Flow Sensors Reasons for Flow Metering Plant control

Flow Sensors

Reasons for Flow Metering • Plant control, for product quality and safety reasons. • Custody transfer, both interplant and selling to outside customers. • Filling of containers, stock tanks and transporters. • Energy, mass balancing for costing purpose and health monitoring of heat exchangers. • Health monitoring of pipelines and on-line analysis equipment, Government and company legislation may dictate the use here of such equipment.

Types of Flow Meters 1. Inferential type flow meters 2. Quantity flow meters a. Positive displacement meters b. Metering pumps 3. Mass flow meters

Inferential Meters The inferential type meters are so-called because rather than measuring the actual volume of fluid passing through them, they “infer” the volume by measuring some other aspect of the fluid flow and calculating the volume based on the measurements 05/12/2020 4

Inferential Meters 1. 2. 3. 4. 5. 6. 7. 8. Variable head or differential meters Variable area meters Magnetic meters Turbine Meters Target meters Thermal flow meters Vortex meters Ultrasonic flow meters 05/12/2020 5

Differential Pressure Meters • • • Orifice Plate Dall Tube Venturi Tube Pitot Tube Rota meter Target mater Averaging Pitot Nozzle Spring Loaded Intake Meter Elbow Meter Bypass Meter

Parts of differential flow meters 1. Primary element (Part of meter used to restrict the fluid flow in pipe line to produce differential pressure) They include • Orifice plate • Venturi tubes • Flow nozzles • Pitot tube etc.

Parts of differential flow meters 1. Secondary element (measure the differential pressure produced by primary elements and convert them to usable forces or signals ) Secondary elements; • Manometers • Bellow meters • Force balance meters etc.

Obstruction Meters • Orifice Meters • Venturi Meters • Flow Nozzles

Flow Through an Orifice Meter P 1 P 2 d P 1 P D

Flow Through an Orifice Meter -Cheapest and Simplest -But biggest pressure drop and power lost (C~0. 6 - 0. 7) -Side Note: Pressure drop caused by friction and turbulence of shear layer downstream of vena contracta Re 0. 85 10 k 5000 100 k 0. 6 0. 1 b=d/D 0. 8

Flow Through an Venturi Meter In a venturi, 0. 95 < C < 0. 98 Advantage: Pressure recovery Uses little power

Flow Through a Nozzle P 1 P 2 P 1 P P 2

Flow Through a Nozzle Shorter and cheaper than venturi But larger pressure drop. Thus, more power lost in operating. 0. 98 C 0. 86 105 103 Re

Flow Through a Nozzle Basic Equations: a. ) Continuity: mass in = mass out b. ) Bernoulli’s Eqn. Total pressure is constant throughout

Flow Through a Nozzle

Flow Through a Nozzle

Flow Through a Nozzle DP

Flow Through a Nozzle Y= Compressibility Factor =1 for incompressible flow or when DP<< Pabs C= Discharge Coefficient =f(Re) and nature of specific flow meter DP P

Elbow Flow Meters

Pitot Tube

Rotameter, variable-area-flowmeter • Force balance – Drag Force – Gravity – Buoyancy • (usually negligible) Derived on next slide

Turbine Flow Meters It consists of a multi-bladed rotor mounted at right angles to the flow and suspended in the fluid stream on a freerunning bearing. Used for measurements of liquid, gas and very low flow rates. It basically works on the principle of turbine. The diameter of the rotor is very slightly less than the inside diameter of the metering chamber, and its speed of rotation is proportional to the volumetric flow rate. The rotational speed is a direct function of flow rate and can be sensed by magnetic pick-up coil. As each rotor blade passes the magnetic pick-up coil, it generates a voltage pulse which is a measure of flow rate. 05/12/2020 23

Turbine Flow Meters Electrical pulses can be counted and totalized and it gives the total flow rate. 05/12/2020 24

Target Flow Meters -- It measures flow by measuring the amount of force exerted by the . flowing fluid on a target suspended in the flow stream -- The fluid flow develops a force on target which is proportional to the . square of the flow --measure the flow of liquids and gases, such as water, air, industrial gases, and chemicals. Q=K(F)1/2 Where Q= flow rate K = a known coefficient F = force 05/12/2020 25

Magnetic Flow Meters The physical principle at work is Faraday's law of electromagnetic induction, it states that whenever a conductor moves through a magnetic field of given strength , a voltage is induced in a conductor which is proportional to the relative velocity between the conductor and the magnetic field.

