Gas Sampling And Monitoring Systems For Hot Water Systems
Authors: Gracey C.M. , Vetter O.J. 
Published: December 2007

A gas monitoring system has been developed by Aerojet Energy Conversion Company to provide a continuous monitoring of entrained and dissolved gases in geothermal fluids. The system can also be adapted for process controlling in other commercial hot water and steam systems. The gas monitoring detector developed is a through-flow system. It operated on a by-pass line either at or below line temperatures and pressures. pressures. Measuring of dissolved and/or entrained gases in high temperature aqueous fluids is compulsory in geothermal and boiler plant operations but is extremely difficult. The principles applied in the developed monitor allow the direct measuring of the volumetric concentration of entrained gases under in-line conditions but also permit the direct measurement of the concentrations of entrained gases developed from the aqueous phase at temperatures and pressures below line conditions. This feature is a sizeable advantage over other monitoring systems. It will allow actual measurements in a single phase, high pressure and temperature fluid (e.g., geothermal wellhead fluids or boiler feed water) to determine the concentration of gases breaking out of solutions at lower temperatures and pressures. Thus, the phase distribution of the various dissolved and/or entrained gas between liquid and gas phases as a function of temperature and pressure can be determined in the field without relying pressure can be determined in the field without relying on theoretical evaluations.

Introduction

The performance of hydrothermal geothermal wells is hampered by entrained gases in the brine. When the level of entrained gases becomes excessive, brine flow is erratic and difficult to control. Too, knowledge of the gas behavior is important in designing and sizing process components within the power plant. Typical process components within the power plant. Typical entrained gases are carbon dioxide, nitrogen, argon, methane, and hydrogen sulfide. This paper concentrates on geothermal fluids where carbon dioxide is the major component of the entrained and dissolved gases.

Sampling gases entrained in geothermal brine has been difficult because the sample must be taken at operating temperatures and pressures. If the pressure decreases, dissolved gases come out of solution and the sample is no longer representative. If the temperature is permitted to drop during sampling, gases will either go into solution or come out of solution depending on their solubility characteristics. Again, the resulting data will not be representative. Another problem with entrained gases is that their extent in the brine is very variable. Significant variation in the quantity of entrained gases can occur from day-today and hour-to-hour. Apparently, this variability is sensitive to production rate, local seismic activity and brine chemistry variation.

The gas monitoring and sampling system discussed herein provides representative gas samples of entrained gases in geothermal brines and monitors the volume of entrained gases during field operations. This is a self-contained automatic on-line device which accepts a small by-pass stream from the main flow, separates the entrained gas from the liquid, measures the time necessary to accumulate a calibrated volume of gas and records the results along with time-date information.

DESCRIPTION OF THE GAS MONITORING AND SAMPLING SYSTEM

Specific details on the gas monitor and its control system cannot be discussed in this paper since patent coverage is presently pending. patent coverage is presently pending. The principle of operation involves:Setting steady liquid flow conditions leaving the detector,Collecting a gas sample in a specified volume,Timing that collection period, andRecording collection period.