Flow sensors

Types of flow sensors used in industries to measure fluid velocity and volumetric flowrates.

1. FLOW SENSORS

2. I ntroduction Flow is defined as the quantity of fluid (gas, liquid, vapour or sublimate) that passes a point per unit time It can be presented by a simple equation: Flow (Q) = quantity/time It is the rate of change of a quantity. It is either volumetric or mass flow rate.

3. Properties Affecting Fluid Flow Velocity of Fluid – the fluid speed in the direction of flow. The fluid velocity depends on the head pressure that is forcing the fluid through the pipe. The greater head pressure, the faster fluid will flow. Pipe size – the larger the pipe, the greater the potential flow rate Pipe friction – reduces the flow rate through the pipe. Flow rate of the fluid is slower near walls of the pipe that at the center . Fluid viscosity – its physical resistance to flow.

4. The specific gravity of fluid – at any given operating condition, the higher fluid’s specific gravity, the lower its flow rate. Fluid condition – the condition of fluid (clear or dirty) is one of the limitations in flow measurement, some measuring devices blocked/plugged or eroded if dirty fluids are used. Velocity Profiles – it has major effect on the accuracy of most flow meters. It can be laminar, transitional or turbulent flow. Properties Affecting Fluid Flow

5. Importance of Fluid Measurement Measuring flow is one of the most important aspects of process control The most diverse substances are transported and distributed in piping system The fluid flowing through pipes have different properties, so different flow measuring devices are used The maintenance of definite rates of flow is important for maximum efficiency and production

6. Types of Flow Meters Differential pressure flow meters Coriolis Flow meter Vortex Flow meter Ultrasonic Flow meter Electromagnetic Flow meter Thermal Flow meter

7. 1. Differential Pressure Flow Meters It works on the principle of partially obstructing the flow in a pipe. This creates a difference in the static pressure between the upstream and downstream side of the device. The difference between static pressure (referred to as differential pressure) is measured and used to determine the flowrate .

8. where: = ratio of meter diameter to pipe diameter ≈ 0.5 usually S = cross sectional area of orifice V = bulk velocity through the orifice/ venturi or nozzle C = discharge coefficient ≈ 0.61 for Re > 30,000

9. Some of the most common types of differential flow meters are: orifice flow meter venturi flow meter nozzle flow meter pitot tube flow meter

10. Orifice Flow Meters Orifice plates are the most common type of Δp meter and are basically a machined metal plate with a hole, as shown below. The plate has a sharp upstream edge and usually a bevelled edge downstream of the flow. Upstream face Downstream face

11. The components of a typical orifice flow meter installation are: Orifice plate and holder Differential pressure transmitter Flow indicator or recorder

12. Advantages and Disadvantages of Orifice Flow Meters Advantages: Lower cost and easy to install Smaller physical size Flexibility to change throat to pipe diameter ratio to measure a larger range of flow rates Disadvantage: High pressure loss Large power consumption in the form of irrecoverable pressure loss Susceptible to erosion or damage

13. Venturi Tubes Venturi tube consists of a section of pipe with a conical entrance, a short straight throat, and a conical outlet. The velocity increases and the pressure drops at the throat. The differential pressure is measured between the inlet (upstream of the conical entrance) and the throat.

14. Advantages and Disadvantages of Venturi Flow Meters Advantages Moderately low pressure loss ( about 10% of Δ P ) No moving parts Less susceptible to damage/wear Suitable for wet gas flow Disadvantages Large physical size Greater cost to manufacture Greater susceptible to “tapping errors” in high Reynolds number gas flows owing the high velocity fluid passing the pressure tapping at the throat.

15. Flow Nozzles Flow nozzles have a curved entry and a cylindrical throat, but no divergent outlet section. Therefore, the discharge coefficient is similar to Venturi tube, but the overall pressure loss is similar to that of orifice plate.

16. Pitot Tubes  a.)it sense two pressures simultaneously, impact and static. The impact unit consists of a tube with one end bent at right angles toward the flow direction. b.) These sensing tubes sense various velocity pressures across the pipe, which are then averaged within the tube assembly to give a representative flowrate of the whole cross section. a. b.

17. Features: Easy to install No pressure loss Sensitive to up stream disturbance Not used for sticky and dirty fluids

18. Coriolis Meter Coriolis meters operate on the principle that, if a particle inside a rotating body moves in a direction toward or away from the center of rotation, the particle generates inertial forces that act on the body.

19. Coriolis meters create a rotating motion by vibrating a tube or tubes carrying the flow, and the inertial force ( Coriolis force) that results is proportional to the mass flow rate.

20. During a no flow condition, there is no Coriolis effect and the sine waves are in phase with each other. When fluid is moving through the sensor's tubes, Coriolis forces are induced causing the flow tubes to twist in opposition to each other. The time difference between the sine waves is measured and is called Delta-T which is directly proportional to the mass flow rate.

