Flow Control: Technology information & new products for fluid handling engineers

June 2004

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Real World Solutions:
Geothermal Plant Improves Efficiency with Vortex Meter

By Scott Rouse

TABLE 1. STEAM MASS FLOW RATE ERROR DUE TO PRESSURE VARIATION

Pressure
(psig)
50
60
70
80
90
100
110
120
130
140
150

Density
(lbm/ft³)
0.1496
0.1713
0.1928
0.2143
0.2357
0.2569
0.2782
0.2994
0.3205
0.3416
0.3627

Error*
( +/- %)
-41.79
-33.35
-25.00
-16.65
-8.39
1.50
8.43
16.61
24.80
33.00
41.21

* Mass flow rate error for volumetric flowmeter, calibrated for saturated steam at 100 psig, density = 0.2569 lbm/ft³

Since the dawn of humankind, geothermal resources have been used for healing and physical therapy, cooking, and heating. In modern times, the use of geothermal energy for the production of electricity commenced in 1904 when Prince Piero Ginori Conti invented the first geothermal power plant at the Larderello dry steam field in Italy. Today, geothermal power plants are producing over 8,200 megawatts of electricity in 21 countries, supplying about 60 million people — mostly in developing countries.

The first geothermal power plants in the United States were built in 1962 at The Geysers dry steam field in northern California, which is still the largest producing geothermal field in the world. The fastest growth in U.S. geothermal capacity occurred between 1980 and 1990, following enactment of federal laws that compelled utilities to purchase electricity from independent power producers. The goal of the geothermal industry and the U.S. Department of Energy is to achieve a geothermal energy lifecycle cost of electricity of $0.03 per KWh. In an effort to reduce cost and lower risk, some geothermal power producers are upgrading steam flow measurement and control technology to improve their bottom line.

Because of high temperatures and varying pressures, measuring steam flow is a challenging task. In geothermal power generation, the measurement difficulty is compounded by fluid composition, varying flow conditions, and application constraints. For the last two years, the Sierra Instruments Innova-Mass multivariable vortex mass flowmeters has been used in five major geothermal steam fields in California and Nevada. In these applications, the meters have demonstrated an ability to reduce maintenance costs and improve the operating efficiency of steam turbine generators.

The Problem
Geologic processes, known as plate tectonics, have broken Earth’s crust into huge plates that move apart or push together at a rate of millimeters per year. Where two plates collide, one plate can thrust below the other, producing extraordinary phenomena such as ocean trenches or strong earthquakes. At great depth, just above the down-going plate, temperatures become high enough to melt rock, forming magma. Because magma is less dense than the surrounding rocks, it moves up toward Earth’s crust and carries heat from below. Sometimes magma rises to the surface through thin or fractured crust as lava.

However, most magma remains below Earth’s crust and heats the surrounding rocks and subterranean water. Some of this water comes all the way up to the surface through faults and cracks as hot springs or geysers. When this rising hot water and steam is trapped in permeable rocks under a layer of impermeable rocks, it is called a geothermal reservoir. These reservoirs are sources of geothermal energy that can be tapped for electricity generation or direct use. A typical geothermal power plant withdraws heated geothermal fluid, and the injection well returns cooled fluids to the reservoir.

In the California and Nevada steam fields, both naturally occurring and man-made steam is mined from the hot rock. A typical field has a productive area of 1,000 to 1,500 acres and includes dozens of steam wells, several injection wells, and miles of steam transmission, water injection, and condensate collection pipelines. The geothermal reservoir rock consists of fractured greywacke and greenstone heated to a temperature of 460 F to 480 F. The fractures are filled with superheated steam at pressures that, depending on location, can vary from 100 to 240 PSIG.

This superheated steam is collected at the wellheads and sent to an electrical generating station that consists of a heat exchanger and a turbine and generator set. Steam is typically delivered to the turbine at 70 PSIA and 300 F to 350 F, at a flow rate of approximately 870,000 lbs/hr at maximum capacity.

Monitoring the flow rate of the steam delivered to the generating plants is necessary to properly control the turbines and monitor the overall efficiency of the process. Unlike boiler-generated steam, geothermal steam contains large quantities of dissolved solids leached from the reservoir rock, which can be deposited as scale whenever the steam experiences a permanent pressure drop and/or cooling effect.

