In this 30 min session you will learn the fundamental requirements for measuring gas density, specific gravity and hydrogen purity using a vibrating element analyzer. The goal is to provide participants with a simple understanding of how a vibrating element analyzer compares and contrasts with other measuring devices, such as fan differential and thermal conductivity. We will also discuss applications within the power generation and refining industries and show how a vibrating element analyzer can be beneficial.
Transcription
Slide 1: Introduction by host.
Slide 2: Good Morning and thank you for attending this introduction to Vibrating Element technology where we will discuss how this technology works and where you may find it useful in your facility. We will also briefly touch on the other technologies used to make these measurements, so we can compare and contrast.
Slide 3: The primary measurement of the vibrating element analyzer is “gas density”. From this primary density value, we are able to calculate a number of secondary values. In any application we are able to calculate specific gravity and molecular weight from density. In certain applications, specifically binary gas streams or pseudo-binary gas streams, we can also calculate caloric value (or BTU) or a % concentration of a constituent. We will discuss later a specific example of % concentration applications which is hydrogen purity in a power generator.
Slide 4: These are the components of a basic vibrating element system. Starting at the bottom, we have the detector where the actual vibrating element resides and where the gas being measured is connected. Also connected to the sample gas is a pressure transmitter that is used for compensation. These elements are connected via cables to the analyzer which receives the inputs, calculates and transmits the values via 4-20 mA signals.
Slide 5: Now we will discuss the primary measurement of the vibrating element analyzer, which is density.
Slide 6: Here we have a very basic illustration of the vibrating element detector. It’s comprised of a cylinder, the resonator, within another cylinder, all of which is inside a protective cover. The gas being measured is introduced and flows between the two cylinders. A piezoelectric actuator forces the resonator to vibrate. The resonant frequency at which the cylinder vibrates is dependent on the density of the gas that is surrounding it. This frequency is measured by the detector and transmitted to the analyzer which calculates the gas density. You will also notice a temperature sensor within the gas stream that is used for compensation.
Slide 7: Some vibrating element systems utilize multi-mode oscillation. In these systems, the cylinder is forced to vibrate in two different modes: an axial mode and a circumferential mode. This of course results in two different frequencies that are measured and the density is calculated not by the magnitude of the frequencies, but by their ratio. As the gas density increases or decreases, the ratio between the two frequencies will increase or decrease.
Slide 8: This illustrates the advantage of using a vibrating element system with multi-mode oscillation. The magnitude of the resonant frequency is affected by gas density, but also by the accumulation of dust, oil, or other contaminates. If a system only measures the magnitude of a single frequency, this will create a measurement error as contaminates accumulate. In a multi-mode oscillation system, the accumulation of contaminates will affect the both frequencies equally, leaving the ratio largely unaffected. This makes a multi-mode system much more resilient in harsh applications that may contain contaminates.
Slide 9: Most vibrating element detectors have an integral temperature sensor to measure the temperature of the gas sample. This allows the system to measure the raw density of the gas at process temperature, as well as calculate a density value at a standard reference temperature (typically 0 degrees Celsius). This also means that the neither the gas sample, nor the detector itself has to reside in a temperature controlled environment. This illustration shows a test where the detector in a climate controlled chamber is exposed to temperatures that are ramped up and down, with very little effect on the density measurement.
Slide 10: Earlier we noted that some vibrating element systems feature a pressure transmitter to monitor the pressure of the sample gas. Once again, this allows the calculation of the raw density at process pressure, as well as the density value at a standard reference pressure (typically 1 atmosphere). This also allows the sample gas to be introduced at a fairly large range of process pressure, reducing the sample conditioning requirements. The illustration shows a gas stream, whose constituents remain constant, but whose pressure is gradually decreased. This results in a decrease of raw density at process conditions, but the analyzer calculates the compensated density which remains unchanged.
Slide 11: The correlation between the output of a vibrating element system versus gas density is very linear in nature. This allows the instrument to cover a very wide range of density values based upon a single calibration. The analyzer is capable of accurately interpolating and extrapolating density values within and beyond the scope of the calibration gasses used. This allows the use of safe, inexpensive calibration gasses that are readily available.
Slide 12: Now we are going to discuss how the system can calculate a % concentration based on gas density. We are going to talk specifically about a power plant application where the purity of hydrogen gas used to cool generators is measured.
