Flow Meter Types Based on Measurement Techniques

Flow meter is an instrument used for the measurement of the amount of liquid, gas or vapor that passes through them. In some flow meter the flow is measured as the amount of fluid passing through the flow meter during a time period (such as 100 liters per minute) whereas other flow meters measure the totalized amount of fluid that has passed through the flow meter (such as 100 liters).

Transducer and transmitter are the primary devices of a flow meter. The fluid that passes through the primary device are sensed by the transducer while the transmitter produces a usable flow signal from the raw transducer signal. These components are often combined, so the actual flow meter may be one or more physical devices.

Selection of flow meter:

One specific flow meter does not fit all the applications. Each type of flow meter has its own applications and constraints.

Generally particular flow meter is selected based on application and not the technology.

Depending upon the techniques of measurement the flow meters are generally categorized as below:

Volumetric flow meters

• 1. Differential Head type
  • A. Orifice plates
  • B. Venturi meters
  • C. Annubar
• 2. Differential Area type (Rotameters)
• 3. Electromagnetic flowmeter
• 4. Vortex flowmeter
• 5. Ultrasonic flowmeter
• 6. Turbine flowmeter
• 7. Positive displacement flowmeter

Mass flow meters

• 1. Coriolis Mass flowmeter
• 2. Thermal Mass flowmeters

1. DIFFERENTIAL HEAD TYPE FLOWMETERS

A. ORIFICE PLATES

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
–▲P = Differential pressure between taps

 

Types of Orifice plates

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

Tappings for the Orifice plates

1. Corner taps (< 1 inch)
2.  D and D/2 taps ( 2 to 16 inch)
3.  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 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 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 cost of DPT is fixed.
• No process interruption for 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.

B. VENTURI METERS

a.    A venturi tube also measures flow rates by constricting fluids and measuring a differential pressure drop.
b.    In the upstream cone of the Venturimeter, velocity is increased, pressure is decreased.
c.    Pressure drop in the upstream cone is utilized to measure the rate of flow through the instrument

Features of Venturimeters

a.    Design Pressure: No limitation. Limited by DP transmitter/ pipe press ratings.
b.    Design Temperature: No limitation. Limited by DP transmitter/ pipe pressure ratings
c.    Sizes: 25 mm to 3000 mm
d.    Fluids/ Applications: Clean Liquids/ clean gases
e.    Limited applications: Dirty /corrosive/viscous Liquids & Dirty gases
f.     Flow range: limited only by pipe size and beta ratio.
g.    MOC: No limitation (cast iron/ carbon steel/ SS/Monel, Titanium, Teflon, Hastelloy, Naval Bronze / haste alloy)
h.    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.
i.      Rangeability is 3:1 to 5:1.
j.      Upstream length/ Downstream straight length is 20 / 5

Advantages of Venturimeters

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

Disadvantages of Venturimeters

a.    Highly expensive
b.    Larger and heavier to handle.

C. ANNUBAR FLOWMETER

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

Features of Annubar

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

Advantages of Annubar

a.    Integral manifold head allows direct mounting of DP transmitters
b.    Hot tapping: Insertion/ installation without system shutdown
c.    Very low pressure drop

Disadvantages of Annubar

1. Not suitable for viscous and slurry applications
2. Can be used for only for clean fluids

2.    VARIABLE AREA FLOWMETER (ROTAMETERS)

1. A free moving float is balanced inside a vertical tapered tube
2. As the fluid flows upward the float remains steady when the dynamic forces acting on it are zero.
3. The flow rate indicated by the position of the float relative to a calibrated scale.

Features of Rotameters

a.    Design Pressure: Upto 350 PSIG (GLASS TUBE) / 720 PSIG (METAL TUBE)
b.    Design Temperature: Upto 400 deg.C (GLASS TUBE) / 538 Deg.C (METAL TUBE)
c.    Sizes: upto 75 mm
d.    Fluids/ Applications: Clean liquids, gases and vapours
e.    Flow range: upto 920 cub.m/hr for liquids & 2210 cub.m/hr for gases
f.    MOC: Borosilicate glass/ brass / steel/ stainless steel/ Hastelloy
g.    Accuracy: It varies from ±1% to ±2% of actual flow.
h.    Rangeability is 10:1
i.    Upstream length/ Downstream straight length is 10 / 5

Advantages of Rotameters

a.    Simple, robust and linear output
b.    Does not require external impulse or lead lines.
c.    Pressure drop is minimal and fairly constant.

Disadvantages of Rotameters

a.    Vertical installation only.
b.    Glass tubes limit pressure & temperature and subject to breakage from hydraulic & thermal shock
c.    Glass tubes eroded by undissolved solids & unsuitable for metering alkaline solutions
d.    Metal tube meters more expensive.
e.    Foreign particles can accumulate around the float & block the flow

3.    ELECTROMAGNETIC FLOWMETERS

a.    Operate on Faraday’s Law of magnetic induction.
b.    When a conductive fluid moves in a magnetic field, a voltage is generated between two electrodes at right angles to the fluid velocity and field orientation.
c.    The flow tube has a fixed area & field intensity so the developed voltage is linearly proportional to the volumetric flow rate.

