Electric motors are used throughout the refinery, and represent over 80% of all electricity use in the refinery. The major applications are pumps (60% of all motor use), air compressors (15% of all motor use), fans (9%), and other applications (16%). The following part discusses opportunities for motors in general. When available, specific examples are listed detailing the refining process to which the measure has been applied and to what success.
Motors | Energy Conservation In Plants
Using a “systems approach” that looks at the entire motor system (pumps, compressors, motors, and fans) to optimize supply and demand of energy services often yields the most savings. For example, in pumping, a systems approach analyzes both the supply and demand sides and how they interact, shifting the focus of the analysis from individual components to total system performance. The measures identified below reflect aspects of this system approach including matching speed and load (adjustable speed drives), sizing the system correctly, as well as upgrading system components. However, for optimal savings and performance, the systems approach is recommended. Pumps and compressors are discussed in more detail in their respective sections, below.
Sizing of Motors
Motors and pumps that are sized inappropriately result in unnecessary energy losses. Where peak loads can be reduced, motor size can also be reduced. Correcting for motor over sizing saves 1.2% of their electricity consumption (on average for the U.S. industry), and even larger percentages for smaller motors.
Higher Efficiency Motors
High efficiency motors reduce energy losses through improved design, better materials, tighter tolerances, and improved manufacturing techniques. With proper installation, energy efficient motors run cooler and consequently have higher service factors, longer bearing and insulation life and less vibration.
Typically, high efficiency motors are economically justified when exchanging a motor that needs replacement, but are not economically feasible when replacing a motor that is still working. Typically, motors have an annual failure rate varying between 3 and 12%.
Sometime though, according to a case study by the Copper Development Association, even working motor replacements may be beneficial. The payback for individual motors varies based on size, load factor, and running time. The best savings are achieved on motors running for long hours at high loads. When replacing retiring motors, paybacks are typically less than one year from energy savings alone.
To be considered energy efficient in the United States, a motor must meet performance criteria published by the National Electrical Manufacturers Association (NEMA). However, most manufacturers offer lines of motors that significantly exceed the NEMA-defined criteria. NEMA and other organizations have created the “Motor Decisions Matter” campaign to market NEMA approved premium efficient motors to industry. Even these premium efficiency motors may have low a payback period. According to data from the CDA, the upgrade to high efficiency motors, as compared to motors that achieve the minimum efficiency, as specified by the Energy Policy, have paybacks of less than 15 months for 50 hp motors. Because of the fast payback, it usually makes sense not only to buy an energy efficient motor but also to buy the most efficient motor available.
Replacing a motor with a high efficiency motor is often a better choice than rewinding a motor. The practice of rewinding motors currently has no quality or efficiency standards.
To avoid uncertainties in performance of the motor, a new high efficiency motor can be purchased instead of rewinding one.
Inductive loads like transformers, electric motors and HID lighting may cause a low power factor. A low power factor may result in increased power consumption, and hence increased electricity costs. The power factor can be corrected by minimizing idling of electric motors, avoiding operation of equipment over its rated voltage, replacing motors by energy efficient motors and installing capacitors in the AC circuit to reduce the magnitude of reactive power in the system.
Voltage unbalance degrades the performance and shortens the life of three-phase motors. A voltage unbalance causes a current unbalance, which will result torque pulsations, increased vibration and mechanical stress, increased losses, motor overheating reducing the life of a motor. Voltage unbalances may be caused by faulty operation of power correction equipment, unbalanced transformer bank or open circuit. It is recommended that voltage unbalance at the motor terminals does not exceed 1%. Even a 1% unbalance will reduce motor efficiency at part load operation. If the unbalance would increase to 2.5%, motor efficiency will also decrease at full load operation. By regularly monitoring the voltages at the motor terminal and using annual thermo-graphic inspections of motors, voltage unbalances may be identified. Furthermore, make sure that single-phase loads are evenly distributed and install ground fault indicators. Another indicator for a voltage unbalance is a 120 Hz vibration.
Adjustable Speed Drives (ASD) / Variable Speed Drives (VSDs)
SDs better match speed to load requirements for motor operations. Energy use on many centrifugal systems like pumps, fans and compressors is approximately proportional to the cube of the flow rate. Hence, small reductions in flow that are proportional to motor speed can sometimes yield large energy savings. Although they are unlikely to be retrofitted economically, paybacks for installing new ASD motors in new systems or plants can be as low as 1.1 years. The installation of ASDs improves overall productivity, control and product quality, and reduces wear on equipment, thereby reducing future maintenance costs.