eFunda: Introduction to Positive Displacement Flowmeters (PD meters)
engineering fundamentals Positive Displacement Flowmeter
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Overview

Positive displacement flowmeters, also know as PD meters, measure volumes of fluid flowing through by counting repeatedly the filling and discharging of known fixed volumes. A typical positive displacement flowmeter comprises a chamber that obstructs the flow. Inside the chamber, a rotating/reciprocating mechanical unit is placed to create fixed-volume discrete parcels from the passing fluid. Hence, the volume of the fluid that passes the chamber can be obtained by counting the number of passing parcels or equivalently the number rounds of the rotating/reciprocating mechanical device. The volume flow rate can be calculated from the revolution rate of the mechanical device.

Many types of positive displacement flowmeters are used in the industry. They are named after the mechanical device inside the chamber. They all share the same principle of operation and are volumetric flow measuring instruments. Common positive displacement flowmeters are:

  • Nutating Disc:

  • Rotating Valve:

  • Oscillating Piston:

  • Oval Gear:

  • Roots (Rotating Lobe):

  • Birotor:

  • Rotating Impeller:

The accuracy of positive displacement flowmeters relies on the integrity of the capillary seal that separates incoming fluid into discrete parcels. To achieve the designed accuracy and ensure that the positiive displacement flowmeter functions properly, a filtration system is required to remove particles larger the 100 µm as well as gas (bubbles) from the liquid flow.

Positive displacement flowmeters, although simple in principle of operation and widely used in the industry, all cause a considerable pressure drop which has to be considered for any potential application.

Common Specifications

Common specifications for commercially available positive displacement flowmeterss are listed below :

  Fluid Phase: Please refer to the flowmeter selection for specific sub-categories

Score Phase Condition
 Liquid  Clean 
  Viscous 
 Liquid  Corrosive  
  Dirty 
: Recommended
: Limited applicability

  Line Size: 6 ~ 300 mm (1/4 ~ 12 inch)
  Turndown Ratio: 5 ~ 15 : 1, might go as high as 100 : 1

Pros and Cons

  • Pros:
  - Low to medium initial set up cost
  - Can be used in viscous liquid flow
  • Cons:
  - Higher maintenance cost than other non-obstructive flowmeters
  - High pressure drop due to its total obstruction on the flow path
  - Not suitable for low flow rate
  - Very low tolerance to suspension in flow (particles larger than 100 µm need to be filtered before the liquid enters the flowmeter)
  - Gas (bubbles) in liquid could significantly decrease the accuracy

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