II Mod 5 - Lecture notes 11 PDF

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Summary

Anemometers: Hot wire/hot film anemometer, laser Doppler
anemometer (LDA), electromagnetic flow meter, turbine and
other rotary element flow meters, ultrasonic flow meters,
doppler flow meters, cross correlation flow meters, vortex
flow meters....

Description

MODULE 5-MASS FLOW METERS Impeller-Turbine Mass Flow Meters: The impeller, turbine type mass flow meter uses two rotating elements in the fluid stream, an impeller and a turbine. Both elements contain channels through which the fluid flows. The impeller is driven at a constant speed by a synchronous motor through a magnetic coupling and imparts an angular velocity to the fluid as it flows through the meter.

The

turbine located downstream of the impeller eliminates all the angular

momentum of the fluid and, therefore, receives a torque proportional to the angular momentum. This turbine is constrained by a spring that deflects through an angle that is proportional to the torque exerted on it by the fluid, which gives a measure of mass flow. Twin-Turbine Mass Flow Meter:

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In this instrument two turbines are mounted on a common shaft.

• As shown in the above figure, a twin-turbine mass flow meter in which two turbines are connected with a calibration torsion member. • A reluctance type pick up is mounted over each turbine and a strong magnet is located in each turbine within the twin turbine assembly. • Each turbine is designed with a different blade angle; therefore, there is a tendency for the turbines to rotate at different angular speeds. • However, since the movement of the turbines is restricted by the coupling torque, the entire assembly rotates in unison at an average speed, and an angular phase shift between the two turbines develops. This angle is a direct function of the angular momentum of the fluid • The angular momentum is a function of mass flow. • In the double turbine assembly, the turbines are not restricted by a spring, but the torsion member that holds them together is twisted. • Therefore, the angle developed between the two turbines is a direct function of the torsion or torsion exerted by the system.

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Coriolis Mass Flow Meter A Coriolis meter works on Coriolis Effect, hence it is named so.

• In this meter unit, the liquid passes through a U-shaped tube which vibrates in an angular harmonic oscillation. • Coriolis forces will then deform the tube and a further vibration component gets added to the already oscillating tube. • This added vibration element results in a phase shift or twist in few parts of the tubes. This resulting phase shift which is directly proportional to the liquid mass flow rate is measured with the help of sensors. • This measured information is further transferred to the electronics unit where it gets transformed to a voltage proportional to mass flow rate. A Coriolis meter is shown in the figure below:

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Principle of Operation:• The flow is guided into the U-shaped tube. • When an oscillating excitation force is applied to the tube causing it to vibrate, the fluid flowing through the tube will induce a rotation or twist to the tube because of the Coriolis acceleration acting in opposite directions on either side of the applied force. For example, when the tube is moving upward during the first half of a cycle, the fluid flowing into the meter resists being forced up by pushing down on the tube. • On the opposite side, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. • When the tube is moving downward during the second half of the vibration cycle, it twists in the opposite direction. • This twist results in a phase difference (time lag) between the inlet side and the outlet side and this phase difference is directly affected by the mass passing through the tube. • An advantage of Coriolis flow meters is that it measures the mass flow rate directly which eliminates the need to compensate for changing temperature, viscosity, and pressure conditions.

ADVANTAGES:1 Coriolis flow meter is capable of measuring a wide range of fluids that are often incompatible with other flow measurement devices. The operation of the flow meter is independent of Reynolds number; therefore, extremely vicious fluids can also be measured. A Coriolis flow meter can measure the flow rate of Newtonian fluids, all types’ non-Newtonian fluids, and slurries. Compressed gases and cryogenic liquids can also be measured by some designs.

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Coriolis flow meters provide a direct mass flow measurement without the addition of external measurement instruments. While the volumetric flow rate of the fluid will vary with changes in density, the mass flow rate of fluid is independent

of density changes. 3

Coriolis flow meters have outstanding accuracy. The base accuracy is commonly on the order of 0.2%. In addition, the flow meters are linear over their entire flow range.

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The range of flow meters is usually on the order of 20:1 or greater. Coriolis flow meters have been successfully applied at flow rates 100 times lower than their full scale flow rate.

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A Coriolis flow meter is capable of measuring mass flow rate, volumetric flow rate, fluid density and temperature — all from one instrument.

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The operation of the flow meter is independent flow characteristics such as turbulence and profile. Therefore, upstream and downstream straight run requirement and flow conditioning are not necessary. They can also be used in installations that have pulsating flow.

