How to choose a gas flow meter (counter)?

Compressed air flow meters (gas meters) are devices that show how much air has passed through a pipe over a certain period of time. They are needed for industry to monitor the operation of compressors and account for the enterprise's energy resources. Usually, such devices are installed on pipes to measure dry and wet compressed air, carbon dioxide (CO2), nitrogen (N2), argon, and other gases (non-aggressive).

Gas meter

Industrial compressed air flow meters come in various types. They differ in pipe diameter, measurement method, type of output signal, presence of a built-in recorder, and other parameters. For example, there are devices that measure based on pressure fluctuations — these are vortex flow meters. They can be used for different tasks.

Also popular are devices that are not affected by ambient temperature. These are thermal anemometric flow meters. They are characterized by accuracy, speed, and reliability. A thermal anemometric compressed air flow meter is installed at enterprises to control the flow of various gases.

Gas flow meter specifications

Among the general characteristics of gas flow meters are:

• Measurement method (thermal mass, vortex, differential pressure are often used);
• Digital or analog output signal; suitable pipeline diameter;
• Range of gases the gas meter can work with (usually compressed air, oxygen, nitrogen, helium, argon, CO2, etc.);
• Flow meter measurement channels (some equipment can measure not only the amount of compressed air passing through but also pressure and flow temperature);
• Permissible pipeline diameter.
• Capability to operate in two directions.

Additional features of modern gas flow measurement equipment include a convenient display and USB interface, the ability to connect to a network for comparing readings, generating reports, and detecting possible gas leaks.

Choosing a gas meter

The choice of a gas flow meter depends on the conditions of use and the tasks assigned to the device.

• Type of gas
• Pressure
• Temperature
• Approximate flow rate
• Installation method
• Pipeline diameter
• Data retrieval. Presence of display and type of output signal

The first thing to consider when selecting a flow meter is the characteristics of the measured medium:

• type of gas,
• pressure,
• temperature.

Then decide on the installation method of the device and take into account related parameters such as pipeline diameter. Finally, consider how data will be retrieved from the device. Let's look at all these steps in more detail.

Type of gas

When choosing a flow meter, you should immediately select devices capable of measuring the specific gas you need*. Some flow meters, such as VA 400, can measure various gases (air, nitrogen, natural gas, etc.), but for gases with physical properties significantly different from air, the devices must be calibrated in the corresponding medium.

* In the case of aggressive or explosive gas environments, flow meters with additional protection should be selected.

Pressure

Next, specify the pressure of the measured medium. Usually, different types of flow meters are used for measuring compressed air (e.g., in compressor rooms) and for measuring air flow at near-atmospheric pressure (e.g., in ventilation systems). Flow meters for ventilation (e.g., SS 20.260 LED) are significantly cheaper than compressed air flow meters (e.g., SS 20.261) because they are designed for less demanding operating conditions.

The maximum allowable pressure limit varies among different flow meters, so if you need to measure gas flow under pressure, you should clarify the working pressure of the medium. For example, the SS 20.261 flow meter can be used at pressures up to 10 bar, SS 20.600 — up to 16 bar (optionally up to 40), and VA 400 — up to 50 bar.

Temperature

Most flow meters are designed for moderate temperatures of the measured medium (for example, from -30 to +120° with SS 20.600)

Attention should also be paid to the ambient temperature. The temperature ranges for electronic components (located outside the pipeline) are usually narrower than for the sensitive element. Therefore, if the sensor is intended to be used, for example, outdoors in winter, it is necessary to ensure that the lower limit of the allowable temperature range will allow the device to withstand severe frost.

Approximate Flow Rate

All compressed air flow meters have a certain measurable flow range. When this range is exceeded, the devices stop providing reliable readings, so when choosing a device, the maximum possible flow rate at the given section should be taken into account.

In the case of thermal flow meters, the measurement range limits are set not by the volume of passing air (since for the same flow meter, the maximum allowable volumetric flow values will vary depending on the pipeline diameter), but by the flow velocity normalized to standard conditions.

Thus, the maximum allowable velocity for the SS 20.260 LED flow meter is 50 m/s, for SS 20.261 – 90 m/s, and for VA 400 – 220 m/s. It is not necessary to use a flow meter with the widest velocity range, as the larger the range, the greater the measurement error (and often the price). Therefore, it is very important to know the maximum possible flow velocity in a specific case.

Flow velocity depends, firstly, on the volume of gas passing through, i.e., the flow rate itself, and secondly, on the internal diameter of the pipeline. The greater the flow and the smaller the diameter, the higher the velocity. We will explain in more detail later why it is necessary to know the diameter of the section where the flow meter will be used when selecting a flow meter.

