Stable small flow mass flowmeter

Measuring principle

A mass flowmeter uses a thermal measurement method to measure flow rate by determining the amount of molecular weight carried away by the separated molecules. Since it uses a thermal measurement method, changes in gas temperature and pressure will not affect the measurement results. It is a relatively accurate, fast, reliable, efficient, stable, and flexible flow measurement instrument that will be widely used in fields such as petroleum processing and chemical engineering. It is believed that it will show great potential in promoting flow measurement.
Mass flow meters cannot control flow rates. They can only detect the mass flow rate of liquids or gases and output flow values through analog voltage, current, or serial communication. However, a mass flow controller is an instrument that can detect and control at the same time. The mass flow controller itself, in addition to the measuring part, also has an electromagnetic or piezoelectric valve, so that the mass flow control itself constitutes a system for controlling the mass flow of the fluid. The set value of the mass flow controller can be provided through analog voltage, analog current, or by a computer or PLC.

technical indicators

Main parameters

Quality flow accuracy: ± 0.002 × flow rate ± zero drift

Density measurement accuracy: ± 0.003g/cm3

Density measurement range: 0.5~1.5g/cm3

Temperature measurement accuracy: ± 1 ° C

sensor

Environmental temperature: -40~60 ° C

Medium temperature: -50~200 ° C

Explosion proof type: iB Ⅱ BT3

Associated equipment: matching transmitter

transmitter

Working temperature: 0~60 ° C

Relative humidity: below 95%

Power supply: 220 ± 10% VAC, 50Hz or 24 ± 5% VDC, 40W

hardware failure

If there is a large error, the integrator does not light up or increase in value, or there are blank spaces on the display, the reasons are:

a. Improper installation can directly cause zero drift of the flow meter, such as installing the mass flow meter close to the pump outlet, insufficient sensor support strength, improper welding of the connecting flange generating stress signals, and electromagnetic interference on the cable.

b. Wiring issues

If the monitor does not light up, check the power connection of the integrator. If the fuse is blown, confirm that the input voltage is consistent with the nominal value of the standard voltage and whether the AC/DC form is *. If the accumulator does not increase with the flow rate, the wiring of the accumulator should be checked. If the accumulator is equipped with forward/reverse programs, the wiring of the flow meter should be checked. Incorrect wiring of the flow meter can cause the accumulator to not increase in reverse flow.

c. Change in process medium

If the measuring medium exhibits phenomena such as gas entrapment, gasification, or two-phase flow, the transmitter will display an alarm, and in severe cases, the sensor will stop working.

d. The transmitter is malfunctioning.

e. Sensor failure.

f. Pipeline blowing problem.

software issue

Troubleshooting of Quality Flow Meter - Before installing and using a flow meter that has just been calibrated, it is important to pay attention to zero calibration under current operating conditions. It is necessary to ensure that the flow meter is filled with medium and close the shut-off valves at both ends before zero calibration can be performed. There are many specific methods, such as panel operation, manual operation, and using software II.

a. Zero calibration is incorrect.

b. Parameter settings are incorrect.

c. Pulse fluctuations of the power supply.

d. The I/O configuration is incorrect (be sure to pay attention to the range).

e. Incorrect operation

Main Categories

Mass flow meters can be divided into two categories: one is direct type, which directly outputs mass flow rate; The other type is indirect or deductive, such as using a combination of ultrasonic flowmeter and density meter, and multiplying their outputs to obtain the mass flow rate.

