All instruments that measure fluid flow are referred to flow meters or flowmeters. The mass or volume of a fluid flowing through a cross-section of a pipe or facility per unit time is called flow rate.
Gas flow meters generally reflect gas flow by gas volume measurement.
To implement the measurement energy saving service project, we should give full play to the technical support role of flow meter measurement. Gas measurement, especially the accurate measurement of expensive gas, is used in custody transfer, pipeline network transmission and distribution management and monitoring of gas equipment. It is a necessary means to reduce energy consumption, reduce operating costs, and eliminate conflicts between settlement parties.
The main gas flow measurement instruments are classified in terms of working principle:
① Velocity flowmeter;
② Positive displacement flowmeter;
③ Differential pressure flowmeter;
④ Mass flow meters and other categories.
Velocity flow meters include turbine gas flowmeters, vortex flowmeters, swirl flowmeters and ultrasonic flowmeters that have developed rapidly in recent years. With the development of large pipeline flowmeters, ultrasonic flowmeters have been used more and more for their features of no moving parts and no pressure loss. Gas Turbine flowmeters are often used to measure natural gas, CO2, air .etc. The differential pressure flowmeter and vortex flowmeter are more used for the measurement of steam flow or natural gas flow measurement. Velocity flow meters are digital gas flow meters with current 4-20mA output or pulse output, digital display to show gas instant flow and total flow; they are also possible with Modbus or HART protocol.
Volumetric flow metering instruments include traditional membrane gas meters, waist wheel flowmeters, wet gas flowmeters, etc.
The differential pressure flowmeter with the throttling device as the detection part is a kind of flowmeter with a long history, rich theoretical and practical data, and widely used in many applications. Because of its simple structure, convenient installation, reliable operation, low price cost and certain accuracy. It is widely used in the flow measurement of large and medium diameter pipes.
Mass flow meters can directly obtain gas mass flow; they are more used in the measurement, detection and control of production process parameters.
Variable area flow meter or rotor flow meter has simple structure; low requirement of straight pipe section, constant pressure loss, etc., the rotor flowmeter is mostly used for gas low flow measurement.
Various types of flow meters can be used to measure gas flow rate, each flowmeter is suitable for certain limited conditions and occasions, and has its advantages.
There are many kinds of flow meters, and there are many classifications. According to their different working principles, they can be divided into the following categories:
1. Differential pressure flowmeter
Differential pressure flowmeter is equipped with a special device (such as orifice plate, throat, etc.) in the pipeline of fluid flow. When the fluid flows, a differential pressure is generated between the front and rear parts of the device, and the differential pressure has a certain functional relationship with the flow rate. That is, the differential pressure is large, the flow is large; the differential pressure is small, the flow is small. The flow rate can be calculated by measuring the differential pressure.
Differential pressure flow meters are divided into two types: constant differential pressure type and throttling type (or variable differential pressure type). The rotor flowmeter belongs to the constant differential pressure flowmeter; the flowmeter composed of the throttling device and the differential pressure transmitter belongs to the DP flowmeter. Normal used DP flow meters include orifice plate flow meter, V-cone flow meter, wedge flow meter, long radius nozzle flow meter.,etc.
Differential pressure flow meter can be used to measure natural gas, compressed gas, biogas, flue gas, steam.etc.
2. Volumetric flowmeter
Positive displacement flowmeters are the most accurate flowmeters. Inside the positive displacement flowmeter sensor, there is a space that constitutes a certain standard volume, which is usually called the "metering space" or "metering chamber". This space is formed by the inner wall of the meter housing and the flowmeter rotating parts. When the fluid passes through the flowmeter, the fluid continuously fills the metering chamber with a certain volume. Since there is a certain pressure difference between the inlet and outlet of the flowmeter, the rotating parts of the flowmeter will rotate under the action of this pressure difference, and the fluid will be transferred by the flow rate. During this process, the fluid fills the "metering space" of the flowmeter again and again, and is then continuously sent to the outlet. Under the conditions of a given flow meter, the volume of the metering space is determined, and pushes the piston (or drum, gear, diaphragm, etc.) to swing (or rotate) back and forth. As long as the number of rotations of the rotating part is measured, that is, the number of times the fluid fills the metering chamber is accumulated by the counter, the total accumulated flow value of the fluid volume passing through the flow meter can be obtained.
PD flowmeter is generally used to measure low-pressure gas flow, it is gas consumption meter.
Diaphragm gas meter, piston meter, gear meter, waist wheel meter, wet meter, etc. belong to positive displacement flowmeter. The positive displacement flowmeter has the advantages of insensitivity to the upstream flow state (no need to install front and rear straight pipe sections), high measurement accuracy, and high cost performance, and is mostly used for custody transfer such as gas and oil.
3. Velocity flowmeter
The output of velocity gas flow sensor is proportional to the flow rate, and one uses the gas velocity flows through the pipe to impel the flowmeter impeller or turbine, causing them to turn. When the gas speed is fast, the number of impeller or turbine revolutions per unit time is more; when the speed is slow, the number of impeller or turbine revolutions is less. The number of revolutions of the impeller or turbine has a relatively stable functional relationship with the flow. The fluid flow rate can be obtained by measuring the number of revolutions of the impeller or rotor per unit time. These type flow meters are normally digital gas flow meters.
4. Mass flow meter
Mass flow meters are used to measure the gas mass flow of fluids flowing through pipes. Mass flow meters mainly include:
(1) Coriolis mass flowmeter
This kind of flowmeter uses the Coriolis force generated by the fluid in the vibrating tube, and adopts the method of directly measuring the Coriolis force to obtain the mass flow of the fluid.Coriolis mass flow meter is normally used to measure gas with large density, high pressure or high flow rate. The Coriolis mass flow meter sensor can be 1/2”, 3/4”,1”, 2”, 3”, 4 inch, 6 inch, DN200.,etc
(2) Thermal mass flowmeter
Thermal flowmeter sets a heat source outside the wall of the fluid pipe, uses the relationship between the heat and mass of the flowing gas, and measures the temperature change in its upstream and downstream to obtain the mass flow of the gas. It can be used to measure Nitrogen ( N2), oxygen gas ( O2), compressed air, natural gas .,etc. it can be inline type mass flow meter or insertion type mass flow meter for large pipeline or ducts gas flow measurement , the flow unit is Nm3/h or SCFH.
(3) Impulse mass flowmeter
This kind of flowmeter uses the force generated by the impulse of the material fluid falling at a certain height, and adopts the direct force measurement method to obtain the fluid mass flow.
Mass flow meters can directly obtain mass flow, and are mostly used in production process parameter measurement, detection and process control. Generally, gas mass flow meters are mainly based on calorimetric mass flow meters.
We have many choice for gas flow measurement devices, Each type of gas flowmeter has its own advantages and disadvantages. The velocity type flowmeter has a simple structure, small size and low cost, but the accuracy is low when measuring small flow, even if the impeller (or turbine) does not rotate, it is easy to cause a "dead meter" . DP flowmeter is more suitable for the measurement of high-pressure fluids. It has strong performance and is easy to produce in large quantities, but it is difficult to use for fluids with small diameter and small flow, and the accuracy is not very good. Positive displacements flowmeters have high accuracy, but are large in size, require high technical requirements for assembly testing, strict requirements on test environment conditions.
