CHEM-E7140 - Process Automation Lecture 2: Basic measurements of process industry Jukka Kortela

CHEM-E7140 - Process Automation

Lecture 2: Basic measurements of process industry

Jukka Kortela

Contents

4.1 Basic measurements4.1.1 Temperature4.1.2 Pressure4.1.3 Level4.1.4 Flow

4.2 Instrumentation symbols

Background 1/3

• Process control and process management requires continuous measurements

Background 2/3

• The sensor, converts the measured property to a property that can be detected

• Electrical magnitude is usually weak and must by strengthened with an amplifier

• The transmitter converts the signal to electrical standard messages

Background 3/3

• General efforts when measuring industrial process variables:– Continuous operation of measurement– Measurement accuracy– Measurement repeatability– Fast dynamics of measurements– Usability of gathered information

Measurement technology terminology

• Uncertainty of measurement - describes the expected variation in the measurement values– All systematic errors known have been corrected

• Repeatability - the deviation between the measurement results when the measurement source is the same

• Drift, instability - change in the measurement signal with time when the measured source value is constant

• Nonlinearity - deviation from the linear characteristic curve

Contents



4.1.1 Temperature Measurement

• Sensor types– Mechanical

• Liquid column• Capillary• Bimetallic

– Electrical• Resistance thermometers• Thermo elements• Quartz crystal

– Optical• Pyrometers.

Temperature sensors measuring ranges

Mechanical temperature sensors1/2• Liquid column

– The liquid will expand in the tank and a thin glass tube– Local meter or calibration meter

• Capillary– Industrial application of liquid column– Liquid-filled dome and leaving capillary tube, which ends to the

pointing device– Based on thermal expansion of the fluid, gas pressure or liquid

to the vapor pressure

Mechanical temperature sensors2/2• Bimetal sensor

– Are based on the different thermal heat expansion characteristics of two interconnected metal strips

– Bimetal is often used in thermostats, or devices that control the on/off function of an electric contact at certain temperatures

Electrical temperature sensors,resistance sensor 1/3• Based on the temperature-

dependent resistance changes of the material

• Probe:– Metal (RTD) tai semiconductor

(thermistor)– For probe, a material is selected whose

resistance is highly temperature-dependent and linear at as wide a temperature range as possible

– The actual probe is made so that the resistance wire is twisted around a glass core and sealed in a metal pipe

Electrical temperature sensors,resistance sensor 2/3• Measurement regions

– Platinum -260…+850 °C– Nickel -200…+350 °C– Copper -50…+250 °C

• Platinum sensors are the most common– In general Pt-100 sensor– Marking Pt-100 means that the sensor is made of

platinum and that its resistance value is 100 ohm / 0 °C

Electrical temperature sensors,resistance sensor 3/3• Amplifier, Transmitter

– In practice, temperature measurement has changed into resistance measurement

– The measurement signal is transferred to the amplifier and further to the transmitter where the resistance information is converted into a standard signal

• For example. 4-20 mA

Electrical temperature sensors, thermoelement 1/2• A thermoelement (thermocouple, thermopile)

consists of two wires of different metals– The wires have been connected to each other as a

close circuit, where electromotive force is produced– The magnitude of the force depends on the

differential temperature between the measurement joint ("hot end") and the reference joint ("cold end").

– One point must either be at a standard temperature or the temperature must be measured.

Electrical temperature sensors, thermoelement 2/2• There is a current in the circuit if temperatures

T1 and T2 are not equal• The current in the circuit depends of the

temperature differential T1-T2 and the used conductor materials

Comparison of thermoelement and resistance measurement• The price is about the same for both circuits• Applicable to the same temperature range• A thermoelement has a better vibration resistance than

a resistance sensor• Thermoelement sensor does not need an external

power supply• A resistance sensor is more accurate than a

thermoelement

Electrical temperature sensors,Quartz crystal• The temperature resistance of the quartz crystal

is based on changes in the vibration frequency• The method is highly accurate• Narrow measurement range restricts its use

Optical temperature sensors,pyrometers 1/2• Measures thermal radiation

– Everything, with temperature over the absolute zero (-273,15°C) , emits electromagnetic radiation, called thermal radiation

• Wide bandwidth radiation, strength and frequency of which increase as temperature rises

– At around +700°C radiation is visible• At higher temperatures the temperature may be determined

on the basis of the color of the radiating body

Optical temperature sensors,pyrometers 2/2• Thermometers based on radiation are

called pyrometers• Emissivity of an object must be known• Suitable especially for:

– Radiation methods are used when measuring moving or extremely hot targets

Contents



4.1.2 Pressure measurement,Common• After the temperature measurement, the most

common measurement in the process industry• The pressure is monitored, controlled and it

provides a means of indirectly measure the level, the flow, density, etc..