The differential pressure drop devices that were previously used for steam flow measurement — such as orifice plates, venturis, and pitot tube arrays — were particularly sensitive to the formation of scale and frequently became plugged. The power station operator reported that monthly maintenance and instrument calibration were not unusually performed.

Furthermore, since the pressure of the steam delivered to the generator varies, depending on load conditions, compensating devices were required in order to measure mass flow rate. These devices added to the complexity and maintenance demands of the system. Another disadvantage of the volumetric devices that were being used was that they did not provide the turndown the operator needed to optimize turbine performance.

The Solution
Vortex flowmeters provide the high temperature and pressure capabilities, durability, accuracy, and turndown required for steam flow monitoring, and they are considered the “instrument of choice” in most steam generation and distribution applications. In a geothermal steam application, the multivariable vortex meter has clearly demonstrated its reliability and performance claims and has helped improve power plant efficiency, as described hereafter.

Vortex-Shedding Sensor Reduces Maintenance Requirements
A major advantage of the vortex-shedding sensor in the geothermal steam application is its low permanent pressure loss. The relatively small surface area of the insertion-type sensor eliminates the large pressure drops associated with differential pressure metering devices, and ensures that any dissolved solids stay in solution. The sensor’s small surface area also equalizes quickly with the surrounding steam temperature, thereby eliminating scaling due to temperature differentials.

The vortex meter’s piezoelectric sensor is virtually maintenance-free, as it has no moving parts to replace due to normal wear. Also, because the shedder bar has no ports or taps exposed to the process, fluid cannot clog the sensor, as it can in an annubar or venture device.

Since the existing annubar devices were replaced with Sierra’s Innova-Mass steam flow meters in the California and Nevada steam fields, there has been a dramatic reduction in maintenance requirements. According to Richard Hutsell, operations and maintenance supervisor for Caithness Energy Corporation, “The occurrence of scaling has virtually disappeared since we switched to multivariable vortex metering. System reliability is up, and maintenance costs are down, which is exactly what we were trying to accomplish.”

Better Turndown Improves Turbine Control
Besides being able to handle high pressures and temperatures, vortex meters provide wide rangeability, enabling accurate steam flow measurement at a variety of velocities.

The single biggest weakness of differential pressure measurement is turndown. A typical differential pressure meter’s operating range or turndown range can vary from 4-to-1 to 15-to-1, depending on the technology, whereas most vortex meters offer 30-to-1 turndown.

The vortex meter’s rangeability has also improved efficiency at the Caithness plants. The changing grid requirements of utility customers dictate generator output and frequently result in load restriction on the generating plant. This is accomplished by venting off the continuous steam flow to control turbine input flow rate.

Traditional differential pressure devices do not perform well under these low-velocity conditions and may deliver erroneous and inconsistent readings. Vortex meters, however, maintain specified accuracy over a wide range of flow, temperature, and pressure conditions.

According to Hutsell, “The vortex transmitter’s turndown capability provides a much more accurate and consistent measurement — over a wider range of process dynamics — than the annubar devices we were using. The direct input of mass flow rate from a single instrument has helped us improve accuracy and efficiency. ”

Single Point Measurement of All Flow Variables Improves Accuracy
Multivariable flow technology is unique in that it provides an accurate reading of mass flow rate from a single entry point in the process line. Systems that use external process measurements to calculate mass flow often fail to consider that process conditions can change radically between the point of velocity measurement and the point where upstream or downstream pressure and temperature measurements are being made, resulting in gross measurement errors.

Pressure variation can affect steam mass flow rate measurement. By measuring all of the flow variables in a single location, the multivariable vortex meter minimizes opportunities for error and optimizes turbine control efficiency.

Scott Rouse earned his bachelor’s of science in Chemical Engineering at the University of Texas. He served as a nuclear engineer with the U.S. Navy for 15 years. Since 1999, Mr. Rouse has been working for fluid handling provider Sierra Instruments. Currently, he is the company’s industrial product manager. Mr. Rouse can be reached at s_rouse@sierrainstruments.com or 800 866-0200, ext. 115.

www.sierrainstruments.com

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