The Positive Displacement Flow Meter, a High Precision Flow meter in Hydraulic Field
Time:2023-01-05 Read:391
Reading guide: This is an ultimate guide about positive displacement flow meters. What is the working principle of the positive displacement flow meter? What are the application and development of positive displacement flow meters? Read on and get what you want to learn.

What is hydraulic technology?

Hydraulic technology is a kind of technology that takes flowing liquid as a working medium and takes the pressure and flow as characteristic parameters to realize energy conversion, transmission, and distribution. Pressure and flow characteristics directly characterize the quality of hydraulic components and hydraulic systems. Therefore, in the hydraulic field, the measurement of pressure and flow is particularly important. 

Since the measurement of flow is often affected by the viscosity and density of the medium itself, the temperature, the flow distortion of the incoming flow, and the rotating flow, the flow measurement is more complex compared with the pressure, which is why there is no flow instrument that is applicable to any fluid, any range, flow state, and any use conditions, although there are hundreds of types of flow measurement instruments. And with the development of the application of hydraulic technology in the industrial field, the requirement for the measurement accuracy of fluid medium flow is becoming higher and higher. The research of high-precision flow meters has been paid more and more attention to.

What is the working medium of the hydraulic system?

The working medium of the hydraulic system is petroleum-based hydraulic oil. Since 1930, the positive displacement flow meter has been formally applied to the measurement of petroleum liquids. Up to now, it has been recognized as the most accurate flow meter in the measurement of petroleum liquids. The positive displacement flow meter (volumetric flow meter) uses a precise standard container to continuously measure the measured fluid, so there are a few factors that affect the measurement accuracy. It is not affected by fluid viscosity, density, flow state, and Reynolds number, and the measurement accuracy is also high. Especially in recent years, with the great improvement in material selection, structural design, processing technology, and other aspects, its performance is also improving.

What is a positive displacement flow meter?

The positive displacement flow meter is mainly composed of a metering body and a measuring part. The metering body is composed of a shell and a movable body (Rotor of Rotameter). The core part is the moving body, which is the moving part in the measurement process. Each measuring chamber composed of the moving body and the shell is a standard volume unit, which is used to measure the volume of the fluid to be measured. 

The measuring part is actually a speed or counting sensor. The pinion of one of the screws on the screw flow meter is embedded with magnetic materials. When the screw rotates, the sensor can identify the speed of the screw through the magnetic effect and convert it into a standard pulse signal or current signal. In this way, the flow can be directly displayed by a secondary instrument or collected and analyzed by a computer.

What are the classifications of positive displacement flow meter (volumetric flow meter)?

There are many types of positive displacement flow meters, which can be classified according to different principles. Generally, according to the structure of measuring elements, they are divided into rotor type, scraper type, rotary piston type, reciprocating piston type, and diaphragm type. Among them, the rotor-type flow meter is the most commonly used, and the sales volume of the rotor-type flow meter accounts for about 90% of the total sales volume of the volumetric flow meter. The common gear type, screw type, waist wheel type, and oval wheel type flow meters are all rotor type.

What is the working principle of the positive displacement flow meter?

There are many types of positive displacement flow meters, all of which adopt the measurement method of fixed displacement, but there are still big differences in details. The rotor flow meter is still common in the hydraulic field. Its working principle is to use mechanical measuring elements to continuously divide the incoming flow into single or several parts of known volume, and measure the volume of fluid passing through the chamber according to the number of times the chamber is filled in unit time. In the rotor type, gear type and screw type are more common. The following two examples are used to introduce their principles.

(1)Gear type flow meters

There are oval gears and circular gears in the gear type. The gear shapes are different, but the working principle is similar. There is a pair of intermeshing oval gears in the cavity of the oval flow meter as the rotor, and the two gears and the cavity can respectively form a fixed volume, which is called the standard volume. The flow rate is measured by calculating the number of units that flow through the standard volume within a certain period of time. When the fluid passes through the flow meter and the gears are in position a, obviously, relative to their respective fixed shafts, the torque borne by gear A is balanced and does not rotate. And the B gear obviously receives the torque in the clockwise direction and starts to rotate clockwise in the direction of the arrow. The rotation of B just drives the A that engages with it to rotate counterclockwise. B is the driving wheel, and A is the driven theory. When the gear turns to the B position, the two gears are subjected to the torque that makes them turn, and A and B actively rotate counterclockwise and clockwise respectively.

