Positive displacement flowmeters are devices used to measure the volumetric flow rate of liquids. They operate by repeatedly filling and emptying a chamber or chambers of known volume and then counting the number of times this process occurs. Here's how they work:

1. Principle of Operation:

   • Positive displacement flowmeters operate based on the principle that the volume of fluid displaced by a moving part of known volume is directly proportional to the flow rate of the fluid.
   • These flowmeters consist of a chamber or chambers with moving parts such as rotors, pistons, gears, or vanes that alternately fill and empty with fluid as it flows through the meter.
   • The movement of the internal components creates discrete volumes of fluid, which are then measured to determine the flow rate.

2. Measurement Technique:

   • As the fluid flows through the meter, it causes the moving parts to rotate, reciprocate, or oscillate.
   • Each cycle of movement corresponds to a known volume of fluid being displaced.
   • The flowmeter counts the number of cycles or revolutions of the moving parts over a specific time period to determine the total volume of fluid passed through the meter.
   • By dividing the total volume by the elapsed time, the volumetric flow rate of the fluid is calculated.

3. Types of Positive Displacement Flowmeters:

   • There are several types of positive displacement flowmeters, including:
     • Rotary Vane Meters: These flowmeters use rotating vanes or blades to trap and measure discrete volumes of fluid.
     • Oval Gear Meters: These flowmeters use two oval-shaped gears that rotate to trap and measure fluid between the gear teeth.
     • Piston Meters: These flowmeters use a reciprocating piston to displace fluid through the meter.
     • Nutating Disk Meters: These flowmeters use a disk that oscillates within a chamber to displace fluid.

4. Advantages:

   • Positive displacement flowmeters offer several advantages, including:
     • High accuracy and repeatability, especially at low flow rates.
     • Direct measurement of volumetric flow rate, independent of fluid properties such as viscosity and density.
     • Suitable for measuring a wide range of fluids, including viscous and non-conductive liquids.
     • Wide turndown ratio and rangeability.

5. Applications:

   • Positive displacement flowmeters are commonly used in various industries for flow measurement applications, including:
     • Custody transfer of petroleum products such as gasoline, diesel, and crude oil.
     • Chemical processing, including measuring the flow of acids, solvents, and other liquids.
     • Water and wastewater treatment for measuring the flow of potable water, sewage, and industrial wastewater.
     • Food and beverage processing, including measuring the flow of milk, beer, and other liquids.

6. Considerations:

   • While positive displacement flowmeters offer many advantages, they may have limitations in certain applications, such as:
     • Limited suitability for high-flow applications due to mechanical limitations and potential for wear.
     • Susceptibility to damage from particulates or contaminants in the fluid.
     • Higher pressure drop compared to some other flow meters.

Overall, positive displacement flowmeters are reliable and widely used flow measurement devices suitable for a variety of liquid flow applications, offering accurate and repeatable measurements with minimal maintenance requirements.