Elbow flow meter measuring principle

I. Introduction

The elbow flowmeter is widely used in the measurement of liquid, gas and steam flow in petroleum, chemical, power, metallurgy, steel and other industries. It can accurately measure the flow of various fluids in a wide range of pipes from Φ10mm to Φ2000mm. The bend sensor can withstand High temperature, high pressure, normal operation in wet, dust, vibration and other harsh environments, no inserts, no additional resistance loss, simple structure, easy installation.

Second, the working principle of the elbow flowmeter

1, principle

The fluid flows in the pipeline. When flowing through the elbow, the fluid is similar to flowing through a rectifier. Due to the diversion of the curved wall, the fluid inside the fluid is accelerated about 2D before entering the elbow, and the fluid is decelerated to the outside until entering. The flow rate of the elbow fluid is rectified to approximate the trapezoidal velocity flow pattern described by the free swirling theory, and reaches the maximum at the 45[deg.] section of the elbow, this process will continue throughout the elbow. At the exit of the elbow and in the 2D range downstream, the change in the velocity model is the inverse of the import change. The velocity distribution of the elbows in the 45° cross section of each particle is shown in Figure 1.

According to the law of conservation of mass, the law of conservation of energy, and the law of conservation of momentum, the energy of each particle in the same overcurrent section is constant. Due to the change of the velocity of each particle, the pressure difference ΔP between the inside and outside of the elbow is formed. This pressure difference reaches the maximum at the 45° cross section and is the most stable. The average velocity v of the fluid at the 45° elbow section is proportional to the pressure difference DP. The larger the flow, the greater the differential pressure. The flow coefficient of the fluid flowing through the elbow is closely related to the geometry of the elbow (bending radius R and internal diameter D), ie, the flow coefficient a=f(R,D). Therefore, when the geometric size of the elbow sensor is determined, As long as the inner and outer pressure difference DP of the 45° section of the elbow and the density r of the fluid are measured, the average flow velocity v of the fluid can be determined.

2, mathematical expressions

(1)

Where a(R.D)—flow coefficient;
DP-45 ° cross section, outside pressure difference;
D—the inner diameter of the elbow;
R—bending radius;
R—fluid density.

According to the relationship between flow velocity v and flow Q of the pipeline fluid, the following flow calculation formula can be obtained:

Q=p D2/4. V3.6r t/h (2)

Third, the composition of the measurement system

The basic composition of the elbow flowmeter includes the elbow sensor and the mainframe. It also needs to configure the differential pressure transmitter, pressure transmitter and temperature transmitter. The system composition is shown in Figure 2.

1. The differential pressure transmitter is used to detect the differential pressure value generated by the elbow sensor, so it is an indispensable accessory for the elbow flowmeter measurement system.

2. Whether the system is equipped with pressure and temperature transmitters depends on the specific measurement object. For systems that measure steam or other gaseous media, in principle, temperature and pressure transmitters must be configured because the differential pressure of the elbow sensor is measured. At the same time, the temperature and pressure of the medium must be measured in order to make necessary real-time temperature and pressure compensation for the steam or gas. See Figure 3.

Fourth, measurement system characteristics

1, the sensor structure is simple

The elbow sensor utilizes the pipe system elbow as the detection element. No additional pressure loss and special installation of throttling elements are its advantages. The elbow pressure inlet is open at 45o, and the pressure inlet structure is the same as the standard orifice plate, in two planes. The two pressure inlets are aligned so that they can be on the same line, as shown in Figure 4.

The elbow sensor is a machined 90o elbow with high geometric accuracy. It is the core component of the elbow flowmeter. Each flow sensor has a different flow coefficient due to its different geometry. Using curved pipe sensors machined by CNC machine tools, the inner wall of the elbow pipe should be kept as smooth as possible, the dimensions of various geometrical structures can be guaranteed, and the accuracy level of the flowmeter can reach 1.0.

