Volumetric flowmeter is an instrument that directly accumulates flow based on the volume of the discharged fluid. It consists of a measuring room, moving parts, transmission and display components. If the fixed standard volume of the measuring room is V and the number of fixed standard volumes of the fluid discharged through the flowmeter during a certain time interval is n, then the total volume Q of the measured fluid is Q=nV. By using a counter to measure the rotational speed n of the moving parts through the transmission mechanism, the flow rate Q of the measured fluid can be displayed. The moving parts of a volumetric flowmeter have two types: reciprocating motion and rotational motion. The reciprocating type includes household gas meters, piston type oil gauges, etc.

Manufacturer of positive displacement flowmeter

Volumetric flow meters can be classified according to their measuring components, including elliptical gear flow meters, scraper flow meters, dual rotor flow meters, rotary piston flow meters, reciprocating piston flow meters, disc flow meters, liquid sealed rotary cylinder flow meters, wet gas meters, and membrane gas meters.

Volumetric flow measurement is the use of a fixed small volume to repeatedly measure the volume of fluid passing through a flow meter. Therefore, there must be a space inside the volumetric flowmeter that constitutes a standard volume, commonly referred to as the “measuring space” or “measuring chamber” of the volumetric flowmeter. This space is composed of the inner wall of the instrument housing and the rotating parts of the flow meter. The working principle of a volumetric flowmeter is that when fluid passes through the flowmeter, a certain pressure difference will be generated between the inlet and outlet of the flowmeter. The rotating component of the flowmeter (referred to as the rotor) rotates under this pressure difference and discharges the fluid from the inlet to the outlet. During this process, the fluid repeatedly fills the “measuring space” of the flowmeter and is then continuously sent to the outlet. Under the given flow meter conditions, the volume of the measuring space is determined as long as the number of rotations of the rotor is measured. The cumulative value of fluid volume passing through the flowmeter can be obtained.

Advantages of positive displacement flowmeter

⑴ High measurement accuracy;
The installation conditions of the pipeline have no impact on the measurement accuracy;
⑶ Can be used for measuring high viscosity liquids;
⑷ Wide range;
The direct reading instrument does not require external energy and can directly obtain cumulative, total, clear and easy to operate information.

Selection of positive displacement flowmeter

The selection of flow meters requires careful consideration of the instrument conditions provided by the foundation. At the beginning, it is necessary to consider the static pressure and temperature of the medium. The static pressure and temperature resistance of the flowmeter are also necessary factors in ensuring the service life and roughness of the instrument. The static pressure of the instrument is the pressure resistance level, which should be slightly higher than the task pressure of the measured medium, usually 1.25 times, to ensure that no leakage or accidents occur. The selection of the second range scale is mainly based on the choice of the upper limit of the instrument scale. Choosing too small can lead to overload and poor performance of the instrument; Choosing too large may affect the accuracy of the measurement. The national standard stipulates that the maximum flow rate shall not exceed 95% of the full scale; Abnormal flow is 70% to 85% of the full scale; The minimum flow rate shall not be less than 30% of the full scale. At the end of the comparison, the crucial instrument condition is the diameter of the base pipe. Some flow meters do not have a rated large diameter, and some large diameter flow meters have very low value. Currently, flow meters that are comparable to large pipe diameters include flute shaped uniform velocity tubes, inserted vortices, inserted turbines, electromagnetic flow meters, Venturi tubes, and ultrasonic flow meters.

The choice of positive displacement flowmeter performance

The choice of positive displacement flowmeter performance
The following five factors should be considered in the performance selection of positive displacement flowmeters: (1) flow range; (2) Physical properties of the measured medium; (3) Measurement accuracy; (4) Pressure resistance (working pressure) and pressure loss; (5) Purpose of use.
⑴ Flow range
The flow range of the positive displacement flowmeter is related to the type of medium to be measured (mainly determined by the viscosity of the fluid), the characteristics of use (continuous or intermittent work), measurement accuracy and other factors. The same positive displacement flowmeter is used for higher viscosity for medium types
When fluid, its flow range is large (mainly because the lower limit flow can be extended to a lower quantity value); For the use of characteristics, when used for intermittent measurement, the flow range is large (mainly the upper limit flow can be larger than continuous work); For measurement accuracy, the flow range is larger when used for low accuracy measurement, while the flow range is smaller when used for high accuracy measurement. This can also be clearly seen from the characteristic curve of the flowmeter.
In order to maintain the good performance and long service life of the instrument, the maximum flow rate should be selected at 70%-80% of the maximum flow rate of the instrument.
Due to the large volume of the general positive displacement flowmeter, it will produce larger noise in the large flow rate, so it is generally suitable for small and medium-sized flow measurement. Combined with the performance of the positive displacement flowmeter, when it is necessary to measure large flow rates, the flowmeter with 45 degree combined waist wheel structure can be used. In the need for low noise work, you can choose a dual rotor flowmeter.
(2) Physical properties of the measured medium
The physical properties of the measured medium mainly consider the viscosity and corrosiveness of the fluid. Consider viscosity to choose the right type of flow meter, and corrosion is one of the main factors in the selection of flow meter material, for example, used to measure various petroleum products, can choose cast steel, cast iron flow meter; Flowmeters made of copper alloy can be used for slightly corrosive chemical liquids and cold and warm water; Flowmeters made of stainless steel should be used for pure water, high temperature water, crude oil, asphalt, high temperature liquids, various chemical liquids and food. In addition, the flowmeter used in the food industry, in addition to the parts in contact with the fluid must be made of stainless steel and meet the requirements of health conditions, in the structure should be easy to disassemble and clean, there is no liquid storage parts in the flowmeter, such as elliptical gear flowmeter, rotary piston flowmeter, etc.
⑶ Measurement accuracy
Positive displacement flowmeter is one of the most accurate flowmeters at present. The measurement accuracy given by the manufacturer on the product sample refers to the basic error obtained under the laboratory reference conditions, and in actual use, due to the deviation of the field conditions, it will inevitably bring additional errors, and the actual error should be the synthesis of basic errors and additional errors. Therefore, measures should be taken according to the problems that may occur on site when the instrument is selected.
The viscosity and temperature of the measured medium are the main factors that influence the measurement accuracy.
The influence of fluid viscosity on flowmeter error characteristics has been discussed in detail in Section 3. When there is a large difference between the fluid viscosity in actual use and the fluid viscosity in laboratory verification, the corresponding viscosity correction should be carried out. If there is no significant change in the parameters other than the fluid viscosity, the viscosity can be corrected using the following two methods.
(4) Pressure resistance (working pressure) and pressure loss
The working pressure of the flow meter should be borne by the flow meter housing, and the different requirements of the working pressure should be selected from different materials to avoid unsafe use.
Pressure loss is also an important issue that must be considered when choosing flow timing. Especially when large flow is used, more attention should be paid to whether the pressure loss of the flow meter can meet the requirements of users.
(5) Purpose of Use
The purpose of use refers to whether the flow meter is used for metering handover and cost accounting, or for process parameter control. The flowmeter used for measurement accounting mainly considers its measurement accuracy, and can be a local indicating instrument; The flowmeter used for process control mainly considers its reliability, and it should have various supporting equipment such as transmitter, counter, adjusting display instrument.