The principle and application of vortex flowmeter

According to the working principle of vortex street flowmeter, a vortex generator (blocking fluid) is set in the fluid, and regular vortexes are generated alternately from both sides of the vortex generator, which is called karman vortex street, as shown in figure 1.Vortex columns are arranged asymmetrically downstream of the vortex generator.It is assumed that the occurrence frequency of the vortex is f, the average velocity of the incoming flow of the measured medium is U, the head-on width of the vortex generator is d, and the diameter of the table body is d. According to the karman vortex street principle, the following equation can be obtained

Vortex street flowmeters can be used for a variety of fluids, that is, liquids, gases and steam.They are considered shou selections and need to be verified to meet the requirements of a particular application.

Vortex flow meter principle

Vortex flowmeters are essentially frequency meters, as they measure the frequency of eddies generated by "blunt bodies" or "disengaging rods."The vortexes will only start at a certain velocity (RE- number), so the vortexmeter will have an elevated zero point called the cut-off point.The meter output will be cut to zero before the speed becomes zero.

At a certain backflow (top cut-off point), some vortexmeters may produce an output signal, which may lead to a misinterpretation.

Vortex flowmeter is an actual volume flowmeter, such as orifice flowmeter.These intrusive instruments, like orifice meters, can cause pressure drops as the flow rate increases, leading to yong durability losses.Thus, a liquid near its boiling point will introduce cavitation when the pressure on the meter drops below the vapor pressure of the liquid.Bubbles form as soon as the pressure returns above the vapor pressure.Cavitation leading to instrument failure should always be avoided.

Characteristics -

Vortex street flowmeters provide a linear digital (or analog) output signal, eliminating the need for a separate transmitter or converter and simplifying installation.Although this range depends on operating conditions, instrument accuracy is good within the possible wide flow range.Shedding frequency is a function of the size of the resistance fluid and is a natural phenomenon, ensuring good long-term stability and repeatability of calibration over a rate of ± 0.15%.There is no drift, because this is a frequency system.

The meter features any moving or wearing components, providing higher reliability and reduced maintenance.The lack of a valve or manifold leads to leakage problems, which further reduces maintenance.The absence of a valve or manifold results in a particularly safe installation, which is an important consideration when process fluids are hazardous or toxic.

If sensors are sensitive enough, the same vortex flowmeter can be used for both gases and liquids.In addition, instrument calibration is virtually independent of operating conditions (viscosity, density, pressure, temperature, etc.), whether the instrument is used for gas or liquid.

Vortex flowmeters also have lower installation costs, especially in tubes less than 6 inches (152 mm) in diameter, and are competitive with the installation costs of orifice plates and differential pressure transmitters.

Limits include gauge size range.Instruments less than 0.5 in (12 mm) diameter are not practical, and instruments larger than 12 in (300 mm) have limited applications due to their high cost compared to orifice systems and their limited output pulse resolution.With the increase of pipe diameter, the number of pulses per cubic volume decreases with the increase of cubic law.Thus, the vortex street flowmeter with a diameter of 24 inches (610 mm) has a typical blocking rate of 0.3, with a full range frequency output of only about 5 Hertz at fluid speeds of 10 ft/s (3 m/s).

Selection and size:

As a first step in the selection process, operating conditions (process fluid temperature, ambient temperature, line pressure, etc.) should be compared with instrument specifications.The compatibility of the instrument fluid receiving material (including adhesives) and the sensor with the process fluid for chemical erosion and safety should then be checked.For example, in oxygen, nonferrous materials or extremely careful access should be avoided.The minimum and maximum flow rates of the flowmeter for a given application should then be established.

The minimum meter flow is determined by Reynolds number 10,000 to 10,500, fluid density, and minimum acceptable shedding frequency for electronic equipment.The maximum flow rate is determined by the instrument pressure loss (usually two velocity heads), the beginning of liquid cavitation, and the sound velocity flow (obstruction) of the gas.Therefore, the flow range for any application depends entirely on the working fluid viscosity, density and steam pressure, as well as the maximum flow rate and line pressure applied.For low-viscosity products, such as water, gasoline, and liquid ammonia, the maximum application speed is 15ft/s (4.6m/s), the vortex flow meter has an adjustable range of about 20:1, and the pressure loss is about 4PSIG (27.4 kpa).

The instrument's good (" rate ") accuracy and digital linear output signal make its application in a wide range of flows a practical proposition.Adjustability decreases proportionally with increasing viscosity, decreasing density, or decreasing the maximum flow rate of the method.Therefore, vortex street flowmeter is not suitable for high viscosity liquid.