Slide 13: First, a little background on why hydrogen is used to cool power generators. Hydrogen is very efficient at conducting heat which means it will readily absorb heat from the equipment that is to be cooled. Hydrogen has a very low viscosity compared to other gasses. Low viscosity means less friction or windage will be created by rotating equipment which allows electricity to be generated more efficiently. Also hydrogen is one of the most abundant substances on earth which translates to relatively low procurement costs. The purity of the hydrogen is measured for two main reasons: safety and efficiency. The explosive range of hydrogen in air is 4-75%. The hydrogen concentration must be controlled outside of this range. Also, the higher the hydrogen purity, the more efficiently electricity is generated.
Slide 14: This illustrates the effect hydrogen purity has on the efficiency of generators. As the hydrogen purity decreases, thousands of dollars are lost due to the decrease in efficiency. You can also see that this effect is multiplied as the capacity of the generator increases.
Slide 15: Three gasses are involved in generator applications, each with their own distinctive density. Hydrogen which flows throughout the generator to cool in normal operation, air which is a contaminate due to leakage during normal operation, and carbon dioxide which used to purge the generator of hydrogen when the generator is shut down for maintenance activities. Once the generator is purged of hydrogen, the carbon dioxide is then purged out with air so the environment is safe for technicians. This is considered a binary gas application as only two gasses are of concern at one time: Hydrogen in air during normal operation, hydrogen in carbon dioxide during the first purge cycle and air in carbon dioxide during the second purge cycle.
Slide 16: By measuring density in a binary gas stream, we can calculate percent concentration based on a simple calculation. We have known values for the constants, which are the zero and span concentrations and their associated densities. The only variable is the actual density of the gas which is dependent on the concentrations of the two gasses in the process stream.
Slide 17: As an example, if we want to measure the purity of hydrogen in the range of 85-100% in air, we know the densities associated with the span and zero concentrations. We program these constants into the analyzer. The analyzer measures the density of the process stream, and based on that density, it calculates the % concentration of hydrogen.
Slide 18: This illustrates what the output of a vibrating element system typically looks like in these applications. One 4-20 mA output represents the % concentration of hydrogen in air. The second output represents the concentration of hydrogen in carbon dioxide or the concentration of air in carbon dioxide depending on which purge cycle is being performed. This set-up allows one vibrating element system to act as a tri-gas analyzer, where all three situations can be monitored by one analyzer.
Slide 19: Here is a typical configuration for these applications. The sample from the generator is brought in through a system of valves configured to allow selection of calibration gasses. The sample is passed through a coalescing filter to drop out moisture and pressure regulator to knock the pressure down below 70 psi. A needle valve and rotameter is used to control the flow of the gas between 0.1 and 1 liter per minute. From there it flows through the detector, its pressure is measured, and then it can be exhausted to a vent line or back to the generator.
Slide 20: Calibrations for these applications are performed with two gasses: 100 % Hydrogen and 100% Carbon dioxide as they bracket the full range of densities the analyzer will measure. Since we are calibrating the density measurement, the very light hydrogen gas is the zero cal gas and the much heavier carbon dioxide is the span cal gas. It is recommended that instrument or calibration grade gasses are used to ensure the most accurate density measurement possible.
Slide 21: In a perfect world, the detector would only see a clean, dry sample. In the real world we know that is not the case. These samples can often be contaminated with seal oil, sometimes to the point where the detector is completely flooded with oil. Vibrating element detectors can be salvaged from such an event, on site. The detector can be cleaned with a detergent based solution, rinsed with water, then with pure isopropyl alcohol, then dried with instrument air. A calibration can then be performed and the analyzer placed back in service. Typically there is no need to send the unit back to the factory for repair.
Slide 22: Next we are going to discuss applications for the vibrating element system found in refineries. These are typically specific gravity applications, but in some cases hydrogen purity can be calculated. The applications where hydrogen purity can be calculated are multi-component gas streams that act as pseudo-binary gas streams. In these cases, as the hydrogen concentration increases or decreases, the concentration of the other components increase or decrease at the same rate. The density of the background gas can be calculated and treated as the second gas in a binary system.
