Flow measurement and common flow meters are briefly introduced

Concept of flow: refers to the amount of fluid flowing through a certain section of the pipeline per unit time. Divided into volume flow and mass flow, mass flow M = volume flow Q * fluid density ρ。 The common units of mass flow are: kg / h, t / h, etc., and the common units of volume flow are: L / h, m3 / h, etc.

Classification of fluid flow state: A. laminar flow (Reynolds number Re ＜ 2300) B. transitional flow (2300 ＜ re ＜ 4000) C. turbulent (turbulent) flow (Reynolds number Re ＞ 4000). Reynolds number refers to the ratio of inertial force to viscous force.

Physical parameters related to fluid: temperature, pressure, density, viscosity, speed, flow, etc.

Relationship between density of fluid and temperature and pressure: the density of gas decreases with the increase of temperature and increases with the increase of pressure. The density of liquid mainly decreases with the increase of temperature and has little relationship with pressure.

The types of flow measuring instruments include: vortex flowmeter, metal tube rotameter, orifice throttling device flowmeter, cone tube flowmeter, Weiba flowmeter, Wedge Flowmeter, mass flowmeter, electromagnetic flowmeter, etc.

Classification of flowmeter

There are many kinds of flow measurement methods and instruments, and many classification methods. Up to now, there are as many as 60 kinds of flow instruments available for industry. The reason why there are so many varieties is that we have not yet found a flow meter suitable for any fluid, any range, any flow state and any use condition.

According to the measurement principle, there are mechanical principle, thermal principle, acoustic principle, electrical principle, optical principle, atomic physics principle, etc.

According to the most popular and extensive classification at present, it can be divided into: positive displacement flowmeter, differential pressure flowmeter, rotameter, turbine flowmeter, electromagnetic flowmeter, fluid oscillation flowmeter (including vortex flowmeter and mass flowmeter) and plug-in flowmeter.

General knowledge of mass flowmeter

Application of mass flowmeter: since the mass flowmeter measures the mass flow of fluid, it is not affected by the changes of fluid temperature, pressure, density and other parameters, and has high measurement accuracy (up to 0.1%), there is no requirement for straight pipe section. In some occasions where accurate measurement and strict control of feed are required and when used for trade settlement for measurement, the mass flowmeter is often used for flow measurement, But because of its high price, it is not widely used.

Composition of mass flowmeter: it includes three parts: sensor, transmitter and display unit. The sensitive element of the sensor is the measuring tube, which is installed on the pipeline. The transmitter and display unit (simple type without display unit) are separately installed at the place convenient for observation and maintenance next to the sensor. There are special multi-core shielded cables between them for electrical connection.

Structure classification of Coriolis force mass flowmeter: it can be divided into straight tube type and elbow type according to the shape of measuring tube, and single tube type and double tube type according to the number of measuring tubes (double tube type is commonly used).

Installation requirements of mass flowmeter:

A. The installation site shall not have large vibration sources, and reinforcement measures shall be taken to stabilize the pipeline near the instrument;

B. It cannot be installed near large transformers, motors, pumps and other equipment that generate large magnetic fields to avoid interference from strong magnetic fields;

C. There should be no stress when the sensor is connected with the pipeline;

D. It is better to install the straight tube mass flowmeter vertically. If it is to be installed horizontally, the two measuring tubes shall be in the same horizontal plane. E. When the elbow type mass flowmeter measures liquid, the elbow part shall face downward, and when measuring gas, the elbow part shall face upward.

Commissioning and shutdown methods of mass flowmeter:

A. Before the mass flowmeter is put into use, check whether its installation meets the requirements, whether the pipeline is purged and whether the pressure test has been completed;

B. Power on and preheat for 30 minutes, then start the fluid operation until the sensor temperature is equal to the operating temperature of the fluid. When the downstream valve is cut off and there is no leakage and the pipe is full, adjust the flowmeter to 0;

C. Open the upstream and downstream valves, close the bypass valve, and put the flowmeter into use;

D. When the flowmeter is out of service, open the blowdown valve for the medium that is easy to scale and drain the medium in the flowmeter. For the medium that is easy to solidify, drain the medium and use low-pressure steam or factory air to sweep the line to ensure that there is no medium left in the measuring pipe.

