Laminar Flow Meter Working Principle – InstrumentationTools

Advantages & Disadvantages of Different Flow Meters like electromagnetic flow meter, orifice flow meter, turbine flow meter, Coriolis mass flow meter, Thermal mass flow meter, variable area flow meter, vortex flow meter etc.

Advantages and Disadvantages of Flow Measurement Techniques

Electromagnetic Flow meters

Advantages:

  • Unobstructed flow passage without projecting parts
  • No moving parts
  • No additional pressure drop
  • Essentially flow profile insensitive, only short inlet and outlet sections required
  • Unaffected by changes in temperature, density, viscosity, concentration and electrical conductivity
  • Favorable choice of materials for chemically aggressive or abrasive measuring media
  • Unaffected by contamination and deposits
  • Especially suitable for hydraulic solids transport
  • Linear relationship between flow rate and measured variable
  • Operates in both flow directions (forward and reverse)
  • Measuring range setting can be optimized
  • Low maintenance, but still easy to maintain

Limitations:

  • For liquids only
  • Lower conductivity limit 0.05 μS/cm
  • Gas inclusions cause errors

Ultrasonic Flow meters

Advantages:

  • Unobstructed flow passage
  • No moving parts
  • No additional pressure drop
  • Favorable choice of materials for chemically aggressive liquids
  • Linear relationship between flow rate and measured variable
  • Low maintenance
  • Operates in both flow directions (forward and reverse)
  • Transit time meters unaffected by temperature, density and concentration
  • Later installation in existing pipe possible with individual elements, but onsite calibration required

Limitations:

  • Still problematic for liquid and gas measurements
  • Sound beam must traverse a representative cross section, therefore flow profile dependent. Long inlet and outlet sections required
  • Errors due to deposits
  • Transit time meters require clean liquids
  • Doppler meters only for slight contamination or few gas bubbles
  • Doppler meters affected by sound velocity changes due to temperature, density and concentration
  • Unsuitable for heavily contaminated liquids
  • Gas bubbles cause errors

Coriolis Mass Flow meters

Advantages:

  • True mass flow measurement
  • Additional temperature and density measurements
  • Very high accuracy for mass flow measurements
  • Highly accurate density measurement
  • Unaffected by pressure, temperature and viscosity
  • No inlet and outlet sections required
  • Operates in both flow directions (forward and reverse)
  • Measuring range settings can be optimized for flow rate and density
  • Self-draining

Limitations:

  • Affected by gas inclusions
  • Vibration sensitive when improperly installed
  • Limited choice of materials
  • Nominal diameter limited at the top

Thermal Mass Flow meters

Advantages:

  • Direct gas mass flow measurement
  • No pressure and temperature compensation required
  • Very low pressure drop
  • High measuring accuracy
  • Large span
  • No moving parts
  • Rugged construction
  • Short response time
  • Easily sterilized

Limitations:

  • For gases only
  • Inlet and outlet sections required

Differential Pressure Flow meters

Advantages:

  • Universally suitable for liquids, gases and steam
  • Also usable in extreme situations, e.g. viscosity, due to variety of versions
  • Calculations possible for unusual situations
  • Suitable for extreme temperatures and pressures
  • Range changes possible
  • Low pressure drop for nozzles
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Limitations:

  • Square root relationship between flow rate and differential pressure, therefore smaller span
  • Affected by pressure and density changes
  • Pressure drop for orifice plates
  • Edge sharpness for orifice plates must be assured, therefore no solids or contamination
  • Very long inlet and outlet sections
  • Expensive installation requiring differential pressure lines, fittings and sensors
  • Installation and maintenance experience advantageous
  • High maintenance requirements

Turbine Flow Meters

Advantages:

  • No external power supply for Rotating vane and Woltman meters
  • Turbine flow meters suitable for cryogenic liquids
  • Turbine flow meters usable at extreme temperatures and pressures

Limitations:

  • Limited choice of materials
  • Only for low viscosities
  • Moving parts, wear
  • Sensitive to contamination
  • Axial flow totalizers are flow profile sensitive
  • Inlet and outlet sections required (not for rotating vane meters)
  • Affected by overloading and quick changes at high differential pressure, danger of over speeding
  • Vibration sensitive

Variable Area Flow meters

Advantages:

  • Inexpensive
  • No external power supply required for local indication
  • Suitable for liquids, gases and steam
  • No inlet and outlet sections required
  • Simple meter design, therefore easy to install and maintain
  • Indication also with opaque liquids
  • Metal cone meter with transmitter
  • Metal cone meter can be sterilized, CIP tested

Limitations:

  • Vertical mounting position
  • Constant pressure drop
  • Affected by density, temperature and viscosity changes
  • Solids damage metering edge, otherwise slight contamination allowed
  • Affected by pulsation and vibration
  • Expensive when exotic materials are required

Vortex Flow meters

Advantages:

  • No moving parts
  • Rugged construction
  • Suitable for liquids, gases and steam
  • Easily sterilized
  • Unaffected by pressure, temperature and density changes
  • Linear relationship between flow rate and measured value

Limitations:

  • Inlet and outlet sections required
  • Minimum Reynolds number required

Swirl Flow meters

Advantages:

  • No moving parts
  • Short inlet and outlet sections → 3 x D/1 x D
  • Suitable for liquids, gases and steam
  • Excellent repeatability
  • Unaffected by pressure, temperature and density changes

Limitations:

  • Pressure drop
  • Minimum Reynolds number required

Weirs

Advantages:

  • Simple design
  • Minimum space requirements at the measuring point
  • Low construction costs

Limitations:

  • Damming, therefore higher space requirements upstream of the measuring point
  • Risk of deposit build up upstream of the weir, not suitable for waste water
  • Stream separation through ventilation must be assured
  • Affected by large floating items

Venturi Flumes

Advantages:

  • No potential energy differences compared to the weir
  • Low pressure drop
  • Suitable for unclean waste water
  • Easy to maintain

Limitations:

  • Nonlinear flow characteristic
  • Channel constriction resulting in damming of the head water and risk of deposit build up in the event of velocity decrease
  • Risk of plugging through larger floating items
  • Measurement impossible when backflow exists in tail water up to Venturi flume
  • Quality and reliability of the measurement depending on connected sensor
  • Installation costs

Oval Gear and Oscillating Piston Flow Meters

Advantages:

  • High measuring accuracy
  • Suitable for measuring media with high viscosity
  • Operates in both flow directions (forward and reverse)
  • No flow profile effects, thus no inlet and outlet sections required
  • No external power supply

Limitations:

  • Volume totalizer
  • For liquids only
  • High pressure drop
  • Moving parts, wear
  • Accuracy decrease for lower viscosities due to gap losses
  • Sensitive to contamination, filter required
  • Flow blockage at zero flow through solid impurities
  • Sensitive to overloading
  • Monitoring and maintenance

Lobed Impeller Flow Meters

Advantages:

  • Excellent measuring accuracy for gas measurements
  • No inlet and outlet sections required
  • No external power supply

Limitations:

  • Volume totalizer
  • For gases only
  • Moving parts, wear
  • Flow blockage at zero flow through solid impurities
  • Sluggish toward quick changes
  • Also affected by quick changes at high differential pressure, danger of over speeding
  • Monitoring

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Orifice Flow Meter

An orifice in a pipeline is shown in below figure with a manometer for measuring the drop in pressure (differential) as the fluid passes thru the orifice. The minimum cross sectional area of the jet is known as the “vena contracta.”

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Fundamentals of Orifice Flow Measurement

How does it work?

As the fluid approaches the orifice the pressure increases slightly and then drops suddenly as the orifice is passed. It continues to drop until the “vena-contracta” is reached and then gradually increases until at approximately 5 to 8 diameters downstream a maximum pressure point is reached that will be lower than the pressure upstream of the orifice.

The decrease in pressure as the fluid passes thru the orifice is a result of the increased velocity of the gas passing thru the reduced area of the orifice.

When the velocity decreases as the fluid leaves the orifice the pressure increases and tends to return to its original level. All of the pressure loss is not recovered because of friction and turbulence losses in the stream.

The pressure drop across the orifice ( ΔP in Fig.) increases when the rate of flow increases. When there is no flow there is no differential.

The differential pressure is proportional to the square of the velocity, it therefore follows that if all other factors remain constant, then the differential is proportional to the square of the rate of flow.

Orifice Construction

Orifice Construction

Inlet Section

A linearly extending section of the same diameter as the inlet pipe for an end connection for an incoming flow connection. Here we measure the inlet pressure of the fluid / steam / gas.

Orifice Plate

An Orifice Plate is inserted in between the Inlet and Outlet Sections to create a pressure drop and thus measure the flow.

Outlet Section

A linearly extending section similar to the Inlet section. Here also the diameter is the same as that of the outlet pipe for an end connection for an outgoing flow. Here we measure the Pressure of the media at this discharge.

As shown in the adjacent diagram, a gasket is used to seal the space between the Orifice Plate and the Flange surface, prevent leakage.

Sections 1 & 2 of the Orifice meter, are provided with an opening for attaching a differential pressure sensor (u-tube manometer,differential pressure indicator).

