Types of Flowmeters and their Applications: Part 1 of 3

Flowmeters are used to measure the flow in a piping system. This article will provide the following information in short:

  • TYPES OF FLOWMETERS
  • PRINCIPLE OF OPERATION
  • FEATURES OF FLOWMETERS
  • ADVANTAGES AND DISADVANTAGES
  • SELECTION/ APPLICATION OF A FLOWMETER

TYPES OF FLOWMETERS

VOLUMETRIC FLOWMETERS

  • Differential Head type
    • Orifice plates
    • Venturi meters
    • Annubar
  • Differential Area type (Rotameters)
  • Electromagnetic flowmeters
  • Ultrasonic flowmeters
  • Turbine flowmeters
  • Vortex flowmeters
  • Positive Displacement Meters

MASS FLOWMETERS

  • Coriolis Mass flowmeter
  • Thermal Mass flowmeters

DIFFERENTIAL HEAD TYPE FLOWMETERS (ORIFICE, Fig. 1)

  • The difference in pressure exists between the upstream & downstream sides of a restriction in a confined fluid stream, which related to the square of fluid velocity.
    • Q α √ ▲P

Where Q = Volume flow rate and ▲P = Differential pressure between taps

Differential head type flowmeters
Fig. 1:Differential head type flowmeters

Types of Orifice plates (Fig. 1)

  • Concentric orifice plate: Most commonly used
  • Segmental & Eccentric orifice plate: Used for fluids containing suspended solids.

Tappings for the Orifice plates:

  • Corner taps (< 1 inch)
  • D and D/2 taps ( 2 to 16 inch)
  • Flange taps (> 16 inch)

FEATURES OF ORIFICE PLATES

  • Design Pressure: No limitation. Limited by DP transmitter
  • Design Temperature: No limitation. Limited by DP transmitter
  • Sizes: Maximum size is the pipe size
  • Flow range: limited only by pipe size.
  • Fluids/ Applications: Cryogenic / clean gases & liquids/ Steam (saturated/superheated)
  • MOC: No limitation (Steel/ monel/nickel/ haste alloy)
  • Accuracy : It varies from ±0.25% to ±0. 5% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
  • Rangeability is 3:1 to 5:1.
  • Upstream length/ Downstream straight length is 20 / 5

ADVANTAGES OF ORIFICE PLATES

  • Easily installed between flanges.
  • Fabrication simple and inexpensive.
  • No limitations on the materials of construction, line size and flow rate
  • Cost relatively independent of pipe diameter since the cost of DPT is fixed.
  • No process interruption for the exchange of DP transmitter.

DISADVANTAGES OF ORIFICE PLATES

  • High permanent pressure loss & hence high energy consumption to overcome pressure loss.
  • Impractical for systems with low static pressure.
  • Measuring range to about 3:1 to 5:1.
  • Accuracies decrease with Beta ratios above approximately 0.7.
  • Subject to damage by water hammer and foreign objects.

VENTURI METERS (Fig. 2)

  • A venturi tube also measures flow rates by constricting fluids and measuring a differential pressure drop.
  • In the upstream cone of the Venturi meter, velocity is increased, the pressure is decreased.
  • Pressure drop in the upstream cone is utilized to measure the rate of flow through the instrument
Figure showing Venturi meter and Annubar Flowmeter
Fig. 2: Figure showing Venturi meter and Annubar Flowmeter

FEATURES OF VENTURI METERS:

  • Design Pressure: No limitation. Limited by DP transmitter/ pipe press.ratings.
  • Design Temperature: No limitation. Limited by DP transmitter/ pipe pressure ratings
  • Sizes: 25 mm to 3000 mm
  • Fluids/ Applications: Clean Liquids/ clean gases
  • Limited applications: Dirty /corrosive/viscous Liquids & Dirty gases
  • Flow range: limited only by pipe size and beta ratio.
  • MOC: No limitation (cast iron/ carbon steel/ SS/Monel, Titanium, Teflon, Hastelloy, Naval Bronze/haste alloy)
  • Accuracy : It varies from ±0.25% to ±0. 75% of actual flow. The accuracy of DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
  • Rangeability is 3:1 to 5:1.
  • Upstream length/ Downstream straight length is 20 / 5

ADVANTAGES OF VENTURI METERS

  • Lower head losses than orifice plates reducing the capital expenditure on pumping eqpt. / save pump energy costs
  • No process interruption for the exchange of DP transmitter.
  • Can be used for temperature extremes
  • Cryogenics or High Temperatures

DISADVANTAGES OF VENTURI METERS

  • Highly expensive
  • Larger and heavier to handle.

ANNUBAR FLOWMETER (Fig. 2)

  • The probe is installed in the media line as a pressure sensor.
  • With the flow, the probe records both the static and the dynamic pressure via the probe openings.
  • In the minus chamber of the annubar, lying on the opposite side, only the static pressure has any effect
  • The differential pressure corresponds to the dynamic pressure in the pipeline & the flow can is calculated directly.

FEATURES OF ANNUBAR:

  • Design Pressure: Upto 97 bars (38 Deg.C) / 55 bars (370 Deg.C)
  • Design Temperature: Upto 400 deg.C
  • Sizes: 50 mm to 3000 mm
  • Fluids : Clean Liquids, gases and steam
  • MOC: Brass / steel/ stainless steel/ Hastelloy
  • Accuracy : It varies from ±1% to ±2% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
  • Rangeability is 3:1 to 5:1.
  • Upstream length/ Downstream straight length is 20 / 5

ADVANTAGES OF ANNUBAR

  • The integral manifold head allows direct mounting of DP transmitters
  • Hot tapping: Insertion/ installation without system shutdown
  • Very low-pressure drop

DISADVANTAGES OF ANNUBAR

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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