DEVICES FOR FIELD DETERMINATION OF WATER VAPOR IN NATURAL GAS Betsy Murphy MNM Enterprises 801 N. Riverside Drive Fort Worth, Texas 76111

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1 INTRODUCTION Water vapor in natural gas has more than a substantial effect on the quality of the gas stream. Without quality measurement of water vapor the gas is basically not saleable. Contracts are written around it and companies spend a considerable amount of time researching the type of device to be used for a particular application. This paper will attempt to discuss the types of devices used for the measurement of water vapor in natural gas streams. Dew Point The temperature at which water vapor condenses is called the dewpoint. This temperature varies with the pressure of the gas stream, and some researchers are examining a theory that barometric pressure also has an effect. Many charts are available to show the calculated dewpoint at temperatures ranging from -40F to 250F at pressures from 14.7 psia to 5000 psia. Charts are available as the Institute of Gas Technology Research Bulletin No. 8 and ASTM Method D-1142, and are useful if the dewpoint and pressure are known in order to determine the concentration. Bureau of Mines (Chilled Mirror) The Bureau of Mines instrument consists of a chamber that can be pressurized with pipeline gas. One end of the chamber has a copper rod with a mirror on it inserted into it. The opposite side of the chamber has a window through which said mirror can be observed. A gauge on top of the chamber measures the pressure. The accuracy of this gauge should be considered of utmost importance. A thermometer is inserted into the rod for temperature measurement. Gas expanded into the expansion chamber on the right side will lower the temperature of the rod. As more gas is expanded into the chamber the rod will cool more rapidly. As the temperature of the rod is lowered, water vapor in the gas will condense and show as a fog on the mirror. At this moment when the fog appears it is defined as the dewpoint temperature. If any time elapses, a speck will form in the center of the fog and that then becomes the frost point. Also, if a colored ring appears around the fog, it signifies a hydrocarbon interference which is usually methanol. Low levels of methanol do not cause a measurement error of any concern. However, as the methanol level increases the error will increase until it is significant at higher methanol levels. The Bureau of Mines method is usually not effective below 40F ambient temperature and is directly dependent on the skill of the operator. It is known as the industry standard. Colorimetric Detector Tubes Detector tubes are used throughout the pipeline industry for a quick ballpark analysis of gas quality. Tips of the tube are snapped off using a built in tool a hand pump matched to the tube manufacturer. The tube is inserted into the end of the hand pump which is inserted into the pipeline. The pump is then used to pull gas through the tube. The tube changes color as the gas flows through it. The length of the color change in the tube is proportional to the moisture concentration. If the reading is above contract limits and methanol is present, it is advisable to use a methanol detector

2 tube to determine if there might be methanol interference causing an inordinately high reading. The tubes are discarded after use. The manufacturing quality of the tubes and the skill of the operator will determine the quality of the measurement. ABSOLUTE VS. RELATIVE All sensor types will fall into one of two categories Absolute or Relative. Absolute sensors are based on primary laws of physics and do not require periodic calibration against known moisture standards, nor can the sensors actually be calibrated. Relative sensors are based on a comparative measurement and will require a known, certified moisture source for calibration. Calibration frequency will vary depending on the individual sensor characteristics. ELECTROLYTIC SENSORS Electrolytic sensors are unique in that they can be either absolute or relative measurement devices. In either case, the basic operation of the sensor is the same. The typical electrolytic sensor consists of two precious metal electrodes, wound around a support mandrel or imbedded in a hollow glass tube. These electrodes are then coated with a thin layer of phosphorus pentoxide (P205). In operation, a controlled amount of gas is allowed to constantly flow through or across the sensor allowing sufficient time for the P205 coating to adsorb all of the moisture from the gas stream. A voltage potential is applied across the electrodes, splitting (or electrotyzing) the water molecules that have been collected on the coating. Once equilibrium conditions are attained, the rate at which moisture molecules enter the cell will exactly match the rate at which the molecules are electrolyzed. Each etectrolyzed molecule causes two electrons to be displaced from the anode to the cathode. The electrolysis second. Since the elementary charge of an electron is known, by measuring the current, we can determine the rate at which water molecules are entering the sensor. Combined with a known flow rate through the sensor, the moisture content of the gas can be determined. The critical aspect here is the controlled measurement version of electrolytic based analyzers. Electrolytic sensor based moisture analyzers are available as portable and stationary devices with flow control mechanisms built into the unit. As with all analyzers, it is important that the proper sampling techniques and sample conditioning criteria be strictly adhered to. Failure to do so may cause premature failure of the sensor. Liquid intrusion or the presence of conductive particles will also cause premature sensor failure. CAPACITANCE SENSORS Capacitance sensors fall into the category of relative measurement devices. Periodic calibration of the sensor against a known moisturestandard is an absolute requirement. Of all the different sensor technologies, capacitance sensors are the choice of a majority of manufacturers. There are numerous types of capacitance sensors, including aluminum oxide, silicon oxide, polymer base and thin film, but they all share the same basic principle. Regardless of the sensor type, the core of the sensor consists of two electrodes and a dielectric material that absorbs the water vapor in the gas stream and achieves an equilibrium condition based on the partial pressure of water vapor in the specific gas stream. In operation, moisture in the sample stream

