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מדי זרימה גלגלי שיניים

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.

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