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Introduction to the application of flowmeters in gas power plants

With the rapid development of the national economy, the demand for electricity is increasing, especially in the summer, the phenomenon of power shortage is more and more prominent. In order to alleviate the problem of power shortage, many companies have come up with many ways to save electricity. On the other hand, capable manufacturers can generate electricity to solve the problem of power shortage. For example, in the sewage treatment industry, they use the biogas generated in sewage treatment to generate electricity. At present, biogas power generation technology is only in a few countries in the world. The enterprise is concerned about the benefits. The concern is that 1m3 gas can emit several degrees of electricity. The traditional methods of measurement include the use of vortex flowmeters, ultrasonic flowmeters, and rotameters. The results are not satisfactory. Now we are using a thermal gas mass flowmeter. Its successful application has solved the problem of long-term distressing in our measurement problems (the difference between theoretical design and actual consumption). Because of the wide range ratio, we have different powers. The unit can be selected with the same medium specification to achieve high precision measurement. The characteristics, principles, use of thermal gas mass flow meters and the performance of various other flow meters are now compared.
2, thermal mass flowmeter and vortex flowmeter, ultrasonic flowmeter, rotor flowmeter, etc.
2.1 vortex flowmeter (gas)
▲ Vortex flowmeter Features: no moving parts, high measurement accuracy, low pressure loss
The detecting element is not in contact with the fluid to be measured, and the output signal is independent of parameters such as temperature, pressure, density, composition, viscosity of the fluid.
▲ Principle of vortex flowmeter: It is the application of the Karman vortex principle in fluid mechanics to measure fluid flow. A vortex generator (a non-streamlined symmetry body such as a cylinder or a triangular prism) is vertically inserted into the pipeline. When the fluid flows in the pipeline, vortices are alternately generated on the left and right sides of the vortex generator to form a vortex row. The two rows of vortices form a parallel shape with each other, and the left and right alternately appear, and the rotation directions are opposite. The frequency f (Hz) of the vortex has the following relationship with the average flow velocity v (m/s) of the fluid and the width (m) of the vortex generator: f = St * v / d (St is the Strouhal coefficient, and the vortex The body width d is related to the fluid Reynolds number)
▲ The use of vortex flowmeter is relatively simple and easy to install. It is divided into plug-in type and duct type. When installing, be aware that the flow direction of the fluid should be the same as the direction indicated by the flow meter.
▲ Disadvantages of vortex flowmeter: small range ratio (15:1); greatly affected by vibration; pipeline size is proportional to price; temperature and pressure compensation is required for metering; small range is insensitive, unstable, almost impossible to measure .
2.2, ultrasonic flow meter (gas)
▲ Ultrasonic flowmeter features: no pressure loss; simple installation; independent of the fluid's temperature, pressure, viscosity and other properties; no moving parts; can measure dirt, corrosive gases and multi-component gases.
▲ Ultrasonic flowmeter principle: Measure the propagation time of ultrasonic waves in the forward and reverse flow directions in the pipeline, and then calculate the flow velocity of the fluid. The flow rate of the fluid can be obtained by adjusting the flow rate of the medium, the diameter of the tube, and the kinetics of the fluid with Reynolds number.
Specifically, a pair of ultrasonic sensors are mounted in the "Z" shape in the pipe, and the distance L between the two sensors is used as the propagation travel of the ultrasonic waves. The downstream and countercurrent propagation times of the ultrasonic waves between the two sensors are expressed as:
Ts=L/(C+Vcosθ); Tn= L/(C-Vcosθ)
Where C is the velocity of the sound wave in still air as a function of the nature of the gas, in m/s. V is the gas medium flow rate in m/s. θ is the angle between the sonic stroke and the axis of the pipe.
▲ Ultrasonic flowmeter is also convenient to use. Install the “Z” type on the surface of the pipe. It should be noted that the sensor installation should be polished and buttered, and the sensor and pipe should be tightened with the fixing device.
▲ Disadvantages of ultrasonic flowmeter: small range ratio (15:1); maintenance is relatively troublesome; high gas precision is high; metering requires temperature and pressure compensation;
2.3 Rotameter (gas)
▲Rotor flowmeter features: local display, no power supply; the price is relatively cheap;
It is available in two materials, the glass tube rotor flow meter and the metal tube mass flow meter. Both local display and remote transmission display are available. The interface has HART, standard current signal, PROFIBUS and other forms.
▲The principle of the rotor flowmeter: The rotor flowmeter is also a speed flowmeter. It consists of two parts, one is a large and small conical tube, and the other is a rotor (also called a float) placed in a conical tube. The weight of the rotor is balanced by the differential pressure ΔP generated when the fluid passes through the gap (throttle area) between the rotor and the wall of the tapered tube. During operation, the fluid to be measured flows in from the lower end of the conical tube and flows out from the upper end. The magnitude F of the fluid that exerts an upward force on the rotor is always equal to the weight G of the rotor, ie
F=△P*A; G=V*(ρt-ρf)g
Since F=G, △P*A= V*(ρt-ρf)g
△P=V*(ρt-ρf)g/A (1)
Where V-rotor volume; A-rotor cross-sectional area; g-local heavy acceleration; ρt-ρf is the rotor material and the density of the measured fluid; ΔP is the vertical direction of the rotor The pressure difference. It is known from equation (1) that the pressure difference remains constant throughout the entire work process. When the flow rate is increased, of course, the flow rate through the throttle area is also increased, and only the throttle area is increased, and the flow rate is decreased to maintain the pressure difference ΔP. Therefore, the flow rate can be read according to the height of the balance position of the rotor. Can be expressed by equation (2)
Q=k*h(2△P/ρf)1/2 (2)
Where k is the meter constant; h—the height at which the rotor floats. Substituting formula (1)
Q=k*h(2gV(ρt-ρf)/(ρf*A))1/2
▲ Rotor flowmeter is convenient to use. It is ok to tighten the corresponding bolts during installation, but it should be noted that the direction of fluid flow must be vertical.