ANGULAR MOMENTUM-TYPE MASS FLOWMETERS H = angular momentum (lbf-ft-sec) I = moment of inertia (lbf-ft2) ω = angular velocity (rad/sec) α = angular acceleration (rad/sec2) Υ = torque (ft-lbf) r = radius of gyration (ft) Newton’s second law of angular motion states that H = Iω …………….eqn 1

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But because, by definition, I = mr2 and Y = mr2α .Therefore H = mr2ω Υ = (m r2ω )/t………………eqn 2 m/t=y/r2w eqn1 /t H/t= (m/t)r2w since r2 is a constant for any given system, the mass flow of fluid can be determined if an angular momentum is introduced into the fluid stream and measurements are made of the torque produced by this angular momentum and of the fluid’s angular velocity. Constant Torque-Hysteresis Clutch mass flowmeter eliminates the necessity of making a torque measurement after imparting a constant torque to the fluid stream. The relationship between mass flow and torque is m/t=y/r2w Therefore, if Υ is held at a constant value, and since r2 is a physical constant of the system, m/t=k/w This relationship is used in designing a mass flowmeter as follows: 1. A synchronous motor is placed in the center of the flowmeter 2. assembly. 2. This motor is magnetically coupled to an impeller that is located within the flowing process stream. 3. The magnetic coupling between the motor and the impeller is provided by means of a hysteresis clutch that transmits a constant torque from the motor to the impeller. Thus, a measurement of the rotational speed of the impeller is inversely proportional to the mass flow rate.

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RADIATION-TYPE MASS FLOWMETERS One of the earliest methods of mass flow determination was to install two separate sensors one to measure the volumetric flow and the other to detect the density of the flowing stream and then to use the two transmitter signals as inputs into a mass flow computing module. • This approach was feasible, but it required coordination between the products of different suppliers and corrections for such process variables as temperature, pressure, viscosity, particle size, and velocity profile changes. • The key working component in these combinational designs is the multiple-input transmitter, which, in addition to a radiation-type density input, accepts a flow measurement signal from any volumetric flowmeter. • Based on these two inputs, the microprocessor-based transmitter generates an output signal that relates to mass flow. • A further improvement occurred in the design of these density mass flow systems in which the density and volumetric flow sensors were combined in a single package • These units are composed of a either a Doppler ultrasonic flowmeter or a magnetic flowmeter and a gammaradiation-based densitometer, all in a single unit including a microcomputer.

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TARGET FLOW METERS-VCONE FLOW METER • The V-Cone meter is a differential pressure (or “DP”) meter. • These meters work according to the same principle of DP flow devices. That is an obstruction in the pipe (i.e., a reduction in the cross sectional area available to the flow) causes an increase in flow velocity and a corresponding reduction in pressure. • Hence be measuring the upstream pressure, the temperature and the difference in the static pressure between the upstream and the minimum cross sectional areas the flow rate can be determined as long as the fluid properties are known. • The flow rate determination is done by applying the laws of conservation of mass and energy.

•

The principal theory is Bernoulli’s theorem for the conservation of energy in a closed pipe.

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• This states that for a constant flow, the pressure in a pipe is inversely proportional to the square of the velocity in the pipe. Simply, the pressure decreases as the velocity increases. For instance, as the fluid approaches the V-Cone flow meter, it will have a pressure of P1 .

• • As the fluid velocity increases at the constricted area of the V-Cone, the pressure drops to P2 , as shown in Figure 1. • Both P1 and P2 are measured at the V-Cone flow meter’s taps using a variety of differential pressure transducers. • The Dp created by a V-Cone flow meter will increase and decrease exponentially with the flow velocity. • As the constriction takes up more of the pipe cross-sectional area, more differential pressure will be created at the same flow rates. The beta ratio equals the flow area at the largest cross section of the cone (converted to an equivalent diameter) divided by the meter’s inside diameter

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Advantages: ▪

Lower sensitivity than orifice plates and Venturi tubes to installation effects

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Shorter installation lengths

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Less pressure loss than orifice plates

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Wafer (between flange) versions

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Effective for wet gas flow measurement applications

Disadvantages: ▪

Lack of standards

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Pressure loss higher than Venturi tubes

• Factors affecting the selection of flow meter • its size and measuring range of the flowmeter • Chemical compatibility • Process accuracy requirements • Pressure requirements • Acceptable pressure drop

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• Cleaning requirements (i.e., do you need, and does the unit offer, cleanin-place capabilities?) • Desired measurement units (such as volume, velocity or mass) • Uni-directional or bi-directional mea-surement • Fluid viscosity limitations • Necessary approvals for use in hazardous areas, sanitary applications • Custody-transfer approvals • Data-output requirements (i.e., 4–20 mA, relay, digital or simple display) • Calibration and recalibration requirements • Maintenance issues • Operating costs • Connection styles (flanged, wafer, threaded, weld-on and so on)accuracy and turndown ratio

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