PRINCIPLE OF MASS FLOW METER

PRESENTED BY

PREM BABOOSr. Manager(Prod)National Fertilizers Ltd. IndiaAn Expert for www.ureaknowhow.comFIE institution of Engineers India

Mass Flow Operating Principle:

Contents:1. How does a Mass flow meter measure mass

flow?2. Why do the tubes vibrate?3. How does the sensor detect mass flow

measurement?4. What is the flow calibration factor and Delta-

T and how do these relate to the mass flow measurement?

5. How does a flow meter measure volume flow?

PRINCIPLE OF MASS FLOW METERA Mass Flow Meter operating on the "Coriolis principle" contains a vibrating tube in which a fluid flow causes changes in frequency, phase shift or amplitude. The sensor signal is fed into the integrally mounted pc-board. The resulting output signal is strictly proportional to the real mass flow rate, whereas thermal mass flow meters are dependent of the physical properties of the fluid. Coriolis mass flow measurement is fast and very accurate.A mass flow meter, also known as an inertial flow meter is a device that measures mass flow rate of a fluidtraveling through a tube. The mass flow rate is the mass of the fluid traveling past a fixed point per unit time.The mass flow meter does not measure the volume per unit time (e.g., cubic meters per second) passing through the device; it measures the mass per unit time (e.g., kilograms per second) flowing through the device. Volumetric flow rate is the mass flow rate divided by the fluid density. If the density is constant, then the relationship is simple. If the fluid has varying density, then the relationship is not simple. The density of the fluid may change with temperature, pressure, or composition, for example. The fluid may also be a combination of phases such as a fluid with entrained bubbles. Actual density can be determined due to dependency of sound velocity on the controlled liquid concentration.[

If the fluid has varying density, then the relationship is not simple. The density of the fluid may change with temperature, pressure, or composition, for example. The fluid may also be a combination of phases such as a fluid with entrained bubbles. Actual density can be determined due to dependency of sound velocity on the controlled liquid concentration.

AMMONIA MASS FLOW METER

In a Coriolis mass flow meter, the “swinging” is generated by vibrating the tube(s) in which the fluid flows. The amount of twist is proportional to the mass flow rate of fluid passing through the tube(s). Sensors and a Coriolis mass flow meter transmitter are used to measure the twist and generate a linear flow signal.

Coriolis mass flow meters measure the mass flow of liquids, such as Ammonia, water, acids, caustic, chemicals, and gases/vapors. Because mass flow is measured, the measurement is not affected by fluid density changes

Coriolis mass flowmeters measure the force resulting from the acceleration caused by mass moving toward (or away from) a centre of rotation. This effect can be experienced when riding a merry-go-round, where moving toward the centre will cause a person to have to “lean into” the rotation so as to maintain balance. As related to flow meters, the effect can be demonstrated by flowing water in a loop of flexible hose that is “swung” back and forth in front of the body with both hands. Because the water is flowing toward and away from the hands, opposite forces are generated and cause the hose to twist.

There are two basic configurations of coriolis flow meter: the curved tube flow meter and the straight tube flow meter. This article discusses the curved tube design. The animations on the right do not represent an actually existing coriolis flow meter design. The purpose of the animations is to illustrate the operating principle, and to show the connection with rotation Fluid is being pumped through the mass flow meter. When there is mass flow, the tube twists slightly. The arm through which fluid flows away from the axis of rotation must exert a force on the fluid, to increase its angular momentum, so it bends backwards. The arm through which fluid is pushed back to the axis of rotation must exert a force on the fluid to decrease the fluid's angular momentum again, hence that arm will bend forward.In other words, the inlet arm is lagging behind the overall rotation, and the outlet arm leads the overall rotation.

