Natural gas flow measurement and instrument selection

With the widespread use of natural gas, the requirements for flow measurement under various usage conditions and needs are very different. At the same time, due to the compressibility of gas, the flow measurement of gas is much more difficult than that of liquid. If the gas metering is required to achieve a fairly high accuracy, the economic investment in the metering instrument must be huge. Therefore, it is very necessary to reasonably select the measurement method and the function, characteristics, and economical flow meter. The following is a practical introduction to the measurement standards, measurement methods and instrument selection of natural gas. 

1 Natural gas measurement standards and temperature and pressure correction methods

(1) Natural gas measurement standards

Natural gas measurement is expressed in volume, unit: m3. Since gas has the characteristics of being able to fill any space and compressibility, it can be seen from the gas state equation that as a standard volume for measurement, the pressure and temperature must be clearly and uniformly stipulated. It is meaningless to talk about gas volume without state.

The national petroleum industry standard has clearly stipulated that the state at a temperature of 293.15K (20°C) and a pressure of 101.325kPa is the standard state for natural gas measurement; any actual state that does not conform to this standard state should be corrected according to the gas state equation.

In the formula: Qn---gas volume under standard conditions, m3;
Q---gas volume under working conditions, m3;
t---gas temperature under working conditions, ℃;
P---gauge pressure under working conditions, kPa;
Pa---local atmospheric pressure during measurement, kPa;
z---gas compression coefficient.

The compression coefficient z varies with temperature and pressure. It has a clear functional relationship with the ratio of the working pressure P of the gas to the critical pressure PC of the gas (P/PC), the working temperature T (K) of the gas to the critical temperature TC (K) of the gas (T/TC). The compression coefficient z of natural gas = 1/F2z, and the supercompression factor Fz can be found in Appendix A of the national petroleum industry standard SY/T6143-1996; when the pressure is less than 0.1MPa, z = 1. The actual situation

of natural gas pipeline transmission and distribution is complex and often inconsistent with the standard state. Therefore, a certain correction method needs to be adopted for compensation correction.

(2) Fixed manual correction coefficients for different regions

When there is no temperature and pressure automatic compensation device and pressure recorder, and when the pressure fluctuation is not large, a fixed manual correction coefficient K reference table applicable to the local area can be formulated according to the different atmospheric pressures in different regions (Beijing is 101.32 kPa, Chongqing is 98.32 kPa, Shanghai is 101.61 kPa, and Lhasa is 65.18 kPa) and different temperatures.

According to formula (1), we can get:

Where: KT---uniform temperature correction coefficient;
KP---uniform pressure correction coefficient.

In a certain area and a period (winter or summer or all year round), the uniform temperature t and the uniform atmospheric pressure Pa can be constants; while z changes with P, so K only changes with P. From this, a reference table of local series pressure correction coefficients with the correct relationship between KP can be listed. According to the actual pressure P (approximately read by the pressure gauge in front of the meter), K solid can be directly found, and the volume flow rate under standard conditions can be calculated according to the following formula:

Qn=K solid×Q

Where: Q---the indicated reading of the flow meter, m3

(3) Manual timing correction coefficient using pressure recorder

Install a pressure recorder in front of the flow meter to continuously record the pressure of the gas within 24 hours. Adjust the starting point of the pressure recorder to the local atmospheric pressure, use 100% circular scale recording paper, and use the arithmetic uniform method or equivalent radius method (proportional integrator) to calculate the static pressure uniform grid number XP, then:

Where: Pmax---upper limit of pressure recorder, kPa.

Use a thermometer to record the temperature every 2 hours and calculate the average temperature of the day. KT can be obtained from this. Qn can be obtained according to formulas (2) and (3).

