Most chemical plants are working to become more energy efficient. Companies are implementing energy management software, installing occupancy sensors throughout plants to help lower electricity bills, and even changing times of operation to use less power at peak load to avoid the associated higher rates. One of the best ways to save energy is to focus on motor-driven pumps.
Pumps consume more energy in chemical plants than any other category or type of rotating equipment. The average annual spending on pump maintenance and operations is approximately 50% greater than that of any other rotating machine, according to a recent study by the FiveTwelve Group. Companies that operate large numbers of pumps usually recognize the high energy costs as well as the impact pumps have on reliability and process control. However, too many organizations focus on these factors separately when, in fact, they are closely linked.
A recent report on the use of motor efficiency technologies by the U.S. Department of Energy’s Industrial Technologies Program (ITP) contained an in-depth analysis of energy use and savings potential by market segment and industry. The report identified centrifugal pumps as the largest consumers of motor energy (Figure 1). Also, among all rotating assets in the plant, process pumps had the highest overall potential for electrical energy savings.
Figure 1. Pumps consume more motor energy than any other type of rotating equipment at plants. Source: U.S. Dept. of Energy Industrial Technologies Program
Figure 2. Energy and maintenance costs account for more than half of long-term expenses. Source: Pump costs from a top 10 global chemical manufacturer, FY 2006.
Figure 3. Multiple pumps can cope with fluctuating rates that a single pump can’t handle efficiently.
Source: U.S. Dept. of Energy ITP.Another mechanical option is installing a parallel system to cope with fluctuation in flow or demand rates (Figure 3). By installing two similar pump systems with parallel piping to handle variable flow rates, plant operators can deal with rates that one pump can’t support alone. Although this may seem like an obvious solution, if the load increases more than the pump was designed to handle, the original pump may be running inefficiently from the beginning. Electronic control systems. Digital technologies, such as electronic inverters, electronic-only variable frequency drives (VFDs) and variable speed drives (VSDs)that include mechanical devices in addition to electronics, canalter the speed of the pump motor to help improve efficiency. A VFD (Figure 4) is an electrical system that controls motor speed by varying the frequency supplied to the motor. The drive also regulates the output voltage in proportion to the output frequency to provide a relatively constant ratio of voltage to frequency (V/Hz), as required by the characteristics of the AC motor to produce torque. In closed-loop control, a change in power and frequency supplied to the motor alters its speed, thus compensating for a change in process demand. This means greater process control and system efficiency, with even more intelligence integration.Modern VFDs are the most efficient method to change pump speed, with up to 98% electronic efficiency. The simplest of these is the soft starter, a solid-state motor starter that’s used to start or stop a motor by reducing the voltage to each of its phases, gradually increasing the voltage at a fixed frequency until the motor gets up to full voltage/speed. Motors, especially low-voltage ones, have a high initial current (amp draw) when first turned on; this can cause voltage fluctuations and affect the performance of other circuits. Voltage spikes also can damage motor windings. To counteract this issue, you can add components in series to control current in-rush upon startup. In addition, it’s crucial for electronic systems to reside in a climate-controlled environment, which is becoming more common as plants integrate digital systems. More-advanced VFDs or VSDs utilizing frequency inverters can vary the speed of the pump to match the process flow demand. VFDs offer tremendous benefits, including pump size reductions, lower energy costs and improved efficiency. The U.S. Department of Energy estimates that up to 25% of installed motor systems can benefit from retrofitting VSD technology. Despite the known benefits of the technology, initially VFD adoption was relatively slow, primarily due to perceived complexity, reliability and electrical issues. However, today’s more mature VFD technology and cable installation practices largely have mitigated these concerns. As a result, VFD implementation on pump systems is on the rise.
Figure 4. Units such as these control motor speed by changing the frequency supplied to the motor.
ROBERT LAX is global product manager for PumpSmart Control Solutions at ITT PRO Services, Seneca Falls, N.Y. MIKE PEMBERTON is the energy &reliability program manager for ITT Goulds Pumps/Plant Performance Services, Seneca Falls, N.Y. E-mail them at [email protected] and [email protected].