A differential pressure is a flow measurement instrument that uses the differential pressure of a fluid to calculate the flow. Differential pressure flow measurement is widely used in most industrial processes.
This post provides you with a detailed understanding of differential pressure flow meters.
What is a Differential Pressure Flow Meter?
A differential pressure flow meter calculates flow rate based on the differential pressure generated by a restriction installed in a pipeline, known fluid conditions, and the geometry of the restriction and pipeline. A differential pressure flow meter generally consists of a restriction, a differential pressure transmitter, a flow totalizer, a temperature sensor, a pressure sensor, and a valve manifold. Differential pressure flow meters can be used to measure the flow of most liquids, gases, and steam.
- Differential pressure flow meters have a wide range of applications.
- Fluids: Single-phase, mixed-phase, clean, dirty, viscous, etc.
- Operating Environment: Normal pressure, high pressure, vacuum, normal temperature, high temperature, low temperature, etc.
- Pipe Diameter: Applicable to a wide range of pipe diameters, from a few millimeters to several meters
- Flow Conditions: Subsonic, sonic, pulsating, etc.
Advantages:
- High measurement accuracy;
- long-term stability;
- Simple structure;
- Support customization and OEM;
- Easy maintenance;
- Wide application, suitable for measuring various fluid media;
- High response speed and measurement accuracy.
Disadvantages:
- Suitable for wear due to installation and fluid impact (such as orifice plate and nozzle flow meters);
- Narrow range, generally 3:1-4:1;
- High on-site installation requirements; inaccurate installation can significantly affect measurement accuracy;
- Generates high pressure drop.
How does a Differential Pressure Flow Meter Work?
A differential pressure flow meter is based on the throttling principle of fluid flow. It measures flow rate by utilizing the pressure difference generated when the fluid flows through the throttling device. In a fluid-filled pipe, when the fluid flows through a restrictive device, the flow velocity creates a localized constriction at the device. This condition increases the flow velocity and reduces the static pressure. It creates a pressure differential across the device.
The greater the flow rate, the greater the pressure differential. This pressure differential can be used to measure flow rate. This measurement method is based on the flow continuity equation and Bernoulli’s equation. The differential pressure signal generated by the restrictive device is converted to a corresponding standard signal by a differential pressure transmitter. This signal is transmitted to a flow totalizer, and the flow totalizer calculates the flow rate using a formula.
Differential Pressure Flow Meter Formula
Diagnostic pressure flow meters typically measure volume flow rate. But it is possible to calculate the mass flow rate from the volume flow rate. Flow rate can be expressed as mass flow and volume flow. You can determine mass flow and volume flow based on your application scenario or required data.
Volume flow is expressed in units such as m³/s or l/min. Mass flow rate, on the other hand, uses units such as kg/s or g/min to describe the rate at which a specific mass is moved. Calculating volume flow using a differential pressure flow meter:
Q=C*A*√(2ΔP/ρ)
Where
- Q is the flow rate, expressed in units of volume/time;
- C is the flow meter’s flow coefficient. It represents the actual flow rate per unit differential pressure;
- A is the cross-sectional area of the pipe through which the fluid flows;
- ΔP is the differential pressure.
- ρ is the density of the fluid.
It should be noted that the calculation formula for a differential pressure flow meter is theoretical. In actual use, certain corrections and coefficients must be considered. The specific corrections and coefficients should be determined based on the specific flow meter parameters and operating conditions. If you need to measure mass flow, multiply the volume flow rate by the density to obtain the mass flow rate.
Differential Pressure Flow Meter Structure and Types
A differential pressure flow meter consists of a standard throttling device, a pressure guide tube, a differential pressure transmitter, and a flow totalizer. The standard orifice plate throttling device is the most commonly used. Therefore, we will use an orifice plate flow meter to analyze the functions of different structures. The pressure guide tube transmits the differential pressure signal to the differential pressure transmitter.
A differential pressure transmitter converts the differential pressure signal into a standard output signal. In extreme environments, pressure and temperature sensors can be added to the piping of the differential pressure transmitter. Connecting the pressure and temperature sensors to the flow meter allows for temperature and pressure compensation. It results in more accurate measurement results.
We can classify differential pressure flow meters according to different principles, results and applications.
Principal types include
- Throttling type;
- Dynamic pressure type;
- Hydraulic resistance type;
- Centrifugal type;
- Dynamic pressure boost type;
- Jet type.
Structural type includes:
- Standard orifice plate;
- Standard nozzle;
- Classic Venturi tube;
- Standard Venturi nozzle;
- Segmented orifice plate;
- Wear-resistant orifice plate;
- Annular orifice plate;
- Conical inlet orifice plate.
Application Scenario includes:
- Standard throttling device;
- Dirty flow throttling device;
- Low Reynolds number throttling device;
- Low pressure loss throttling device;
- Wide range throttling device;
- Small pipe throttling device;
- Critical flow throttling device, etc.
