Flow meters have their own advantages. How to select vortex flowmeters?

The vortex flowmeter installs one or more non-streamlined bluff bodies in the fluid. The fluid alternately separates and releases two series of regular vortices on both sides of the bluff body. This vortex is called Karman vortex street. Within a certain flow range, the vortex separation frequency is proportional to the average flow velocity in the pipeline. The vortex frequency can be measured by using various forms of detection elements to deduce the flow rate of the fluid.

The vortex flowmeter consists of two parts: the sensor and the converter. The sensor consists of the bluff body (i.e., vortex generator), detection element, shell, etc.; the converter consists of the preamplifier, filter shaping circuit, D/A converter, output interface circuit, etc. In addition to the basic circuit, the intelligent type also combines functional modules such as microprocessor, display, HART communication, etc. in the converter.

Flow meters have their own advantages. The purpose of selection is to select the most suitable meter from many varieties. To correctly select the flow measurement method and meter, you must be familiar with the measured object:

1. What type of fluid is the measured fluid?

(1) Is the measured fluid a liquid, gas, steam, liquid-liquid mixture, or liquid-gas mixture?

(2) Is the measured fluid clean or dirty?

(3) Does it contain wet gas flow or slurry?

(4) Is the measured fluid corrosive?

(5) Is it conductive?

2. What are the working conditions of the process? What are the temperature and pressure limit values?

(1) High temperature or low temperature?

(2) Small flow or large flow?

(3) Is there pulsating flow?

3. What are the installation conditions of the flow meter?

(1) Is the planned flow measurement in an open channel or in a closed pipe?

(2) What is the inner diameter of the pipe?

(3) What is the pipe Reynolds number of the fluid under working conditions?

(4) How long are the straight pipe sections upstream and downstream of the planned flow meter? What kind of spoiler is upstream?

(5) Is it necessary/can a flow conditioner be used?

(6) Is there excessive vibration in the process pipeline?

(7) Is the flow of the fluid steady/stable? Or pulsating?

(8) What are the ambient or indoor temperature and humidity conditions?

4. What are the general requirements for the performance and flow measurement capabilities of the flowmeter?

(1) What is the required overall accuracy? What is the required flow measurement range to ensure the above overall accuracy?

(2) Is the flowmeter used at a specific flow rate? Or is it used within a flow range?

(3) Is the flowmeter used only for flow control? If it is only for flow control, what is the necessary response frequency?

(4) What is the range of the measured flow? What are the maximum and minimum flow values that may be encountered occasionally?

After understanding the object to be measured, select the appropriate flow measurement method and instrument according to the process conditions, mainly based on instrument performance, fluid characteristics, installation conditions, environmental conditions and life cycle cost factors.

Usually, vortex flowmeters are mainly used for flow measurement of industrial pipeline media fluids, such as gas, liquid, steam and other media. So how to judge the performance of vortex flowmeters? It can be reflected by several important indicators such as its accuracy, reproducibility, sensitivity, response time, etc.

1. Accuracy

Accuracy is also called precision, which is the accuracy of the measurement result of the vortex flowmeter close to the true value. It is generally expressed by absolute error or relative error:

Absolute error = measured value - true value Relative error = absolute error / true value

Any type of flowmeter cannot measure the true value of the measured parameter absolutely accurately during measurement, and can only strive to make the measured value close to the true value. In practical applications, we generally use the indication value of a standard instrument with higher accuracy as the true value of the measured medium, and the difference between the indication value of the measuring flowmeter and the indication value of the standard instrument is the measurement error. The smaller the error value, the higher the reliability of the measuring flowmeter.

2. Repeatability

Repeatability refers to the percentage of the difference between each measured value and the average value relative to the maximum scale range when the same vortex flowmeter is used for repeated measurements under the condition that the measurement conditions such as pipe diameter, medium flow, temperature, and pressure remain unchanged. This is an important indicator of the stability of the vortex flowmeter, which generally needs to be tested when the instrument is put into operation and daily calibration.

3. Sensitivity

Sensitivity refers to the sensitivity of the vortex flowmeter measurement. The industry often uses the ratio of the change in the output of the vortex flowmeter to the change in the measured parameter that causes these changes to express it. We know that the integrated vortex flowmeter with temperature and pressure compensation function can simultaneously measure temperature, pressure, and flow, and the sensitivity of multiple parameters often makes it easier to reflect the performance of the flowmeter.

4. Response time

When the parameters of the measured medium change, the display value of the vortex flowmeter will always take a period of time to be accurately expressed. This period of time that lags behind the change in the measured parameter is the response time of the vortex flowmeter. In fact, other instruments also have response time, some of which are expressed by time constants, such as thermal resistance temperature measurement, and some are expressed by damping time, such as ammeter resistance measurement, etc.