What Is a Differential Pressure Flow Meter?
A differential pressure flow meter is a device that measures flow rate by measuring the pressure difference. The differential pressure transmitter is the key component in differential pressure flow measurement. Using a differential pressure transmitter, the pressure difference before and after a pipe throttling device can be measured. Then, based on the measured value, the flow rate is calculated using Bernoulli’s equation.
The measuring elements of a differential pressure transmitter are carefully divided into multiple levels, resulting in extremely small measurement errors, thus enabling accurate flow measurement.
Differential Pressure Flow Measurement System
Sometimes, people are accustomed to dividing the structure of a differential pressure flow meter into primary elements, secondary elements, and sometimes even tertiary elements. Here, we briefly explain that the differential pressure flow measurement system is mainly composed of the following parts:
- Throttling element: such as the throttling orifice plate, Annubar, etc.
- Differential pressure transmitter: detects the differential pressure value before and after the throttling element.
- Flow Totalizer: calculates the flow value based on the differential pressure. Temperature and pressure compensation can also be performed.
- Accessories: pressure pipes, valves, connectors, etc.
Advantages of DP Flow Measurement Systems
High Accuracy: Differential pressure flow meters measure flow rate by measuring the pressure difference generated as fluid flows through a pipe. They offer high accuracy, meeting precise flow measurement requirements.
Wide Applicability: Differential pressure flow meters are suitable for fluids of various viscosities, including single-phase, multiphase, clean, dirty, and viscous flows. They are also suitable for different operating conditions (e.g., normal pressure, high pressure, vacuum, normal temperature, high temperature, low temperature) and pipe diameter ranges (from millimeters to meters). Their wide range of applications is unmatched by any other type of flow meter.
Good Stability: Differential pressure flow meters have a simple structure and stable, reliable operation. They are highly adaptable to changes in fluid pressure, temperature, and other parameters, maintaining stable measurement performance over long periods.
Easy Installation: Differential pressure flow meters are relatively easy to install. They typically use flange or threaded connections, facilitating installation and maintenance.
High Reliability: Differential pressure flow meters have no moving parts. They are not easily damaged or worn, resulting in a long service life. Furthermore, it is unaffected by suspended solids, particulate matter, or other impurities within the pipeline.
Lower Cost: Compared to other flow meters, differential pressure flow meters have lower manufacturing costs. Their price is relatively suitable for large-scale applications.
Multiple Models Available: Differential pressure flow meters are available in a variety of models, including orifice plate, nozzle, venturi, and annubar types. Suitable for different flow ranges and fluid types, the appropriate model can be selected based on specific requirements.
Featured DP Transmitter for Flow Measurement
Sino-Inst produces and supplies a full range of differential pressure transmitters. Including compact type, intelligent type, high static pressure type, high accuracy type, etc.
Benefits
- Differential pressure transmitters are available in a variety of ranges for a wide range of applications;
- Even at high static pressures, differential pressure transmitters can measure small differential pressures of only a few millibars.
- An economical alternative to flow transmitters;
Featured Types of Primary Elements
Primary elements are devices that convert the flow rate of a measured fluid into a differential pressure signal; they are also called throttling devices. They include the throttling element and the pressure tapping device. Primary elements have a simple structure and can measure the flow rate of single-phase fluids under various operating conditions, including high temperature and high pressure.
However, the selection, design, and production of primary elements require rigorous calculations based on flow parameters.
Read more about: A Practical Guide to 5 Different Types of Flow Elements
Selecting a DP Transmitter for Flow Measurement
Selecting a DP Transmitter requires matching the instrument’s capabilities to the requirements of the application.
Key specifications to consider include:
- Line size: Determines full-size flow and primary component design;
- Fluid type: Liquid, gas, steam compatibility;
- Maximum working pressure: Pressure rating of valve body/flange;
- Temperature limit: Temperature limits of the primary element, sensor, and transmitter;
- Viscosity range: Higher viscosity fluids may require special considerations;
- Required accuracy and turndown ratio;
- Maximum allowable permanent pressure loss;
- Required output: 4-20mA, HART, Modbus;
- Explosion-proof certification: ATEX, IECEx, FM, etc.;
- Other factors include pipeline layout, straight pipe requirements, flow profile, and environmental conditions.
It is recommended to contact our technical engineers for confirmation when selecting a DP Transmitter for Flow Measurement. We will perform a rigorous differential pressure calculation based on your measurement parameters and issue a differential pressure calculation report, similar to the document below.
Technical Support
The working principle of a differential pressure flowmeter is primarily based on two fundamental principles of fluid mechanics: the law of conservation of mass (continuity equation) and the law of conservation of energy (Bernoulli’s equation).
Law of Conservation of Mass (Continuity Equation):
In a closed pipe, when fluid flows through a throttling device (such as an orifice plate or nozzle), although the flow velocity changes, the mass of the fluid remains constant before and after the throttling device. This principle can be expressed by the continuity equation: V1A1ρ1 = V2A2ρ2 (for liquids, the density ρ can be considered a constant, simplifying to V1A1 = V2A2). Where V1 and V2 are the flow velocities before and after the throttling device, A1 and A2 are the cross-sectional areas before and after the throttling device, and ρ1 and ρ2 are the fluid densities before and after the throttling device.