(4) Electronic correction integrator

Use microelectronic processing technology to process and calculate the collected signals to achieve accurate correction of the measured volume. Collect the pulse signal from the gas flow meter, calculate the uncorrected actual flow volume Q according to the set pulse equivalent value, and combine it with the measured working pressure P (set the uniform pressure fixed value or the measured current pressure introduced by the pressure transmitter), working temperature T (set the uniform temperature fixed value or the measured current temperature introduced by the temperature sensor), compression z (set to a fixed value or enter the formula for automatic calculation), and local atmospheric pressure Pa. According to formula (1), the software is designed to automatically calculate the standard volume. It can be used in conjunction with gas turbine flowmeters, rotary flowmeters, vortex flowmeters, membrane gas meters, etc.

2 Natural gas standard orifice flowmeter and supporting devices

The standard orifice measures the flow rate when the gas flows through the orifice, and the pressure generated before and after the orifice is different. The flow rate is calculated by this pressure difference. The standard orifice is a standard throttling device (standard orifice, nozzle, venturi tube) that complies with GB/T2624-1993 and has reliable experimental data. The standard orifice designed, processed and installed according to GB/T2624-1993 does not need to be calibrated or calibrated separately.

(1) Flow calculation formula of throttling device:

Where: Q---volume flow rate under working conditions, m3/s;
ε---expansion coefficient;
△P---pressure difference before and after the throttling device, Pa;
ρ---density at a certain temperature and pressure on the upstream side of the throttling device, kg/m3;
d---diameter of the throttling device opening, m;
β=d/Ｄ---diameter ratio of the throttling device, D is the inner diameter of the pipe, m;
C---outflow coefficient. The correction coefficient related to the friction resistance, eddy loss, contraction and pressure hole position of the flow beam under working conditions can be obtained by experimental methods or calculated using the Stolz equation.

(2) Practical formula for measuring the flow rate of natural gas with a standard orifice plate

When natural gas uses a standard orifice plate for flow measurement, the flow rate can be calculated according to SY/T6143-1996. The practical formula for calculating the standard volume flow rate is:

Where: Qn---natural gas volume flow rate under standard conditions, m3/s;
AS---second measurement coefficient, depending on the measurement unit used, in this formula AS=3.1794×10-6, if the unit is hour H, it needs to be multiplied by 3600.
C---outflow coefficient;
E---asymptotic velocity coefficient,

d---orifice opening diameter, mm, d=d20[1+λd(t-t20)], d20 and t20 are the orifice opening detection diameter (mm) and indoor temperature (℃) at 20℃±2℃, λd is the linear expansion coefficient of the orifice material (mm/mm.c), t is the temperature of the natural airflow passing through the orifice (℃).
FG---relative density coefficient, , Gr is the true relative density;
P1---absolute static pressure of the pressure-taking hole on the upstream side of the orifice, MPa;
△P---differential pressure generated when the airflow passes through the orifice, Pa;
ε---expansion coefficient, ε=1-(0.41+0.35β4) , k is the isentropic index;
Fz---supercompression factor, zn and z1 are the compression coefficients under standard state and flow state respectively;
FT---activity temperature coefficient,

The parameters C, ε, λd, Gr, FG, FT, Fz, z1, etc. are all correction coefficients for converting actual working conditions into standard conditions. The specific data can be found in the relevant appendix of the standard.

(3) Flow integration of natural gas measured by standard orifice plate

a) Integration using double bellows differential pressure flowmeter

The double bellows differential pressure flowmeter is composed of a measuring orifice plate (differential pressure, static pressure) and a display part and an accessory integrator. Guangzhou Junkai Electronic Technology Co., Ltd. uses 100% circular scale recording paper (i.e. square root card) to record the differential pressure △P and static pressure P1 of the orifice plate. The integrator calculates the flow rate of the day. According to formula (6), the practical formula can be obtained:

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Where: Pmax (MPa), △Pmax (Pa) are the upper limits of the instrument static pressure and differential pressure, respectively; XP, X△P are the average grid numbers of the actual static pressure and differential pressure recorded on the square root card, respectively.