Direct type

There are various types of direct type, such as calorimeter, angular momentum, gyroscope, and double bladed wheel. This type of instrument is suitable for measuring small flow gases, but its disadvantage is that it has high inertness, and the measured value is related to the specific heat at constant pressure of the gas. The measuring element is in contact with the medium and is prone to contamination and corrosion. It is a dual orifice differential pressure mass flowmeter. Install two identical orifice plates at locations A and B. Two identical constant volume flow rates with opposite directions are installed in the diversion pipelineqA fixed flow pump. Pressure difference △ between front and rear of two orifice platesP=P1-P3=4Kr, withρTheQDirectly proportional. in the formulaKAs a constant coefficient,ρFor density,QFor the volumetric flow rate of the pipeline,ρ QIt is the quality flow rate. A double bladed mass flowmeter is a device that installs two impellers with inclination angles x1 and x2 on the same straight line. The impellers are connected by torsion springs, and when fluid passes through, an offset angle x is generated between them. Therefore, the torque difference △ M between the two impellers is related to the mass flow rate Qm, flow velocity u, and inclination angles x1 and x2 by △ M=Qm * u * (k1 * tgx1-k2 * tgx2) (k1 and k2 are blade structure size constants), △ M=k3 * u * Qm, (k3=k1 * tgx1-k2 * tgx2). The offset angle x=k4 * △ M=k4 * k3 * Qm * u; The rotational speed U of the impeller group is directly proportional to the flow velocity of the fluid, U=k6*u， The time required for the entire impeller group to rotate through the two impeller angles x is Δ t=x/U=k7 * Qm. By measuring Δ t with a counter, the mass flow rate Qm can be obtained.

Indirect type

There are three main types of indirect mass flow meters: combinations of velocity flow meters and density meters, combinations of throttling (or target) flow meters and volumetric flow meters, and combinations of throttling (or target) flow meters and density meters.

There is also a method of converting the measured value of a volumetric flowmeter into the volumetric flow rate under standard conditions based on the working pressure and temperature of the fluid. However, when the type or composition of the medium changes, it cannot provide an accurate mass flow rate. Output density, specific gravity, volumetric flow rate, mass flow rate, mass energy flow rate, etc., with multiple functions such as indication, analog output, printing, over limit alarm, instrument fault alarm, etc.

thermal

The basic principle of a thermal mass flowmeter is to use an external heat source to heat the measured fluid inside the pipeline. The thermal energy flows along with the fluid, and the mass flow rate of the fluid is reflected by measuring the changes in heat (temperature) caused by the fluid flow.

When the fluid composition is determined, the specific heat at constant pressure of the fluid is a known constant. Therefore, as can be seen from the above equation, if the heating power is kept constant, the mass flow rate can be calculated by measuring the temperature difference; If the constant temperature difference method is used, that is, keeping the temperature difference between two points constant, the mass flow rate can also be calculated by measuring the heating power. Due to its simplicity and ease of implementation, the constant temperature difference method is widely used in practical applications. This type of flowmeter is commonly used for measuring large gas flow rates.

To avoid contamination and corrosion of temperature measuring and heating elements by the measured fluid due to direct contact, non-contact measurement methods can be used, where the heater and temperature measuring element are installed outside the thin-walled tube, and the fluid passes through the inside of the thin-walled tube. Non contact measurement method, suitable for measuring small flow rates in small-diameter pipelines. When used for high flow measurement, the method of diversion can be adopted, that is, only measuring the diversion part of the flow and then calculating the total flow to expand the range.

differential pressure

Differential pressure mass flowmeter is a flowmeter based on Magnus effect, which uses a combination of orifice plate and quantitative pump to achieve mass flow measurement in practical applications. There are two common structures: double orifice plate and four orifice plate combined with a quantitative pump.

The double orifice plate structure is shown in Figure 6. Two orifice plates A and B with the same structure and size * are installed on the main pipeline. Two quantitative pumps with opposite flow directions and fixed flow rates are installed on the diversion pipeline. The differential pressure gauge is connected at the inlet of orifice plate A and the outlet of orifice plate B.

Coriolis

Coriolis mass flowmeter (abbreviated as Coriolis force flowmeter) is an instrument that directly measures mass flow by utilizing the principle of Coriolis force proportional to mass flow generated by fluid flow in a vibrating tube.

There are various forms of Coriolis flowmeter structures, generally consisting of a vibrating tube and a converter. Vibration tube (measuring pipeline) is a sensitive device, which has several shapes such as U-shaped, Ω - shaped, ring-shaped, straight tube shaped, and spiral shaped, as well as double tube shaped methods, but the basic principle is the same.