▲ Rotor flowmeter shortcomings: the range ratio is 10:1; the pressure loss is relatively large; the dirt is easy to block; the installation and maintenance are relatively complicated; the vibration is greatly affected; the temperature and pressure compensation are required; the diameter of the pipe is proportional to the price, generally Below DN200.
2.4, thermal gas mass flowmeter
▲Characteristics of thermal mass flowmeter: The range ratio is up to 1000:1; the small range is sensitive; the temperature is not directly affected by temperature and pressure; the pressure loss can be neglected; the large diameter and small flow can be measured with high precision. The price is not much different from the size of the pipeline; the precision is up to 1%; the temperature range is as wide as -70°C to 450°C; it is not sensitive to dust and particulate matter.
▲ Principle of thermal mass flowmeter: It is a flowmeter based on the principle of thermal diffusion. Generally speaking, the heat source placed in the fluid, when the fluid passes through it, the heat of the heat source itself will be lost, the greater the mass flow rate of the fluid, the greater the heat lost by the heat source. Thus the flow of fluid and the heat lost by the heat source should ideally be equal. So knowing the amount of heat, we can get the flow of the fluid.
Specifically for the implementation of a thermal gas mass flow meter (hereinafter referred to as a thermal flow meter), the probe of the thermal flow meter has two probes, one of which is a reference point for measuring the temperature of the fluid; the other is only A heating source used to act as a heat source. According to the above we can have the following formula (3)
P/△T=A+B*(Q)m (3)
Where P—the power supplied by the electronic module for the heating source; ΔT—the temperature difference between the heating source and the reference point; A, B, and − are constants related to the nature of the fluid itself; m—is related to the nature of the fluid itself The index coefficient; Q—the mass flow rate of the fluid. From equation (3) we can see that there are two ways to achieve flow measurement: P fixed is the constant power method; △ T fixed is the constant temperature difference method. Usually we use the constant temperature difference method to measure, the characteristics of the constant temperature difference method: the small-range segment is very sensitive, and the reaction speed is also fast. Currently, the products circulating on the market adopt this method.
▲The thermal mass flow meter is easy to use and install. It has two types: plug-in type and pipeline type. Whether it is plug-in or pipeline type, maintenance and operation are extremely simple. When installing, please note that the direction of the fluid should be the same as that indicated by the thermal flowmeter.
▲The disadvantages of the thermal mass flowmeter: it is not suitable for installation in a high viscosity environment; it is not suitable for installation in places where the water content is above 40%.
3. Application of thermal gas mass flowmeter in gas generator set
The whole system is designed according to the above diagram. The pressure and composition requirements of the generator set are not high. For the Gaobeidian sewage treatment plant, the pressure of the biogas is 2KPa, the temperature is normal, and the biogas is desulfurized. Composition ratio:
H2S 0.653% CO2 33.7% CH4 62.3% O2 0.47%H2 0.063% N2 1.46%
Due to the complex composition and low pressure of the gas source, the traditional flowmeters have not reached our design specifications. After our research on the performance of the traditional flowmeter, we conclude that the components with complex composition and low pressure are not enough. Conditions, traditional flowmeters have been unable to measure due to their limitations. Therefore, after our investigation, we found that the thermal gas mass flow meter is fully in line with our working conditions. We use a gas mass flow meter with a flow rate range of 0.05 to 120 m/s. Drill a ¢20 port on the DN150 pipe during installation, then weld a M27X1.5 internal threaded base and screw the flow meter with the locking joint into the base. Installation is also very convenient. Because it outputs the standard current signal, it can directly access the computer's acquisition card, relying on special software to achieve integrated management of the system, so that our energy loss and power output have an accurate calculation basis.
Actual test data:
The meter is a thermal gas mass flow meter with a gas pressure of 2 kPa, an ambient temperature of 33 ° C, and a relative humidity of 50%. The gas pipe diameter is φ150.
The flowmeter measurement value is the gas flow rate S (m/s), and the flow rate value is a calculated value.
The calculation formula is: Q = S × πR2 × 3600, and R is a pipe diameter of 0.075 m.
The flow rate measured by the flowmeter is calibrated to the air component and should be corrected when the gas composition changes. The biogas components are as follows:
Ingredients H2S CO2 CH4 O2 H2 N2
Percentage % 0.653 33.7 62.3 0.47 0.063 1.46
Correction formula: 1/Fmix=V1/F1+V2/F2+...Vn/Fn
Where: Fmix is the correction factor, Fn is the coefficient of each component, and Vn is the percentage.
The composition coefficients are as follows:
H2S CO2 CH4 O2 H2 N2
Coefficient 0.933 0.795 0.823 0.946 1.019 0.946
According to the calculation of the above formula, the correction coefficient is 0.827.
It can be seen from the above calculation that the air consumption rate of the unit at 450 kW is 1.97 kW/m3.
4 Conclusion
According to our practical application, the advantages of thermal gas mass flow meters in gas measurement are particularly obvious, especially in the case of small pressure and complex gas composition. Compared with other flowmeters, the thermal gas mass flowmeter has the advantages of small pressure loss, wide range ratio and sensitive small range, which is more conducive to energy conservation. Its successful application provides equipment for measuring gas generators in the future.

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