Magnetic Flow Meters (Cont‘d) For high corrosive applications Induced voltage is given by • E=CBLv • v=E/CBL Equation of continuity: • Q=v. A so • Q=EA/CBL • Q = KE WHERE • K= A/CBL=CONSTANT • So induced voltage is directly and linear proportional to the volumetric flow rate.

Thermal Flow Meters Based on specific heat equation which is given as Q=WCP(T 2 – T 1) W= Q/CP(T 2 – T 1) Where Q= heat transfer W=mass flow rate of fluid Cp=specific heat of fluid T 1=initial temperature of the fluid after heat has been transferred T 2= final temperature after heating the fluid

Vortex Flow Meters Swirl Meter §Operates on principle of vortex precession. §It gives an output in the form of pulses whose frequency is proportional to the fluid flow rate. §In the area where expansion occurs , the swirling flow precceds or oscillates at a frequency proportional to the fluid flow rate. §Each high velocity vortex passed thermistor, changes the resistance and since a constant current is applied, the resistance changes is converted into voltage pulses which are amplified , filtered and transformed into constant amplitude high level pulses of square waveform.

Vortex Flow Meters (Cont‘d) Vortex Shedding Meter • Based on the phenomenon of Vortex shedding. • The frequency at which the vortices are formed is directly proportional to the fluid velocity. • The velocity and pressure distribution in the fluid around the sluff body change at the same frequency as the vortex shedding frequency.

Ultrasonic Flow Meters Time difference Type Doppler Type • TAB-TBA=2 LVcosθ/C • V= Δf. C/2 fo cosθ • • • Where L=acoustic path length between A and B C=velocity of sound in fluid Θ=angle of path wrt to pipe axis V=velocity of fluid in pipe • • • Where C=velocity of sound in fluid Θ=angle of transmitter and receiver wrt to pipe axis fo = frequency of transmission Δf = difference between transmitted and received frequency

Inferential Meters Advantages Very good repeatability • Reduced susceptibility to fouling and deposits • Less sensitive to viscosity changes Available in large sizes, good value for high flow rates • Low maintenance Registers near zero flow rate Disadvantages High pressure drop that increases drastically with viscosity Relatively high cost Indirect measurement 32

Assignment

Coriolis Mass Flowmeter In the Coriolis meter the fluid is passed through a tube. The tubes are available in different design like tubes of U-shape or horseshoe-shaped. The tubes can either be curved or straight. When two tubes are used the flow is divided when entering the meter and then recombined. The flow when enters the tube encounters oscillating excitation force that causes the tubes to vibrate at a fixed frequency. The vibration is induced in the direction that is perpendicular to flow of fluid. This creates the rotation frame of reference. Consider the tube during oscillation moving up and downward, when the tube is moving upward the fluid flowing in it tends to resist this and forces it downward. When the tube moves in the opposite direction, so does the fluid and a twist in introduced in the tube. All this might not be visible by directly observing. The twist at inlet of fluid and outlet of fluid results in phase difference or time lag and that is dependent on the fluid mass passing through the tube.

