Environmental Engineering Reference
In-Depth Information
the total head increases and the capacity or volume discharged by the
pump decreases. In many cases, this method is employed to reduce the
discharge of a centrifugal pump. It should be noted, however, that this
does increase the load on the pump and drive system, causing addi-
tional energy requirements and additional wear.
The total closure of the discharge control valve increases the fric-
tion head to the point where all the energy supplied by the pump is
consumed in the friction head and is not converted to pressure head.
Consequently, the pump exceeds its cut-off head and the pump discharge
is reduced to zero. Again, it is important to note that, although the oper-
ation of a centrifugal pump against a closed discharge may not be haz-
ardous (as with other types of pumps), it should be avoided because of
the excessive load placed on the drive unit and pump. Our experience
has shown that on occasion the pump can produce pressure higher than
the pump discharge piping can withstand. Whenever this occurs, the
discharge piping may be severely damaged by the operation of the pump
against a closed or plugged discharge.
3.3.4.2 Efficiency
Every centrifugal pump will operate with varying degrees of effi-
ciency over its entire capacity and head ranges. The important factor in
selecting a centrifugal pump is to select a unit that will perform near its
maximum efficiency in the expected application.
3.3.4.3 Brake Horsepower Requirements
In addition to the head capacity and efficiency factors, most pump
literature includes a graph showing the amount of energy in horsepower
that must be supplied to the pump to obtain optimal performance.
3.3.5 advantages and disadvantages of the Centrifugal Pump
The primary reason why centrifugal pumps have become one of the
most widely used types of pumps is that they offer several advantages,
including:
•
Construction
—The pump consists of a single rotating element and
simple casing, which can be constructed using a wide assortment
of materials. If the fluids to be pumped are highly corrosive, the
pump parts that are exposed to the fluid can be constructed of lead
or other material that is not likely to corrode. If the fluid being
pumped is highly abrasive, the internal parts can be made of abra-
sion-resistant material or coated with a protective material. Also,
the simple design of a centrifugal pump allows the pump to be con-
structed in a variety of sizes and configurations. No other pump
currently available offers the range of capacities or applications
that the centrifugal pump does.