working principle of the positive displacement flow meter

And a standard volume of a fluid formed by the B gear and cavity is expelled. Then turn to position c, the torque that B is subjected to is balanced, and A is subjected to a counterclockwise rotational torque. Now the gear has rotated 1/4 circle, B is similar to the state of A at position a, and A is similar to the state of B at position a. Therefore, for the next 1/4 week, a standard volume of a fluid formed by the A gear and the cavity is discharged. Then return to position a, and so on. If the sum of the two half-moon-shaped standard volumes are V, the gear rotates once, and the passing fluid is 2V. Therefore, as long as the number n (ie, the rotational speed) of the oval gear rotates per unit of time is recorded, the flow rate of the fluid per unit of time can be calculated. Q=2nV.

There are many ways to measure the speed, such as photoelectric, electric contact, and so on. Some oval gear meters use the magnetoresistive effect of semiconductors to measure rotational speed. Four magnets are equally embedded in the end face of the oval gear, and the magnets rotate synchronously with the gear. The magnetic induction probe (magnetic metal thin film resistor) on the flow meter sensor is correspondingly installed on the trajectory of the magnet movement. This thin film resistor can form a bridge amplifier with related electronic components. When a magnetic field with a magnetic field strength B is close, if the magnetic field is perpendicular to the current direction of the probe, the electrons will be deflected by the influence of the Lorentz force, and the resistivity in the probe will increase. 

This phenomenon is called the magnetoresistance effect. Therefore, when the magnet on the gear passes the probe, the bridge is unbalanced and the output is a high point. The magnet turns across the probe, and the bridge returns to balance, causing the output to return to its original low potential. In this way, every revolution of the gear produces 4 pulses, and the continuous rotation produces a pulse sequence proportional to the gear speed, which can accurately display the instantaneous and cumulative flow of the measured liquid after being received and processed by the display instrument or computer.

In addition, the circular gear flow meter rotor with higher precision and repeatability than the oval gear uses the circular gear instead of the original oval gear, but there is no essential difference in the working principle.

 2) Screw type flow meters

The screw flow meter is mainly composed of a pair of intermeshing screws, a metering box and a measuring sensor. The tooth groove of the screw and the measuring box form a standard volume, which is used to measure the volume of the passing fluid. When the fluid passes through the flow meter, the screw is driven to rotate, and the fluid is discharged after passing through the standard cavity. The flow rate of the fluid can be obtained by measuring the number of the fluid passing through the standard chamber per unit time.

The measurement of screw speed is similar to the measurement of gear speed, and photoelectric, electric contact and magnetoresistance effects can also be used. Accurate flow values can be obtained more easily.

Pros and Cons of Positive Displacement Flow meters

Positive displacement flow meters have been widely used in flow measurement in industrial production processes for a long time, and have rapidly expanded to measurement in medical, precision chemical, food industry, and other fields. This shows that positive displacement flow meters have many excellent characteristics (Pros).

(1) The measurement accuracy is high, the repeatability is good, and there are a few factors affecting the measurement accuracy. The viscosity of the liquid, the change of the fluid velocity field, and other factors have little influence on the measurement accuracy.

(2) Since the swirling flow and velocity field distortion have no effect on the accuracy, the installation is simple and does not require a flow stabilization device and a long straight pipe in front, so it is of great practical significance for practical use.

(3) The viscosity range used is very wide, especially for high-viscosity liquids, and the measurement accuracy is also very good.

(4) The flow range that can be measured is relatively wide. Compared with the measurement ratio of 10:1 and 5:1 of general flow meters, some container flow meters can reach 30:1 or even larger.

(5) Positive displacement flow meters can have direct-reading instruments, without other external energy sources, and the operation is simple and clear.

(6) Because the volume flow meter is directly calculated by volume, compared with other derivation volume flow meters such as velocity type, it is less affected by factors.

Of course, due to the influence of materials, processing technology, and its own structure, at present, positive displacement flow meters also have some defects (Cons).

(1) Since it is directly connected and installed with the pipe to be tested when the caliber is large, the instrument will appear cumbersome, and the handling and installation will be troublesome.

