Energy Efficiency Opportunities for Pumps

There are several factors that affect pump performance and its energy efficiency. The main areas for energy conservation include:

1. 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. 

 

2. 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).

3. 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.

4. 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.

5. 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.

6. 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).

7. 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/.