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Understanding Mechanics. There are a number number of gas properties that affect the speed of sound and they include the composition of the gas, the pressure of the gas and the temperature of the gas. AGA Report No. This is especially especially true since the speed of sound is related to the compressibility of the gas.
The ultrasonic meter determines the speed of sound in the gas as it calculates the flow of gas through the meter. In order for one to check the accuracy of the speed of sound measured by the ultrasonic meter, it was necessary to have an accurate method to calculate the speed of sound in natural gas.
The average average expected uncertainties for gases whose composition are within the normal range column will correspond to the uncertainties uncertainties shown in Figure 1.
When the gas compositions fall within the expanded range, higher uncertainties can be expected, especially outside of Region 1. Therefore the method in this document is only for the gas phase. The application of this method for calculation of the speed of sound outside of the ranges shown in Figure 1 should be verified by experimental tests to insure their accuracy. It is not recommended using this calculation method near the critical point. Calculation Method The method used in AGA Report No 10 to calculate the speed of sound is the detailed characterization of the gas composition.
They are best best solved using programs that are available from various sources. They make the job of calculating the speed of sound a rather simple task once you have the accurate input data outlined in the next paragraph. The flowing pressure has a lesser degree of an effect on the accuracy of the calculations than changes of the same amount amount in either of the other two inputs. In other words an error of 1 psi in the flowing pressure measurement has a much less effect on the accuracy of the speed of sound calculation than an error of 1 degree in the flowing temperature measurement.
A description of the calculations that are required for determining the speed of sound follows. These are included to give one the idea of the complexity of the equations. There are several several partial derivatives solved during the computation. The general procedure for computing the speed of sound at the flowing or operating conditions is as follows: 1. Input the operating temperature T , the operating pressure P and the gas composition 2. Calculate the molar mass of the mixture 3.
Calculate the compressibility and density of the fluid at the operating conditions 4. Calculate the ideal gas constant pr essure heat capacity at the operating temperature 5. Calculate the real gas constant volume heat capacity at the operating conditions 7. They are extremely difficult for people without a degree in mathematics to perform and then it would take a while if they do the calculations without the use of a computer.
It is certainly something that most people would not want to attempt without access to a computer. There are a number of commercially available programs that can be purchased and used for these calculations. It is relatively easy to make the calculations using the computer programs.
The difference in the transit times can be used to calculate the velocity of the gas in the pipe. The path lengths are measured very accurately in the manufacture of the meter. One of the meter diagnostics is comparing the speed of sound determined by the meter to the theoretical speed of sound in the gas as calculated by AGA Report No. As mentioned earlier, the three inputs to the equations are gas composition, flowing pressure and flowing temperature.
The gas composition used for these calculations is the Gulf Coast Gas shown in Table 3. This shows that if a calculated speed of sound is used to verify that the speed of sound determined by your meter is correct, you must have very accurate temperature measurements. The pressure is also also very important in calculating the standard volumes but a change does not affect the speed of sound calculation nearly as much as does a corresponding change in the temperature.
This composition was determined by averaging a large number of gas samples collected by various companies that operate facilities both onshore along the Gulf of Mexico Coast and offshore in the Gulf of Mexico. Components in Mole Percent Speed of Sound The comparison is made at several pressures and temperatures. The composition of the Ekofisk gas has a high Ethane content, nearly 8. The Ekofisk gas also has five times the amount of propane and the other components are also greater except for the hexanes and the Methane which is only As one can can see from the values in Table 4, there can be a significant difference in the speed of sound calculated for the same conditions when the gas composition is changed.
In this figure you can can clearly see how how the speed of sound varies with the changes in temperature and pressures. This covers a large span of the normal operating pressures and temperatures except for most gas storage operations. This demonstrates in graphical form how the speed of sound varies for gasses with different compositional mixtures at various pressures.
Air, being composed of primarily of nitrogen, carbon dioxide and oxygen is included as a reference. One meter manufacturer also has the ability to connect to pressure and temperature transmitters, as well as a gas chromatograph and bring those inputs into the meter meter electronics. The meter software then uses the AGA 10 calculation method, along with pressure, temperature and gas composition to compute a theoretical or calculated speed of sound. This comparison is one of many diagnostics available in the ultrasonic ultrasonic meter.
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Understanding Aga 10