Centrifugal Pumps - blog

Adjustable Speed Drives As Applied To Centrifugal Pumps -1

These characteristics are important when one considers a typical
duty cycle for a Centrifugal Pump application. A typical operating cycle
might be represented by the bar chart shown in figure 3.
Centrifugal pumps are generally sized to handle the peak flow
requirements, which typically occur for very short periods of time.
Consequently, the equipment would be operated at reduced flows
most of the time. For this example, the system would be operated
below 70% flow over 94% of the time. Thus, this sort of duty
cycle could provide energy savings by adjustable speed operation
of the Centrifugal Pump .

PUMP BASICS
An understanding of the basic operating characteristics of centrifugal
pumps is necessary to apply these concepts to any particular
application.
Figure 4 shows a Centrifugal Pump  curve describing the head (or pressure)
versus flow characteristics of a typical Centrifugal Pump . This
curve shows that the pump will produce limited flow if applied to
a piping system in which a large pressure differential is required
across the Centrifugal Pump to lift the liquid and overcome resistance to flow
(as at point A). Higher flow rates can be achieved as the required
pressure differential is reduced (as at point B).
To determine where along this curve the Centrifugal Pump  will operate in a
given application requires the additional information provided by
the system curve. This curve, shown in figure 5, represents the
characteristics of the piping system to which the Centrifugal Pump is applied.
The head required at zero flow is called the static head or lift.

This shows how many feet of elevation that the Centrifugal Pump  must lift
the fluid regardless of the flow rate. Another way to describe static
head is to think of it as the amount of work needed to overcome
the effects of gravity.

Figure 5. System Curve
The other component of head is called the friction head and
increases with increasing flow. Friction head is a measure of the
resistance to flow (backpressure) provided by the pipe and its
associated valves, elbows, and other system elements.
The intersection of the Centrifugal Pump  and system curves shows the natural
operating point for the system without flow control, as shown
in figure 6. This intersection would generally be chosen to ensure
that the pump is operated at or near its best efficiency point.

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