• In general, the pressure sensor produces a small offset, which is changed to electrical signal

4.1.2 Pressure measurement,measurement principles• Relative measurement

– Measurement of pressure relative to atmospheric pressure

• Absolute measurement– In comparision to vacuum

• Pressure difference– In comparision to second pressure

4.1.2 Pressure measurement,pressure sensors• Sensor types

– Capacitivy– Inductive– Pressure switches– Pressure repeaters

Pressure measurement,Capacitive pressure sensor• Measured quantity changes the sensor capacity• The capacity change in a capacitor can be

achieved in two ways:– 1. through changing the distance between the

capacitor plates– 2. through changing the dielectric matter between the

plates

Pressure measurement,Inductive pressure sensor• The principle of an inductive sensor is similar to

the one of a capacitive sensor• Membranes move the coils of the differential

transformer instead of capacitor plates.a

Pressure measurement,Pressure switches• Pressure switches are used in pressure limit

detection• The operation principle is bellows + contact• There can be one or two pressure switches in

one instrument• A pressures switch is tuned for an increasing or

a decreasing pressure• The switch is opening or closing according to

the situation

Pressure measurement,Pressure repeaters• A pressure repeater is used when the transmitter cannot

be installed directly to the process– the transmitter cannot be installed directly to the process– Hydraulic pressure repeater :

Contents



4.1.3 Level measurement,common• Measuring level in containers is one of the most

important and the most common measurements in industry

• Can be divided into the two main groups:– Liquid level measurement– Solids level measurement

4.1.3 Level measurement, methods

• Mechanical methods• Electronic methods• Methods based on hydrostatic pressure

Level measurement,Mechanical methods• Mechanical methods

– Float– Weighing

Level measurement,mechanical methods, float• Float moves in the control with the level

– Float measurement actually means measuring the float location

• The method issuitable for cleanfluids

Level measurement,mechanical methods, weighing• Commonly used method• Accurate

– Provided that the density of the substance does not change

• Weighing sensor carries the tank weight as well• As a further application of this method stretch

gages installed in the tank feet are used– Considerably cheaper and more inaccurate than

installing the actual weighing cells under the tank

Level measurement,Electronic methods• Capacitive conductivity method• Ultrasound method• Microwave method• Radioactive method

Level measurement,Electronic methods 1/4• Capacitive method and conductivity method

– In capacitive measurement a liquid covers the capacitor plates and the capacitance changes as the liquid level changes

• As air and the capacitive properties (permittivity) of the liquid in the tank are different

– If the walls of the tank are made of a conductive material they may act as the second electrode of the capacitor. The second electrode may be e.g. a teflon-coated cable or rod

– Conductivity based measurements measure the conductivity between the rods

Level measurement,Electronic methods 2/4• Ultrasound method

– The transmitter transmits short pulses at a certain frequency– When the sound pulse meets an obstacle, part of it is

reflected and the transmitter in the same instrument registers the echo

– The time of propagation is proportional to the distance between the surface and the measuring instrument

– Disturbing factors include the vapor, dust and solids on the measurement area, changes in temperature, and the clinker on top of the measured surface

Level measurement,Electronic methods 3/4• Radioactive method

– The measurement system includes a radiation source containing radioactive isotope (e.g. cobalt) that emits gamma radiation and a detector that transforms the gamma radiation into electric current

– The radiation source is located at the side of the measured container and the detector is located on the opposite side

– are used in measuring the level of thick stock, high-viscosity substances and solids when other methods are not suitable

Level measurement, Methods based on hydrostatic pressure

• Pressure and differential pressure sensor– Open containers– In pressurized containers

• Bubble pipe

Level measurement, Methods based on hydrostatic pressure 1/4

• Pressure and differential pressure sensor– Hydrostatic pressure is used in pressure or

differential pressure measurement– Density of the liquid should stay constant– Pressure sensors are used in open containers and

differential pressure sensors in pressurized container


• Open containers– The level can be measured in an open tank using a

flange transmitter installed directly on the side of the tankn

– If the material is too rigid or soiling this method cannot be used


• Pressurized containers– The level in a pressurized tank can

be measured in the same way as the level in an unpressurized open tank, but the transmitter must have a differential pressure connection option

– The pressure above the tank level is usually connected to the minus pole of the transmittera


• Bubble pie– bubble pipe is attached to the side of the tank– Adequate amount of air is fed into the pipe to

maintain the pipe free from the fluid in the tank– The pressure in the pipe equals the hydrostatic

pressure of the fluid at the bottom of the pipe– The pressure inside the pipe is measured using a

differential pressure sensor– As there is nothing but air in the pipe this method is

particularly suitable for measuring corrosive or rigid fluids

Flow measurement,Ultrasound measurement• Microwave Method

– Pulse radad• Send a short pulse signal• The time difference between a sent and a received pulse is

proportional to the distance traveled by the pulse

– The frequency-modulated continuous-wave radar• Send a continuous signal• The phase difference is directly proportional to the distance

Contents



4.1.4 Flow measurement,sensors• Differential pressure methods• Inductive measurement• Ultrasonic flow measurements• Measurement with a variable-size opening• Turbine sensors• Vortex meters• Vortex meters

