What flow rate do we measure when a gas flow meter operates?
You may also have this question.
Does the flow meter measure a standard or working flow rate?
Let’s find out together.
First of all, let’s find out the working and standard flow rates for gas flow meters.
The working flow rate refers to the flow rate in the actual working condition. It is measured in m3 /H.
Gases are compressible. The volume of a gas at the same nominal flow rate varies when subjected to different pressures and temperatures in a closed pipe. The gas pressure is higher, and the volume is lower. The higher the pressure, the smaller the volume flow rate. The flow rate at a specific temperature with pressure (relative to an atmosphere) is usually called the service flow rate.
The standard flow rate is the gas flow rate at one atmosphere and at a temperature of 20 degrees Celsius (in some cases, 0 degrees Celsius is also used as a standard condition).
The standard flow rate is measured in Nm3/h.
Note: This is usually when the nominal temperature is 0 degrees Celsius (273.15 Kelvin), and the pressure is 101.325 kpa (one standard atmosphere, 760 mmHg). This distinction is in contrast to the regulations for our industrial gas conditions.
Usually, the flowmeter measures the working volume flow rate. If not, it is also because the flowmeter has added temperature and pressure or density compensation.
It is calculated on the principle of the law of conservation of mass.
The calculation formula is as below :
Service volume flow rate / standard volume flow rate = standard volume density / service volume density flow rate.
The conversion relationship between the standard and service flow rates of gases is calculated according to the law of conservation of mass. It means that the pressure and temperature of the gas are different in the standard and working conditions, so we can check the change in gas density to know the density in the respective state. The working volume flow rate ratio to the standard volume flow rate is inversely proportional to its density.
The formula is as follows.
Working volume flow rate/standard volume flow rate = normal volume density/working volume density
(Standard conditions for industrial gases: temperature is 20 degrees, pressure is 0.101325 MPa)
What is the difference between the standard and working flow rates for gas flow meters?
Working condition: It is the flow rate in the actual working condition. Unit: m³/h
Standard condition: It is the flow rate at a temperature of 20°C and one atmosphere (101.325kPa). Unit: Nm³/h
The units are the same in both states; only the corresponding flow rates are different. In addition, the standard conditions are different in other countries.
The ideal gas state equation is as follows：
PV = nRT.
This equation has four variables.
p is the pressure of the ideal gas
V is the volume of the perfect gas.
n is the amount of gas substance
T represents the geometric temperature of an ideal gas.
There is also a constant: R is the ideal gas regular.
PV/T=nR is a constant, so P1×V1/T1=P2×V2/T2 .
Assuming a volume flow rate of V0 at standard conditions
Volume flow rate at working condition is V, and
Temperature T (degrees Celsius)
The pressure is P(Mpa).
Ignoring the change in compression factor, we have V*(P+0.101325)/(T+273)=V0*P0/T0
Note: Generally, natural gas is delivered at medium and low pressure, with low pressure into the home, all with pressure. It belongs to the working conditions.
Natural gas is measured by standard condition (strictly speaking, quasi-standard condition, we call it normality). General trade metering is based on 20°C, 1 atmosphere (0.1013 MPa) state.
The volume is measured in the standard state. Therefore, it is slightly larger than the volume in the standard state.
International standards measure it at 0°C and one standard atmospheric pressure.
The volume can vary considerably for different pressures for gases (which are very compressible). So, of course, the volume flow rate will be very different, and the flow rate under other working conditions in the same diameter will naturally be very different.
For example, the maximum flow rate for a steam line of the same diameter is different for 10 bar and 3.5 bar.
Whether you use the working conditions or the standard conditions in your process calculations depends on the chart and the constants you use. However, measures for both conditions are possible.
For example, when defining compressor parameters, we often use the parameters in the standard condition to give the manufacturer the conditions. At the same time, we also provide parameters such as temperature and atmospheric pressure for calibration in the operating conditions. The advantage of this is that we can indicate the parameters in the same state. Just as the performance curves for pumps are all in terms of clear water, no one can say what the performance curve is for petrol and what the performance curve is for crude oil.
We use the working conditions in many calculations, mainly when calculating flow rates.
How do we calculate the gas state?
There are three standard states for gases.
- The standard state agreed by the 10th International Conference on Weights and Measures (CGPM) in 1954 is: 273.15 K (0°C) for temperature and 101.325 KPa for pressure. This standard state is widely used in the scientific and technical fields worldwide.
