Common types of flow instruments

There are many kinds of flow measurement methods and instruments, and there are also many classification methods. Before 2011, there were 60 kinds of flow instruments available for industrial use. The reason why there are so many varieties is that there is no flow meter applicable to any fluid, any range, any flow state and any service conditions. However, with the progress of the times, in this era of science and technology explosion, a new product - mass flowmeter has finally emerged. Mass flowmeter is applicable to any fluid, any range, any flow state and any service conditions, but the price is relatively expensive, It cannot be popularized in all industries.

Classifications of current meter flow measurement

There are more than 60 kinds of old flow meters. Each product has its specific applicability and limitations. According to the measurement object, there are two categories: closed pipeline and open channel; According to the purpose of measurement, it can be divided into total amount measurement and flow measurement. Its instruments are called total amount meter and flowmeter respectively.

In addition, according to the measurement principle, it can be divided into the following categories:

1. Principle of Mechanics: Instruments belonging to this principle include differential pressure type and rotor type using Bernoulli's theorem; Impulse type and movable tube type using momentum theorem; Using the direct mass formula of Newton's second law; Target type based on the principle of fluid momentum; Turbine type using angular momentum theorem; Vortex type and vortex street type based on the principle of fluid oscillation; Pitot tube type, positive displacement type, weir and trough type using total static pressure difference, etc.

2. Electrical principle: instruments used for this principle include electromagnetic type, differential capacitance type, inductance type, strain resistance type, etc.

3. Acoustic principle: ultrasonic type, acoustic type (shock wave type), etc. are used for flow measurement.

4. Thermal principle: there are thermal type, direct calorimetric type, indirect calorimetric type, etc. using thermal principle to measure flow.

5. Optical principle: laser type and photoelectric type are instruments belonging to this kind of principle.

6. Principles of atomic physics: nuclear magnetic resonance and nuclear radiation are instruments belonging to such principles

7. Other principles: labeling principle (tracer principle, nuclear magnetic resonance principle), correlation principle, etc.

According to the most popular and extensive classification at present, this paper expounds the principle, characteristics, application overview and the exertion of various flow meters at home and abroad:

Target type

Target flowmeter is a kind of flowmeter based on mechanical principle. It has been developed and applied in industry for decades. The new SBL target flowmeter is a new capacitive force sensing flowmeter developed on the basis of the traditional target flowmeter and with the development of new sensors and microelectronic technology. It not only has the characteristics of orifice, vortex street and other flowmeters without movable parts, but also has high sensitivity, accuracy comparable to that of volumetric flowmeter and wide range.  [3]

In the 1970s, China developed electric and pneumatic target flow transmitter, which is the detection instrument of electric and pneumatic unit combination instrument. At that time, the force converter directly adopted the force balance mechanism of differential pressure transmitter, which inevitably brought many defects caused by the force balance mechanism itself, such as easy drift of zero position, low measurement accuracy, poor reliability of lever mechanism and so on. Due to the poor performance of the force balance mechanism, many advantages of the target flowmeter itself have not been effectively brought into play. So far, the bad impression of users on the old target flowmeter has not been eliminated.

The force converter of the new SBL target flowmeter adopts the strain force converter, which completely eliminates the shortcomings of the above force balance mechanism. The new target flowmeter also applies microelectronic technology and computer technology to the signal converter and display part. The flowmeter has a series of advantages and is believed to play an important role in many flowmeters in the future.

Differential pressure type

Differential pressure flowmeter is an instrument that calculates the flow according to the differential pressure generated by the interaction between the flow detector and the fluid installed in the pipeline, the known fluid conditions and the geometric size of the detector and the pipeline.

Differential pressure flowmeter is composed of primary device (detector) and secondary device (differential pressure converter and flow display instrument). Differential pressure flowmeter is usually classified in the form of detection parts, such as orifice flowmeter, venturi flowmeter, averaging tube flowmeter, pitot tube principle pitoba flowmeter, etc.

The secondary devices are various mechanical, electronic and electromechanical integrated differential pressure gauges, differential pressure transmitters and flow display instruments. It has developed into a large class of instruments with high degree of three Standardization (serialization, generalization and standardization) and complex types and specifications. It can measure not only flow parameters, but also other parameters (such as pressure, material level, density, etc.).

According to its working principle, the detection parts of differential pressure flowmeter can be divided into throttling device, hydraulic resistance type, centrifugal type, dynamic head type, dynamic head gain type and jet type.

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

The so-called standard test piece is designed, manufactured, installed and used according to the standard documents, and its flow value can be determined and the measurement error can be estimated without real flow calibration.

Non standard test pieces are those with poor maturity and have not been listed in international standards. Differential pressure flowmeter is one of the most widely used flowmeters, and its usage ranks first in all kinds of flow instruments. Due to the advent of various new flowmeters, the percentage of its use has gradually decreased, but it is still the most important type of flowmeter at present.

The fluid volume flow formula of differential pressure flowmeter is:

v=aA √2/j(p-q)

V -- volume

J -- liquid density

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

A -- opening area of orifice plate

P-Q -- pressure difference

Advantage:

(1) Orifice plate flowmeter, which is most widely used, has firm structure, stable and reliable performance and long service life;

(2) It has a wide range of applications, and so far there is no type of flowmeter that can be compared with it;

(3) The detection parts, transmitters and display instruments are produced by different manufacturers, which is convenient for large-scale economic production.

