The use of vortex flowmeter and the analysis of common failures

Project Overview

Vortex flow meters are mainly used for flow measurement of industrial pipeline media fluids, such as gas, liquid, steam and other media. It is characterized by small pressure loss, large measuring range, and high accuracy. When measuring the volume flow rate under working conditions, it is almost not affected by parameters such as fluid density, pressure, temperature, and viscosity. There are no moving mechanical parts, so the reliability is high, the maintenance is small, and the instrument parameters can be stable for a long time. The vortex flowmeter uses a piezoelectric stress sensor and can work in the operating temperature range of -20°C to +250°C. It has analog standard signals and digital pulse signal output, which is easy to use with digital systems such as computers. It is a more advanced and ideal measuring instrument.

1 Working Principle and Structure of Vortex Flowmeter

1.1 Working Principle

When the fluid flow is obstructed by a non-cylindrical body perpendicular to the flow direction , obvious vortices will occur on both sides of the downstream side of the cylinder, becoming a Karman vortex street. The formation of the vortex street is related to the Reynolds number of the fluid.

By measuring the separation frequency of the vortex, the fluid flow rate and instantaneous flow rate can be measured. The Strouhal number St is a dimensionless constant that can be determined experimentally. The linear part of the functional relationship between the dimensionless constant St and the Nobel-like number Re is the linear measurement range of the vortex flowmeter. By detecting the frequency F, the velocity of the fluid in the pipe can be obtained, and then the volume flow rate can be obtained from the flow velocity. The ratio of the number of pulses output within a period of time to the volume of fluid (the number of pulses corresponding to a unit volume of fluid flowing through) is called the instrument coefficient (K coefficient).

The frequency of vortex separation used to measure flow varies with flow rate and is not affected by fluid density and viscosity. The pressure pulsation caused by the separation of the vortex is detected by the piezoelectric force-sensitive probe and converted into a pulse signal corresponding to the vortex frequency in the detection circuit. The signal converter converts this pulse signal into a standard current signal of 4-20mA for output.

1.2 Structure

The vortex flowmeter consists of two parts: the sensor and the converter. Sensors include vortex generators (bluffs), detection components and meter bodies, etc.; converters include preamplifiers, filter shaping circuits, D/A conversion circuit output interface circuits, terminals, brackets and protective covers, etc.

The possible causes of this failure and the corresponding solutions are as follows:

(1) The possible cause of the failure is that the power supply fails or the power supply is not connected. Therefore, it is necessary to check whether the power supply voltage meets the requirements and whether the power polarity is correct. For example, if the current output type vortex flowmeter is reversed in polarity of the power supply, the output current will be 0mA instead of 4mA.

(2) The possible cause of the failure is disconnection of the connecting cable or incorrect wiring. It is necessary to check whether the line is smooth. If there is a fault, rewire it and check it.

(3) The possible cause of the failure is that the ripple coefficient of the power supply is too large. During inspection, a filter electrolytic capacitor can be used for testing. Generally, the capacitance is required to be ³100μF, 50V.

(4) The possible cause of the failure is that a certain stage of the amplification board is faulty. You can observe the changes in the output end by observing the waveforms at the input and output ends of the amplifier, or injecting human body induction signals from the input end. If there is no change at the output, the amplifier is not working properly. In actual maintenance, you can also use other types of flowmeter main boards available on site to replace them for inspection. It is determined that the amplification plate is faulty and should be replaced.

(5) The possible cause of the failure is damage to the generator. The detection method is to first increase the flow rate and replace the vortex mainboard with the same type of vortex flowmeter. If the mainboard failure is eliminated, check the generator.

Damage to the vortex generator often occurs in steam measurement systems. Because the craftsman opens the valve too fast, the high-temperature and high-pressure water vapor entrained with condensed water violently impacts the vortex generator, causing its damage, which is often referred to as "Water hammer" phenomenon. Therefore, it is necessary to open the guide shower to drain the condensed water and then slowly open the steam valve to avoid damaging the vortex generator.

(6) The possible cause of the failure is that the pipeline flow rate is too low and does not enter the measurement range. You can increase the flow rate to see the reaction of the flow meter. Tap regularly and continuously next to the pipe near the flow meter, and the flow rate display will appear regardless of the type of flow meter. If the measured medium is a liquid, it is best to test with an empty tube, because the liquid itself is a shock absorber. The troubleshooting method is to replace the flow meter with a smaller diameter or increase the flow rate if the process conditions permit.

(7) The possible cause of the failure is whether the gap between the generator and the shell is stuck by fine particles. This phenomenon usually occurs after normal work.