What is the difference between standard conditions and operating conditions? How to calculate the gas status? How should standard flow and operating flow be changed? Today, Xiao Bian has come to help you easily understand the status of standards and conditions.
The difference between standard conditions and operating conditions
Working condition: Flow under actual working condition, unit: m3/h
Standard conditions: Flow rate at 20°C and one atmosphere pressure (101.325kPa) Unit: Nm3/h
Note: The standard conditions usually referred to are the temperature of 0°C (273.15 Kelvin) and the pressure of 101.325 kPa (1 standard atmosphere, 760 mmHg), which is different from the regulation of industrial gas in China.
The units in both states are the same, but the corresponding traffic is different. In addition, the standard states in different countries are different.
Calculation equation
According to the ideal gas state equation
Its equation is pV=nRT.
This equation has four variables: p is the pressure of the ideal gas, V is the volume of the ideal gas, n is the amount of gas material, and T is the thermodynamic temperature of the ideal gas; there is also a constant: R is the ideal gas constant.
PV/T=nR is constant,
So P1×V1/T1=P2×V2/T2
Set the volume flow rate to V0,
Temperature T0=273+20=293k, pressure P0=101.325Kpa=0.101325Mpa,
Volume flow rate is V, temperature T (degrees Celsius), pressure P (gauge pressure, Mpa),
Ignore the change in compression factor V*(P+0.101325)/(T+273)=V0*P0/T0
Note: In general, natural gas is transported by medium and low pressure, and low-pressure homes are all under pressure.
The measurement of natural gas is measured according to the standard state (strictly, the quasi-standard state, which we call its normality), and the general trade measurement is measured at a volume of 20°C and 1 atmospheric pressure (0.1013MPa), which is lower than the standard state. The volume is slightly larger, which is good for the seller (since it is multiplied by 273, it is multiplied by 273+20 in accordance with 20°C, so it becomes larger).
The standard state in the international standard is 0°C, 1 standard atmospheric pressure.
For gases, different pressures, the volume will be very large (the gas is very easy to compress), of course, the volume flow will be very large difference, under the same conditions, the flow rate will be very different under different conditions, the same diameter steam The line is different for maximum flow at 10 bar and 3.5 bar.
The calculation of the two states can occur when the process is calculated using the operating conditions or the standard conditions depend on the charts and constants you use.
For example, when defining compressor parameters, we use the parameters under normal conditions to provide conditions for manufacturers. At the same time, we also provide parameters such as temperature and atmospheric pressure for correction under operating conditions. The advantage of this is that we can use the same status. Shows the parameters, just as the pump's performance curve is in clear water, no one can say what the performance curve of gasoline is, what is the performance curve of crude oil.
In many calculations, conditions are used, such as when calculating the flow rate.
The standard state of the gas
The standard state of gas is divided into three types:
1. The standard status of the 10th International Conference on Quantification (CGPM) in 1954 was: temperature 273.15K (0°C), pressure 101.325KPa. This standard is widely used in the science and technology world.
2. The International Organization for Standardization and American National Standards stipulate that the temperature is 288.15K (15°C) and the pressure is 101.325KPa as the standard state of the metered gas volume flow.
3. China's "standard orifice plate calculation method for natural gas flow rate" stipulates that the temperature is 293.15K (20°C) and the pressure 101.325KPa is used as the standard state for measuring the volumetric gas flow.
The standard volume conversion formula for natural gas is different from common gas and must meet the standards issued by China National Petroleum Corporation.
The gaseous equation of the gas:
Qn=Zn/Zg*(Pg+Pa)/Pn*Tn/Tg*Qg
In the formula:
Qn - volumetric flow in the standard state (Nm3/h)
Zn - Compression coefficient under standard conditions
Zg - compression factor at work
Pg - gauge pressure (KPa)
Pa - local atmospheric pressure (KPa)
Pg+Pa - Absolute pressure under working conditions
Pn - standard atmospheric pressure (101.325KPa)
Tn - the absolute temperature (293.15K) in the standard state (natural gas national standard 20 °C)
Tg - The absolute temperature of the medium (273.15+t)K
t - Celsius temperature of the measured medium (°C)
Qg - unmodified volume flow (m3/h)
Note: The parameters with n are standard parameters, and the parameters with g are working parameters.
Example of conversion between standard conditions and operating conditions
Usually, the traffic that we are talking about is to describe the traffic of the standard status in order to facilitate the unified unit; and the actual flow recorded by the factory operation is basically the flow under the working condition.
1. Example of standard flow to working flow conversion:
Q
Air compressor rated gas production 2 cubic meters / minute, pipeline pressure 8 kilograms, the actual pipeline conditions of flow is how much?
A
A rough calculation assumes that the compressed air temperature is 20 degrees.
Condition flow rate = 2/(0.8+0.101325)*0.101325
= 0.2248 cubic meters/minute
Where: 0.101325 is the absolute pressure of the atmosphere; 0.8 is the pipeline pressure, the unit is MPa.
2. Example of operating flow to nominal flow conversion:
Q
Oxygen pipeline pressure 12 kg, working flow 10 cubic meters / hour, the standard flow rate is how much?
A
Assume that the temperature is 20 degrees and does not participate in calculations.
Standard flow rate = 10/0.101325*(1.2+0.101325)
= 128.43 cubic meters/hour
Where: 0.101325 is the absolute pressure of the atmosphere; 1.2 is the pipeline pressure, the unit is MPa.
Flow meter selection calculation example
It is known that the actual working pressure of a gas supply pipeline is (gauge pressure) 0.8MPa~1.2MPa, the medium temperature range is -5°C~40°C, and the gas supply volume is 3000~10000Nm3/h (standard flow rate). Considering the natural gas composition, it is required to determine the specification of the flowmeter.
Answer: According to the gaseous state equation, the standard flow is first converted to operating flow, and then the appropriate caliber is selected.
The gas equation is as follows:
Qb=Q×PTb/PbT×Zb/Zg=QCF2
Where: C is the conversion factor; F is the gas compression factor
Calculation:
1 When the medium pressure is the lowest and the temperature is the highest, it should have the maximum standard volume flow rate.
That is Qb=Q×PTb/PbT×Zb/Zg=QCF2=1200.87m3/h
2 When the pressure of the medium is the highest and the temperature is the lowest, it should have the smallest standard volume flow rate,
That is Qmin=213.51m3/h
According to the above calculation results, the flow meter to be selected has a flow rate range of (214-1200) m3/h.
According to the calculated flow range, select the flowmeter that meets the operating conditions.
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