eFunda: Introduction to Differential Pressure Flowmeters (PD meters)
engineering fundamentals Differential Pressure Flowmeter
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Differential pressure flowmeters (in most cases) employ the Bernoulli equation that describes the relationship between pressure and velocity of a flow. These devices guide the flow into a section with different cross section areas (different pipe diameters) that causes variations in flow velocity and pressure. By measuring the changes in pressure, the flow velocity can then be calculated.

Many types of differential pressure flowmeters are used in the industry:

  • Orifice Plate: A flat plate with an opening is inserted into the pipe and placed perpendicular to the flow stream. As the flowing fluid passes through the orifice plate, the restricted cross section area causes an increase in velocity and decrease in pressure. The pressure difference before and after the oriffice plate is used to calculate the flow velocity. A calculator for the orifice plate flowmeters can be found in the fluid mechanics section.

  • Venturi Tube: A section of tube forms a relatively long passage with smooth entry and exit. A Venturi tube is connected to the existing pipe, first narrowing down in diameter then openning up back to the original pipe diameter. The changes in cross section area cause changes in velocity and pressure of the flow. A calculator for the venturi tube flowmeters can be found in the fluid mechanics section.

  • Nozzle: A nozzle with a smooth guided entry and a sharp exit is placed in the pipe to change the flow field and create a pressure drop that is used to calculate the flow velocity.

  • Segmental Wedge: A wedge-shaped segment is inserted perpendicularly into one side of the pipe while the other side remains unrestricted. The change in cross section area of the flow path creates pressure drops used to calculate flow velocities.

  • V-Cone: A cone shaped obstructing element that serves as the cross section modifier is placed at the center of the pipe for calculating flow velocities by measuring the pressure differential.

  • Pitot Tube: A probe with an open tip (Pitot tube) is inserted into the flow field. The tip is the stationary (zero velocity) point of the flow. Its pressure, compared to the static pressure, is used to calculate the flow velocity. Pitot tubes can measure flow velocity at the point of measurement. See Pitot Tube Flowmeters and Pitot Static Tubes for more details.

  • Averaging Pitot Tube: Similar to Pitot tubes but with multiple openings, averaging Pitot tubes take the flow profile into consideration to provide better over all accuracy in pipe flows.

  • Elbow: When a liquid flows through an elbow, the centrifugal forces cause a pressure difference between the outer and inner sides of the elbow. This difference in pressure is used to calcuate the flow velocity. The pressure difference generated by an elbow flowmeter is smaller than that by other pressure differential flowmeters, but the upside is an elbow flowmeter has less obstruction to the flow.

  • Dall Tube: A combination of Venturi tube and orifice plate, it features the same tapering intake portion of a venturi tube but has a 'shoulder' similar to the orifice plate's exit part to create a sharp pressure drop. It is usually used in applications with larger flow rates.

Differential pressure flowmeters, although simple in construction and widely used in industry, have a common drawback: They always create a certain amount of pressure drop, which may or may not be tolerated in a particular application.

Common Specifications

Common specifications for commercially available differential pressure flowmeterss are listed below (These specifications are for differential pressure flowmeters in general. Individual numbers may vary from product to product.):

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

Score Phase Condition
 Gas  Clean 
 Liquid  Clean 
 Steam  Saturated 
 Liquid  Corrosive  
 Slurry  Abrasive 
: Recommended
: Limited applicability

  Line Size: 6 ~ 300 mm (1/4 ~ 12 inch)
  Turndown Ratio: 10 : 1

Pros and Cons

  • Pros:
  - Low to medium initial set up cost
  - Can be used in wide ranges of fluid phases and flow conditions
  - Simple and sturdy structures
  • Cons:
  - Medium to high pressure drop

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