【2025 Flow Meters】Pressure drop requirements for seven types of flow meters and selection strategies

The following reply is not text content and is for reference only. Flow measurement instruments can be classified into various types based on their working principles. For example: 1 Differential pressure flow meters (such as orifice plate flow meters and venturi tube flow meters): These measure flow by utilizing the relationship between the pressure difference generated as fluid passes through a throttling element and the flow rate. 2 Positive displacement flowmeters (such as gear flowmeters and scraper flowmeters): The flow rate is determined by measuring the number of times fluid passes through a space of known fixed volume per unit time. 3 Velocity-type flow meters (such as turbine flow meters, electromagnetic flow meters, ultrasonic flow meters): They measure the flow velocity of the fluid in the pipeline directly or indirectly, thereby calculating the flow rate. 4 Mass flow meters (such as thermal mass flow meters, Coriolis mass flow meters): They can directly measure the mass flow rate of a fluid, unaffected by changes in the physical properties of the fluid. Furthermore, flow measurement instruments can be distinguished based on their functions as either instant flow measurement or cumulative flow measurement: the former reflects the flow rate at a specific moment, while the latter calculates the total amount of fluid that has passed through over a certain period of time.

The working principle of a rotameter is based on the balance between Archimedes’ buoyancy and force. It mainly consists of a conical tube and a rotor that can float up and down inside the conical tube. As the fluid flows in from the bottom of the conical tube and moves upward, the rotor is pushed upward by the impact of the fluid. Depending on the fluid flow rate, the buoyant force acting on the rotor also changes. When the buoyant force is in balance with the rotor’s own weight and the damping force generated by fluid resistance, the rotor stabilizes at a certain position. This position is directly proportional to the flow rate through the pipe; in other words, the greater the flow rate, the higher the rotor rises. Inside the rotor, there is usually a permanent magnet; changes in its height and position can be detected by an external indicator such as a pointer, and transmitted through magnetic coupling to display the current flow rate. Smart rotameters also employ microprocessor control technology, using dual Hall sensors to detect the magnetic field that changes with the rotor’s position, and to measure its vertical and horizontal components. These data are processed and compared with a preset magnetic field calibration table; the instantaneous flow rate is then converted into an actual flow value through interpolation, displayed on the screen, and a standard signal (such as 4-20mA) may be output for remote monitoring or control.

The working principle of the electromagnetic flowmeter (EMF for short) is based on Faraday’s law of electromagnetic induction. Specifically: inside the electromagnetic flowmeter, there is a measurement tube made of non-magnetic material, whose inner wall is coated with an insulating material as a lining to ensure good electrical insulation between the fluid and the measurement tube. An electromagnetic coil is placed on each side of the measuring tube; when electric current flows through these coils, a stable magnetic field is generated around them, and this magnetic field is perpendicular to the axis of the measuring tube. When a conductive fluid (such as water, acid or alkali solutions, slurries, etc., with a conductivity greater than 5 μs/cm) passes through this magnetic field, an electromotive force (E) is induced as the charged particles in the fluid cut through the magnetic field lines; the magnitude of this force is proportional to the flow velocity of the fluid. There are two electrodes in contact with the fluid inside the pipe, which are used to detect this induced electromotive force. This electromotive force is transmitted to the converter section of the instrument. The converter amplifies and filters the weak signals detected, and converts the induced electromotive force into the corresponding flow rate value using pre-defined calculation formulas. Ultimately, the flow meter indicates the volumetric flow rate or mass flow rate of the fluid flowing through the pipeline (after density compensation).

An electromagnetic flowmeter is a device that determines fluid flow by measuring the induced electromotive force generated when a conductive fluid moves in a magnetic field.

A vortex flow meter is a flow measurement instrument that operates on the principle of the Karman vortex street. Its working principle is as follows: Inside a vortex flow meter, there is usually a flow obstructing element of a specific shape (such as a triangular prism or cylinder); this element is known as a vortex generator and is installed vertically on the centerline of the fluid pipeline. When fluid flows past a vortex generator, the separation effects caused by the fluid flowing around the non-streamlined object result in a series of regular vortices being generated alternately on both sides of the vortex generator. These vortices form what is akin to a vortex street, which is why it is called a vortex street flow meter.

