What are Positive Displacement Flow Meters?


How they work, where they excel, advantages and limitations.

How they Work:

Positive displacement flow meters act like a hydraulic pump in reverse. The moving parts are designed so that a finite amount of liquid is alllowed to fill a chamber and then is expelled, before it is filled again by the same amount. The volume per revolution of the internal components is a known value per revolution. The precise motion of the rotating element against the uniform measuring chamber creates a direct volumetric measurement.

There are many different internal configurations for positive displacement flow meters. Regardless of the differences, they all operate on this same principle. It is similar to taking a one cup measuring cup, allowing it to fill up, and then dumping it out, over and over and over again. This is essentially what happens as a precise, discreet amount of liquid is trapped and then released from the meter body. As a result, these meters can handle intermittent flow because they move when there is flow and stop when there is none.

Positive displacement technology is well proven and is the standard technology for certain operations. For example, when they are properly installed and calibrated, they are accepted universally as the technology of choice for applications like custody transfer. This technology is also commonly used in residential water consumption, although allowances and considerations have to be made for using such a low viscosity media.

Positive Displacement Flow Meters
Positive Displacement Diagram
Positive Displacement Flow Meters

Types of Positive Displacement:

  • Oval Gear
  • Helical Gear
  • Spur Gear
  • Reciprocating/Oscillating Piston
  • Nutating Disc, Wobble Plate
  • Rotary/Sliding Vane
  • Bi-rotor or Tri-rotor
  • Multi-piston

Advantages:

  • Can process a wide variety of viscosities
  • Next to no maintenance
  • High pressure capabilities
  • Can measure intermittent flows
  • High turndown ratio
  • Accurate, repeatable, linear, reliable
  • High resolution
  • Can measure low flow rates
  • No power needed for mechanical models
  • Doesn’t require a developed flow profile
  • No straight run requirements
  • Unaffected by noise or vibration
  • Accuracy not affected much by viscosity changes over a wide range
  • Less expensive than other technologies
  • Can measure non-conductive liquids
  • Not affected by changes in temperature

Common Applications:

  • Oil and Gas
  • Water and Wastewater
  • Chemical Industry and Injection
  • Power Generation
  • Pharmaceuticals
  • Food and Beverage
  • Pulp and Paper
  • Metal and Mining
  • Aerospace
  • Test Stands
  • Hydraulic Testing

Considerations:

  • Pressure drop should be considered
  • Can measure liquids or gases
  • Can technically be used for lower viscosity liquids, like water, but it reduces longevity
  • Must be installed in the correct orientation

Limitations:

  • Only for clean media
  • Usually only uni-directional
  • Not for applications with air pockets
  • Heavy
  • Usually requires filtration

Sample Media:

  • Detergents
  • Deoderizers
  • Caustics
  • Acids
  • Fuels
  • Oils
  • Honey
  • Molasses
  • Anti-icing agents
  • Brake fluid
  • Waxes
  • Adhesive coatings
  • Polyurethane foams
  • Combustion modifiers
  • Corrosion inhibitors
  • Kerosene
  • Diesel fuels
  • Inks
  • Dyes
  • Pastes
  • Resins
  • Glues
  • Creams