Material of construction

The Orifice plates in the Orifice meter, in general, are made up of stainless steel of varying grades.

Shape & Size of Orifice meter :

Orifice meters are built in different forms depending upon the application specific requirement, The shape, size and location of holes on the Orifice Plate describes the Orifice Meter Specifications as per the following:Types of Orifice plates

  • Concentric Orifice Plate
  • Eccentric Orifice Plate
  • Segment Orifice Plate
  • Quadrant Edge Orifice Plate

Concentric Orifice Plate

It is made up of SS and its thickness varies from 3.175 to 12.70 mm. The plate thickness at the orifice edge should not be exceeded by any of following parameters:

  • 1 – D/50 where, D = The pipe inside diameter
  • 2 – d/8 where, d = orifice bore diameter
  • 3 – (D-d)/8

*Beta Ratio(β): It is the ratio of orifice bore diameter (d) to the pipe inside diameter (D).

Eccentric Orifice Plate

It is similar to Concentric Orifice plate other than the offset hole which is bored tangential to a circle, concentric with the pipe and of a diameter equal to 98% of that of the pipe. It is generally employed for measuring fluids containing

  • Media having Solid particles
  • Oils containing water
  • Wet steam

Segment Orifice Plate

It has a hole which is a semi circle or a segment of circle. The diameter is customarily 98% of the diameter of the pipe.

Quadrant Edge Orifice Plate

This type of orifice plate is used for flow such as crude oil, high viscosity syrups or slurries etc.

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It is conceivably used when the line Reynolds Numbers range from 100,000 or above or in between to 3,000 to 5,000 with a accuracy coefficient of roughly 0.5%.

Operation of Orifice meter

  • The fluid flows inside the Inlet section of the Venturi meter having a pressure P1.
  • As the fluid proceeds further into the Converging section, its pressure reduces gradually and it finally reaches a value of P2 at the end of the Converging section and enter the Cylindrical section.
  • The differential pressure sensor connected between the Inlet and the and the Cylindrical Throat section of the Venturi meter displays the difference in pressure (P1-P2). This difference in pressure is in direct proportion to the flow rate of the liquid flowing through the Venturi meter.
  • Further the fluid passed through the Diverging recovery cone section and the velocity reduces thereby it regains its pressures. Designing a lesser angle of the Diverging recovery section, helps more in regaining the kinetic energy of the liquid.

Advantages of Orifice meter

  • The Orifice meter is very cheap as compared to other types of flow meters.
  • Less space is required to Install and hence ideal for space constrained applications
  • Operational response can be designed with perfection.
  • Installation direction possibilities: Vertical / Horizontal / Inclined.

Limitations of Orifice meter

  • Easily gets clogged due to impurities in gas or in unclear liquids
  • The minimum pressure that can be achieved for reading the flow is sometimes difficult to achieve due to limitations in the vena-contracta length for an Orifice Plate.
  • Unlike Venturi meter, downstream pressure cannot be recovered in Orifice Meters. Overall head loss is around 40% to 90% of the differential pressure .
  • Flow straighteners are required at the inlet and the outlet to attain streamline flow thereby increasing the cost and space for installation.
  • Orifice Plate can get easily corroded with time thereby entails an error.
  • Discharge Co-efficient obtained is low.

Applications of Orifice meter

  • Natural Gas
  • Water Treatment Plants
  • Oil Filtration Plants
  • Petrochemicals and Refineries

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Advantages of Laminar Flow Meter

A unique advantage of the laminar flowmeter is its linear relationship between flow rate and developed pressure drop.

It is the only pressure-based flow measurement device for filled pipes that exhibits a linear pressure/flow relationship.

This means no “square-root” characterization is necessary to obtain linear flow measurements with a laminar flowmeter.

Disadvantages of Laminar Flow Meter

The big disadvantage of this meter type is its dependence on fluid viscosity, which in turn is strongly influenced by fluid temperature.

Thus, all laminar flowmeters require temperature compensation in order to derive accurate measurements, and some even use temperature control systems to force the fluid’s temperature to be constant as it moves through the element.

Applications of Laminar Flow Meter

Laminar flow elements find their widest application inside pneumatic instruments, where a linear pressure/flow relationship is highly advantageous (behaving like a “resistor” for instrument air flow) and the viscosity of the fluid (instrument air) is relatively constant.

Pneumatic controllers, for instance, use laminar restrictors as part of the derivative and integral calculation modules, the combination of “resistance” from the restrictor and “capacitance” from volume chambers forming a sort of pneumatic time-constant (τ ) network.

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