2, maintenance-free flow sensor

The elbow flowmeter will not wear during operation; it will not be deformed, twisted, or vibrated under the impact of high-speed fluid; it is not sensitive to vibrations, dust, moisture, and electromagnetic field interference that may occur in the environment; after long-term operation, its stability, Sensitivity and accuracy will not change significantly; it will prevent the sensor from being contaminated, crusted, blocked, etc. to the greatest extent. This ensures the long-term high-precision measurement of the flowmeter.

Due to its wear-resistant, maintenance-free and long-term high-precision operation, the elbow sensor can meet the technical conditions of direct welding installation. The installation of welding method solves the headache problems of running, running, dripping, and leaking of the flow measurement device on the spot. .

3, without any additional throttle or insert, no additional pressure loss, save energy

For the hole pull flowmeter, the pressure loss of the fluid on the orifice plate is not recoverable, and the loss can reach 60%~80% of the differential pressure value generated by the orifice plate under the flow, in order to improve the measurement accuracy, the design When selecting the differential pressure value of the orifice flowmeter, the personnel generally take a high value, so that the pressure loss becomes a value that cannot be ignored.

The elbow flowmeter does not have the problem of additional resistance loss in the pipeline. The throttling loss of the orifice plate can be simply regarded as the energy saving effect of the elbow flowmeter. For those large systems, large pipe diameters, large flow, and low pressure The benefits of measuring objects are even more pronounced.

4, can measure easily dirty, easy to plug the sensor fluid

The water quality of the circulating water is poor and the pollution to the flow measuring device is great. Orifice orifice flowmeter or vortex flowmeter inserts are easily contaminated or blocked by the circulating water, especially at the right angle part of the orifice orifice, even slight contamination will greatly affect the measurement accuracy. influences. The bend sensor is not doubtful about the adaptability of a medium such as circulating water (such as cracked high-temperature pyrolysis gas). Even a long-term operation can ensure the normal operation of the elbow flowmeter and ensure its sufficient measurement accuracy. .

5, strong adaptability, wide range, straight pipe requirements are not strict

As long as the fluid can be measured with a flow meter such as an orifice plate, a vortex street, an equal velocity tube, etc., an elbow flow meter can be used to measure the flow rate, and is resistant to high temperatures, high pressure, impact resistance, vibration resistance, moisture resistance, and dust resistance. In other respects, the elbow flowmeter is far superior to these commonly used flowmeters.

High temperature, high pressure, shock and vibration make vortex flowmeters have a limited range of application, which is related to its measurement principle. Vibration and impact have little effect on the normal operation of the elbow sensor. High temperature and high pressure can be solved for the elbow sensor by using a standard elbow of the same material as the process pipe.

Wide range of measurement, on the one hand, means that the elbow flowmeter has a wide range of application for the flow rate of its measuring medium. Such as steam or other gas medium, the flow rate range of 5m/s ~ 70m/s, liquid medium flow rate range of 0.3m/s ~ 5m/s. When a micro-differential pressure transmitter with sufficient accuracy is matched, the lower limit of the fluid flow rate can be achieved. The application of high-quality micro-differential pressure transmitter enables the elbow flowmeter to adapt to the requirements of low flow rate, small signal measurement. In short, the range of the elbow flowmeter can fully meet the flow measurement requirements of different objects.

Another aspect of the wide measuring range is that there is no limitation on the geometry of the elbow sensor. The diameter of the elbow sensor can range from more than ten millimeters to 1 meter or even more than 2 meters. The strict requirements on the straight pipe section are also one of the most important features of the elbow flowmeter in field use. Many flow measurement devices cannot measure or ensure the measurement accuracy because the on-site straight pipe segment cannot meet the requirements. In the practical application, the elbow flow meter only needs to guarantee the first 5D and the latter 2D, which is far lower than the requirements of other flow measurement devices.

6, high measurement accuracy

Experiments show that the accuracy of the elbow flowmeter can reach 1.0%.

7, measurement data can be directly connected to the DCS system

The measurement data can be introduced into the DCS, which can realize the arithmetic functions such as temperature and pressure compensation, and can realize functions such as instantaneous amount, cumulative amount, report, and printing.

8, can achieve two-way measurement