Slide 23: The first refinery applications we are going to discuss are hydrotreators which are found throughout the refining process.
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Slide 26: This simplified layout shows the hydrotreating process and points where specific gravity or hydrogen purity can be measured to improve efficiency and reduce hydrogen consumption. The gas coming off the separator can be measured to indicate hydrogen content. This determines how much make up hydrogen is required to increase hydrogen concentration to the required level. The makeup hydrogen can be monitored to confirm its purity.
Slide 27: Another application in refining is Catalytic Reforming of naphtha.
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Slide 30: Again, the gas coming off the separator can be measured to indicate its hydrogen content. This can be used to determine if the gas needs to be enriched with make-up hydrogen, recycled back to the reformer process, sent to other units such as hydrotreators, or any combination of these scenarios.
Slide 31: Hydrocracking is another refining application that may benefit from vibrating element analyzers.
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Slide 34: This should look familiar. Once again, the mixed, hydrogen rich gas coming off the separator is measured to determine how much make up hydrogen is required to recycle the gas back to the process. The purity of the make-up hydrogen can be measured as well.
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Slide 36: The last application we want to discuss is calculating the BTU content of a binary gas stream.
Slide 37: Many facilities will have a backup propane supply system for times when their normal natural gas supply is unavailable, or when it is economically advantageous to consume propane rather than natural gas. Propane has a higher caloric, or BTU content than natural gas. So the propane must be diluted with air to utilize the same burner settings as those used with their normal natural gas supply.
Slide 38: As with % concentration calculations, we can use a simple calculation to calculate the BTU content of binary gas streams based on a density measurement. We know the densities and BTU contents of the zero and span ranges of the binary gas stream. We program these constants into the analyzer. The analyzer then calculates the BTU content based on the measured density.
Slide 39: These are just a portion of the applications that can benefit from vibrating element systems. Other applications include measuring the density of fuel gas to correct orifice type flow meters. Also, specific gravity can be used in compressor control schemes to prevent slugs of light gasses from damaging expensive equipment. Now we would like to briefly discuss other methods available to make these measurements so we can compare and contrast them with vibrating element technology.
Slide 40: Another method available to measure specific gravity is the fan differential analyzer, also known as kinetic energy, viscous drag type, or by its well known brand name. This analyzer has two chambers, an air chamber used as a reference and a sample chamber for the measured gas. Each chamber has a motor driven impeller which draws the appropriate gas into it. Each gas comes into contact with a stationary impulse wheel and is then vented out of the analyzer. The impulse wheels are linked to each other and the torques created on the wheels are in opposite directions of each other. The magnitude of the torque is dependent on the specific gravity of the gas. This is a purely mechanical means to determine if the sample gas is lighter or heavier than the reference air. One thing you may notice about this method is that it has many moving parts that are subject to wear. There are motors, impellers, impulse wheels and linkages. This requires calibrations to compensate for this wear or a drifting measurement error will occur. This method also consumes reference air that needs to be the same pressure and humidity level of the gas being measured. It also does not take a lot of imagination to understand how a dirty sample containing particulates or oil will affect the accuracy of this method.
Slide 41: Thermal conductivity is another method employed to measure % concentration in binary gas streams. Each gas has the ability to conduct heat at a specific rate. This is known as the thermal conductivity of the gas. In thermal conductivity analyzers, heated metal filaments or thermistors are exposed to reference and sample gasses. The amount of heat carried away by the gasses changes the temperature of the filaments, which changes their resistance. Since the filaments are arranged in a Wheatstone bridge, the resistance change can be converted to an electrical current. The magnitude of this current is dependent on the differential thermal conductivity of the two gasses. The concentration of the sample gas is then calculated based on the thermal conductivity value. Some things to consider about this method. One is that a reference gas is required. Also, the detector must be kept at a stable temperature through the use of heaters or ovens. These ovens require a warm up time to reach the target temperature. It is recommended that the flow rate of the sample gas be held constant to prevent cooling of the filaments due to increased flow. Also, since thermal conductivity versus the concentration of increasingly heavier hydrocarbons is not a linear relationship, multipoint calibrations with specially blended gasses may be necessary in certain applications. Lastly, it would be difficult and rather expensive to restore an analyzer of this type to working order in the field if it is significantly contaminated with oil.
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