Law of Conservation of Energy (Bernoulli’s Equation):
In a closed pipe, there is a certain relationship between the pressure, velocity, and potential energy of a fluid. When fluid flows through a throttling device, the flow velocity increases due to the decrease in the flow cross-sectional area, while the static pressure decreases. There is a specific relationship between this pressure drop (i.e., differential pressure) and the flow rate, which is the basis for differential pressure flowmeters to measure flow rate.
Using Bernoulli’s equation, the pressure drop ΔP can be converted into flow rate units. This equation states that the pressure drop across a pipe is proportional to the square of the flow rate.
P1 + ½ ρv1² + ρgh1 = P2 + ½ ρv2² + ρgh2
- Different Principles
Differential pressure flow meters measure flow based on the principle of fluid throttling. When fluid flows through a throttling element in a pipe, the flow velocity locally contracts at the throttling element, increasing the velocity and decreasing the static pressure, thus creating a pressure difference before and after the throttling element. The larger the fluid flow rate, the larger the pressure difference, so the flow rate can be measured based on the magnitude of the pressure difference.
Other flow meters are based on different physical principles. For example, electromagnetic flow meters are based on the principle of electromagnetic induction. Gear flow meters measure the total volume by measuring the number of times the fluid continuously passes through a fixed-volume flow element.
- Different Applications
Differential pressure flow meters can measure various fluids, including liquids, gases, and steam.
Other types of flow meters are generally designed for measuring one type of medium. For example, electromagnetic flow meters can only measure the flow rate of conductive liquids, not gases. Oval gear flow meters can only measure liquid flow rates. Thermal mass flow meters can only measure the flow rate of dry gases.
Primary components of differential pressure flow meters can be classified by structure as follows:
Standard orifice plates, standard nozzles, classic Venturi tubes, Venturi nozzles, quarter-circle orifice plates, conical inlet orifice plates, segmental orifice plates, eccentric orifice plates, wedge orifice plates, Dor tubes, Rollos tubes, linear orifice plates, small-diameter orifice plates (integrated orifice plates), bends, annular tubes, replaceable orifice plate throttling devices, balanced flow meters, insertion Annubar tubes, etc.
When fluid fills a pipe and flows through a throttling device, the flow stream contracts, the velocity increases, and the static pressure decreases. A significant pressure difference is generated across the throttling device.
Based on Bernoulli’s equation and the fluid continuity equation, a quantitative relationship can be derived between the pressure difference and the flow rate when fluid flows through a throttling device. This relationship conforms to the fundamental equation of flow rate, which states that the flow rate is proportional to the square root of the pressure difference.
A differential pressure flow meter typically consists of a primary device (flow measurement element, such as an orifice plate) and a secondary device (display instrument). The primary device is installed in the pipe containing the fluid being measured, generating a pressure difference proportional to the flow rate. The secondary device receives this differential pressure signal and converts it into a corresponding flow rate value through calculations such as square root extraction for display.
Differential pressure flow meters are the most commonly used flow meters in the metering field for measuring the flow rate of gases, liquids, and vapors. It is estimated that approximately 75% of flow meters used in industrial production systems such as steel mills, oil refineries, and petrochemical plants are differential pressure flow meters. In the entire industrial production sector, throttling flow meters account for more than half of all flow meters.
Our engineers generally recommend choosing a differential pressure flow meter in the following situations:
- High-temperature and high-pressure fluids. Differential pressure flow meters can be customized with materials to meet high-temperature (900℃) or high-pressure measurement conditions.
- Large-diameter pipelines. In large-diameter pipelines, the cost of flow meters such as vortex and electromagnetic flow meters increases significantly with the increase in pipe diameter. Differential pressure flow meters (especially insertion Annubar) have a significant cost advantage in throttling components, offering excellent cost-effectiveness.
- High reliability and long lifespan with extended maintenance cycles.
- Limited initial investment budget with availability for future maintenance.
- The medium is a non-conductive liquid or various gases/vapors.
Based on our years of experience in flow measurement, we recommend considering alternative flow measurement methods in situations such as: differential pressure flow meters not meeting required accuracy; non-unidirectional fluid flow; large flow rate variations; and high-viscosity or highly corrosive fluid media.
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DP Transmitter for Flow Measurement can continuously and accurately generate pressure drop in the pipe. Then use the two pressure measurements to measure the flow in the pipe.
Differential pressure flow transmitters can be easily installed in a variety of applications. The differential pressure transmitters supplied by Sino-Inst support various signal outputs, communication protocols, etc. They can be compatible with the user’s industrial control system.
Sino-Inst is committed to providing users with effective tools for measuring flow. If you need to configure a differential pressure transmitter for flow measurement or have related technical questions, please contact our sales engineer!