Selection of Flow Meters

Compared with other process measuring instruments, there are many kinds of flow instruments. According to statistics, there are more than 100 kinds of flow instruments applied in the industry. Therefore, the selection of flow instruments is complex work, which requires engineers to have good process measurement knowledge and rich experience and be familiar with the types and principles of commonly used instruments.

Type and selection principle of flow instrument

Classification of flow instruments

According to different principles, flow meters have different classification methods. From the perspective of instrument selection, it is reasonable to classify flow instruments according to the measurement principle. According to the flow measurement principle, the flow meters commonly used in industry can be divided into 11 types as shown in Table 1.

Tab.1 Classifications of flowmeters

Selection principle of flow instrument

Before the selection of flow meters, it is the first problem to determine whether flow meters are needed. Because in many cases, users only need to know whether the media flow is fast or slow or needs a high flow alarm (and/or low flow alarm). In these situations, the choice of flow indicator or flow switch can meet the demand at a lower cost. The flowmeter is only needed when the accuracy is required to be higher (usually no more than 5%) or the flow signal is involved in the control.

After the flowmeter is determined to be used, the flowmeter can be selected. One of the basic methods for flowmeter selection is the exclusion method, that is, excluding the types of instruments that do not meet the measurement requirements. In the first step, the flowmeter shall be preliminarily selected according to the type of fluid to be measured (liquid, gas, or mixed fluid), as shown in Table 2.

Tab.2 Application range of flowmeters

After preliminary selection, in-depth analysis and comparison shall be conducted according to the following five factors to eliminate inappropriate instrument types and finally determine the appropriate flowmeter type.

1) Instrument performance

In terms of instrument performance, the main factors to be considered include accuracy, repeatability, linearity, range ratio, pressure loss, output signal characteristics, and response time. The inspection of instrument performance is based on the measurement requirements of the process, such as the high precision requirements for trade handover measurement and the small pressure loss requirements for large-diameter pipeline measurement. The performance of various types of instruments can be obtained by consulting the instrument selection manual. It should be noted that the instruments are constantly updated. Therefore, when selecting instruments, it is necessary to ensure that the instrument performance parameters obtained are up to date.

2) Medium characteristics

The characteristics of the measured medium have different effects on all types of flowmeters. The characteristic parameters of commonly used media can generally be found in the relevant manuals and then compared with the specification parameters of the flowmeter, it can preliminarily determine which instruments can be used. During the selection, the main factors to be considered include density, temperature, pressure, specific gravity, viscosity, isentropic index, compressibility, conductivity, specific heat capacity, thermal conductivity, chemical and mechanical properties of the fluid, etc.

3) Installation and maintenance

Different types of instruments have different requirements for installation. Some instruments need a long straight pipe upstream, while some instruments have very low requirements for the straight pipe upstream. The main factors to be considered include installation direction, medium flow direction, upstream and downstream straight pipe sections, pipe diameter, vibration impact, valve position, electrical connection, accessories (such as filters, voltage stabilization compensation devices, rectifiers, etc.), explosion hazard areas, unstable flow conditions, etc.

4) Environmental conditions

Environmental conditions mainly include environmental temperature, humidity, atmospheric pressure, safety factors, electromagnetic interference, etc.

5) Economic conditions

All costs of flow instruments include purchase cost, accessory cost, installation cost, operation cost, maintenance cost, calibration cost, spare parts cost, etc. When considering economic conditions, we should consider all kinds of factors, not just the purchase cost.

Practical application analysis

In the low-temperature multi-effect distillation seawater desalination device, the main medium for flow measurement includes steam, seawater, and freshwater. In the following, the flowmeter selection analysis is carried out for these three media.

Flow measurement of water vapor

Medium: saturated water vapor; Pipe diameter: 750 mm; Operating temperature: 153 ℃; Operating pressure: 0.6MPa (A); Normal flow: 81200 kg/h; Accuracy requirement: 2.5%; Limit of straight pipe section: ＜ 30 D; Cost limit: None.

According to the steps of flow instrument selection introduced in Part 1, the analysis is as follows:

Step 1 Conduct the primary selection of flowmeter according to Table 2, remove the inappropriate type, and determine the available flowmeter types as Group 1, 2, 4, 5, and 10.

Step 2 Inspect the performance of the instrument. Since the water vapor flow involves trade handover, the accuracy of the flowmeter is not less than 2.5%; Since the downstream of the flowmeter is a steam ejector, the pressure loss of the flowmeter should be as small as possible to ensure the inlet steam pressure. Therefore, it is determined that the available flowmeter types are Group 1, 2, 4, and 5. On the premise of meeting the measurement accuracy, the pressure loss of the flowmeter has a great impact on the model selection, so the selected flowmeter is ranked as follows from the aspect of instrument performance: 1. Group 2, 2. Group 5, 3. Group 4, 4. Group 1.

Step 3 Inspect the medium characteristics, mainly the calculation results of Reynolds number, and determine the available flowmeter types as Group 1, 2, 4, and 5.

Step 4 Inspect the installation and maintenance, mainly focusing on the restriction of the straight pipe length required by the flowmeter on the installation. After comparison, the first, second, fourth, and fifth groups still meet the installation and maintenance requirements.

Step 5 Check the environmental conditions, and whether the candidate instruments can meet the requirements of environmental conditions.

Step 6 Examine the economic conditions. The main influencing factors of the economic conditions are the purchase cost, installation cost, and calibration cost. After comparison, the selected flow meters are ranked as follows: 1. Group 1, 2. Group 2, 3. Group 5, 4. Group 4.

