Energy Conservation Opportunities in Pumping System | Energy Efficient Pumping System

Energy Conservation Opportunities in Pumping Systems: -

The main areas for energy conservation include:

1. Selecting the right pump

2. Controlling the flow rate by speed variation

3. Pumps in parallel to meet varying demand

4. Eliminating flow control valve

5. Eliminating by-pass control

6. Start/stop control of pump

7. Impeller trimming

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 pump operating point is the point where the pump curve and the system resistance curve intersect. However, it is impossible for one operating point to meet all desired operating conditions. For example, when the discharge valve is throttled, the system resistance curve shifts to the left and so does the operating point. (See Figure-1) Figure 1 below shows a typical vendor-supplied pump performance curves for a centrifugal pump where clear water is the pumping liquid.

Fig.1 Typical centrifugal pump performance curve given by suppliers

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. Inefficiencies of oversized pumps can be overcome by, for example, the installation of VSDs, two-speed drives, lower rpm, smaller impeller or trimmed impeller (BEE, 2004).

2. Controlling flow rate by speed variation

2.1 Explaining the effect of speed

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, 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

Q α N

H α N2

P α N3

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.It is relevant to note that flow control by speed regulation is always more efficient than by a control valve. This is because valves reduce the flow, but not the energy consumed by pumps.

In addition to energy savings, there could be other benefits of lower speeds.

• Bearing’s life is increased. This is because bearings carry the hydraulic forces on the impeller (created by the pressure profile inside the pump casing), which are reduced approximately with the square of speed. For a pump, bearing life is proportional to the seventh power of speed (N7)!

• Vibration and noise are reduced and seal life is increased, provided that the duty point remains within the allowable operating range.

2.2 Using variable speed drive (VSD)

As explained earlier, 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). VSDs allow pump speed adjustments over a continuous range, avoiding the need to jump from speed to speed as with multiple-speed pumps. VSDs control pump speeds use two types of systems:

• Mechanical VSDs include hydraulic clutches, fluid couplings, and adjustable belts and pulleys.Electrical VSDs include eddy current clutches, wound-rotor motor controllers, and variable frequency drives (VFDs). VFDs are the most popular and adjust the electrical frequency of the power supplied to a motor to change the motor’s rotational speed.

• For many systems, VFDs offer a means to improve the pump operating efficiency under different operating conditions. The effect of slowing pump speed on the pump operation is illustrated in Figure 14. When a VFD reduced the RPM of a pump, the head/flow and power curves move down and to the left, and the efficiency curve also shifts to the left.

The major advantages of VSD application in addition to energy saving are (US DOE, 2004):

• Improved process control because VSDs can correct small variations in flow more quickly.

• Improved system reliability because wear of pumps, bearings and seals is reduced.

• Reduction of capital & maintenance cost because control valves, by-pass lines, and conventional starters are no longer needed.

• Soft starter capability: VSDs allow the motor the motor to have a lower start-up current.

Fig-2 : Effect of VFD (US DOE, 2004)

3. Pumps in parallel to meet varying demand

Operating two pumps in parallel and turning one of 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. Below figure shows the pump curve for a single pump, two pumps operating in parallel and three pumps operating in parallel. It also shows that the system curve normally does not change by running pumps in parallel. The flow rate is lower than the sum of the flow rates of the different pumps.

Typical performance curves for pumps in parallel (BPMA)

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, it does not reduce the power consumed, as the total head (static head) increases. Figure 16 shows how the system curve moves upwards and to the left when a discharge valve is half closed. This method 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.

Control of Pump Flow by Valve (BPMA)

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 an 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. Similar to the affinity laws, the following equations apply to the impeller diameter. 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 an energy wastage. This option should therefore be avoided.

8. 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).

9. Impeller trimming

Changing the impeller diameter gives a proportional change in the impeller’s peripheral velocity. Similar to the affinity laws, the following equations apply to the impeller diameter

Q α N

H α N2

P α N3

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 and therefore decrease 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.

Figure 4 illustrates the effect of impeller diameter reduction on centrifugal pump performance.

Impeller diameter reduction on centrifugal pump performance

Best Practices in Pumping Systems: -

These most important options to improve energy efficiency of pumps and pumping systems.

• Operate pumps near their best efficiency point (BEP)

• Ensure adequate NPSH at site of installation

• Modify pumping system and pumps losses to minimize throttling.

• Ensure availability of basic instruments at pumps like pressure gauges, flow meters

• Adapt to wide load variation with variable speed drives or sequenced control of multiple units

• Avoid operating more than one pump for the same application

• Use booster pumps for small loads requiring higher pressures

• To improve the performance of heat exchangers, reduce the difference in temperature between the inlet and outlet rather than increasing the flow rate

• Repair seals and packing to minimize water loss by dripping

• Balance the system to minimize flows and reduce pump power requirements

• Avoid pumping head with a free-fall return (gravity), and use the siphon effect

• Conduct a water balance to minimize water consumption, thus optimum pump operation

• Avoid cooling water re-circulation in DG sets, air compressors, refrigeration systems, cooling towers feed water pumps, condenser pumps and process pumps

• In multiple pump operations, carefully combine the operation of pumps to avoid throttling

• Replace old pumps with energy efficient pumps

• To improve the efficiency of oversized pumps, install variable speed drive, downsize /replace impeller, or replace with a smaller pump

• Optimize the number of stages in multi-stage pump if margins in pressure exist

• Reduce the system resistance by pressure drop assessment and pipe size optimization

• Regularly check for vibration to predict bearing damage, misalignment's, unbalance, foundation looseness etc.