Disadvantages:

(1) The measurement accuracy is generally low;

(2) Narrow range, generally only 3:1 ~ 4:1;

(3) High requirements for on-site installation conditions;

(4) Large pressure loss (refer to orifice plate, nozzle, etc.).

Note: a new product: the balance flowmeter developed by NASA is introduced. The measurement accuracy of this flowmeter is 5-10 times that of the traditional throttling device, and the permanent pressure loss is 1 / 3. The pressure recovery is twice as fast, and the minimum straight pipe section can be as small as 1.5D. It is easy to install and use, greatly reducing the capacity consumption of fluid operation.

Application overview of flow meter transducer

Differential pressure flowmeter is widely used in the flow measurement of closed pipeline, all kinds of objects have applications. Such as fluid: single phase, mixed phase, clean, dirty, viscous flow, etc; Working state: normal pressure, high pressure, vacuum, normal temperature, high temperature, low temperature, etc; Pipe diameter: from several mm to several m; Flow conditions: subsonic, sonic, pulsating flow, etc. Its consumption in various industrial departments accounts for about 1 / 4 ~ 1 / 3 of the total consumption of flowmeter.

1. Commonly used standard throttling devices (orifice), (nozzle), (Venturi tube).

2. Common non-standard throttling devices include (double orifice plate), (round missing orifice plate), (1 / 4 round nozzle) and (Venturi nozzle).

3. The common pressure taking methods of orifice plate include (corner connection pressure taking), (flange pressure taking), and other methods include (theoretical pressure taking), (diameter distance pressure taking) and (pipe connection pressure taking).

4. The distance between the front and rear flange end faces is also called the standard hole pressing method, and the distance between the front and rear flange end faces is ± 1.4 mm.

5. The working power supply range of 1151 transmitter is (12) VDC to (45) VDC, and the load is from (0) ohm to (1650) ohm.

6. The measurement range of 1151dp4e transmitter is (0 ~ 6.2) to (0 ~ 37.4) kPa.

7. The maximum positive migration of 1151 differential pressure transmitter is (500%) and the maximum negative migration is (600%).

8. In general, the velocity of fluid in the pipe is the maximum at (pipe centerline) and equal to zero at (pipe wall).

9. If (Reynolds number) is the same, the motion of the fluid is similar.

10. When the fluid filled the pipe flows through the throttling device, the flow beam will occur (local contraction) at (necking), so that (flow rate) increases and (static pressure) decreases.

11. 1151 differential pressure transmitter adopts variable capacitance as the sensitive element. When the differential pressure increases, the measuring diaphragm displaces, so the capacitance on the low-voltage side (increases) and the capacitance on the high-voltage side (decreases)

12. When the minimum calibration range of 1151 differential pressure transmitter is used, the maximum load migration is (600%) and the maximum positive migration is (500%). If the maximum calibration range of 1151 is used, the maximum negative migration is (100%) and the positive migration is (0%).

13. The accuracy of 1151 [4] differential pressure transmitter is (± 0.2%) and (± 0.25%). Note: large differential pressure transmitter is ± 0.25%

14. The commonly used flow unit and volume flow are (m3 / h), (T / h), mass flow is (kg / h), (T / h), and gas volume flow under standard state is (Nm3 / h).

15. The orifice flowmeter is used to measure the steam flow. When the steam density is 4.0kg/m3 in design and 3kg / m3 in actual operation, the actual indicated flow is (0.866) times of the design flow.

16. The gas ammonia flow is measured by orifice flowmeter. The design pressure is 0.2MPa (gauge pressure) and the temperature is 20 ℃, while the actual pressure is 0.15Mpa (gauge pressure) and the temperature is 30 ℃, the actual indicated flow is (0.897) times of the design flow.

17. The straight pipe section in front of the orifice plate is generally required to be (10) d, and the straight pipe section behind the orifice plate is generally required to be (5) d. for correct measurement, the straight pipe section in front of the orifice plate is preferably (30 ~ 50) d, especially when there is a pump or regulating valve in front of the orifice plate.

18. In order to make the flow coefficient of orifice flowmeter α The Reynolds number of the fluid shall be greater than (the limit Reynolds number) when it tends to the fixed value.

19. In the technical requirements for orifice plate processing, the upstream plane shall be perpendicular to the center line of the orifice plate (without visible scars), the upstream and downstream surfaces shall be (parallel), and the upstream inlet edge shall be (sharp and free of burrs and scars).

float

Rotameter, also known as rotameter, metal rotameter and Chengfeng glass rotameter, is a kind of variable area flowmeter. In a vertical conical tube expanded from bottom to top, the gravity of the float with circular cross-section is borne by hydrodynamic force, so that the float can rise and fall freely in the conical tube.

Rotameter is the most widely used type of flowmeter after differential pressure flowmeter, especially in small and micro flow.

In the mid-1980s, the sales volume of flow meters in Japan, Western Europe and the United States accounted for 15% ~ 20%. China's output in 1990 was estimated at 120000 ~ 140000 units, of which more than 95% were glass cone tube rotameter.

characteristic:

(1) The glass cone tube rotameter has the advantages of simple structure and convenient use. Its disadvantage is low pressure resistance and high risk of glass tube fragility;

(2) Suitable for small diameter and low flow rate;

(3) Low pressure loss.