The construction of Positive Displacement Flowmeter

A Positive Displacement (PD) Flowmeter measures flow by capturing and counting the volume of fluid passing through the meter. It is one of the most accurate types of flowmeters, especially for viscous fluids. Here's a general overview of the construction and key components:

Main Components of a PD Flowmeter

Meter Housing (Body):

Typically made of metal (like stainless steel, brass, or aluminum) or durable plastic.

Designed to contain the moving parts and direct the fluid flow.

Has inlet and outlet ports for fluid entry and exit.

Measuring Chamber:

A sealed chamber inside the meter body where fluid is captured.

Divided into sections by internal moving parts (e.g., gears, pistons, or diaphragms).

Displacement Mechanism:

The core of the PD flowmeter; moves in response to fluid flow.

Types of displacement mechanisms include:

Oval Gears: Two oval gears rotate, trapping fluid between the gear teeth and chamber wall.

Rotary Pistons: Piston(s) move in a reciprocating or rotating motion.

Nutating Disk: A wobbling disk captures fluid volume with each nutation.

Lobed Rotors or Helical Screws: Rotate as fluid passes, moving precise volumes.

Each cycle or movement corresponds to a known volume of fluid.

Rotor Shaft/Bearings:

Support and allow the smooth rotation or movement of the displacement element.

Typically low-friction materials to reduce wear and maintain accuracy.

Output/Transducer System:

Converts mechanical motion into readable data (pulse, analog signal, or digital output).

May use:

Magnetic pickups

Optical sensors

Encoders

The signal is proportional to the volume of fluid displaced.

Counter/Display Unit (Optional):

May be local or remote.

Displays flow rate, total volume, or other parameters.

Can include electronics for signal conditioning, data logging, or communication.

Working Principle (Summary)

As fluid flows through the meter, it mechanically moves the displacement components (e.g., gears, pistons), which trap and transport a fixed amount of fluid. The number of these movements is counted to determine the total flow. Since the volume per movement is known, total volume = number of movements × volume per cycle.

Applications

Custody transfer of fuels

Hydraulic fluids

Viscous liquids like syrups, oils, and chemicals

Residential and industrial water/gas metering

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