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PRINCIPLE OF OPERATIONCORIOLIS FLOWMETER

Contents

Coriolis Effect1

Coriolis Flow Meter2

Flow Measuring Principle3

Density Measuring Principle4

Coriolis Effect

Coriolis Effect

Coriolis effect is an apparent deflection of moving object when they are viewed from a rotating frame of reference.

This effect is caused by the Coriolis force, which appears in the equation of motion of an object in a rotating frame of reference.

m: mass of object v: velocity of object in the rotating frame Ω: angular velocity of rotating frame

In the inertial frame of reference (upper part of picture), the black object moves frictionlessly in straight line. However, the observer (red dot) who is standing in the rotating frame of reference (lower part of the picture), sees the object as following a curved path.

vmFC 2

Coriolis Flow Meter

Sensor: detects flow rate, density and temperature.

Transmitter: provides sensor information as outputs, provides a display, basic menu access, and outputs to interface with other systems.

Peripheral devices: provide monitoring, alarm of additional functionality.

A Coriolis Flow meter consists of a sensor, a transmitter and, in many cases, peripheral devices.

Sensor - Overview

A Micro Motion curved tube sensor includes:

Flow Tubes Drive Coil and Magnet Pickoff Coil and Magnet RTD Process Connection Flow Splitter Core Processor Case

Sensor – Flow Tubes

Flow Tubes

There are 2 flow tubes inside a sensor

Sensor – Drive Coil

Drive Coil and Magnet

The drive coil is used with a magnet to produce the oscillation of the Micro Motion Coriolis sensor flow tubes.

The coil is energized to keep the tubes vibrating at their natural frequency.

Sensor – Pickoff Coil

Pickoff Coil and Magnet

The pickoff coils are used to produce the signals that represent the velocity and position at two points on the vibrating tube.

Mass flow is determined by measuring the phase difference between these signals.

Sensor – RTD

RTD

Sensor – Process Connection

Process Connection

Sensor – Flow Splitter

Flow Splitter

Flow Splitter is used to divide process flow evenly between the two flow tubes

Sensor – Core Processor

Core Processor

The wiring for drive coil, pickoff coils, RTD is routed to core processor (CP).

CP is used to controls the sensor, primary signal measurement and processing.

CP executes all necessary calculations to get process variable values and communicates these to the transmitter for interfacing with operator and control system.

CP can be located either on sensor or transmitter.

Sensor – Case

Case

Case is used to protect the electronics and wiring from external corrosion.

Some cases may have purge fittings.

Flow Measuring Principle

Tube Vibrating

By vibrating in opposition, the flow tubes are balanced and isolated from external vibration or movement of sensor.

In Micro Motion flowmeters, during operation, a drive coil is energized to oscillate the tubes in opposition.

Flow Measuring Principle

Signal Generation

Pickoff coils are mounted on the side legs of one flow tube and magnets are mounted on the side legs of the opposing flow tube.

During tubes vibrating, each coil moves through the uniform magnetic field of adjacent magnet. The voltage generated from each pickoff coil creates a sine wave.

These sine waves represent the motion of one tube relative to the other.

Flow Measuring Principle

No flow – No Coriolis effect

During no flow condition, there is no Coriolis effect. The inlet motion and outlet motion is in phase and the sine waves coincide with each other.

Flow Measuring Principle

Flow – Coriolis effect During flow condition, Coriolis

forces are induced in both inlet and outlet of both flow tubes.

These forces cause the flow tubes to twist in opposition to each other.

Flow Measuring Principle

As a result of the twist in the flow tubes, the sine waves generated by pickoff coils are now out of phase with each other.

The Delta-T time delay between these two sine waves is measured. This Delta-T is always directly proportional with the mass flow rate, hence, the mass flow rate can be calculated.

Flow Measuring Principle

Flow Calibration Factor

Each Coriolis flowmeter has a unique flow calibration factor The flow calibration factor format consists of 10 characters. For

example: 4.27454.75 The first five digits is proportionality constant (4.2745). This

constant multiplied by Delta-T measured in ms, yields mass flow rate in grams/s

The last three digits is temperature coefficient to compensate for the effect of temperature on tube stiffness. It is expressed in term of a percent change in the proportionality constant per 100°C

Example:

Given Delta-T = 5ms → Mass flow rate = 5 x 4.2745 = 21.3725g/s

Density Measuring Principle

Density and Mass Relationship

Density is defined as mass per unit volume.

The volume of the fluid contained in the flow tubes remains constant. So, the mass of the full flow tubes indicates the density of the fluid contained.

Density Measuring Principle

Mass and Frequency Relationship

To understand this relationship, consider mass and spring system As the mass decreases, the natural frequency of the system increases As the mass increases, the natural frequency of the system decreases

Density Measuring Principle

In Coriolis sensor, the tubes correspond to the spring, the mass of the tubes and fluid correspond to the mass at the end of the spring, the stiffness of the tubes remains constant. Therefore, the mass of the fluid (density) is the only variable effecting the natural frequency.

During operation, a drive coil causes the tubes to oscillate at their natural frequency.

As the mass of the process fluid increases, the natural frequency decreases.

As the mass of the process fluid decreases, the natural frequency increases.

Density Measuring Principle

Density Measuring Principle

Tube Period

Tube period is the reciprocal of natural frequency. Coriolis flowmeters measure density by measuring tube period. Fluid density is directly related to tube period.

Increase density Decrease density

Density Measuring Principle

Density Calibration

Micro Motion Coriolis flowmeters are calibrated for density prior to shipment using air and water.

The relationship between the fluid density and the tube period squared is linear.

Density Measuring Principle

Determining Process Density

The relationship between the fluid density and the tube period is a well defined function. If any two density/tube period points are known, density at other tube period measured during operation can be measured with great accuracy.

References

References: http://www.emersonprocess.com/micromotion/tutor/index.html

http://en.wikipedia.org/wiki/Coriolis_effect

http://www.alicatscientific.com/Types_of_devices.php

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