A vortex flow meter is a device that indirectly measures fluid flow by detecting the frequency of vortices generated by a vortex generator, and it is suitable for measuring the flow rate of various gases, liquids, and vapors.

The selection of flow measurement instruments is a process that involves considering multiple factors. The following are some basic principles and steps for making such selections: 1 Fluid properties: temperature, pressure, density, viscosity, chemical corrosivity, abrasiveness, tendency to form scale, presence of mixed phases or phase changes, conductivity (important for electromagnetic flowmeters), sound speed (relevant to ultrasonic flowmeters), thermal conductivity, and specific heat capacity (relevant for heat flowmeters), etc. 2 Measurement requirements: Determine whether it is necessary to measure the instantaneous flow rate or the cumulative flow rate, or both. Select the instrument based on the required level of accuracy to ensure that it meets the precision requirements for process control or trade handover. Determine the flow range and select a meter with an appropriate span (range ratio) to ensure accurate measurement throughout the entire operating range. 3 Instrument performance: Consider factors such as the instrument’s accuracy, repeatability, linearity, response time, and pressure loss. Select the appropriate type of instrument based on the characteristics of the output signal, such as analog signals (4-20mA), pulse signals, or digital communication interfaces (such as HART, MODBUS, PROFIBUS, etc.). 4 Installation conditions: The orientation of the pipes, the flow direction, and the length of the straight sections upstream and downstream have a significant impact on the performance of flow meters, especially for differential pressure flow meters, vortex flow meters, etc. Ensure there is sufficient installation space, and take into account the pipe diameter, ease of maintenance, as well as any auxiliary equipment that may be required, such as filters and air eliminators. 5 Environmental conditions: Environmental temperature, humidity, electromagnetic compatibility, safety, explosion protection rating, resistance to pipeline vibration, and so on are all factors that need to be considered when making a selection. 6 Specific application scenarios: Choose the appropriate type of flow meter based on the specific situation; for example, for media that tend to crystallize or are viscous, electromagnetic flow meters or positive displacement flow meters may be suitable. For clean gases or liquids, turbine flowmeters, vortex flowmeters, or ultrasonic flowmeters may be better choices. For steam, vortex flow meters, V-cone flow meters, or orifice flow meters combined with differential pressure transmitters may be chosen. For large-diameter pipes in situations with limited installation space, Anubaa flow meters or venturi tubes may be an ideal solution. The selection of flow meters is a systematic task that requires detailed analysis and evaluation taking into account various factors such as specific process parameters, operating conditions on site, and economic benefits. In practical applications, it is also necessary to consult the detailed product specifications and technical manuals provided by the manufacturer to ensure that the selected flow meter is suitable and meets the project requirements.

Electromagnetic flowmeter: Application scenarios: It is suitable for measuring various conductive liquids, including water, wastewater, corrosive fluids, etc.; it is not suitable for measuring non-conductive liquids (such as petroleum products) or gases. Selection guide: It is determined based on factors such as the conductivity of the fluid to be measured, maximum flow rate, minimum flow rate, fluid pressure, temperature, and pipe material.

Float flow meter (rotary flow meter): Application scenarios: Suitable for measuring small volumes of liquids, gases, and steam with small diameters and low flow rates. Selection guidelines: Consider the physical properties of the fluid (density, viscosity, etc.), operating pressure and temperature range, as well as flow rate range and accuracy requirements.

Vortex flow meter: Application scenarios: Widely used for measuring large flow rates of gases, liquids, and steam in industrial pipelines; it is particularly suitable for applications where pressure loss is low and frequent maintenance is not required. Selection guidelines: Pay attention to the properties of the fluid, flow rate range, pressure rating, and pipeline vibration; also be careful to avoid using it in environments with severe vibration.