Pumps & Systems, April 2008
Production, distribution and refining applications in the oil and gas industry rely heavily on motor-driven pumps and pumping systems. Keeping electric motors driving critical operations at peak performance is vital to ensure maximum profitability.
For continuous motor performance, engineers are focusing on variable frequency drive (VFD) and motor control technology. Because of their ability to regulate motor speed, VFDs are effective for controlling energy use.
Centrifugal loads, such as pumps and fans, offer great energy savings potential when the application requires less than 100 percent flow or pressure. Although very few oil and gas applications require 100 percent flow continuously, many of the motors employed in these pumping systems are started at full speed and remain running at full, fixed speed while in use. In these applications, users can integrate VFDs in both low- and medium-voltage configurations.
Companies can gain energy savings by using VFDs to control flow by modulating motor speed. A 20 percent speed reduction can help save energy use by up to 50 percent, which means VFDs often quickly pay for themselves in energy savings. The higher the level of motor operating time and the larger the variation in load duty cycles, the greater the savings.
For many large energy users, such as refineries, electric utilities often will charge higher peak-demand electricity prices when the plant exceeds a preset limit or base load of electricity. These peak demand charges typically occur when motors draw large peaks of current when started across-the-line. Variable frequency drives help reduce power peaks by gradually ramping the motor up to speed. By automatically controlling motor speed, the drive allows the motor to run at full horsepower only when necessary. The result is lower energy costs and increased operating efficiency.
Numerous oil and gas customers also choose VFDs for tighter process control. Tighter control on variable process means less energy used. For example, a control valve may not be able to properly control a process variable due to range limitations or pressure drop problems, resulting in wide process variations. The use of a VFD can provide tighter control, reduce overall waste and improve plant efficiency.
No Time for Downtime
Equipment reliability is crucial in any industry, but is especially important in oil and gas operations where every minute of uptime counts and lost production time carries costly consequences. Any schedule backup, from crude production to refined delivery, affects the entire process. Since many of these operations need to run continuously, engineers need constant monitoring and maintenance access to pumps, compressors and other system components to provide maximum throughput and uptime.
When looking at a drive investment, look for simplicity. Simplicity will help eliminate excess space demands and weight, while ensuring easy to troubleshoot design. In addition, simple VFDs will result in highly reliable solutions.
One inherent maintenance challenge in many oil and gas applications is remote or difficult-to-access locations, such as oil fields or offshore platforms. Advancements in intelligent motor control technology, along with the proliferation of high-speed Ethernet and wireless networks, have expanded access to component and production information.
Many of today's drives, soft starters and electronic overload relays offer condition-based monitoring capabilities that protect motor assets and warn of impending faults so users can take preventive action. With the ability to monitor motor current and speed, drives can perform protective functions based on that information. With access to more detailed information over longer periods of time, engineers can better use predictive trending to anticipate problems and prevent catastrophic failures.
In more extreme situations, the drive will act to protect the motor. Most drives have a built-in electronic motor thermal overload feature. When a motor is in a state of severe exertion, beyond its safe operating limits, the motor overload feature can help reduce the output current or shut off the motor and protect it from thermal damage or catastrophic failure.
Equally important is the far-reaching impact that drives can have on the overall health of automated systems. In fact, converting a process from fixed speed to variable speed can minimize wear and tear and reduce maintenance requirements for mechanical systems by reducing start/stop cycles and eliminating valves and other mechanical components. In addition, it can also maximize the efficiency of the motor and extend the life of many rotating parts in a typical motor-pump combination.
Recent VFD technology offers expanded communications capabilities, more compact designs and integrated control features driven by powerful, faster microprocessors. Among the biggest breakthroughs are advances in medium voltage drives that save cost and space.