- The International Organization for Standardization (ISO) and the American National Standard (ANSI) specify the following. They use the temperature 288.15K (15℃) and the pressure 101.325KPa as the standard state for measuring gas volume flow.
- China’s 《Standard Orifice Plate Calculation Method for Natural Gas Flow》stipulates that the temperature of 293.15K (20℃) and the pressure of 101.325KPa are used as the standard state for measuring the volume flow of gas.
The natural gas standard state volume conversion formula is different from ordinary gases. It must comply with the China National Petroleum Corporation (CNPC).
The gaseous equation for gases.
Qn = Zn/Zg*(Pg+Pa)/Pn*Tn/Tg*Qg
The equation in which：
Qn – volumetric flow rate in the standard state (Nm³/h)
Zn – compression coefficient in normal condition
Zg – compression coefficient in working condition
Pg – gauge pressure (KPa)
Pa – local atmospheric pressure (KPa)
Pg+Pa – absolute pressure in working condition
Pn – standard atmospheric pressure (101.325KPa)
Tn – absolute temperature in the standard state (natural gas national standard 20°C) (293.15K)
Tg – the absolute temperature of the medium (273.15 + t) K
t – the temperature of the measured medium in Celsius (℃)
Qg – uncorrected volume flow rate (m³/h)
Note: with n is the standard condition parameter, with g is the working condition
And how do we convert the standard flow rate to the working flow rate for gas flow meters?
For uniform units, the flow rates we usually refer to in gas flow meters are the flow rates in the standard condition. However, the actual flow rates recorded in plant operations are the flow rates in the working condition.
- An example of converting standards to operating conditions is as follows.
The air compressor is rated to produce two cubic meters of gas per minute, and the pipe pressure is 8 kg. What is the actual working flow rate of the pipeline at this point?
Let us first assume that the compressed air temperature is 20 degrees.
At this point the working flow = 2/(0.8+0.101325)*0.101325 = 0.2248 cubic metres/minute
In the above formula: 0.101325 is the absolute pressure of the atmosphere; 0.8 is the pipe pressure in MPa.
- Examples of workflow to standard flow conversions are as follows.
If the oxygen pipeline pressure is 12 kg, the working flow rate is ten cubic meters per hour. What is the standard flow rate at this point?
We assume a temperature of 20 degrees; no calculation is involved.
Normal flow rate = 10/0.101325*(1.2+0.101325)=128.43 cubic metres per hour
In the above equation: 0.101325 is the absolute pressure of the atmosphere; 1.2 is the pipe pressure in MPa.
- In selecting a gas flow meter to measure natural gas, the customer provided a flow rate of 240 cubic meters per hour. And the working pressure is 4 bar. So how should we choose and convert at this point?
We can analyze the calculation in the following steps.
Firstly, if the customer is not sure it is a standard or working flow rate.
At this point, we can confirm the caliber with the customer. We can then compare the flow rate with the flow rate to determine whether it is standard or working.
Secondly, 240 cubic meters per hour is the standard flow rate.
At this point, the working flow rate is 240/(4+1) = 50 cubic meters per hour.
Thirdly, 240 cubic meters per hour is the working flow rate.
At this point the standard flow rate is 240 x (4 + 1) = 1200 cubic metres per hour.
The “1” here is one standard atmosphere, and pls note.
Example of flowmeter selection calculation
The actual working pressure of a gas supply line is known to be (gauge pressure) 0.8 MPa~1.2 MPa. The temperature range of the medium is -5℃~40℃. The gas supply volume is 3000~10000Nm³/h (standard flow rate). Without considering the composition of natural gas, we are asked to determine the specification model of the flowmeter.
The standard flow rate is first converted to a working flow rate according to the gas equation, and then a suitable caliber is selected.
The gas equation is as follows.
The equation above is C’s conversion factor; F is the gas compression.
The calculation is as follows.
① When the medium pressure is the lowest and the highest temperature, it should have the maximum nominal volume flow rate.
That is, Qb=Q×PTb/PbT×Zb/Zg=QCF2=1200.87m³/h
② When the medium pressure is the highest, the temperature is the lowest, and it should have the minimum nominal volume flow rate.
That is, Qmin = 213.51m³/h
From the above calculation results, we have to choose the flowmeter for the operating flow range (214-1200) m³/h.
According to the calculated flow range, we can choose to meet the requirements of the working conditions of the flowmeter.