Features of Magnetic flow meters

a.    Design Pressure: 20 BARS to 172 BARS
b.    Design Temperature: Upto 120 deg.C with teflon liners /
c.    180 Deg.C with ceramic liners
d.    Sizes: 2.5 mm to 3000 mm
e.    Fluids: Liquids (clean/ corrosive/dirty/viscous/ slurry)
f.     Velocity range: 0.1 to 10 m/s
g.    MOC: Liners: ceramic/ teflon/rubber
Electrodes: Platinum/ hastelloy/SS
h.    Accuracy: It varies from ±0.5% to ±1% of actual flow.
i.      Rangeability is 10:1
j.      Upstream length/ Downstream straight length is 10 / 5

Advantages of Magnetic flow meter

a.    Flow rate unaffected by fluid density, consistency, viscosity, turbulence, or piping configuration.
b.    Highly accurate due to absence of moving parts/ external sensing lines
c.    Corrosion-resistant using Teflon liner and platinum electrodes
d.    Wide flow measuring ranges & no pressure drop

Disadvantages of Magnetic flow meter

a.    Costly, relative to other flow meter types.
b.    Temperature of the fluids being metered limited by the liner material rating.
c.    Cannot be used for gas flow measurements

4.    VORTEX FLOWMETERS

1. An obstruction is placed across the pipe bore at right angle to fluid flow.
2. As fluid flows, vortices are shed from alternating sides of the body & this shedding frequency is directly proportional to fluid velocity.
3. Detection of the vortices by means of pressure changes in the vortex stream.
4. Rate of creation of vortices directly proportional to the flow rate.

 

Features of Vortex flow meters
a.    Design Pressure: 138 bars
b.    Design Temperature: -200 Deg. C to 400 Deg.C
c.    Sizes: 15 mm to 300 mm
d.    Fluids : Gases (clean/ dirty) and clean liquids
e.    Velocity range: 0.3 to 10 m/s (liquids) and 6 to 80 m/s (gases)
f.    MOC: mostly in stainless steel, some in plastic
g.    Accuracy: It varies from ±0.5% to ±1% of actual flow for liquids and
±1% to ±1.5% for gases
h.    Rangeability is 20: 1
i.    Upstream length/ Downstream straight length is 20 / 5

Advantages of Vortex flowmeters

a.    Minimal maintenance, no moving parts.
b.    Calibration using fluid flow not required & unaffected by viscosity, density, pressure, and temperature within operating specification.
c.    Digital or analog output.

Disadvantages of Vortex flowmeters

a.    At low flows, pulses are not generated and the flow meter can read low or even zero.
b.    Reynolds number should be greater than 10000
c.    Vibration can cause errors in accuracy.
d.    Correct installation is critical as a protruding gasket or weld beads can cause vortices to form, leading to inaccuracy.
e.    Long, clear lengths of upstream pipework must be provided, as for orifice plate flow meters.

5.    ULTRASONIC FLOWMETERS

1. A pair (or pairs) of transducers, each having its own transmitter and receiver, are placed on the pipe wall, one (set) on the upstream and the other (set) on the downstream.
2. The time for acoustic waves to travel from the upstream transducer to the downstream transducer td is shorter than the time it requires for the same waves to travel from the downstream to the upstream tu.
3. The larger the difference, the higher the flow velocity.

Features of Ultrasonic flowmeters

1. Design Pressure: 207 bars (insertion type)/ unlimited (clamp on type)
2. Design Temperature: -180 Deg. C to 260 Deg.C
3. Sizes: 3 mm to 3000 mm
4. Fluids: clean gases, clean/corrosive liquids (with little/no solids/ bubbles)
5. Velocity range: 0.3 to 15 m/s
6. MOC: mostly in stainless steel/ alloyic
7. Accuracy is +0.5% of flowrate for insertion type +1% to +3% of flowrate for clamp on type
8. Range ability is 10 : 1 to 300 : 1
9. Upstream length/ Downstream straight length is 10 / 5
10. Bidirectional flow measurement
11. For insertion type, hot tapping in pressurized pipelines possible

Advantages of Ultrasonic flow meters

a.    No obstruction/ moving parts in the flow path
b.    No pressure drop
c.    Low maintenance cost
d.    Multi-path models have higher accuracy for wider ranges of Reynolds number
e.    Can be used in corrosive fluid flow
f.     Portable models available for field analysis and diagnosis

Disadvantages of Ultrasonic flow meters

a.    Only clean liquids and gases can be measured
b.    Higher initial set up cost

6.    TURBINE FLOWMETERS

1. Consists of a multi-bladed rotor mounted at right angles to the flow & suspended in the fluid stream on a free-running bearing.
2. The diameter of the rotor is slightly less than the inside diameter of the flow metering chamber.
3. Speed of rotation of rotor proportional to the volumetric flow rate.

