Wellfield / Powerplant Coordinated Control to Minimize Emissions

Abstract

The Western GeoPower project is a new nominal 38.5 MW geothermal power project located in the Geysers geothermal area; about 70 miles north of San Francisco, California. The steamfield consists of three wellpads with a total of nine steam-dominated production wells, two injection wells, three condensate collection tanks with forwarding pumps and miscellaneous auxiliary components and systems. The steam flow from each production well is monitored by a local transmitter and controlled by a motor operated valve (MOV). California’s strict emissions regulations requires a carefully coordinated startup/shutdown of the steamfield production well flows, the turbine bypass system (which discharges to the main condenser), the H2S incinerator, and the initial purge of air from the steamfield piping system to the plant’s rock muffler venting system. In this way, atmospheric venting of steam to the rock muffler is kept to a minimum. A key feature of the Western GeoPower wellfield control strategy is the use of Foundation Fieldbus (FF) to implement local PID (proportional, integral, derivative) control in the field. These field devices (valves, transmitters, etc) have sufficient built in intelligence to allow them to communicate with each other via FF and to implement a coordinated PID control strategy with supervisory setpoint control from the central plant’s control system. There is no need for a wellhead PLC (programmable logic controller) or other local dedicated control system. The field devices themselves are the control system. The result is a truly distributed control system. The Western GeoPower Foundation Fieldbus based steamfield control system provides a cost effective and robust distributed control system. The control logic runs in distributed IP68 rated field devices which work together holistically and can continue to function autonomously even in the event of a total loss of communications back to the central plant control system. The plant’s central control system software is used to configure the field device logic strategy, just as though the Foundation Fieldbus IO (input/ output) points were directly hardwired to the central system. The result is a tightly integrated, rugged, steamfield control system that meets the project’s challenging environmental requirements.