New Life for Older Turbine Fuel controllers

The fuel governor is an integral component of turbine control equipment. Its specific functions are based on the particular rotating equipment OEM’s proprietary requirements. Modern fuel governors have been installed on industrial gas turbines since the early 1960s. In the mid-1980s microprocessors became fast enough to perform turbomachinery control functions. Built on proprietary architecture, these systems require specific equipment and training to update. In addition, the older systems presented challenges when updating the software/firmware or modifying the start-up, operating and shutdown sequence. Obtaining spare parts and adding new input and output signals to many legacy controllers was also problematic and typically expensive.

The RDU controller was developed by HPI as a replacement for the obsolete Digicon unit

In 2004, HPI LLC, a Houston, Texas, U.S.A.-based turbine control, maintenance and power plant engineering, procurement and construction (EPC) company, developed novel replacement controller technology. Based on an offthe- shelf programmable logic controller (PLC) architecture, this new technology fits within the same rack space as existing fuel governors. The housing offers a matching backplane for receiving existing D and J cable connections, which avoids the need for rewiring the controller when it is replaced.

In addition to dramatically reducing fuel governor replacement cost, the new replacement controllers are extending the life of existing turbo machinery and associated control systems by 10 to 20 years. Using PLC ladder logic, the replacement unit contains the code necessary for governing fuel and performing the start-up and shutdown
turbine sequence.

From the initial development of microprocessor-based fuel governors, specialized firms often provided proprietary control platforms to OEMs. Later, independent system integrators, specializing in turbine control solutions, began to offer PLC-based controllers that were typically wholesale replacements of entire control systems. These system integrators were competing with the major OEM suppliers of rotating equipment controls.

Recognizing the need for more versatile, off-the-shelf technology, HPI developed two PLC replacement controllers, the RNU and the RDU. The latter was formerly provided by Hawker Siddeley Dynamics Engineering (HSDE), which was eventually acquired by Vosper Thornycroft (VT) Controls. Many of the HPI management team and engineering staff had been employed by HSDE and were familiar with the Digicon controllers.

When VT Controls ceased operating in the United States in 2002, replacement parts for the obsolete Digicon unit became difficult to obtain, prompting HPI to develop a replacement controller.

The use of PLC-based control systems offers commercially available, off-the-shelf replacement components, such as I/O modules, software programming options and multiple communication protocols. Because the new system can be installed directly in place of existing controllers, generally, no significant rewiring or new cables are needed. In addition, the system can be fully commissioned and tuned in a matter of days. Outage time and upgrade costs are dramatically reduced in comparison to that of replacing existing control cabinets and associated hardware.

PLC-based systems host a choice of several commercially available, offthe- shelf, graphic human machine interface (HMI) systems. End users can now troubleshoot and even modify the control program. In addition, the new HMIs offer robust graphical functionality, including graphical representation of data, alarm history, trending (real-time and historical) and user-configurable parameters (times and setpoints).

Shown here is the back c o n n e c t o r layout for the HPI RNU P L C - b a s e d controller
Shown here is the back c o n n e c t o r layout for the HPI RNU P L C - b a s e d controller

The new technology also supports multiple communication protocol options, including Ethernet, Modbus and DeviceNet, enabling interface to existing control and monitoring equipment. In addition, Ethernet TCP/IP is provided, enabling communications between the replacement controller and local or wide area networks, process monitoring systems, and other diagnostic and control equipment.

Systems are now operational in facilities worldwide for multiple turbine applications. Retrofitting existing controllers can be completed in just a few hours, as pre-configured hardware is sent to the site in preparation for installation and programming.

The Allen Bradley CompactLogix PLC family has been used as a basis for the system architecture, as its RSLogix5000 programming language allows the user the flexibility to provide additional functionality to the system, if required. The RSLogix 5000 programming environment also provides good diagnostic tools for fault finding, such as fast real-time trending (10 ms), forcing I/O Signals, etc.

CompactLogix is a small, highly adaptable, distributed control option with local and networked I/O capability. It uses the same Logix control engine and RSLogix 5000 programming software found in other Logix platforms, including the function block library, making this an ideal control architecture.

RSLogix 5000 is a tag-based programming language, enabling documentation and integration with visualization programs. In addition, it has a range of IEC 61131-3 languages, including Ladder Diagram, Function Block Diagram, Sequential Function Chart and Structured Text.

Allen Bradley Flex I/O modules are used in these systems, which support distributed control or internal panel applications. Flex I/O, the compact, DIN-rail mounted I/O found in thousands of industrial applications, is the companion I/O configuration to the controller. The smaller footprint of the I/O offers all of the functionality of larger rack-based I/O without the space requirements. This makes it ideal for cases where the I/O needs to remain within the physical confines of the original controller housing.

The RDU and RNU are now operational in generators, compressors and pump sets on multiple offshore oil and gas facilities in Southeast Asia, the North Sea, and off the coast of California. They have also been installed in turbine generators at an aluminum production facility in Jamaica and in power plants in the United States and Canada. In each application, the customer ported the turbine control logic sequence from the existing system to the new unit in about three hours. Total installation and commission was completed in an average of three to five days, so turbine downtime was minimized.



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