21. Coriolis Meter Advantages • Measure mass flow rate directly • Suitable for applications where temperature and pressures fluctuate • No obstructions in the fluid path • Low cost of ownership Coriolis Meter Limitations • May not be suitable for viscous fluids as the pressure drop increases with fluid viscosity

22. VORTEX FLOW METER The vortex flow meter is used for measuring the flow velocity of gases and liquids in pipelines flowing full . MEASURING PRINCIPLE Based on the development of a Karman vortex shedding street in the wake of a body built into the pipeline.

23. VORTEX FLOW METER

24. VORTEX FLOW METER Von Karman Vortex Street Vortex Sensor

25. Common Applications Custody transfer of natural gas metering Steam measurement Flow of liquid suspensions General water applications Liquid chemicals & pharmaceuticals VORTEX FLOW METER

26. Advantages Vortex meters can be used for liquids, gases and steam Low wear (relative to turbine flow meters) Relatively low cost of installation and maintenance Low sensitivity to variations in process conditions Stable long term accuracy and repeatability Applicable to a wide range of process temperatures Available for a wide variety of pipe sizes Can also measure temperature, density or media levels are commonly available Disadvantages L ow to medium pressure drop due to the obstruction in the flow path Not suitable for very low flow rates VORTEX FLOW METER

27. A type of flow meter that measures the velocity of a  fluid with    ultrasound to calculate volume flow. ULTRASONIC FLOW METER MEASURING PRINCIPLE The flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow ( Transit Time Type ) or by measuring the frequency shift from the Doppler Effect ( Doppler Shift Type ).

28. ULTRASONIC FLOW METER Transit Time Ultrasonic Flow Meter Doppler Ultrasonic Flow Meter

29. ULTRASONIC FLOW METER

30. Advantages Obstruction less flow Unaffected by changes in temperature, density or viscosity Bi-directional flow capability Low flow cutoff Corrosion-resistant Relative low power consumption TRANSIT TIME ULTRASONIC FLOW METER Common Uses Transit Time flow meters work with clean liquids like water, oils and chemicals . Disadvantages This type of meters are highly dependent on the Reynolds number ( the velocity profile) It requires nonporous pipe material (cast iron, cement and fiberglass should be avoided) It requires periodic re-calibration Liquids with entrained gases cannot be measured reliably.

31. Advantages The common clamps-on versions are easily installed without process shutdown . It can be installed bi-directional. Flow measurement is not affected due to change in the viscosity of the process . Generally suitable for measurements in large water pipes The meter produces no flow obstruction Its cost is independent of line size. DOPPLER ULTRASONIC FLOW METER Common Uses Heavily Aerated Liquids Raw Sewage Any Liquid with Over 100 ppm of Suspended Solids Disadvantages The sensor may detect some sound energy travelling in the causing interference reading errors. Its accuracy depends on the difference in velocity between the particles, the fluid, the particle size, concentration, and distribution. The instrument requires periodic re-calibration.

32. Electromagnetic flow meters ( Magnetic flow or Mag meter ) use Faraday’s Law of Electromagnetic Induction to determine the flow of liquid in a pipe. Operating Principle The operation of a magnetic flowmeter or mag meter is based upon Faraday's Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.  ELECTROMAGNETIC FLOW METER

33. ELECTROMAGNETIC FLOW METER

34. ELECTROMAGNETIC FLOW METER

35. Common Applications In order of usage: water/wastewater industry c hemical food and beverage pulp and paper metals and mining pharmaceutical. ELECTROMAGNETIC FLOW METER

36. Advantages Are bi-directional Have no flow obstruction Are available with DC or AC power It can measure multiphase; however, all components should be moving at the same speed It can install vertically or horizontally (the line must be full, however) Can be used with fluids with conductivity greater than 200 umhos /cm. Changes in conductivity value do not, affect the instrument performance. ELECTROMAGNETIC FLOW METER Disadvantages It's above average cost It's large size Its need for a minimum electrical conductivity of 5 to 20 μmhos / cm Its accuracy is affected by slurries containing magnetic solids. The line must be full and have no air bubbles (air and gas bubbles entrained in the liquid will be metered as liquid, causing a measurement error ). In some applications, appropriate mechanical protection for the electrodes must be provided

37. Operates by monitoring the cooling effect of a gas stream as it passes over a heated transducer. Operating Principle The operation of thermal dispersion mass flow meters is based King's Law that reveals how a heated wire immersed in a fluid flow measures the mass velocity at a point in the flow. It is based on the principle of conductive and convective heat transfer. THERMAL FLOW METER

38. THERMAL FLOW METER

39. Common Applications Thermal flow meters are used almost entirely for gas flow applications . THERMAL FLOW METER

40. Advantages T hermal flow meters can measure the flow of some low-pressure gases that are not dense enough for Coriolis meters to measure. Disadvantages Practical for gas flows only Subject to blockage by foreign particles or precipitated deposits due to small openings in flow meter Power requirements excessive in larger pipe sizes Has to be taken out of process line for servicing Accurate field calibration is difficult THERMAL FLOW METER

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