1、 Instrument Introduction:
Stress typevortex flow meterMainly used for flow measurement of industrial pipeline media fluids, such as gases, liquids, vapors, and other media. Its characteristics are small pressure loss, large range, high accuracy, and almost unaffected by parameters such as fluid density, pressure, temperature, viscosity, etc. when measuring volumetric flow rate under working conditions. No movable mechanical parts, therefore high reliability and low maintenance. The instrument parameters can remain stable for a long time. Using piezoelectric stress sensors, it has high reliability and can operate within a temperature range of -20 ℃ to+250 ℃. It has both analog standard signals and digital pulse signal outputs, making it easy to use in conjunction with digital systems such as computers. It is a relatively advanced and ideal flow meter.
2、 Working principle:
Stress type vortex flowmeterWhen a cylindrical object is vertically inserted into the measuring tube, the fluid alternately generates regular vortices on both sides of the object, which are called Karman vortex streets. The release frequency of the Karman vortex street is related to the fluid velocity and the width of the column, and can be expressed by the following equation:
In the formula f=St · v/d, f represents the release frequency of the Karman vortex street
St coefficient (known as Strouhal number)
V - Flow velocity
D - Width of the pillar
The release frequency f of the Karman vortex street is directly proportional to the flow velocity v, so the instantaneous flow rate can be calculated by measuring the release frequency of the Karman vortex street.
Strouhal number is an important coefficient of vortex flowmeter. In the straight part of the curvature with St ≈ 0.17, the release frequency of the vortex is proportional to the flow velocity, so the detection frequency f can be used to obtain the flow velocity v, and the volume flow rate can be calculated from v.
The vortex release frequency of the vortex flow sensor is detected by the alternating action of vortices on the stress on the detection sensor (probe) through piezoelectric elements inside it.
3、 Application scope:
Stress type vortex flowmeterIt has a series of advantages such as wide range ratio, high accuracy, easy installation and maintenance, and wide adaptability to media. It can be widely used in the process measurement and energy-saving management of various low viscosity single-phase fluids such as liquids, gases, steam, etc. in industries such as petroleum, chemical, metallurgy, machinery, food, papermaking, as well as urban pipeline heating, water supply, gas, etc.
4、 Advantages:
1. Adopting a low-power CPU central processing unit and LCD display, with low power consumption.
2. Easy to install, very convenient to maintain, long calibration cycle, usually two years.
3. The structure is simple and sturdy, with no movable parts, making it highly reliable for long-term operation.
4. Wide range: The normal range is 1:10, and the range ratio can reach 1:15.
5. Low pressure loss, low operating costs, and more energy-saving significance.
6. Within a certain Reynolds number range, the output signal is not affected by the physical properties and composition changes of the measured medium, and the instrument coefficient is only related to the shape and size of the vortex generator. After replacing the accessories, there is generally no need to recalibrate the instrument coefficient.
7. It can be displayed on-site, transmitted over long distances, and connected to computer control systems.
8. The detection probe does not directly contact the measured medium, resulting in more stable performance.
9. The integrated design of temperature and voltage compensation and the current output are both of the optoelectronic isolation type, which has good resistance to common mode interference suppression capability.
5、 Performance indicators:
1. Temperature range: Piezoelectric type: -25 ℃~+350 ℃ Capacitive type: -40 ℃~+450 ℃
2. Connection method: DN15~DN300 are flange card type flange connection types, DN300~DN1000 are plug-in type, DN1000 and above can be specially ordered
3. Flow rate range: Steam: 7m/s~70m/s Liquid: 0.7m/s~7m/s Gas: 5m/s~35m/s
4. Pressure specification: PN1.6MPa; PN2.5MPa； PN4.0MPa， Higher pressure specifications can be specially customized
5. Pressure loss coefficient: Cd ≤ 2.6
6. Accuracy: Flange mounted, connection type: 0.5 level, 1.0 level, 1.5 level Insertion type: 1.5 level, 2.5 level
7. Supply voltage: Sensor: DC12V or DC24V Transmission: DC24V
On site display type: DC24V, DC12V or 3.6V lithium battery
8. Output signal: Sensor: Pulse frequency signal 2~3000Hz Low level ≤ 1V High level ≥ 6V
Transmitter: two-wire 4-20 mA DC current signal
9. Anti vibration performance: piezoelectric type ≤ 0.2g, capacitive type ≤ 2.0g
10. Environmental temperature: Sensor: -30 ℃~+65 ℃ Transmitter: On site display: -10 ℃~+50 ℃
11. Environmental humidity: relative humidity of 5-8%
12. Signal transmission distance: ≤ 500m
13. Length of straight pipe section: upstream ≥ 10D, downstream ≥ 5D
14. Explosion proof grade: Exia Ⅱ CT2-T5 intrinsically safe explosion-proof
15. Protection level: Regular IP65, Submersible IP68
16. Instrument material: The instrument housing is made of aluminum alloy, and the body part is made of stainless steel 304 material. Special materials can also be used according to user requirements.
