Among the flow measurement techniques, positive displacement flow meters PDFs stand out as one of the most reliable and versatile solutions for volumetric quantification of liquids.
This article explains the definition of PD Flow meters, Types of PD Flow meters, Principles of operation, Advantages, Applications and selection factors
What are Positive Displacement Flow meters?
PD Flow meters, also known as displacement flow meters, are mechanical devices that measure liquid flow by capturing and quantifying the volume of fluid as it passes through a series of discrete chambers.
What are the Types of Positive Displacement Flow meters?
The basis of a PD Flow meter is its measuring element, which comes in various configurations. Commonly used types include:
- Pistons: Cylindrical or spherical elements that move within a chamber, displacing the enclosed liquid volume.
- Vanes: Rotary or reciprocating components that sweep through a chamber, pushing the liquid ahead.
- Nuts or screws: Rotating elements that trap and transport liquid within a helical cavity.
- Membranes: Flexible dividers that reciprocate or deform, capturing the enclosed liquid volume.
Principles of Operation
The operation of PD Flow meters is predicated on the fundamental principle that the movement of the measuring element within the chamber equates to a corresponding displacement of the liquid.
As the liquid flows through the meter, it enters the measuring chamber and is confined by the rotary motion. This action creates a positive displacement of liquid, hence the name.
The measuring element movement is precisely calibrated to correspond to a specific volume of liquid, These calibrations need meter proving and prover.
Each revolution, rotation, or reciprocal motion of the element represents a known volume that is displayed as flow rate or accumulated total. The output signal from the PD Flow meter can be analog, digital or mechanical (counter register).
Advantages of PD Flow meters
- PD meters provide accurate and repeatable measurements over a wide flow range.
- They are not affected by changes in fluid density or viscosity, making them suitable for measuring various fluids.
- PD flow meters can withstand high pressures, making them suitable for demanding applications.
- PD flow meters can be designed to measure either liquids or gases.
Disadvantages of PD Flow meters
- PD flow meters have moving parts that can wear over time, affecting their accuracy.
- They create a pressure drop across the meter, which can be a concern in low-pressure applications.
- PD flow meters have a limited flow rate range, as their capacity is limited by the size of the cavities.
Applications of Positive Displacement Flow meters
- Petroleum Industry: Custody transfer, product blending, and injection metering.
- Chemical and Petrochemical: Raw material metering, product transfer, and blending processes.
- Food and Beverage: Ingredient dosing, syrup dispensing, and beverage filling.
- Pharmaceutical Industry: Precise metering of active ingredients, solvents, and excipients.
- Automotive Sector: Fuel injection metering and leak detection.
Selection Factors of PD Flow meters
Choosing the appropriate PD Flow meter requires careful consideration of several factors:
- Flow Rate: The flow range of the intended application.
- Pipeline Size: The diameter of the pipe where the meter will be installed.
- Liquid Properties: The viscosity, density, and temperature of the liquid being measured.
- Accuracy Requirements: The desired precision of the flow measurement.
- Operating Environment: Temperature, pressure, and exposure to corrosive or hazardous substances.
Positive Displacement Flow meter Installation Considerations
Proper installation of a PD flow meter is essential for accurate measurements. Here are some key considerations:
- Pipe size: Choose a PD flow meter that matches the pipe size to minimize pressure drop.
- Orientation: Mount the PD flow meter in the recommended orientation, typically horizontal with the flow direction.
- Filtration: Install a filter or strainer upstream of the flow meter to prevent particles from clogging the metering chamber.
- Calibration: Regularly calibrate the flow meter to ensure accuracy over time.
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