General knowledge of vortex flowmeter

Application of vortex flowmeter: vortex flowmeter is a vortex separation flowmeter made by using the principle of natural oscillation of fluid. The vortex frequency is proportional to the flow rate of fluid. The commonly used vortex generator is a triangular column with low output frequency. When the vortex flowmeter is used for measurement, the Reynolds number of the fluid is required to be between 20000 and 7000000, and the flow rate must be within the specified range. Different calibers have different flow rate requirements, and the flow rate requirements for liquid phase, gas phase and steam are different; The instrument has the requirement of straight pipe section, which is generally used for the measurement of clean low viscosity media with a measurement accuracy of 1%.

Installation requirements of vortex flowmeter:

A. Installed on the straight pipe with stable flow velocity distribution, the length of the straight pipe section at the upstream side shall be greater than 20d, and the length of the straight pipe section at the downstream side shall be greater than 5D;

B. Prevent the sensor from generating mechanical vibration;

C. Prevent external electromagnetic interference;

D. The flowmeter is preferably installed on the upstream side of the control valve, temperature measuring point and pressure measuring point;

E. The size of the flowmeter shall be consistent with the inner diameter of the pipeline;

F. The center line of the flowmeter shall be concentric with the center line of the pipeline, and the gasket shall be prevented from being inserted into the pipeline;

Stop valves shall not be used before and after g vortex flowmeter.

Commissioning and shutdown methods of vortex flowmeter:

A. Before the vortex flowmeter is put into use, check whether its installation meets the requirements, whether the pipeline is purged and whether the pressure test has been completed;

B. After putting into use, ensure that the flowmeter runs in full pipe;

C. After the flowmeter is stopped, check whether the measuring element is contaminated, and clean the flow element and its detection hole in time.

General knowledge of metal tube rotameter

Application of metal tube rotameter: rotameter is widely used and is generally used to measure medium, small and micro flows. The measuring medium is generally clean, hard to crystallize and solidify, and liquid, gas and steam with low viscosity. The medium flow rate is required to change slowly.

The metal tube rotameter is divided into local type and remote type. Its measuring principle is constant pressure drop variable flow cross-sectional area flowmeter. Its range ratio is 10:1, and the accuracy is generally 1.6%.

Installation requirements for metal tube rotameter: the conical tube of the metal tube rotameter must be installed vertically and not tilted. During installation, it should be strictly calibrated with a level gauge and should not be stressed during assembly. The medium flow direction of the vertically installed rotameter is from bottom to top. The medium flow direction of the horizontally installed rotameter should be consistent with its marked direction. In order to facilitate use and disassembly and inspection, the valve group is generally required to be installed. The remote transmission part of the remote transmission type metal tube rotameter is coupled with the rotor by magnetism. If the medium contains small particles that are easy to be adsorbed by magnetic substances, the rotor is easy to be worn and jammed, resulting in inaccurate or impossible measurement. The solution is to install a magnetic filter in the front.

Operation and shutdown methods of metal tube rotameter:

A. Before the flowmeter is put into use, check whether its installation meets the requirements, whether the pipeline is purged and whether the pressure test has been completed;

B. After putting into use, ensure that the flowmeter runs smoothly within its effective measurement range, and prevent the instrument from being damaged due to the impact of large flow on the rotor when opening the meter;

C. After the flowmeter is stopped, check whether the measuring element is contaminated and whether the rotor is adsorbed with iron filings, and clean the flow element and rotor in time.

Common sense of orifice throttling device flowmeter

Application of orifice throttling device flowmeter:

A. The fluid must run continuously with full pipe;

B. The fluid must be Newtonian, uniform and single-phase in physics and thermodynamics;

C. No phase change occurs when the fluid flows through the throttling device;

D. The fluid flow basically does not change with time and is not suitable for pulsating flow and critical flow conditions;

E. Before the fluid flows through the throttling device, the flow beam must be parallel to the pipeline axis and there must be no rotating flow;

F. The fluid flow condition shall be turbulent, the Reynolds number shall be within a certain range and there shall be no vortex.