6、 Key selection points:
Proper selection is necessary to ensure better use of vortex flowmeters. What type of vortex flowmeter should be selected based on the physical and chemical properties of the measured fluid medium? What are the diameter, flow range, lining material, electrode material, and output current of the vortex flowmeter? Can adapt to the properties of the measured fluid and the requirements of flow measurement.
1. Precision functional inspection
The accuracy level and function of the instrument are selected based on measurement requirements and usage scenarios to achieve cost-effectiveness. For example, in situations such as trade settlement, product handover, and energy measurement, higher accuracy levels such as 1.0, 0.5, or higher should be selected; For process control applications, choose different accuracy levels according to control requirements; In some cases where only the process flow is detected without the need for control and measurement, a slightly lower accuracy level, such as 1.5, 2.5, or even 4.0, can be selected. In this case, a low-cost plug-in vortex flowmeter can be used.
2. Measurable medium
When measuring medium flow rate, instrument range, and caliber for general medium measurement, the full flow rate of the vortex flowmeter can be selected within the range of 0.5-12m/s for measuring medium flow rate, which is relatively wide. The selection of instrument specifications (caliber) may not necessarily be the same as the process pipeline, and should be determined based on whether the measured flow range is within the flow rate range. That is, when the pipeline flow rate is too low to meet the requirements of the flow instrument or the measurement accuracy cannot be guaranteed at this flow rate, the instrument port diameter needs to be reduced to increase the flow rate inside the pipeline and obtain satisfactory measurement results.
3. Selection of Vortex Flow Transmitter
In saturated steam measurement, VA type piezoelectric vortex flowmeter is used. Due to the wide range of the vortex flowmeter, in practical applications, it is generally considered that the flow rate of saturated steam should not be lower than the lower limit of the vortex flowmeter, which means that the fluid flow velocity must not be lower than 5m/s. Choose vortex flow transmitters with different diameters based on the amount of steam used, rather than selecting the transmitter diameter based on the existing process pipeline diameter.
4. Selection of pressure compensation pressure transmitter
Due to the long saturated steam pipeline and significant pressure fluctuations, pressure compensation must be used. Considering the correspondence between pressure, temperature, and density, only pressure compensation is required for measurement. As the saturated steam pressure in the pipeline is in the range of 0.3-0.7MPa, the range of the pressure transmitter can be selected as 1MPa.
5. Display instrument selection
Intelligent flow display instrument with voltage stabilization compensation, instantaneous flow display, and cumulative flow calculation functions.
7、 On site requirements:
1. Low maintenance volume - Most vortex flowmeters on the market use pressure tapping holes or inserted detection elements to sense vortices. Once impurities in the medium are embedded in the pressure tapping holes or gaps between the sensing element and the meter body, the signal becomes weak or unstable. A good design should have no easily clogged parts, thereby reducing maintenance.
2. Online replacement of sensing elements - Some manufacturers' sensing elements are combined with vortex generators, which may seem simple in design but bring inconvenience to practical use. Because once the sensing part fails, the medium inside the pipeline needs to be emptied and depressurized before replacing the components, which affects production. A perfect design should separate the two, so that the sensing part can be replaced separately without emptying the medium.
3. Vortex flow meters are susceptible to vibration interference, and well-designed vortex flow meters can eliminate interference through hardware and digital signal processing, thereby obtaining stable signals.
4. The installation of a vortex flowmeter requires a long straight pipe section, and some manufacturers can provide a design for reducing the diameter inside the flowmeter, greatly reducing the need for users to equip vortex flowmeters with straight pipe sections.
8、 Issues to be noted during debugging and use:
1. The installation and maintenance of flow meters on site must comply with the warning message 'Do not open the cover when there is explosion-proof gas', and turn off the external power before opening the cover.
2. When using the vortex flowmeter outdoors, it is recommended to add a protective cover.
3. Do not loosen the fixed part of the flowmeter at will. Protection of pressure sensors. When using a flowmeter, users should note that the pressure overload value of the flowmeter is 1.5 times the rated working pressure of the pressure sensor (i.e. 1.5 times the highest pressure of the medium). The selection should be within the specified flow range to prevent overspeed operation, in order to achieve ideal accuracy and ensure normal service life. (Due to overspeed caused by pressure testing, pipeline purging, or exhaust, as well as turbine operation in reverse flow, the flow meter may be damaged)
4. When the vortex flowmeter is put into operation, the valve should be slowly opened to prevent damage to the turbine caused by instantaneous airflow impact. Lubricating oil should be added according to the signs, and the frequency of refueling depends on the cleanliness of the gas, usually 2-3 times a year.
5. It is not allowed to open the back cover or change the internal parameters of the vortex flowmeter during operation, otherwise it will affect the normal operation of the flowmeter.