Since the measurement accuracy of the plug-in flowmeter (vortex street and turbine) is greatly affected by the installation of the instrument, according to the investigation of other devices that have been put into operation, the measurement accuracy of the plug-in flowmeter (vortex street and turbine) for steam measurement is not ideal. As there is no strict limit on instrument purchase cost in this project, plug-in turbines and plug-in vortex flowmeters are excluded from the list to be selected. Based on the above analysis, it is finally concluded that the flowmeter is arranged as follows: 1. Group 2 (averaging tube), 2. Group 5 (vortex street), 3. Group 1 (orifice plate, nozzle). According to HG/T20507, the above three groups of flow meters shall be equipped with temperature and pressure compensation.

Flow measurement of brine

Medium: salt water; Pipe diameter: 450 mm; Operating temperature: 68 ℃; Operating pressure: 0.28 MPa (A); Normal flow: 450 m3/h; Accuracy requirement: 1.5%; Limit of straight pipe section: ＜ 10 D; Cost limit: None.

Step 1 Conduct a preliminary selection of flow meters according to Table 2, remove inappropriate types, and confirm that except for the ninth group, the other nine groups of flow meters can meet the requirements.

Step 2 Inspect the performance of the instrument. The measurement of brine flow does not involve metering but involves in the level control of the evaporator, so there are certain requirements for accuracy. For the low-temperature multi-effect distillation desalination unit, energy consumption is an important indicator. From the perspective of energy conservation, the pressure loss of the selected flowmeter should be as small as possible. After comparison, the selected flow meters are ranked as follows: 1. Group 6, 2. Group 7, 3. Group 2, 4. Group 1.

Step 3 Inspect the medium characteristics, mainly the calculation results of the Reynolds number, and determine that the available flowmeter types are Group 1, 2, 6, and 7.

Step 4 Inspect the installation and maintenance because the installation position of the seawater flowmeter is located on the pipe rack below the equipment. In order to facilitate the daily maintenance of the flowmeter, the flowmeter that the sensor and transmitter can be installed separately should be selected. At the same time, the restriction of the straight pipe length required by the flowmeter on the installation shall be investigated. After investigation, groups 1, 2, 6, and 7 meet the requirements of installation and maintenance.

Step 5 Check the environmental conditions, and whether the candidate instruments can meet the requirements of environmental conditions.

Step 6 Examine the economic conditions. The main influencing factors of the economic conditions are the purchase cost, installation cost, and calibration cost. After comparison, the selected flow meters are ranked as follows: 1. Group 7, 2. Group 2, 3. Group 6, 4. Group 1.

Since the measurement accuracy of a plug-in flowmeter (electromagnetic) is greatly affected by instrument installation, it is not recommended to use a plug-in flowmeter (electromagnetic) to ensure the accuracy of measurement results under the premise of sufficient funds. Ultrasonic flowmeter (Doppler method) is greatly affected by pipeline vibration, on-site installation, and other factors in use, so it should be comprehensively considered when selecting the type. Based on the above analysis, it is finally concluded that the flowmeter is arranged as follows: 1. Group 6 (electromagnetic), 2. Group 7 (Doppler method), 3. Group 2 (averaging tube), 4. Group 1 (Venturi tube).

Flow measurement of product water

Medium: water; Pipe diameter: 600 mm; Operating temperature: 40 ℃; Operating pressure: 0.5 MPa (A); Normal flow: 1450 m3/h; Conductivity: ＜ 10 μ s； Accuracy requirement: 1.5%; Limit of straight pipe section: ＜ 10 D; Cost limit: None.

According to the steps of flow instrument selection introduced in Part 1, the analysis is as follows:

Step 1 Conduct a preliminary selection of flow meters according to Table 2, remove inappropriate types, and confirm that except for the sixth and ninth groups, the other eight groups of flow meters can meet the requirements.

Step 2 Inspect the performance of the instrument. Since the measurement of product water flow involves trade handover, the measurement accuracy is required to be no less than 1.5%. In addition, as with seawater flow measurement, the pressure loss of the selected flowmeter should be as small as possible from the perspective of energy conservation. After comparison, the selected flow meters are ranked as follows: 1. Group 7, 2. Group 5, 3. Group 4, 4. Group 1.

Step 3 Inspect the medium characteristics, mainly the calculation results of the Reynolds number, and the candidate instruments can meet the requirements.

Step 4 Inspect the installation and maintenance because the product water flow meter is installed on the pipe rack below the equipment. To facilitate the daily maintenance of the flowmeter, select the flowmeter that can be installed separately from the sensor and transmitter; At the same time, the restriction of straight pipe length required by the flowmeter on the installation shall be considered. After investigation, the candidate instruments meet the requirements of installation and maintenance. 

Step 5 Check the environmental conditions, and whether the candidate instruments can meet the requirements of environmental conditions.

Step 6 Examine the economic conditions. The main influencing factors of the economic conditions are the purchase cost, installation cost, and calibration cost. After comparison, the selected flowmeters are ranked in the order of cost from low to high as follows: 1. Group 7, 2. Group 1, 3. Group 5, 4. Group 4.

Based on the above analysis, it is finally concluded that the flowmeter is ranked as 1. Group 7 (propagation velocity difference method), 2. Group 5, 3. Group 4 (turbine), 4. Group 1 (venturi, nozzle).

Conclusion