Quantity Flow Meters • Used for the measurement of small percentage of industrial flow rates. • These meters operate by passing the fluid to be measured through the meter in separate and distinct increments of alternately feeling and emptying containers of known fixed capacity. • The number of times the container is filled and emptied gives the quantity of flow. • Types are: 1. Positive displacement meters 2. Metering pumps

PD Rotary Meters ( Displacement Meters) Positive displacement flow meters, also know as PD meters, measure volumes of fluid flowing through by counting repeatedly the filling and discharging of known fixed volumes. 05/12/2020 36

PD Rotary Meters ( Displacement Meters) Principle of Operation POSITION 1. As the bottom impeller rotates in a counterclockwise direction towards a horizontal position, fluid enters the space between the impeller and cylinder. POSITION 2. At the horizontal position, a definite volume of fluid is contained in the bottom compartment. 05/12/2020 37

PD Rotary Meters ( Displacement Meters) Principle of Operation POSITION 3. As the impeller continues to turn, the volume of fluid is discharged out the other side. POSITION 4. The top impeller, rotating in opposite direction, has closed to its horizontal position confining another known and equal volume of fluid. 05/12/2020 38

PD Rotary Meters ( Displacement Meters) • • • Oval Gear Nutating Disk Oscillating Piston Multi Piston Rotating Impellers Rotating Valve Birotor Roots Meter Helix Meters 05/12/2020 39

PD Rotary Meters ( Displacement Meters) Nutating Disk A nutating disc meter has a round disc mounted on a spindle in a cylindrical chamber. By tracking the movements of the spindle, the flowmeter determines the number of times the chamber traps and empties fluid. 05/12/2020 40

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PD Rotary Meters ( Displacement Meters) Oval Gear Two identical oval rotors mesh together by means of slots around the gear perimeter. The oval shaped gears are used to sweep out an exact volume of the liquid passing through the measurement chamber during each rotation. 05/12/2020 42

PD Rotary Meters ( Displacement Meters) Oval Gear The flow rate can be calculated by measuring the rotation speed. 05/12/2020 43

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PD Rotary Meters ( Displacement Meters) Roots Meter The roots meter is similar in many respects to the oval gear meter. Two-lobed impellers rotate in opposite directions to each other within the body housing. 05/12/2020 45

PD Rotary Meters ( Displacement Meters) Roots Meter • These peanut-shaped gears sweep out an exact volume of liquid passing through the measurement chamber during each rotation. • The flow rate can be calculated by measuring the rotation speed. 05/12/2020 46

PD Rotary Meters ( Displacement Meters) Rotating Impeller 05/12/2020 47

Rotary Meters ( Displacement Meters)

PD Rotary Meters ( Displacement Meters) Advantages High accuracy over a wide range of viscosities and flow rates up to 2000 c. P with proper clearances. Extremely good repeatability on high viscosity fluids, very low slippage, long life if little or no abrasive material in the fluid Low pressure drop 05/12/2020 49

PD Rotary Meters ( Displacement Meters) Advantages Special construction available for high viscosities and temperatures Can register near zero flow rate Measures directly, not an inferential device, for more consistent results Easy to repair and economical. 05/12/2020 50

PD Rotary Meters ( Displacement Meters) Disadvantages Increased maintenance compared to other meters, more moving parts • May become damaged by flow surges and gas slugs Chance of corrosion and erosion from abrasive materials Relatively high cost for large sizes 05/12/2020 51

Metering Pumps 1. It is a positive displacement pump which is used to provide a predictable and accurate of process fluid flow. 2. Reciprocating Piston Pumps 3. Peristaltic Pumps 4. Diaphragm pumps

Reciprocating Piston Pumps • Used in heavy chemical and manufacturing industry. • It contains a piston or plunger with the inlet and outlet check valves and the piston moves with a reciprocating motion within a chamber. • As the piston retracts from its cylinder, the inlet check valve opens and the cylinder is filled. • When the piston re-enter the cylinder , the inlet check valve closes and liquid is forced throughout the outlet check valves and enters into the discharging pipe.

Reciprocating Piston Pumps

Diaphraghm Pumps • Same as reciprocating piston pump except that the process fluid is separated by a flexible diaphragm. • Consists of a diaphragm which is directly flexed by a piston

Diaphraghm Pumps

Peristaltic Pumps • Fluid is moved forward by progressively squeezing a flexible container from the entrance to the discharge. • This container is usually a tube that can be made out of any material that possesses a property to recover its original shape immediately after compression. • The flow rate is adjusted by changing the speed of squeezing mechanism.

Peristaltic Pumps