Features of Turbine flow meters
a.    Design Pressure: 1500 PSIG
b.    Design Temperature: 150 Deg. C
c.    Sizes: 5 mm to 600 mm (Full bore type)/ > 75 mm for insertion type
d.    Fluids: Clean liquids/ gases and vapours
e.    Velocity range: 0.3 to 15 m/s
f.     MOC: mostly in stainless steel/ hastelloy
g.    Accuracy is +0.25% to + 0.5% of flowrate for full bore type +1% to +3% of flowrate for insertion type
h.    Range ability is 10 : 1
i.    Upstream length/ Downstream straight length is 15/ 5
j.    Bidirectional flow measurement
k.    For insertion type, hot tapping in pressurized pipelines possible

Advantages of Turbine flow meters

a.    Very accurate. Commonly used to prove other meters.
b.    Digital output provides for direct totalizing, batching, or digital blending without reducing accuracy.
c.    There is less tendency to read high in pulsating flow than in head or variable-area type meters.

Disadvantages of Turbine flowmeters

a.    Not usable in dirty streams or with corrosive materials.
b.    Subject to fouling by foreign materials -fibers, tars etc.
c.    Bearings subject to wear or damage. Shift in calibration if bearings replaced
d.    Can be damaged by overspeeding (over 150 percent) or by hydraulic shock.
e.    Pressure loss at rated flow varies & can be high.

7.    POSITIVE DISPLACEMENT METERS

a.    This meter repeatedly entraps the fluid into a known quantity and than passes it out.
b.    The quantity of the fluid that has passed is based on the number of entrapments.
c.    The volume flow rate can be calculated from the revolution rate of the mechanical device.

Features of PD flow meters

a.    Design Pressure: 1500 PSIG (liquids)
: 100 psig (gases)
b.    Design Temperature: 293 Deg. C (liquids)
: -34 to 60 Deg. C (gases)
c.    Sizes: 6 mm to 400 mm
d.    Fluids: Clean Liquids/ gases
e.    Flow range: 0 – 20000 GPM (liquids)
i.    : 0 – 3000 cub.m/hr (gases)
f.     MOC: mostly in aluminum, stainless steel, plastics, hastelloy
g.    Accuracy is + 0.5% to + 1% of flowrate
h.    Range ability is 15: 1

Advantages of PD flow meters

a.    Good accuracy and high range ability
b.    Can be used in viscous liquid flow
c.    Low to medium initial set up cost
d.    Require no power supply and available in wide variety of read out devices

Disadvantages of PD flow meters

a.    Maintenance required at frequent intervals because of the `moving parts.
b.    High pressure drop due to obstruction
c.    Not suitable for low flow rate
d.    Not suitable for fluids with suspended solids
e.    Gas (bubbles) in liquid could significantly decrease the accuracy

Applications of Flowmeters

The selection of the flowmeters primarily depends upon the application or service for which it is to be used. Based on accuracy, cost, durability and reliability you can select the technology you wish to use. So the guide for selection of right flowmeter is the application rather than the technology.

Based on the application below is the list of flowmeter which can be used.

1. Clean Liquids/Gases

• Orifices
• Venturi meters
• Annubar
• Variable Area
• Magnetic (only liquids)
• UltrasonicFlowmeter
• VortexFlowmeter
• Coriolis Mass Flowmeter
• Thermal mass flowmeter (only gases)
• PD Flowmeter

2. Dirty Liquids

Most suited

• MagneticFlowmeter
• Coriolis Mass Flowmeter

Limited applications

• Venturi meters

3. Dirty Gases

Most suited

• Vortex Flowmeters

Limited applications

• Venturi meters
• Thermal mass Flowmeter
• Variable area Flowmeter
• Magnetic Flowmeter
• Ultrasonic Flowmeter
• Coriolis Mass Flowmeter
• Coriolis Mass Flowmeter
• Magnetic
• Positive Displacement Meter
• Magnetic Flowmeter
• Coriolis Mass Flowmeter
• Magnetic Flowmeter
• Coriolis Mass Flowmeter (limited applications)

4. Corrosive Liquid

5. Non-Newtonian Liquids

6. Viscous Liquids

7. Abrasive Slurries

8. Fibrous Slurries

9. Saturated Steam

Most suited

• Orifice DP meter
• Vortex Flowmeter

Limited applications

• Venturi meters
• Variable area flowmeter

10. Superheated Steam

Most suited

• Orifice DP meter

Limited applications

• Venturi meters
• Venturi meters
• Orifice plates

11. Cryogenic