What is a Positive Displacement Flow meter?
Positive displacement flow meters are referred to as PD flow meters or PDF. They are the most accurate type in flow meters. Volumetric flow measurement uses a fixed small volume to repeatedly measure the volume of fluid passing through the flowmeter. It uses mechanical measuring elements to continuously divide the fluid into a single known volume part. The total flow volume is measured according to the number of times that the metering chamber is filled and discharged with the volume part of the fluid successively and repeatedly.
How does a positive displacement flow meter work?
In principle, PDF is a hydraulic engine that absorbs a small amount of energy from the fluid. This energy is used to overcome the frictional force of the flow detection element and the rotation of the accessories. At the same time, a pressure drop is formed at the inflow and outflow ends of the meter.
The working principle of a typical PDF (gear type) is shown in Figure 1.
The two shank round gears have a special shape that rolls to each other and rotates in contact.
P1 and p2 represent the inlet pressure and the outlet pressure respectively, obviously p1>p2.
Figure 1(a) The lower gear is under the action of the two pressure differences. Produce counterclockwise rotation, which is the driving wheel;
The upper gear has equal pressure on both sides. It does not produce a large rotating moment and is a driven wheel. Driven by the lower gear, it rotates clockwise.
In the position of Figure 1(b), both gears generate a rotational torque under the action of differential pressure and continue to rotate.
When rotating to the position in Figure 1(c), the upper gear becomes the driving wheel. The lower gear becomes the driven wheel. Continue to rotate to the same position as Figure 1(a) to complete a cycle.
One cycle action discharges the fluid volume of the four crescent-shaped cavities enclosed by the gear and the shell wall. This volume is called the “circulation volume” of the flow meter.
Suppose the circulation volume of the flow meter is u, and the number of gear rotations in a certain period of time is N. Then the volume of fluid flowing through the flowmeter during this time is V,
The rotation of the Nanyuan gear is transmitted to the counter through the magnetic seal coupling and the transmission reduction mechanism to directly indicate the total amount flowing through the flowmeter.
If the sending device is attached and then equipped with an electric display instrument, it can realize that the remote transmission is only the instantaneous flow or the cumulative flow.
Although there are many segmentation methods to form various forms of PDF, most of them have similar basic characteristics.
The main reason for the PDF error is the leakage of the gap between the movable measuring piece and the static measuring chamber that divides a single fluid volume.
One of the reasons for leakage is to overcome the frictional resistance of moving parts. The second is the effect of pressure drop formed by the hydraulic resistance of the instrument.
Converter Tool: Volumetric Flow Rate & Pipe Diameter to Flow Speed Calculator
Positive displacement flow meter types
There are many types of PDF, which can be classified according to different principles. According to the structure of the measuring element, there are the following types:
1) Rotor type
2) Scraper type
3) Rotating piston type
4) Membrane type, etc.
Read more about: Types of positive displacement flow meter
Positive displacement flow meter advantages and disadvantages
PDF measurement accuracy is high, the basic error is generally ±0.5%R, and the special one can reach ±0.2%R or higher. Usually used inexpensive media or occasions requiring precise measurement.
PDF has no effect on the measurement accuracy when the flow velocity field of the rotating flow and the pipe choke is distorted, and there is no requirement for the front straight pipe section. This is of great significance in on-site use.
PDF can be used for the measurement of high-viscosity fluids.
The range is wide, generally 10:1 to 5:1, and special can reach 30:1 or more.
PDF is a direct-reading instrument without external energy. The cumulative total can be directly obtained, which is clear and easy to operate.
In the indirect mass flow measurement combined with volumetric flowmeters, PDF is compared with volumetric flowmeters derived from velocity equations. The volume is a direct geometrical quantity. The influencing factors of the volumetric quantity are simpler.
In the measurement of high-pressure natural gas that is not suitable for density meter measurement, the compression coefficient of difficult-to-handle gas can be obtained indirectly with PDF.
The PDF structure is complex, bulky, and heavy, especially the larger-diameter PDF is bulky. Therefore, it is generally only suitable for small and medium-sized.
Compared with several other types of general flow meters (such as differential pressure, float, electromagnetic). PDF’s measured medium types, medium working conditions (temperature, pressure), and caliber limitations are larger, and the scope of application is narrow.
Due to the thermal expansion and deformation of parts at high temperatures, and the brittleness of materials at low temperatures, PDF is generally not suitable for high and low temperature applications. At present, the usable temperature range is roughly -30~+160℃, and the maximum pressure is 10MPa.
Most PDF instruments are only suitable for clean single-phase fluids. When particles and dirt are contained, a filter must be installed upstream, which will increase the pressure loss and increase maintenance work. For measuring liquids containing gas, a gas separator must be installed.
PDF security is poor. If the detection moving parts are stuck, the fluid cannot pass through, and the cut-off piping system cannot be used. However, some structural designs (such as Instromet’s waist wheel flowmeter) have a built-in bypass in the housing. When the detection moving element is stuck, the fluid can pass through the bypass.
Read more about Mass Flow Rate vs Volumetric Flow Rate
Use and Maintenance
- The volumetric flowmeter should be installed and used in accordance with the standard flow range, nominal pressure and flow direction marks on the nameplate.
- The positive displacement flowmeter should work under the condition that the fluid temperature is -20~+120℃ and the ambient temperature is -20~+55℃ and the relative humidity of the environment is not more than 80%.
- When the positive displacement flowmeter leaves the factory, it is calibrated with water at room temperature. If the measured fluid is different from the water at room temperature,the constant should be corrected or re-calibrated with the actual measured fluid, but for fluids with a viscosity less than 5×10-6m2/s (5Mpa.s), there is no need to re-calibrate.
- Under normal circumstances, the service life of positive displacement flowmeters is generally from six months to one year, depending on the severity of the working conditions. And should be removed and washed regularly. If the shaft or bearing is found to be severely worn, it should be replaced and re-calibrated.
Positive displacement flow meter vs turbine
Turbine flowmeter has the characteristics of simple structure, light weight, simple maintenance, and adaptability to high parameters (high temperature, high pressure and low temperature). Turbine flowmeters are currently mainly used in the measurement of natural gas, coal gas, petroleum, organic liquids, inorganic liquids, cryogenic fluids, liquefied gas and other media.
So is the turbine flowmeter a positive displacement flowmeter?
Turbine flowmeter is not a positive displacement flowmeter, but a velocity flowmeter. Turbine flow meters, positive displacement flow meters and Coriolis mass flow meters are three types of flow meters with excellent repeatability and accuracy.
Read more about: Turbine Flow Meters
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