3 is absorbed into the dielectric material creating impedance within the sensor. An excitation voltage is applied to the electrodes and a return signal, proportional to the water vapor content, is transmitted back to the base electronics. Capacitance sensor based analyzers are available in both portable and stationary configurations. Although the sensor probe can be mounted directly in the process line, glycol and other contaminants in the gas stream can cause false readings and premature sensor failure. The sensor should be mounted in an independent sample conditioning system adjacent to the sample point in order to protect against sensor contamination and to facilitate calibration and maintenance. VIBRATING CRYSTAL SENSORS Piezo-Electric sensors are commonly referred to as vibrating quartz crystal sensors. The crystal is coated with a hygroscopic material in order to allow absorption of the water vapor from the gas stream. The analyzer electronics monitor the vibration frequency change of the crystal as water vapor is absorbed onto the coating. During operation, the crystal is alternately exposed to the sample gas stream and a dry reference stream. The reference stream is the actual sample gas, passed through an on-board gas dryer. In operation, the sample gas flows across the crystal for a fixed time period. During this time, moisture in the gas stream is absorbed onto the coating of the crystal causing a change in the vibration frequency. This frequency is read, stored and compared against a sealed crystal. The sample gas is then diverted through the on board dryer and again is passed across the crystal. Once again, the frequency is read, stored and compared against the sealed crystal. Using the differential in the vibration frequency of the sample gas and the dried sample gas, one can determine the water vapor content of the sample gas. These analyzers are strictly stationary devices and are not suitable for portable applications. They are also considered relative measurement devices that require periodic calibration against a known moisture source. LASER CELL If a water molecule collides with a photon that has a to its wavelength), the molecule will absorb the photon. Absorption spectroscopy is a relatively simple method of passing light through a gas sample and measuring the amount of light absorbed at that specific wavelength. Traditional spectroscopic techniques have not been successful at doing this in natural gas because methane absorbs light in the same wavelength regions as water. However, these regions are actually made up of groups of narrow peaks. If you use a very highresolution spectrometer, you can find some water peaks that are not overlapped by other gas peaks path; therefore this technique is a direct measurement of moisture. In order to achieve a long enough path length of light, a mirror is used in the instrument. The mirror may become partially blocked by liquid and solid contaminants, but since the measurement is a ratio of absorbed light over the total light detected, the calibration is unaffected by the partially blocked mirror (if the mirror is totally blocked, it can be easily cleaned in the field). Because a light beam in the volume of the gas is measuring the moisture, the instrument can respond as quickly as the moisture concentration in the beam path changes.

4 SAMPLE CONDITIONING No discussion of natural gas water vapor analyzers is complete without mention of sample conditioning. Regardless of what type of analyzer is used bad sample conditioning can give bad readings. To get an accurate measurement of the water vapor content in a natural gas pipeline it is essential that the gas in the analyzer sample cell be representative of the gas in the pipeline. The gas in the pipeline must be pressure reduced and transported to the analyzer without changing its composition. We will examine each step of this process and suggest the best approach. 1 )Extract a representative sample of gas from the pipeline. A probe should be used to sample gas from near the center of the pipeline. Sampling from a valve on the wall of the pipeline is a bad technique because contaminants that reside on the walls can be pushed through the valve and into the sampling system. The American Petroleum Institute recommends sampling from the wall on large diameter pipelines). Turbulence in the gas flow caused by elbows, valves, diameter changes, etc, can stir up liquids and particles from the pipeline walls into the center of the pipe. For this reason the probe should be located in a straight section of pipe at least 5 pipe diameters upstream and downstream of pipe discontinuities. If liquids are potentially present in the pipeline for any reason, a liquid separating membrane tipped probe should be used to reject liquids. The reason for this is that liquids cannot be reliably sampled from the pipe so the industry practice is to sample only the gas portion of the stream. 2) Reduce pressure without condensing liquids. When gas pressure is reduced without the introduction of heat the temperature of the gas will drop, typically about 7F per 100 psi. This temperature drop could cause hydrocarbon or moisture condensation to occur in the regulator. To prevent condensation, it may be necessary to add heat. This can be done with a regulator that has its pressure-reducing orifice in the gas stream where it is warmed by the flowing gas, or with an external regulator that is heated with electricity or steam. A phase diagram can be calculated for any particular gas composition to determine if heating may be required. In extremely cold environments it may be necessary to heat trace the tubing between the pressure regulator and the analyzer. 3) Maintain the integrity of the gas sample. Moisture can permeate through plastic tubing. Always use stainless steel tubing from the pipeline to the analyzer. The tubing and all components in the line must be dry and free of contamination Liquid water in the tubing can take a long time to evaporate, especially at low ambient temperatures. Needless to say, the tubing connections must all be leak free because the moisture concentration in the air is typically a hundred times higher than in the pipeline. 4) Prevent contaminants from entering the analyzer. Use a membrane separator close to the analyzer to prevent liquids and particles from getting into the analyzer. These separators must be checked on a regular basis to make sure they do not become plugged. It is also necessary to be sure that the maximum flow rate limitations of the separator are observed. Membrane separators will increase the sensor life or cleaning interval on any type of moisture analyzer. Paying close attention to the sample extraction and conditioning will result in analyzer readings that are representative of the

5 actual gas composition. increase the sensor life or cleaning interval on any type of moisture analyzer. Paying close attention to the sample extraction and conditioning will result in analyzer readings that are representative of the actual gas composition. CONCLUSION There are a number of different types of moisture analyzers on the market today. Some work better in certain applications than others. Application research is very important to match manufacturer specifications with the gas stream to be monitored. Operator training will determine the quality of the measurement after matching specifications with applications. 100% accuracy is not really attainable with any instrument in any application. But with quality analyzers, proper operator training, and application research, a good representation of moisture concentration can be determined.