There
are several factors that affect pump performance and its energy
efficiency.
The
main areas for energy conservation include:
Selecting
the right pump
In
selecting the pump, suppliers try to match the system curve supplied
by the user with a pump curve that satisfies these needs as closely
as possible. The operating point is where the system curve and pump
performance curve intersect (as explained in the introduction).
The
Best Efficiency Point (BEP) is the pumping capacity at maximum
impeller diameter, in other words, at which the efficiency of the
pump is highest. All points to the right or left of the BEP have a
lower efficiency. The BEP is affected when the selected pump is
oversized. The reason is that the flow of oversized pumps must be
controlled with different methods, such as a throttle valve or a
by-pass line. These provide additional resistance by increasing the
friction. As a result the system curve shifts to the left and
intersects the pump curve at another point. The BEP is now also
lower. In other words, the pump efficiency is reduced because the
output flow is reduced but power consumption is not.
This
figure shows a typical vendor-supplied pump performance curves for a
centrifugal pump where clear water is the pumping liquid.
Controlling
the flow rate by speed variation
A
centrifugal pump’s rotating impeller generates head. The impeller’s
peripheral velocity is directly related to shaft rotational speed.
Therefore varying the rotational speed has a direct effect on the
performance of the pump.
The
pump performance parameters (flow rate, head, and power) will change
with varying rotating speeds. To safely control a pump at different
speeds it is therefore important to understand the relationships
between the two. The equations that explain these relationships are
known as the “Affinity Laws”:
Flow
rate (Q) is proportional to the rotating speed (N)
Head
(H) is proportional to the square of the rotating speed
Power
(P) is proportional to the cube of the rotating speed
As
can be seen from the above laws, doubling the rotating speed of the
centrifugal pump will increase the power consumption by 8 times.
Conversely a small reduction in speed will result in a very large
reduction in power consumption. This forms the basis for energy
conservation in centrifugal pumps with varying flow requirements.
Controlling
the pump speed is the most efficient way to control the flow because
when the pump’s speed is reduced, the power consumption is also
reduced. The most commonly used method to reduce pump speed is
Variable Speed Drive (VSD).
Pumps
in parallel to meet varying demand
Operating
two or more pumps in parallel and turning some off when the demand is
lower, can result in significant energy savings. Pumps providing
different flow rates can be used. Parallel pumps are an option when
the static head is more than fifty percent of the total head.
Eliminating
flow control valve
Another
method to control the flow by closing or opening the discharge valve
(this is also known as “throttling” the valves).
While
this method reduces the flow, disadvantages are -
It
does not reduce the power consumed, as the total head (static head)
increases. The figure shows how the system curve moves upwards and
to the left when a discharge valve is half closed.
It
increases vibration and corrosion and thereby increases maintenance
costs of pumps and potentially reduces their lifetimes
VSDs
are a better solution from an energy efficiency perspective.
Eliminating
by-pass control
The
flow can also be reduced by installing a by-pass control system, in
which the discharge of the pump is divided into two flows going into
two separate pipelines. One of the pipelines delivers the fluid to
the delivery point, while the second pipeline returns the fluid to
the source. In other words, part of the fluid is pumped around for no
reason, and thus is energy wastage. This option should therefore be
avoided.
Start/stop
control of pump
A
simple and reasonable energy efficient way to reduce the flow rate is
by starting and stopping the pump, provided that this does not happen
to frequently. An example where this option can be applied is when a
pump is used to fill a storage tank from which the fluid flows to the
process at a steady rate. In this system, controllers are installed
at the minimum and maximum level inside the tank to start and stop
the pump. Some companies use this method also to avoid lower the
maximum demand (i.e. by pumping at non-peak hours).
Impeller
trimming
Changing
the impeller diameter gives a proportional change in the impeller’s
peripheral velocity
Changing
the impeller diameter is an energy efficient way to control the pump
flow rate. However, for this option, the following should be
considered:
This
option cannot be used where varying flow patterns exist.
The
impeller should not be trimmed more than 25% of the original
impeller size, otherwise it leads to vibration due to cavitation. It
decreases the pump efficiency.
The
balance of the pump has to been maintained, i.e. the impeller
trimming should be the same on all sides.
Changing
the impeller itself is a better option than trimming the impeller,
but is also more expensive and sometimes the smaller impeller is too
small.
To know more details
about pump performance and its energy efficiency, contact us today at
+91 99819 92838 or visit our website at http://www.darlingpumps.in/.