For a certain metering device, when the natural gas composition is constant, d, Pmax, △Pmax, Gr can be regarded as constants; when a throttling device with a certain β value (i.e., d and D values ​​are constant) is used in a region with a high Reynolds number, C can be regarded as a constant; for a specific environment, the Fz (=1/z) value can basically be regarded as a constant (when the gas supply pressure is lower than 0.5MPa, z can be taken as approximately 1); when the pressure is relatively stable and the gas volume does not change much, ε can also be regarded as a constant, so K is a constant under specific circumstances. Then:

According to this formula, the daily gas consumption can be manually calculated.

The double-corrugated differential pressure gauge is often used in medium and high pressure, large and medium flow industrial pipelines and total measurement and temporary measurement due to its good performance stability and low economic investment. However, it is inconvenient to manually calculate and the accuracy of the calculation is affected by human factors, so it is gradually replaced by more advanced instruments. [page]

b) Microcomputer flow metering device

The microcomputer flow metering device consists of an orifice plate (or other primary flow meter and turbine, vortex, etc.), a differential pressure and pressure transmitter (or turbine, vortex transmitter, etc.), a temperature transmitter and an industrial control microprocessor and its corresponding supporting facilities. The differential pressure, pressure and temperature signals of the orifice plate are converted into analog 4-20mA by the transmitter, and transmitted to the microprocessor which has been pre-input and strictly programmed according to formula (6) (or other flow meter transmitter corresponding calculation formula) through the A/D converter. Guangzhou Junkai Electronic Technology Co., Ltd. thus obtains the gas flow through the display; it can realize real-time sampling of pipeline operation data (once every two seconds or every few seconds), which is much more accurate than manual sampling per hour, and can install multiple sets of metering pipelines and signals of various flow meters, and record, calculate and display them at the same time. Its principle diagram is shown in Figure 1.

It can receive 4-20mA (or 1-5V) analog signals (as well as pulse signals from turbine, vortex and other flowmeters, with a frequency range of 0-5000Hz and an amplitude of 12V) from the orifice plate through pressure, differential pressure and temperature transmitters. It can be designed to display instantaneous flow, hourly and daily cumulative flow, differential pressure and pressure, temperature and its upper and lower limits, density, alarm, record parameter changes, time, pressure, differential pressure, flow curve and other functions.

3 Diaphragm gas meter and lumbar flowmeter

These two instruments belong to volumetric flowmeters. They are characterized by high measurement accuracy, wide range ratio, small influence of changes in density and viscosity of the measured gas on the instrument display value and accuracy, and low requirements for the straight pipe section before and after the instrument, but the instrument transmission mechanism is complex, the manufacturing requirements are high, and key parts are easy to wear. The lumbar flowmeter needs to be cleaned and lubricated regularly.

Working principle of diaphragm gas meter: Under the pressure difference between the inlet and outlet of the meter, the gas passes through the slide valve and the distribution chamber, causing the diaphragms of the two metering chambers to form a reciprocating motion of alternate intake and exhaust; the volume of each reciprocating intake and exhaust is a rated value, that is, a rotation volume (V1); at the same time, the transmission conversion mechanism connected to the diaphragm main shaft continuously inputs the number of diaphragm rotations into the counting mechanism to accumulate the flow, thereby displaying the total amount of gas outflow. As shown in Figure 2.

The rotary flowmeter is also called Roots or rotary flowmeter. When the gas flows in from the inlet, under the action of the inlet and outlet pressure difference, it is in the position of Figure 3a. The synthetic torque on the rotary wheel A is unbalanced, so the rotary wheel A cannot rotate. The synthetic torque on the rotary wheel B is unbalanced, so the rotary wheel B rotates clockwise, and the gas in the metering chamber is discharged to the outlet. At the same time, the driving gear on the rotary shaft of the rotary wheel B drives the driving gear on the rotary shaft of the rotary wheel A, so that the rotary wheel A rotates counterclockwise, gradually reaching the position of Figure 3c from the position of Figure 3b. Similarly, the torque and rotation process on the two rotary wheels form the position of Figure 3d. The two rotary wheels rotate alternately as the master and the slave. When the rotary wheel rotates one circle, four gases with the volume of the shaded part in the figure are discharged. Through the number of revolutions of the rotary wheel and the gear reduction system, they are input into the indicating mechanism to display the total flow of the gas. As shown in Figure 3.

