Conversion between standard conditions and operating conditions

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What is the difference between standard conditions and operating conditions? How to calculate the state of a gas? How can standard-condition flow rate be converted to operating-condition flow rate? Today, the editor will help you easily understand the knowledge related to standard conditions and operating conditions. (1) Difference between standard conditions and operating conditions: Operating conditions refer to the flow rate under actual working conditions, with the unit being m3/h. Standard conditions refer to the flow rate at a temperature of 20°C and one atmosphere of pressure (101.325 kPa), with the unit being Nm3/h. Note: The standard conditions commonly referred to are those with a temperature of 0°C (273.15 Kelvin) and a pressure of 101.325 kPa (1 standard atmosphere, 760 mmHg), which differs from the standards specified for industrial gases in China. The units are the same in both states; it’s just the corresponding flow rates that differ. Furthermore, the standard states referred to by different ** are also different. (II) Equation of calculation: According to the ideal gas law, pV = nRT. This equation has 4 variables: p represents the pressure of the ideal gas, V is the volume of the ideal gas, n denotes the amount of substance of the gas, and T represents the thermodynamic temperature of the ideal gas ； There is another constant: R is the ideal gas constant. Since PV/T = nR is a constant, it follows that P1×V1/T1 = P2×V2/T2. Let the volumetric flow rate under standard conditions be V0, with temperature T0 = 273 + 20 = 293 K, and pressure P0 = 101.325 Kpa = 0.101325 Mpa. For other operating conditions, the volumetric flow rate is V, the temperature is in degrees Celsius, and the pressure is in MPa (gauge pressure). Ignoring any changes in the compression factor, we have V*(P + 0.101325) / (T + 273) = V0×P0/T0. Note that natural gas is generally transported at medium to low pressures and delivered to homes at low pressures; it is always under pressure, which means it falls under these operating conditions. Natural gas is measured under standard conditions (more precisely, quasi-standard conditions, which we refer to as normal conditions). In general commercial transactions, the measurement is based on the volume at 20°C and 1 atmosphere of pressure (0.1013 MPa); this volume is slightly larger than that under standard conditions, which is advantageous for the seller (since the calculation involves multiplying by 273, while at 20°C it involves multiplying by 273 + 20, resulting in a larger value). In international standards, the standard state is 0°C and 1 standard atmosphere of pressure. For gases, different pressures result in significantly different volumes (gases are highly compressible). Naturally, the volume flow rates also vary greatly. Under the same pipe diameter, the flow velocities under different operating conditions also differ considerably. For instance, for steam pipelines of the same diameter, the maximum flow rates at 10 bar and 3.5 bar are not the same. When performing process calculations, it depends on the charts you refer to and the constants you use whether to use operating conditions or standard conditions; calculations under both conditions are possible. For example, when defining compressor parameters, we usually use the parameters under standard conditions to set requirements for manufacturers; at the same time, we also provide parameters such as temperature and atmospheric pressure for adjustments under actual operating conditions. The advantage of this approach is that we can use the same set of conditions to represent the parameters. Just as pump performance curves are defined in terms of clean water, no one talks about the performance curves of gasoline or crude oil. In many calculations, operating conditions are used, such as when calculating flow velocity. Image (III) Standard state of gases. There are three types of standard states for gases: (1) The standard state defined by the 10th General Conference on Weights and Measures (CGPM) in 1954 is a temperature of 273.15 K (0°C) and a pressure of 101.325 KPa. This standard state is widely adopted in the field of science and technology around the world. (2) The International Organization for Standardization and American **standards specify 288.15 K (15°C) as temperature and 101.325 KPa as pressure as the standard conditions for measuring the volumetric flow rate of gases. (3) China’s \"Standard Orifice Plate Calculation Method for Natural Gas Flow Rate\" specifies 293.15 K (20°C) and 101.325 KPa as the standard conditions for measuring the volumetric flow rate of gas. The formula for converting the volume of natural gas at standard conditions differs from that of ordinary gases, and it must comply with the standards issued by China National Petroleum Corporation. The gas equation of state: Qn = Zn/Zg * (Pg + Pa) / Pn * Tn/Tg * Qg. Where: Qn – volumetric flow rate under standard conditions (Nm3/h); Zn – compression coefficient under standard conditions; Zg – compression coefficient under operating conditions; Pg – gauge pressure (KPa); Pa – local atmospheric pressure (KPa); Pg + Pa – absolute pressure under operating conditions; Pn – standard atmospheric pressure (101.325 KPa); Tn – absolute temperature under standard conditions (20°C per national natural gas standards) (293.15 K); Tg – absolute temperature of the medium (273.15 + t) K; t – Celsius temperature of the medium being measured (°C); Qg – uncorrected volumetric flow rate (m3/h). Note: Parameters marked with ‘n’ refer to standard conditions, while those marked with ‘g’ refer to operating conditions. (IV) Examples of conversion between standard conditions and operating conditions: Usually, the flow rates mentioned by us are those under standard conditions, in order to use a unified unit of measurement ； The actual flow rates recorded in the factory operation logs are basically the flow rates under operating conditions. 1. Example of converting flow rate under standard conditions to flow rate under actual operating conditions: Question: The air compressor has a rated gas production capacity of 2 cubic meters per minute, and the pipeline pressure is 8 kilograms. What is the actual flow rate in the pipeline? Answer: Roughly calculated, assuming the compressed air temperature is 20 degrees. Operating flow rate = 2/(0.8+0.101325)*0.101325 = 0.2248 cubic meters per minute. Here, 0.101325 represents the absolute pressure of the atmosphere ； 0.8 is the pipeline pressure, in MPa. 2. Example of converting flow rate under operating conditions to flow rate under standard conditions: Question: The pressure in the oxygen pipeline is 12 kg, and the flow rate under operating conditions is 10 cubic meters per hour. What is the flow rate under standard conditions? Answer: Assuming the temperature is 20 degrees, it is not used in the calculation. Standard condition flow rate = 10 / 0.101325 * (1.2 + 0.101325) = 128.43 cubic meters per hour. Here, 0.101325 represents the absolute atmospheric pressure ； 1.2 is the pipeline pressure, in MPa. (V) Example of calculation for selecting a flow meter: Problem: It is known that the actual operating pressure of a certain gas supply pipeline is (gauge pressure) 0.8 MPa to 1.2 MPa, the medium temperature ranges from -5°C to 40°C, and the gas flow rate is 3000 to 10,000 Nm3/h (at standard conditions). Without considering the composition of the natural gas, it is necessary to determine the appropriate model of flow meter. Answer: First, convert the flow rate under standard conditions to the flow rate under actual operating conditions using the gas equation, and then select an appropriate diameter. The gas equation is as follows: Qb = Q × PTb / PbT × Zb / Zg = QCF2. Here, C is the conversion factor ； F is the gas compressibility factor. The calculations are as follows: ① When the pressure of the medium is at its lowest and the temperature is at its highest, the maximum standard-condition volume flow rate is achieved; that is, Qb = Q × PTb / PbT × Zb / Zg = Q × CF2 = 1200.87 m3/h. ② When the pressure of the medium is at its highest and the temperature is at its lowest, the minimum standard-condition volume flow rate is achieved; that is, Qmin = 213.51 m3/h. Based on these calculation results, the flow rate range for the flow meter to be selected should be (214–1200) m3/h. Select a flow meter that meets the operational requirements based on the calculated flow range.