The approximate flow rate, if we are talking about compressed air, can be found in the compressor's technical documentation. Methods for calculating velocity based on diameter and flow rate are usually provided in the flow meter's user manual. For example, this table shows the maximum flow values for various versions of the VA 400 flow meter:

Gas Meter Installation Methods

Taking into account the characteristics of the measured medium, attention should also be paid to the installation conditions of the flow meter. Three main installation methods can be distinguished.

• Insertion flow meters. Such devices represent a ready-made small section of pipeline with a flow meter installed on it. To install such a device, it is necessary either to remove a section of the pipe and install the flow meter in its place or to mount it on a bypass pipeline. The advantage of insertion flow meters is their relatively low cost (but only if we are talking about small pipeline diameters). The disadvantage is the inconvenience of installation – insertion requires certain efforts, takes a lot of time, and, of course, requires production shutdown. In addition, insertion flow meters are not suitable for use on large-diameter pipelines. This type of flow meter includes, for example, the VA 420 device.
• Immersion flow meters. To install these devices, it is not necessary to cut out an entire section of the pipeline or install a bypass connection. Installation is carried out by drilling a small hole in the pipeline wall, inserting the flow meter rod into it, and securing the device in this position. More details about installing an immersion flow meter can be found in the corresponding article. The advantages of this type of device are ease of installation and relatively low cost. In addition, these devices can be easily used on large-diameter pipelines. For example, the rod length of some versions of the SS 20.600 flow meter allows its use in pipelines up to 2 meters in diameter. The disadvantage is that these devices are not very convenient to use on very small pipelines – for diameters of 1/2'' and less, insertion flow meters are preferable.

• Clamp-on flow meters. The operating principle of these flow meters does not require direct access to the measured medium – measurement is carried out through the pipeline wall, usually by ultrasonic method. Installation of these flow meters is the most convenient and simple, but their cost is usually several times higher than that of immersion and insertion devices, so it makes sense to use them only if there is no possibility to breach the integrity of the pipeline.

Pipeline Diameter

Regardless of whether an insertion, immersion, or clamp-on flow meter is used, it is necessary to specify the diameter of the pipeline section where the flow meter is to be installed.

When choosing an insertion flow meter, the pipeline diameter is one of the main parameters, as these devices differ by the diameter of the built-in measuring section. As for immersion flow meters, it may seem that the diameter does not matter since the probe can be immersed in the flow at any diameter, but because the sensor element (located at the end of the probe) must be placed exactly in the center of the pipeline, it is necessary to ensure that the probe length is sufficient for installation at the specific site. When calculating the minimum required probe length, it should also be remembered that part of it will be used for mounting components: union and ball valve.

Suppose the outer diameter of the pipeline is 200 mm. Then the probe must be immersed 100 mm. Another 100-120 mm is required for installation. Thus, the minimum probe length for this diameter should be 220 mm. Most flow meters are available in various versions differing in probe length. For example, the VA 400 flow meter is available with probe lengths of 120, 220, 300, and 400 mm.

Data Acquisition. Display Availability and Output Signal Type

Finally, you need to decide how you want to receive measurement results. Most flow meters use an analog or digital output signal to transmit measurement data. If the enterprise has its own automated process control system (APCS) that can accept these output signals, then an analog or digital signal will most likely be sufficient. However, if there is no ready control system, it may be necessary to read data from the display. Some flow meters (for example, the VA400) have a built-in display or offer it as an option. For other devices, a separate indicator must be purchased and the sensor output signal connected to it.

The data displayed on the screen usually includes current and accumulated flow. In some cases, it may be necessary to record data over different time intervals and process it to generate reports and present information in tabular or graphical form. If the enterprise lacks a ready control system capable of performing these functions, it makes sense to purchase a device with a built-in data logger and accompanying software that allows quick and convenient data processing. An example of such a device is the DS 400.

If the flow meter does not have a built-in display and data retrieval requires an output signal, you need to determine the type of this signal. The most common analog signals are 4…20 mA and 0…10 V. Some flow meters, such as the SS 20.600, can generate either of these signals depending on the connected resistance value. In some cases, a digital output signal may be required, for example, using Modbus or Profibus protocols.

The parameters listed above should be sufficient for selecting a flow meter. At the same time, if you want a more comprehensive understanding of different types of flow meters, as well as the advantages and disadvantages of each type, you can also read articles on the classification of flow sensors by measurement principle.