Rockwell automates dual drive compressor

  • December 17, 2014
  • Rockwell Automation
  • Rockwell Automation
  • Case Study

December 17,2014 --The world depends on natural gas to fuel its manufacturing centers and power its cities. And natural gas production relies on a complex supply chain that begins with exploration and drilling – and ends at the treatment facility. Midstream companies, which transport natural gas from the drilling sites to treatment facilities and on to distribution, play a key role in the process. To pump natural gas through pipelines and processing plants, these companies rely on a network of compressors. In 2004, Dallas-based ETT developed its patented dual drive compressor that toggles between electric motors and gas engines to capture significant energy savings – and meet emissions requirements. The system design received a Texas Environmental Excellence Award for Innovative Technology in 2009. Even so, ETT prepared for an equipment upgrade in 2011 to improve performance – and increase overall reliability. ETT is a subsidiary of Energy Transfer Partners, a midstream company that operates approximately 69,000 miles of pipelines. ETT supplies its dual drive compressors to its parent company and other midstream companies and producers. 

Challenge Before ETT developed its dual drive compressors, the industry had just two options: electric-driven or gas-driven systems. “Depending on the model, a company’s future energy costs were intrinsically tied to the electricity or gas market,” said Kenneth Tinkle, director of operations, ETT. “Since gas compression is energy-intensive, any volatility in those markets could translate to significantly higher energy costs.” In addition, while gas-driven compression can be more cost-effective, continuous gas operation may not meet emissions requirements. Alternatively, electric-driven compressors run cleanly, but are increasingly subject to “rolling blackouts” and peak demand charges. The dual drive system allows companies to control emissions – and take advantage of the lowest energy costs at any given time. Companies can also negotiate better rates with electric utilities, since they can switch to gas compression during peak demand periods.   “The key to our dual drive compressor is its ability to easily transfer from one energy source to the other,” Tinkle explained. “Our original model had performed well, but we knew there was room for improvement.” The compressor redesign centered on both the hardware – updated engines and motors – and the control system. “The drive transfer control system was particularly in need of improvement,” Tinkle said. “With the old equipment, we had to reduce the load on the compressor and slow the gas volume to transfer between the drives. This volume change can impact the process within gas treatment facilities. Solution To design a control system that would ease the drive transition, ETT called on Prime Controls, a Rockwell Automation Solution Partner. Based in Lewisville, Texas, Prime Controls is a systems integration and Instrument and Controls (I&C) construction firm with expertise in the oil and gas industry. The old control system was based on Allen-Bradley SLC 500 controllers running on a Data Highway Plus network. After evaluating the existing system, Tim Shepardson, engineering manager at Prime Controls, discovered that the network was overloaded at many of the compressor sites. In addition, Shepardson found that the gas engines were prone to stall during the drive transfer because too much load was being placed on the gas engines too quickly. “First, we needed to design a transfer sequence that would work with the new engines,” Shepardson explained. “In addition, we needed a sequence that would gradually shift load from one driver to the other – instead of dropping off the initial driver immediately.” Prime Controls installed a new control system based on the Rockwell Automation Integrated Architecture platform. The system includes Allen-Bradley CompactLogix controllers and medium voltage Allen-Bradley SMC Flex soft starters, running on a ControlNet network that’s specifically designed to meet the demands of real-time, high throughput applications.  

The SMC Flex starter enables the fast, smooth transfer between the dual drives – and reduces the overall wear and tear on the compressor. The system also includes an Allen-Bradley PanelView Plus interface networked through EtherNet/IP. The CompactLogix controller is programmed with Rockwell Software RSLogix 5000 software. “The RSLogix software incorporates a number of features that enabled us to write ladder logic that is easier to troubleshoot,” Shepardson said. “For example, the software provided add-on instructions (AOIs) and allowed us to utilize user-defined tag names – instead of addresses. “With ControlNet, we also were able to improve the reliability of the system’s I/O,” Shepardson continued. To enable communication with a third-party power monitoring system, Prime Controls also integrated a ProSoft Technology protocol converter into the CompactLogix chassis. ProSoft Technology is a Rockwell Automation Encompass Product Partner. “No doubt, the overall ease of use of the Rockwell Automation system streamlined our development,” said Shepardson. “We started the project in January 2011, and commissioned the first system about two months later.”  

Results The Rockwell Automation technology has now been installed by Prime Controls on 25 new dual drive compressors.  “We couldn’t be more pleased with the operation of the new compressors,” said Tinkle. “In fact, we were so happy with the control system, we asked Prime Controls to retrofit our older models.” To date, Prime Controls has completed retrofits on more than 30 units. “Thanks to the seamless transition between the two drives, we have been able to improve non-stop performance,” said Tinkle. “And enable true remote operation.” Downtime on the compressors has decreased by about 5 percent. In fact, ETT now boasts an industry-leading 99 percent runtime on the units. In the past, technicians were often called upon to troubleshoot the system during drive transfer. Now, ETT can confidently control any of its systems remotely from plant control rooms – or its Dallas headquarters. “We provide energy management as well as compression technology for our customers,” Tinkle explained. “Based on energy usage parameters, peak demand cycles and current energy rates, we remotely transfer the system from one drive to the other to maximize energy savings.” ETT continues to count on Prime Controls for control system expertise throughout their product’s lifecycle. “Prime Controls handled all phases of the control upgrade,” said Tinkle. “They designed and built our panels – and were here for start-up and system checks. And if we have any problems with the equipment, we’ll call on Tim and his team. We know they’ll handle it.” 

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