Foxboro I/A Series DCS for Feedwater Control Systems

Foxboro I/A Series DCS for Feedwater Control Systems

THE COMPETITIVE ADVANTAGE

Invensys Operations Management offers premiere feedwater control systems using the Foxboro® I/A Series® Distributed Control System (DCS). Recent implementations have offered operation philosophy enhancements, improved startup and runtime performance and significant reduction of unplanned unit trip outages.

These advanced control techniques allow for less operator required interaction, more time of automatic control - including during start-up and shut-down when most unit trips are caused - making the entire control system more capable of handling process upset conditions along with normal operation.

BEFORE I/A SERIES AT THE OMAHA PUBLIC POWER DISTRICT

Before the I/A Series DCS was installed on the feedwater controls at the Omaha Public Power District at Fort Calhoun Station, there were many operational challenges. The controls were poorly tuned. Level perturbations were not uncommon during changes in power, with poor low level control.

John Steinke, Senior Nuclear Design Engineer at the facility states that “Having splitrange 3-Element control on the bypass feed water valve as well as the main valve allowed for automated controls to handle the switchover from the bypass to the main valve. This reduced operator direct manual interaction, and thereby reduced operator challenges. We also rotate our Steam Generator Feedwater Pumps. The split range 3-element controls greatly reduce the perturbations associated with rotating those pumps. More precise tuning and control have maximized our unit’s capability factor (ability to produce electricity) while removing undue burden from the Licensed Operators who are operating the system. To date, we have not tripped based on the DCS or garnered extra down time.”

There was a plant transient soon after the controls were installed in which the Hotwell Makeup Valve failed to open. The operators, by that time, overcame their instinct to place the feedwater system in manual, and instead let the controls take action. The feedwater DCS response was called “outstanding.” It would have been a much more stressful event for operations using the pre-DCS controls. There have been multiple similar incidents after which Operations commented that the unit would have tripped if the steam generator level control was not on the DCS.

AFTER I/A SERIES AT TENNESSEE VALLEY AUTHORITY

Scott Gladney, an Electric Engineer at the Tennessee Valley Authority’s Sequoyah facility, attributes smoother operations and the reduction of single points-of-failure to installation of the DCS and feedwater controls.

Mr. Gladney states that “We were able to eliminate over 40 single points-of-failure per unit using the DCS feedwater controls. It also simplified the startup of our second feedwater pump. When starting the second pump, speed balancing used to be an operator-intensive manual procedure. The I/A Series block features allow for the controls to completely balance the pumps in automatic. Our swap over from startup valve to main valve was greatly simplified. It used to be an operator manual swap that took about an hour to perform. It is now an automatic event that occurs on they fly with no specific interaction required. I have seen a valve transfer occur as they were tying a turbine online (and in single element) with no significant level perturbations in the steam generators.”

FEEDWATER CONTROL SYSTEM ENHANCEMENTS

The following are brief descriptions of some of the control enhancements delivered for various feedwater control systems throughout the nuclear industry by the nuclear control system engineering team of Invensys Operations Management.

Enhancement #1 − Redundant Sensor Algorithms

Redundant Sensor Algorithms (RSAs) are utilized to eliminate single point of failure vulnerabilities within the control application and improve reliability and robustness of the system and its ability to automatically control the plant over its full power range for the entire fuel cycle. These allow the system to suffer a partial or complete loss of one of the redundant input signals with minimal or no upset to plant operation or loss of vital plant process information. The redundancy starts at the process measurement transmitters. Multiple, mutually exclusive field devices are used to measure the same plant process variable. These measurements are brought into the control system on separate I/O devices (FBMs), further increasing the reliability of these signals. Care is taken to also ensure that the transmitters are on separate process connections, otherwise they are still vulnerable to a single failure, such as plugged piping or failure of the sensing line piping itself.

Enhancement #2 − Redundant Valve Outputs

Redundant Valve Outputs are utilized to eliminate single point of failure vulnerabilities within the control application and improve reliability and robustness of the system and its ability to automatically control the plant over its full power range for the entire fuel cycle. The I/A Series Fieldbus Modules include the FBM218, which is capable of providing redundant, channel isolated outputs to field devices. If a failure is detected in one of the Fieldbus Modules, its output is driven to 0 mA and the corresponding channel in the tracking module automatically continues supplying the proper current to the output current loop.

Enhancement #3 − Fully Integrated Single Element/Three Element Control Philosophy

Traditionally, the feedwater control system operated in Single Element control at low power using only the Startup Bypass valve to maintain Steam Generator Level. Above ~25% power, a Three Element system was employed using only the Main Feedwater Regulating (MFWR) valve. Steam generator level is more robustly controlled using the three elements of steam flow, feedwater flow, and steam generator level. As reactor power increased from 20% to 25%, operators would perform a manual transfer from the Single Element Start-up Bypass valve to the Three Element MFWR valve. Similarly, as reactor power would decrease from 25 to 20%, operators manually transferred from the Three Element MFWR valve back to the Single Element Start-up Bypass valve. These manual transitions require intense operator involvement, and have been the cause of many unplanned unit trips.