Remote Control of a Geothermal Steam Pipeline
Authors: Kumataka Mark K. , Bayard William B. 
Published: December 2007

Aminoil U.S.A. Inc. (AUSA) operates a pipeline delivering geothermal steam to a Geysers. Lake County, CA, power plant by means of a distributed control system. Microprocessors located in the field squire data and transmit it digitally to a control building adjacent to the power plant. From this building the shift operator monitors and controls every well in the system, regulates the delivery pressure to the geothermal power plant, and controls the condensate reinjection system.

This paper discusses design, operation, and future objectives of this control system. The design covers selection of the control system, data acquisition and control requirements for individual wells, pipeline over pressure relief system control, and condensate reinjection system.

The operation of the steam pipeline since power plant start-up has been very favorable. Steam delivery pressure fluctuations are controlled to less than +0.5 psi (+/-4 kPa) under normal conditions. Shift operators are capable of responding to power plant upsets (turbine trips and load reductions) in minutes, minimizing steam venting to the atmosphere.

Currently the geothermal steam pipeline is manually remote operated by a shift operator. This operation has provided valuable information for conversion to an unmanned computer control operation. This proposed unmanned computer control is discussed along with our methods of approach.

Introduction

Our project is in the Castle Rock Springs area of The Geysers field in California, approximately 100 miles (160 km) northeast of San Francisco. The operator supplies geothermal steam to a public utility's geothermal power plant rated at 138 MW gross, 133 MW net delivered to the transmission line. Twenty-three geothermal steam wells deliver a total of approximately 2,700,000 lbm/hr (340 kg/s) of steam at rated capacity. Individual well deliverabilities range from 40,000 to 325,000 lbm/hr (5 to 41 kg/s).

Approximately 5 miles (8 km) of insulated pipe is required to deliver the necessary steam flow to the power plant. Individual wellhead pipelines vary in diameter from 10 to 18 in. (25 to 46 cm), depending on steam flow capability. The wellhead pipelines feed into 24- to 30-in. (61- to 76-cm) branch lines that connect into two 36-in. (91 -cm) main steam lines with separators for condensate and particle removal. These two 36-in. (91-cm) pipelines join to form a 48-in. (122-cm) header that delivers steam to the power plant. Along the pipeline, drip legs are positioned at low points to collect condensate. These drip legs are vented to the atmosphere to remove condensate during start-up or shutdown of the pipeline.

To provide electrical power for the control system. a three-point 480-V electrical distribution system parallels the pipeline. Transformers step down the line voltage to 120 Vac at the wellheads.

The remote control system used to monitor and control the geothermal steam pipeline was selected primarily as a mechanical H2S-abatement method for compliance with the Lake County Air Pollution Control Dist. (LCAPCD) regulations. During power plant outages and start-up conditions, all steam is vented to the atmosphere. The LCAPCD H S-abatement requirement for an unscheduled power plant outage is given in Fig. 1, Under these conditions the vented steam rate must be limited to 35 % of full flow within 1 hour after an outage. Further reduction to 10% of full flow is required if the power plant outage is expected to be greater than 7 hours. AUSA determined that only a remote control system capable of controlling all wells in the system could accomplish this flow reduction in the short time frame required. This paper presents our control system structure, design. operation, and proposed computer control.

Control System Structure

A Honeywell TDC-2000 control system combined with a Honeywell 4500 computer (super station) was selected and installed.

JPT

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