About Positive Displacement Flow Meters

Positive displacement flow meters measure the volumetric flow rate of a moving fluid or gas by way of precision-fitted gears or rotors containing cavities through which precisely known volumes of fluid pass. A basic analogy would be holding a bucket below a tap, filling it to a set level, then quickly replacing it with another bucket and timing the rate at which the buckets are filled (or the total number of buckets for the “totalised” flow).

Positive displacement flow meters are very accurate and have high turndown. They work best with clean, non-corrosive, and non-erosive liquids and gases, although some models will tolerate some impurities. They require no straight runs of pipe for fluid flow stream conditioning though pressure drop can be an issue. They are widely used in custody transfer and are applied on residential home natural gas and water metering.

There are several types of positive displacement flow meters available. Trimec Flow Products supplies the following flow meters:

Oscillating Piston meters operate when a liquid enters into a precision machined chamber which contains an oscillating piston. The position of the piston divides the chamber into compartments containing an exact volume. The liquid pressure causes the piston to oscillate and rotate its centre hub. The movements of the hub are sensed through the meter wall by a series of magnets. Each revolution of the piston hub is equivalent to a fixed volume of liquid, which is indicated as flow by an indicator or totaliser. Close clearances between the piston and the chamber ensure minimum slippage of the liquid for highly accurate and repeatable measurement of each volume cycle.

Oval gear meters use oval shaped gear-toothed rotors that rotate within a chamber. As these rotors turn, they sweep out and trap a very precise volume of fluid between the outer oval shape of the gears and the inner chamber walls. The flow rate is then calculated based on the number of times these compartments are filled and emptied.

Selecting a Flow Meter

The basis of good flow meter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process fluid and of the overall installation.

  1. What is the fluid being measured by the flow meter(s) (air, water, etc…)?
  2. Do you require rate measurement and/or totalization from the flow meter?
  3. If the liquid is not water, what viscosity is the liquid?
  4. Is the fluid clean?
  5. Do you require a local display on the flow meter or do you need an electronic signal output?
  6. What is the minimum and maximum flow rate for the flow meter?
  7. What is the minimum and maximum process pressure?
  8. What is the minimum and maximum process temperature?
  9. Is the fluid chemically compatible with the flow meter wetted parts?
  10. If this is a process application, what is the size of the pipe?

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