Coriolis type Mass Flow Meters

The animation on the right represents how curved tube mass flow meters are designed. When the fluid is flowing, it is led through two parallel tubes. An actuator (not shown) induces a vibration of the tubes. The two parallel tubes are counter-vibrating, to make the measuring device less sensitive to outside vibrations. The actual frequency of the vibration depends on the size of the mass flow meter, and ranges from 80 to 1000 vibrations per second. The amplitude  of the vibration is too small to be seen, but it can be felt by touch.When no fluid is flowing, the vibration of the two tubes is symmetrical, as shown in the animations.

The animation on the right represents what happens during mass flow. When there is mass flow, there is some twisting of the tubes. The arm through which fluid flows away from the axis of rotation must exert a force on the fluid to increase its angular momentum, so it is lagging behind the overall vibration. The arm through which fluid is pushed back towards the axis of rotation must exert a force on the fluid to decrease the fluid's angular momentum again, hence that arm leads the overall vibration.The inlet arm and the outlet arm vibrate with the same frequency as the overall vibration, but when there is mass flow the two vibrations are out of sync: the inlet arm is behind, the outlet arm is ahead. The two vibrations are shifted in phase with respect to each other, and the degree of phase-shift is a measure for the amount of mass that is flowing through the tubes.

Coriolis forces Fc are generated in oscillating systems when a liquid or a gas moves away from or towards an axis of oscillation.A Coriolis measuring system is of symmetrical design and consists of one or two measuring tubes, either straight or curved.A driver sets the measuring tube (AB) into a uniform fundamental oscillation mode.When the flow velocity v = 0 m/s / 0 ft/s, the Coriolis force Fc is also 0. At flowing conditions, i. e. flow velocity v > 0 m/s / 0 ft/s, the fluid particles in the product are accelerated between points AC and decelerated between points CB.

The Coriolis force Fc is generated by the inertia of the fluid particles accelerated between points AC and of those decelerated between points CB.This force causes an extremely slight distortion of the measuring tube that is superimposed on the fundamental component and is directly proportional to the mass flow rate.This distortion is picked up by special sensors. Since the oscillatory characteristics of the measuring tube are dependent on temperature, the temperature is measured continuously and the measured values corrected accordingly.

Density and volume measurements

The mass flow of a u-shaped coriolis flow meter is given as: 

where Ku is the temperature dependent stiffness of the tube, K a shape-dependent factor, d the width, τ the time lag, ω the vibration frequency and Iu the inertia of the tube. As the inertia of the tube depend on its contents, knowledge of the fluid density is needed for the calculation of an accurate mass flow rate.If the density changes too often for manual calibration to be sufficient, the coriolis flow meter can be adapted to measure the density as well.

The natural vibration frequency of the flow tubes depend on the combined mass of the tube and the fluid contained in it. By setting the tube in motion and measuring the natural frequency, the mass of the fluid contained in the tube can be deduced. Dividing the mass on the known volume of the tube gives us the density of the fluid.An instantaneous density measurement allows the calculation of flow in volume per time by dividing mass flow with density.

CalibrationBoth mass flow and density measurements depend on the vibration of the tube. Calibration is affected by changes in the rigidity of the flow tubes.Changes in temperature and pressure will cause the tube rigidity to change, but these can be compensated for through pressure and temperature zero and span compensation factors.Additional effects on tube rigidity will cause shifts in the calibration factor over time due to degradation of the flow tubes. These effects include pitting, cracking, coating, erosion or corrosion. It is not possible to compensate for these changes dynamically, but efforts to monitor the effects may be made through regular meter calibration or verification checks. If a change is deemed to have occurred, but is considered to be acceptable, the offset may be added to the existing calibration factor to ensure continued accurate measurement.

Application Cautions for Coriolis Mass Flow meters

If the pressure drop is acceptable, operate a Coriolis mass flow meter in the upper part of its flow range because operation at low flow rates can degrade accuracy. Note that high viscosity fluids increase the pressure drop across the flow meter. For liquid flows, make sure that the flow meter is completely full of liquid. Be especially careful when measuring gas/vapor flow with Coriolis mass flow meters. Pay special attention to installation because pipe vibration can cause operational problems.

THANK YOU