The orifice plate throttling device flowmeter consists of three parts: orifice plate, pressure tapping flange or ring chamber and differential pressure transmitter. The orifice plate is used to throttle to generate differential pressure, the pressure tapping flange or ring chamber is used to take out the fluid pressure before and after the orifice plate, and the differential pressure transmitter is used to measure the pressure difference before and after the orifice plate to achieve the purpose of measuring the fluid flow through the orifice plate.

Installation requirements of orifice throttling device flowmeter:

A. The installation of throttling device requires the straight pipe section with the first 10 times and the last 5 times of the pipe diameter;

B. The front end surface of the throttle and its clamping flange shall be perpendicular to the pipeline axis, and the opening of the throttle and the clamping flange shall be concentric with the pipeline;

C. The sealing gasket for clamping the throttling part shall not protrude into the inner wall of the pipe, and the gasket thickness shall not exceed the specified value;

D. The newly installed pipeline system must be purged to be qualified before the orifice plate can be installed, and the acute angle of the orifice plate should face the flow direction;

E. When measuring gas, the pressure tap shall be within the 45 degree angle between the upper part of the pipeline and the vertical line of the pipeline; When measuring steam, the pressure tap shall be within the 45 degree angle between the upper part of the pipeline and the horizontal centerline of the pipeline; When measuring liquid, the pressure tap shall be within the 45 degree angle between the lower part of the pipeline and the horizontal centerline of the pipeline.

F. The horizontal pressure pipe for measuring gas (or liquid) shall have a pipe slope of 1:10 ~ 1:100 for discharging condensed liquid (or gas).

Commissioning and shutdown methods of orifice throttling device flowmeter:

A. Before the flowmeter is put into use, check whether its installation meets the requirements, whether the pipeline is purged and whether the pressure test has been completed;

B. After putting into use, ensure that the flowmeter runs smoothly within its effective measurement range;

C. The operation steps of the flowmeter are as follows: check that the secondary valve and blowdown valve should be closed and the balance valve should be opened → slightly open the root valve, check whether the impulse pipe system leaks, and fully open the primary valve if there is no leakage → respectively open the blowdown valve for blowdown and then close the blowdown valve → loosen the screw plug of the positive and negative pressure chambers of the differential pressure transmitter to drain the air → open the positive pressure valve of the transmitter, close the balance valve, open the negative pressure valve of the transmitter, and start the differential pressure transmitter;

D. Operating principle of instrument three valve group:

1. Condensate or isolating liquid in the impulse pipe shall not be lost;

2. The measuring capsule of the transmitter shall not be pressurized or heated; The starting sequence of the three valve group is: open the positive pressure valve → close the balance valve → open the negative pressure valve;

E. The shutdown steps of the flowmeter are as follows: close the negative pressure valve of the transmitter, open the balance valve, close the positive pressure valve of the transmitter → close the secondary valve → open the blowdown valve for blowdown, and then close the blowdown valve; The shutdown sequence of the three valve group is: close the negative pressure valve → open the balance valve → close the positive pressure valve.

Common sense of using Weiba flowmeter

Application of Weiba flowmeter: it is used to measure the medium flow in a large process pipeline. Its measurement principle is to measure the average flow rate of the fluid on the cross-section of the pipeline. It is required that the Reynolds number of the measured fluid in the operating state is greater than 20000, and the fluid is free of impurities, dirt and scale. The flow rate range is 0.5 ~ 6m / s for liquid, 10 ~ 60m / s for gas and 5 ~ 30m / s for steam.

Composition of Weiba flowmeter: it includes two parts: pressure measuring tube and differential pressure transmitter. The pressure measuring tube is used to generate differential pressure, and the differential pressure transmitter is used to measure dynamic and static pressure difference to measure fluid flow.

