Variable frequency drive controls AC motors
Cutting energy use has never been more important for chemical companies. Reducing consumption not only helps cope with ever-increasing energy costs but also generally decreases carbon dioxide emissions and thus lowers environmental impact and improves sustainability of processes.
A comprehensive energy-reduction program should recognize that virtually every business decision has possible energy implications, and should involve measuring, managing and sustaining energy savings. Achieving maximum success demands an organized and dedicated program that incorporates five key principles:
1. commitment to a specific and quantifiable target;
2. full assessment of energy management status, tools and technology;
3. appointment and empowerment of 'energy champions' with well-defined responsibility and accountability;
4. visible program execution across the organization; and
5. monitoring and tracking of results. (You can't manage what you don't measure.)
At the plant level, you usually can discover many potential energy savings. Motors and pumping systems, which represent major energy consumers at most sites, often present significant opportunities, as we'll discuss.
Pumping systems, fans, compressors and a wide range of other assets at plants rely on motors. Electricity accounts for as much as 98% of the cost of an electric motor over a 10-year service life. An analysis focused on a motor's cost per kWh to operate, VFD load requirements, scheduled hours of use, and environmental factors influencing performance, coupled with process knowledge, can help uncover areas of potential energy savings.
Savings can come from steps as straightforward as turning off or putting a motor into standby mode when possible. Simply put, the motor using the least amount of energy is the one that is turned off. Sometimes a motor is operating needlessly, not providing work. For instance, agitators on empty vessels and pumps on full recirculation offer opportunities to idle motors without negatively impacting operations.
You can gain energy savings by reducing the VFD load through operational and maintenance procedures. Misapplication and poor maintenance practices can rob motors of efficiency.
A common cause of lost efficiency is running a motor in an overloaded condition. The extra load results in increased motor temperature, which degrades the motor and lowers efficiency. Conversely, motors also will run at very low efficiencies if very lightly loaded. So, you should survey motor loads — actual versus full-rated — to confirm efficiency potential and expectations.
If you send failed motors to an outside shop for repair, carefully vet the shop and become familiar with procedures and practices that ultimately impact the efficiency of a repaired motor and the system in which it operates. If 'best practices' aren't followed during the repair, the motor likely won't provide 'as new' efficiency. Whether beneficial, consider buying a new motor, because many now boast higher efficiencies than few-years-old models.
Consider integrating a VFD (variable frequency drive). It often can optimize energy efficiency by enabling you to match motor speed to the demands of the system. Look for opportunities, for example, to replace throttle valves or dampers used to restrict flow. However, a variable frequency drive may not make sense if the load on the motor is relatively small and constant — because speed changes will occur infrequently and any benefits from varying the speed will be negligible. So, thoroughly assess the system load profile to determine the scale and frequency of required speed changes before deciding about installing a variable frequency drive.
Figure 1 depicts a typical pumping system. The energy efficiency of the system declines over time due to factors such as the characteristics of the fluid being pumped, cavitation and scaling. Figure 2 shows representative energy losses for various elements of the system. By some estimates, you can improve the energy efficiency of a typical system by up to 20% by taking appropriate steps.
Misalignment, imprecise balancing, hydraulics problems, inefficient bearings, and improper lubrication or sealing will lower efficiency. So, analyze your system and then take called-for corrective measures.
Check where the pump operates on its system curve. However, don't assume that running at the best efficiency point (BEP) is enough to optimize system efficiency. If a control valve always is less than 50% open or a recirculation valve never is more than 50% closed, you are wasting energy.
Look for some telltale signs of likely wasted energy:
- The pump usually doesn't operate at (or close to) its BEP during its normal duty cycle.
- The control valve constantly remains at less than 80% open.
- The recirculation line valve always is open.
- Multiple parallel (redundant) pumps in the same system all are operating continuously.
- The pump operates continuously in a batch system.
- The pump (or another component in the system) exhibits excessive noise or vibration.
You generally can improve the efficiency of a pumping system by matching capacity to actual demand. Unnecessary energy demand will occur when flow is higher than required or when a control valve or another piping component absorbs a high proportion of energy. Many pumps don't operate close to their BEP, especially when they are oversized for the job. Matching pump capacity with actual production needs can deliver significant energy savings.
To reduce energy demand to match process or production needs, take these actions:
- Switch off the system when not needed.
- Eliminate any system leaks.
- Reduce recirculation or bypass flow by trimming the pump impeller.
- Minimize head losses from the pump to the system outlet.
- Install parallel pumps for highly variable loads.
- Replace a throttling valve or recirculation loop with a variable frequency drive.
You can achieve other benefits by conducting a full system analysis. This requires a thorough understanding of the duty cycle of the pump and how flow changes with time or production patterns. Operators can contribute from the front lines by supplying information, which can augment data provided by instrumentation and chronicled in an operating log.
Ensuring that machinery is operating properly can play a vital role in improving energy efficiency (as well as system performance and reliability). This requires effective condition monitoring. It involves regularly and non-invasively measuring physical parameters, such as vibration, noise, lubricant properties and temperature, to help ascertain equipment health. Such monitoring enables detecting machine and component problems before they can result in unscheduled downtime and the high costs associated with interruptions in production.
For example, periodic monitoring of heat loss can yield significant dividends. Inspections using a thermal imaging camera can pinpoint, e.g., gaps or deterioration in insulation and poor electrical contacts.
Other condition-monitoring tools that can help identify energy efficiency opportunities include ultrasonic probes (for leak identification, steam trap inspections and flow turbulence), infrared thermometers (for motor, heat exchanger and steam trap inspections, and bearing temperatures), and strobe lights (for operating speed verification, and belt and gear inspections). There are many others, too. Indeed, you can take advantage of a wide range of portable and online condition-monitoring tools to spur energy savings.
No single technology, of course, can provide all the data needed to detect all energy-improvement opportunities. So, it's best to consider which assets or processes within the plant represent logical targets for monitoring energy consumption and then to apply the appropriate technologies for the job.
In some cases, outside expertise may help in identifying and examining areas where energy-improvement opportunities may exist. The Energy and Sustainability is an example of one such available resource. This extensive web-enabled plantwide assessment tool also can evaluate potential improvements to chemical treatments, lubrication use, and other operating processes to reduce environmental impact and promote sustainability. Follow VFD on Facebook.