(2) Because mechanical materials become brittle at low temperatures and expand and deform at high temperatures, the temperature range and pressure rating used by positive displacement flow meters are limited.

(3) Because there are some moving parts, it is not suitable for the measured liquid that is dirty and contains solid impurities, and the filtration accuracy is required to be high. In addition, the fluid must be used to drive the moving parts to make the flow meter work properly, so the pressure loss is large.

What are the key technologies of positive displacement flow meter?

(1) Structural design of moving parts

The moving parts are the core components of the positive displacement flow meter, such as the rotor of the rotor type and the piston of the piston type. The design of the structure often requires repeated demonstrations and calculations. It and the housing constitute the volume cavity of the volumetric flow meter. If the volume is too large, the force and moment it receives will increase; if the volume is too small, the moving parts will move too frequently and the flow rate will be very small. The structural design of the moving parts will directly determine the leakage, pressure, and precision of the flow meter. Therefore, the structure of the moving parts is the most critical part of the positive displacement flow meter technology. Therefore, the structural design of the moving parts is particularly critical.

(2) Clearance of moving parts

The volume cavity of the positive displacement flow meter is not airtight and fixed, and the volume cavity should rotate with the rotating parts. Therefore, there must be a gap between the moving parts and the flow meter housing. The gap should not only ensure the flexible rotation of the movable parts, reduce the pressure loss and wear, but also make the leakage of the gap as small as possible because the leakage directly affects the accuracy of the measurement. Theoretically, this gap should be zero, but it is difficult to achieve in practice. For example, the gap between the rotor and the housing of the waist wheel flow meter and oval gear flow meter is generally set at about 0.1mm. Generally not more than 0.15 mm.

(3) Material of the positive displacement flow meter

Positive displacement flow meters rely on precise volumes to measure flow, and the rotors and materials that make up the volumes will affect the performance and quality of the flow meter. First of all, the mechanical properties of the material restrict the pressure rating used by the flow meter. In a high-pressure fluid, the rotor or housing may be deformed by the high pressure, and the volume of the cavity will change accordingly; the deformation is likely to cause the gap between the rotor and the cavity wall to deviate from the original design value. If it becomes larger, leakage will occur. Bigger, smaller, and the rotor wears out. These will affect the accuracy of the flow meter. This is why early positive displacement meters were generally designed for low pressures. Secondly, due to the presence of moving parts in the volumetric flow parts, the moving parts run continuously during the metering process, and they also need to run at high speed when the flow rate is large, so the problem of wear and tear is highlighted. 

The anti-wear performance of the material has become an important factor affecting the life and accuracy of the flow meter. Again, the temperature rise of the hydraulic oil during continuous operation is very serious, so the thermal deformation of the material is also a problem that needs to be considered in the design of the flow meter. Also, when the ambient temperature is extremely low, attention should also be paid to the cold and brittleness of the material, because the brittleness limits the working pressure of the flow meter. It can be seen that the material of the volumetric flow meter is also one of the key factors determining its performance.

Application and Development of positive displacement flow meter (Volumetric Flow Meter)

This fixed-displacement flow test method of positive displacement flow meters can be traced back to the 18th century, and entered the general commercial application stage in the 1930s. In recent years, the sales volume of PD flow meters in some industrialized countries has exceeded 20%, and it also accounts for about 20% in our country. It is mainly used in the metering of petroleum-based media. However, due to its precise metering characteristics, in recent years, it has been rapidly expanded to chemical, food, medical and other departments to accurately measure the total amount and flow of expensive media. 

In petroleum measurement, an international standard (ISO/DIS2714) 0101 for a measurement system using a positive displacement flow meter has been established. Moreover, the volumetric flow meter with high precision, long-term performance retention and high repeatability is also used as a standard instrument for industrial flow measurement to calibrate and calibrate industrial instruments.

With the advancement and development of hydraulic technology, the requirements for flow measurement accuracy are getting higher and higher. As the most accurate type of flow meter, the positive displacement flow meter will continue to play an advantage in this field. Moreover, with the improvement of the processing technology level and the development of material science, through continuous improvement of its key technology, the positive displacement flow meter will be more widely used.

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