Installation requirements of Weiba flowmeter:

A. The installation of the measuring device requires the straight pipe section with the first 20 times and the last 5 times of the pipe diameter;

B. The measuring pipe shall be installed through the pipe center and perpendicular to the pipe center line;

C. Its installation orientation on the pipeline and the laying of the impulse pipe are similar to the requirements of orifice throttling device.

The commissioning and shutdown methods of Weiba flowmeter are the same as those of orifice throttling device flowmeter.

General knowledge of cone tube flowmeter

Application of conical tube flowmeter: it is used to measure the flow of liquid, gas and steam media in small process pipelines. Its measurement principle is that the pressure generated by the flowing media in the pipeline is proportional to the square of the flow rate, and the additional pressure (pdynamic - pstatic) generated by the media flow is obtained through the V-shaped pressure tap, so as to obtain the instantaneous flow.

The conical tube flowmeter consists of two parts: a measuring tube and a differential pressure transmitter. The pressure measuring tube is used to generate the differential pressure, and the differential pressure transmitter is used to measure the dynamic and static pressure difference to measure the fluid flow.

Installation requirements of cone tube flowmeter:

A. The installation of the measuring device requires the straight pipe section with the first 2 times and the second 5 times of the pipe diameter;

B. Its installation orientation on the pipe: it is used for horizontal pressure measurement when measuring liquid and steam, and the transmitter is installed below the measuring pipe; It is used for horizontal pressure measurement when measuring gas, and the transmitter is installed above the measuring pipe.

The commissioning and shutdown methods of conical tube flowmeter are the same as those of orifice throttling device flowmeter.

General knowledge of electromagnetic flowmeter

Application of electromagnetic flowmeter: it is used to measure the flow of conductive liquid medium. The medium temperature should not exceed 120 ° C, the pressure should not exceed 1.6Mpa, it should not be used under negative pressure, the flow rate should not be less than 0.3m/s, the measured medium should not contain more magnetic substances and bubbles, the measured fluid basically has no pressure loss, the measurement accuracy can reach 0.5%, and the range ratio is 1:20. Its measurement principle is Faraday's electromagnetic induction theorem.

Composition of electromagnetic flowmeter: it includes three parts: sensor, transmitter and display unit. The sensor is installed on the pipeline, and the transmitter and display unit are separately installed at the place convenient for observation and maintenance beside the sensor. There are special multi-core shielded cables for electrical connection between them, or they can be combined into one.

Installation requirements of electromagnetic flowmeter:

A. The installation site shall not have large vibration sources, and reinforcement measures shall be taken to stabilize the pipeline near the instrument;

B. It should not be installed near large transformers, motors, pumps and other equipment that generate large magnetic fields to avoid interference from electromagnetic fields;

C. When the sensor is connected with the pipeline, it should be ensured that the pipeline runs full, and it is better to install it vertically;

D. The transmitter shell, shielded cable, measurement body and pipes at both ends shall be grounded. The grounding electrode shall be set separately, and the grounding resistance shall be less than 10 ohms. It cannot be connected to the electrical or public grounding network;

E. It is required to have straight pipe sections with the first 5 times and the last 3 times of the pipe diameter.

Operation and shutdown methods of electromagnetic flowmeter:

A. Before putting the electromagnetic flowmeter into use, check whether its installation meets the requirements, whether the pipeline is purged and whether the pressure test has been completed;

B. Open the valve to fill the system with liquid. After the residual gas is removed, turn on the power supply of the instrument for preheating. Close the valve to fill the flowmeter with static liquid. Check and adjust the zero point. Reopen the valve to make the flow reach 100%. Check whether the output is correct and stable;

C. Open the upstream and downstream valves, close the bypass valve, and put the flowmeter into use;

D. When the flowmeter is out of service, open the blowdown valve for the medium that is easy to scale to drain the medium in the flowmeter. For the medium that is easy to solidify, drain the medium and use low-pressure steam or factory air to sweep the line to ensure that there is no scaling and solid impurities in the measuring pipe.