Diaphragm gas meters are widely used for residential gas, and rotary flow meters are often used for medium-pressure users. Both instruments generally do not have temperature and pressure compensation devices, so one of the three correction methods described in the previous "Measurement Standards" must be used for compensation correction. When an electronic correction integrator is used, temperature, pressure, and indicating sensors must be installed. One is to reserve signal tubes (pressure, temperature) and indicating sensors before the instrument leaves the factory; the other is to lead out pressure and temperature signal tubes on the natural gas distribution pipeline at the instrument site and equip them with corresponding sensors and indicating sensors.

Both instruments can realize remote data transmission by adding indicating sensors, thereby realizing centralized management of microcomputer data.

4 Gas turbine flowmeter, Karman vortex flowmeter and swirl flowmeter

All three instruments are velocity flowmeters. The characteristics of gas turbine flowmeter are compact structure, high accuracy, good repeatability, wide range, quick response, and small pressure loss, but the installation and use (filter and rectifier need to be installed before the meter) and the wear resistance of bearings are high; vortex flowmeter is an instrument with no moving parts inside, long life, high accuracy, and wide range ratio, but the flow velocity distribution and pulsating flow and vibration have different degrees of influence on the measurement.

Working principle of gas turbine flowmeter: When the gas flows through the turbine body, the gas drives the impeller with spiral magnetic blades to rotate. When the blades made of ferromagnetic material rotate through the magnetic induction or signal detector fixed on the shell, it causes the periodic change of magnetic resistance in the magnetic circuit and generates an electric pulse signal of approximately sine wave in the induction coil. Guangzhou Junkai Electronic Technology Co., Ltd. Within the flow range specified by the turbine instrument, the frequency f (Hz) of the pulse signal is proportional to the flow velocity, that is, the flow rate. The pulse signal can be displayed by the integral display part after calculation and processing, as shown in Figure 4.

The working principle of Karman vortex flowmeter is: a nonlinear column (cylinder or triangular column) is inserted into the fluid pipeline. When the gas flows through and reaches a certain Reynolds number range, two asymmetric and regular alternating vortices are generated downstream of the column. The frequency f (Hz) of the vortex is proportional to the flow velocity, that is, the flow rate. The flow rate of the fluid can be obtained through the detector, amplifier and converter. As shown in Figure 5.

Working principle of vortex flowmeter: When the gas enters the inlet of the flowmeter along the axial direction, the spiral cone forces the fluid to rotate, forming a vortex flow, which swirls, throttles, and accelerates through the Venturi tube, forming a high-speed rotating vortex core at the central axis. When it reaches the diffuser, the pressure change causes backflow, causing the vortex core to deviate from the axis, forming a cone spiral precession. If the relevant geometric shape and size of the instrument are reasonably designed, the spiral vortex precession frequency f (Hz) at the detection point is proportional to the flow velocity, that is, the flow rate within a wide flow range. After amplification and shaping into an electrical pulse signal, it is transmitted to the display part, and the flow rate of the fluid can be calculated. As shown in Figure 6.

Three representative working flow formulas:

In the formula, K is the instrument coefficient, which is related to the specific shape, size, detection point, production process and other factors of the instrument design, and needs to be tested and calibrated. SHIMPO, Japan | ATAGO, Japan | KEM, Kyoto Electronics | MINOLTA | FLUKE | RION, Japan | TPI, USA

Combining formula (1) and formula (10), the three instruments can also be corrected by one of the three correction methods mentioned above to obtain the flow rate under the standard state.

In addition to the above correction method, formula (1) and formula (10) can also be integrated into the intelligent integrated design so that the instrument can directly display the flow rate under the standard state. For example: the intelligent vortex flowmeter, its structure diagram is shown in Figure 6, and its flow integration principle diagram is shown in Figure 7; gas turbine flowmeter and Karman vortex flowmeter can be integrated according to this structure and principle.