As energy costs and competition rise, putting pressure on margins and profitability, energy efficiency in plant operations is even more important now than it was in the past. Experience shows that energy savings are usually best achieved by performing an energy audit. When deciding to carry out an energy audit, a good starting point is to assume that you will save at least 10 percent of your current energy consumption. Reviewing utility bills can bring an indication of the savings to be made and investment that you should be prepared to put into the auditing process.
In most industrial sites, about two-thirds of the energy is consumed by electric motors. In its lifetime, the cost of energy consumed by an electric motor may be 100 times its purchase cost. Many pumps and motors are operated at full power constantly, irrespective of process needs, and in some sites this offers the potential of large cost reductions. It is also frequently true that power factor correction equipment does not adequately compensate for inductive loads at all times. In some energy supply agreements, surcharges arise from poor power factor correction.
An energy audit will identify these issues and calculate the potential savings to be made. An energy audit is a significant investment, and many man-hours will be needed to find out where energy is being consumed. It is important that a thorough log of consumption on each spur is made because excessive energy is sometimes used in the most unlikely places. In the case of fluctuating work or process loads, the monitoring of each energy consuming piece of plant equipment may require several days to complete.
An energy audit should identify the savings to be made by resizing or retrofitting pumps that are operating away from their best efficiency point or the savings to be made by installing variable speed motors in place of motors driven at full power all the time. A good energy audit will calculate from the process requirements what energy consumption should be needed and compare this with actual energy consumption. By ensuring that every pump and motor is working at the maximum efficiency great savings can be made.
To carry out an audit, the performance data for the equipment as originally supplied will be required. In some cases there might not be a list of pumps installed, and an initial walk-through will be required to identify these. Many pumps have nameplates, so data from these should be recorded. Determining actual flows and pressures can sometimes be difficult where pressure gauges and flow meters are not fitted, and power readings on site can often be inaccurate.
However, the variations from the originally specified duty is usually so large that estimated actual performance from data that is available is often sufficient to guide actions for energy reduction. Saving energy may be as simple as turning a pump off rather than recycling flow. The cost of trimming a pump impeller instead of throttling can be recovered in a few days.
For large pumps, where the savings can be very high, it is often worth employing a specialist who can install temporary instrumentation to accurately measure performance. The cost of doing this is usually recouped quickly.
Aside from performing an energy audit, here are seven other ways to save energy with the pumping system.
1. Design systems with lower capacity and total head requirements. Do not assume these requirements are fixed. Flow capacity, for example, can be reduced through use of lower velocity in heat exchangers and elimination of open bypass lines. Total head requirements can be reduced by lowering process static gage, pressure, elevation rise from suction tank to discharge tank, static elevation change by use of siphons, spray nozzle velocities, friction losses through use of larger pipes and low-loss fittings. Eliminating throttle valves is also another option.
2. Avoid allowing for excessive margin of error in capacity and/or total head. It typically will be less expensive to add pumping capacity later if requirements increase. Small differences in efficiency between pumps are not as important as knowing and adjusting to the service conditions. Energy savings may be as high as 20 percent if pumps are sized based on reasonable system heads and capacity requirements. Savings result from operating at a more efficient point on the pump curve, and, in some cases, this also avoids the need to throttle pump capacity or operate at a higher capacity than necessary.
3. Despite the tendency to emphasize initial cost, save in the long run by selecting the most efficient pump type and size at the onset. The choice of a pump depends on the service needed from the pump. Considerations are flow and head requirements, inlet pressure or net positive suction head available, and the type of liquid to be pumped. Maximum attainable efficiency of a centrifugal pump is influenced by the designer’s selection of pump rotating speed as it relates to “specific speed.” Purchasers need to be aware of this, as well as the decision criteria for determining the type of pump to use.
4. Use variable-speed drives to avoid losses from throttle valves and bypass lines, except when the system is designed with high static heads. In such instances, extra concern must be shown when calculating the savings, since the pump affinity laws cannot be used without regard to the change of pump (and motor) efficiency along the system curve. Take care to ensure that the operating point of the pumps remains within the allowable/recommended limits specified by the pump manufacturer.
5. Use two or more smaller pumps instead of one larger pump so that excess pump capacity can be turned off. Two pumps can be operated in parallel during peak demand periods, with one pump operating by itself during lower demand periods. Energy savings result from running each pump at a more efficient operating point and avoiding the need to throttle a large pump during low demand. An alternative is to use one variable-speed pump and one constant-speed pump.
6. Use pumps operating as turbines to recover pressure energy that would otherwise be wasted. Practically all centrifugal pumps will perform as turbines when operated in reverse. A hydraulic power recovery turbine can recover pressure energy when used to drive a generator or assist the driver of a pump or a compressor.
7. Maintain pumps and all system components in virtually new condition to avoid efficiency loss. Wear is a significant cause of decreased pump efficiency. Bearings must be properly lubricated and replaced before they fail. Shaft seals also require consistent maintenance to avoid premature mechanical failures. Most important is the renewal of internal wearing ring clearance and the smoothness of impeller and casing waterways.
This article was adapted from information provided by the British Pump Manufacturers Association Ltd. (BPMA) and the Hydraulic Institute. At www.pumps.org, the Hydraulic Institute is a value-adding resource to member companies, engineering consulting firms, and pump users worldwide by developing and delivering comprehensive industry standards and expanding knowledge by providing education and tools for the effective application, testing, installation, operation, maintenance, and performance optimization of pumps and pumping systems.
At www.bpma.co.uk, the BPMA is a trade association representing the interests of U.K. suppliers of liquid pumps and pumping equipment. BPMA’s prime mission is to influence the business environment in the interests of the competitiveness and profitability of the UK Pump Industry through a range of services including commercial, marketing, technical, environmental, educational and energy together with regular meetings of members.