Working principle of differential pressure flow meter

The differential pressure flowmeter is based on the throttling principle of fluid flow, and uses the pressure difference generated when the fluid flows through the throttling device to achieve flow measurement. It is one of the most mature and commonly used methods for measuring flow in production. It is usually composed of a throttling device that can convert the measured flow into a pressure difference signal, a differential pressure gauge that can convert the pressure difference into a corresponding flow value for display, and a display instrument. In the unit combination instrument, the pressure difference signal generated by the throttling device is often converted into a corresponding standard signal (electrical or pneumatic) through a differential pressure transmitter for display, recording or control.
                                                                                                                                                                                                       

1 Application Overview

For example, in terms of fluid: single phase, mixed phase, clean, dirty, viscous flow, etc.; in terms of working conditions: normal pressure, high pressure, vacuum, normal temperature, high temperature, low temperature, etc.; in terms of pipe diameter: from a few mm to a few meters; in terms of flow conditions: subsonic, sonic, pulsating flow, etc. Its usage in various industrial sectors accounts for about 50%~60% of the total usage of flow meters (about one million units per year).

2. Instrument calibration

The calibration of a pair of differential pressure flowmeters are:

⑴Geometric test method

⑵ Coefficient verification

KERNB Annubar flow meter

For details about the geometric verification method, please refer to the introduction of the verification procedures, which will not be repeated here.

The following is a brief introduction to the relevant calculation formulas and methods involved in the coefficient verification method

Second calculation formula

v=aA√2/j（p-q）

v--volume

j--Liquid density

a--Flow coefficient, which is related to the flow channel size, pressure taking method and flow rate announcement

A--Orifice opening area

pq--pressure difference

Formula for calculating the orifice plate outflow coefficient C (edited by the author)

The Reynolds number can be calculated according to the relevant formula, such as:

Common calculation formula for Reynolds number eDR

9 The basic error limits of the sensor should comply with the requirements of Table 8

Measurement accuracy: ±0.2% ±0.5% ±1.0%

Repeatability: ±0.2%

Sensor error requirements

The above verification information is for reference by users who have verification devices. If there is any violation with the verification regulations of JJG640-1994,

The regulations shall prevail.

There are many types of differential pressure flow meters. The differential pressure flow meters commonly used in the market are: Bitobar flowmeter, orifice flowmeter, V-cone flowmeter, Annubar flowmeter, Vidibar flowmeter, Diamond bar flowmeter, Deltabar flowmeter, porous balanced flowmeter, etc.

3 Development

The development of flow measurement can be traced back to ancient water conservancy projects and urban water supply systems. In the era of Caesar in ancient Rome, orifice plates were used to measure the amount of drinking water for residents. Around 1000 BC, ancient Egypt used the weir method to measure the flow of the Nile River. In the 17th century, Torricelli laid the theoretical foundation for the differential pressure flowmeter, which was a milestone in flow measurement. Since then, the prototypes of many types of flow measurement instruments began to take shape in the 18th and 19th centuries, such as pitot tubes, venturi tubes, volumetric, turbine and target flowmeters. In the 20th century, the rapid growth of demand for flow measurement in process industries, energy metering, and urban utilities led to the rapid development of instruments. The leapfrog development of microelectronics technology and computer technology has greatly promoted the replacement of instruments, and new flowmeters have sprung up like mushrooms after rain. So far, it is said that hundreds of flowmeters have been put on the market, and many difficult problems in field use are expected to be solved.

Differential pressure flowmeter is a type of flowmeter with a long application history, rich and mature practical experience, perfect standards and specifications, and a full range of varieties and specifications. Before the 1950s, it was probably the only natural gas flowmeter. It has many shortcomings: such as general measurement accuracy, high requirements for on-site installation conditions, large pressure loss, narrow range, etc. In response to the above shortcomings, domestic manufacturers have introduced a series of improvement measures in recent years, such as integrated differential pressure flowmeter, fixed value throttling device, replaceable orifice plate throttling device, standard nozzle, etc., plus the two components of differential pressure flowmeter after the 1980s: differential pressure transmitter and flow display instrument have made outstanding progress. It is estimated that this type of flowmeter will continue to occupy an important position in the future.

China started working on modern flow measurement technology relatively late, and the flow meters needed in the early days were all imported from abroad.

Flow measurement is the science of studying the change of mass. The law of mutual transformation of mass is the basic law of the connection and development of things. Therefore, its measurement object is no longer limited to the traditional pipeline liquid. Wherever the change of mass is required, there is the problem of flow measurement. Flow, pressure and temperature are listed as the three major detection parameters. For a certain fluid, as long as these three parameters are known, its energy can be calculated. These three parameters must be detected in the measurement of energy conversion. Energy conversion is the basis of all production processes and scientific experiments. Therefore, flow and pressure and temperature instruments are the most widely used.

Basic requirements for annual calibration The basic requirements that calibration should meet are as follows:

1. Environmental Conditions If the calibration is carried out in a verification (calibration) room, the environmental conditions should meet the temperature, humidity and other requirements of the laboratory. If the calibration is carried out on-site, the environmental conditions shall be based on the conditions that can meet the on-site use of the instrument.

2. The error limit of the instrument used as a standard instrument for calibration should be 1/3~1/10 of the error limit of the instrument being calibrated.

3. Although personnel calibration is different from verification, the personnel who perform calibration should also undergo effective assessment and obtain the corresponding certificate of qualification. Only certified personnel can issue calibration certificates and calibration reports based on the inspection results, and only such certificates and reports are considered valid.

Calibration can be done by local metrology institutes or third-party calibration units, which must be accredited by the China National Accreditation Service for Conformity Assessment (CNAS) laboratory. If you want to solve the problem of industrial production flow leakage detection accuracy, according to the market application feedback, you should choose KEDTB Deltabar flowmeter, KEDTB Wedibar flowmeter or KERNB Annubar flowmeter, and the matching KEDCB multi-parameter differential pressure flow transmitter can accurately measure various steam, air and liquid media, and is used in energy management control. It is the most advanced differential pressure measurement equipment at present.

4 Application composition

The differential pressure flowmeter consists of a primary device (detection part) and a secondary device (differential pressure converter and flow display instrument). Differential pressure flowmeters are usually classified by the detection part, such as orifice flowmeter, venturi flowmeter, average velocity tube flowmeter, Pitot tube principle-Pitobar flowmeter, etc.

Secondary devices include various mechanical, electronic, electromechanical integrated differential pressure gauges, differential pressure transmitters and flow display instruments. It has developed into a large category of instruments with a high degree of three-dimensionalization (serialization, generalization and standardization) and a wide variety of specifications. It can measure flow parameters as well as other parameters (such as pressure, level, density, etc.).

The detection parts of differential pressure flowmeters can be divided into several categories according to their working principles: throttling device, hydraulic resistance type, centrifugal type, dynamic pressure head type, dynamic pressure head gain type and jet type.

Test parts can be divided into two categories according to their degree of standardization: standard and non-standard.

The so-called standard test parts are designed, manufactured, installed and used in accordance with standard documents, and their flow value and measurement error can be determined and estimated without actual flow calibration.

Non-standard test parts are test parts that are less mature and have not yet been included in international standards.