Sino-Inst Featured Differential Pressure Flow Meters
Differential Pressure Flow Meter Installation
Sino-Inst has summarized the following precautions to note when installing a differential pressure flow meter. Let’s take the installation of an orifice plate flow meter as an example.
1. The straight pipe section before and after the throttle must not be bent.
2. When installing the throttle, the straight pipe section should be smooth. If it is not smooth, the flow coefficient should be multiplied by the roughness correction factor.
3. To ensure a fully developed turbulent velocity distribution 1D before the throttle. And to ensure a uniform, axisymmetric distribution of the fluid flow, the following requirements must be met:
1) The straight pipe section must be circular. The roundness requirements for the 2D section before the throttle are very strict, with specific roundness specifications.
Specific measurement method:
a. Measure at least four single values of pipe diameters at equidistant angles in four vertical pipe sections before the throttle: d/2, d, and 2d. Take the average value, d. The difference between any single internal diameter measurement and the average value must not exceed ±0.3%.
b. After the throttling device, measure eight single internal diameter measurements at the 0D and 2D locations using the above method. The maximum deviation of any single measurement from D must not exceed ±2%.
2) Long straight pipe lengths are required before and after the throttling device. This is related to the shape of the local resistance plate before the throttling device and the diameter ratio β.
3) The straight pipe length between the resistance plate upstream of the throttling device and the second resistance plate can generally be 1/2 of the listed value. It is based on the economic second development resistance plate shape and β = 0.7 (regardless of the company’s actual β value).
4. When the upstream of the throttling device is an open space or container with a diameter ≥ 2D, the straight pipe length between the open space or large container and the throttling device should be no less than 30D (15D). If there are other local resistance components between the throttling device and the open space or large container, the minimum straight pipe length between the throttling device and the local resistance component, and the straight pipe length between the open space and the throttling device, must not be less than 30D (15D).
5. Connect the signal and power cables correctly.
6. Open the inlet and outlet valves. Ensure they open to the same degree.
7. Open the three-valve manifold balancing valve and slowly open the high- and low-pressure valves on the orifice plate. The fluid can flow through one flow meter before closing and closing the stainless steel three-valve manifold balancing valve.
8. The straight pipe section requirement is generally 10D followed by 5D.
Therefore, when purchasing an orifice flow meter, it is important to select one suitable for the site’s application. During installation, follow the product’s operating manual. If you encounter any problems, consult the supplier and relevant professionals.
Differential Pressure Flow Meter Maintenance
Diagnostic differential pressure flow meters require maintenance during use. Regular maintenance helps maintain measurement accuracy and extend the life of the device. You can refer to the following maintenance procedure.
Regularly inspect the appearance and operating condition of the differential pressure flow meter. This includes checking for leaks, loose electrical connections, and dust accumulation. Check the power supply and connecting cables of the differential pressure transmitter for proper function. Ensure there are no broken or corroded wires to ensure accurate measurement and safety.
Regularly clean the instrument case and connecting components. To prevent dust and dirt from affecting measurement accuracy. Components such as the impulse piping and throttling devices must be regularly inspected and cleaned to prevent blockage or leaks.
Regularly calibrate and verify that the differential pressure flow meter meets design accuracy. During calibration, carefully check that the throttling device’s installation position, orientation, and orifice plate dimensions meet standards.
When used in cold regions or during winter, take antifreeze measures to prevent the differential pressure gauge and the hydraulic fluid in the impulse piping from freezing.
How accurate is a differential pressure flow meter?
Differential pressure flow meters have an average accuracy level among flow meters. In general, their accuracy ranges from 0.5% to 1.0%. But the specific value can be affected by various factors.
How to calculate gpm from differential pressure?
Pressure is the driving force for fluid flow in any flow system and is measured in GPM (gallons per minute). Calculating GPM based on differential pressure is based on the measurement principle of a differential pressure flow meter.
The core formula for this calculation is:
Q = k × √(ΔP / ρ)
Where:
Q is the volumetric flow rate;
ΔP is the measured differential pressure;
ρ is the fluid density;
k is the meter factor or flow coefficient. It is determined by the calibration of the specific flow meter.
What are the methods of measuring flow?
There are some methods to measure flow. The most common method is using a flow meter. We can use many devices to measure flow, including electromagnetic flow meters, differential pressure flow meters, and ultrasonic flow meters.
In short, there are many different types of differential pressure flow meters. This article provides a general understanding of differential pressure flow meters. We hope this article is helpful.
Sino-Inst supplies many differential pressure flow meters. We have over two decades of experience in flow measurement. If you need to purchase a differential pressure flow meter, please feel free to contact us. We will provide you with the most cost-effective service. We will not raise prices due to tariffs. We will protect the interests of our customers. We provide a one-year warranty. You can buy with complete confidence.
