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Measuring the critical temperature and critical pressure error of the gas

In the gas flow measurement, the compression coefficient correction is required because the relationship between the gas density and its temperature and pressure always deviates from the ideal gas equation. For gases that are not seriously deviated, such as air, nitrogen, etc., the compression coefficient can meet the accuracy requirements by setting method. However, for gases such as gaseous ammonia, propylene and carbon dioxide, the temperature and pressure are different, and the corresponding compression coefficients are also significantly different. In this case, the calculation method should generally be used.
The formula for calculating the compression factor is more complicated, but there are not many independent variables [5], only the fluid temperature, pressure, and fluid critical temperature t and critical pressure. . The calculation is automatically done by the smart meter, but T and p. The instrument personnel must obtain the instrument according to the gas name and check the manual.
Other gases t and p are sometimes borrowed at the flow meter use site. The phenomenon, as well as the calculated compression factor (there is a special window display in the secondary table) appears to be disparity. For example, ammonia Te = 132.3 gas, p. =11.278MPa, when Wang 20C, P=IMPa (absolute value), the meter calculates Z=0,9122 according to the formula, but if the data of air is borrowed, it is Te=- 140.7 mouth C,p. = 3.766 MPa, the calculated compression factor is 0.9947, resulting in a large error.
The error in the compression factor gives the corresponding error to the flow measurement. See Section 15.4.2 for details.
(5) Design pressure setting error
The example in (3) above is to mistake the property of the design pressure to cause an error. The example given here is to revise the value of the design pressure.
In a pressure-measured flow measurement system, for a differential pressure flow meter, p. From the calculation device of the throttle device; for the analog output vortex flowmeter, the density is calculated from the consultation table or according to the mass flow rate, the volume flow rate, the enthalpy coefficient and the frequency in the factory calibration record, and the pressure is calculated, which cannot be arbitrarily given.
In the field of flowmeter use, there are occasions when the pd is randomly given, often for two reasons. One is that the calculation book or the consultation form, the checklist cannot be found, and the "head" is used to determine the pd; the second is because of the process or The actual operating conditions of the equipment have changed a lot with the previous mention of the purchase of the flowmeter, so that it is wrong to mistake the use of the original pd.

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