discharge ports of varying size for added versatility. The impellers used on
these pumps are a closed design (see figure below) and not open like those used
on other types of centrifugal pumps. Similarly the diffuser is more compact than
a regular volute in order to generate the high discharge pressures. 
These pumps by design are not capable of handling any types of solids or even sandy
water, Silt, sand or debris would almost immediately clog the pump if allowed to
enter into the casing. Additionally, the impeller and diffuser may be made of
aluminum rather than weather resistant cast iron since they are not subject to
abrasive materials. It is recommended that a mesh net always be placed over the
suction strainer if the pump is being used in dirty water.
Trash Centrifugal Pumps
Trash centrifugal pumps get their name
from their ability to handle large amounts of debris and are the preferred
choice of contractors and the rental industry. The most common sizes are in the
2 to 6 inch range producing flows from 200 to 1,600 GPM and heads up to 150
feet.
The rule of thumb is that a trash pump will generally handle spherical solids up to 1/2 the diameter of the suction inlet. Solids (sticks, stones and debris) flow through without cloggin making themideal for the water conditions typically found on job sites. Trash pumps handle up to 25% suspended solids by volume.
Trash pumps offer another benefit in that they can be quickly and easily disassembled for service or inspection. While standard pumps require special tools that are not always available the inside of a trash pump gousing can usually be accessed with common tools.
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2012-05-23