Flow measurement,differential pressure methods 1/2• Based on Bernoull equation

– In all of the tube cross sections, the sum of the potential, the pressure and the kinetic energy remains same

– If the pipe cross-section of a pipe changes (horizontal pipe),

– Flow rate will remain the same in each cross-section

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Flow measurement,differential pressure methods 2/2• The pressure difference can

be measured and from itthe flow rate can becalculated

Flow measurement,Differential pressure methods• Measurement flange• Venturi pipe• Nozzle• Bended pipe• Pitot pipe• Annubar• V-cone

Differential pressure methods, Measurement flange• Measurement flange

– Measurement flange is the simplest and most commonly used pressure reducing device

– Cheap– Causes of a relatively large pressure drop

Differential pressure methods, venture pipe• Venture pipe

– Due to the flow-following shape of the venturi pipe the pressure losses are small

– In a venturi pipe, the pressure is measured before the narrow part of the pipe, in the straight pipe section and at the venturi part

– More expensive thanmeasurement flange

Differential pressure methods,nozzle• Nozzle

– A nozzle is considerably more expensive to produce than a measurement flange

– They do not have erodable edges like the measurement flange

• Nozzles are suitable for measuringhigh-pressure vapor and abrasive materials

– Lower pressure drop than inmeasurement flange

– dirt and wear affect it significantlyless

Differential pressure methods, Bended pipe• Bended pipe

– Flow can be measured in a readymade process pipe bend or in a specially constructed bended pipe

– The centrifugal force produced by the flow creates a differential pressure between the pipe and the inner curve

Differential pressure methods,pitot pipe• Pitot pipe

– The operation of a Pitot pipe is based on measuring the difference between the dynamic pressure produced by the flow rate and the static pressure in the process piping

– The measurement pipes are installed so that one pipe is perpendicular to the flow

– The second pipe measures the static pressure in the process pipe as well as the dynamic pressure produces by the flow

Differential pressure methods,Annubar• Annubar flow sensor

– Annubar pipe is a more advanced version of the Pitot pipe

– The measurement pipe is installed in the process pipe so that the four holes in the measurement pipe are against the direction of the flow

Differential pressure methods,V-Cone• V-Cone

– The structure of the sensor is a conical pipe resembling a pitot pipe

– The sensor is installed in the process pipe so that flow is directed towards the pipe and causes a dynamic pressure inside the pipe

– The static pressure in the process pipe is connected to the plus chamber of the transmitter and the internal pressure, which is the difference between the static pressure and the flow-induced dynamic pressure, is connected to the minus chamber of the transmitter

4.1.4 Flow measurement,sensors• Differential pressure methods• Inductive measurement• Ultrasonic flow measurements• Measurement with a variable-size opening• Turbine sensors• Vortex meters• Vortex meters

Flow measurement, Inductive flow measurement• An inductive flow meter is based

on electromagnetic induction– When a conductor moves in a magnetic

field, a voltage is induced– An inductive meter is suitable for

measuring conductive fluids– Measurement sensor consists of a pipe

made of insulating material• Magnetizing windings outside the pipe• The voltage induced in the connector

on the is measured using electrodes at a 90° angle to the windings

Flow measurement, ultrasonic flow measurement• Doppler principle

– Any solid particles or air bubblescontained by the fluids

• Reflect the signal at a higher or lower frequency depending on the speed of the flowing fluid and the angle between the transmitted signal and the moving particle

• Time of propagation principle– They use two ultrasonic sensors that take turns in transmitting

and receiving• A pulse or a short pulse sequence of ultrasound energy is

transmitted through the pipe at a specified angle, first down-stream then up-stream

Flow measurement, measurement with a variable-size opening

• ” Rotameter”– The flow or part of it through

a conical pipe that broadens upwardsi– The flow rises the float enought to create a balance

between the upward force created by the flow and the weight of the float

Flow measurement, turbine sensors• Turbine sensors

– The sensor is spinning rotor• Mechanically or electronically to counter or to display

– The rotor spinning rate is almost linearly proportional to flow rate

Flow measurement, Vortex measurement• Vortex measurement

– Vortices of a specified frequency are formed behind an obstacle, whose shape differs from the shape of the flow line

– The take-off frequency of the vortices is directly proportional to the take-off speed

– An ultrasonic beam is transmitted through the vortex pattern downstream from a small vortex strut

• As the vortices travel through the beam, they modulate the carrier wave

Flow measurement, Coriolis meter• Are based on the Coriolis force• Bended pipe is vibrated by a crystal• When the fluid arrives at the measurement pipe it tries to

bend the pipe in one direction, while the outgoing fluid tries to bend the other end of the pipe in the opposite direction– The angular rotation is directly proportional to the mass flow

• Vibration is measured using an inductive or optical method

Contents




• Standards SFS-ISO 14617, SFS-ISO 14617-5, SFS-ISO 14617-6


• Process and instrumentation (P & I diagram) presents the process and the associated measurements and controls

Letter Codes in Instrumentation

• The functions of an instrument are defined by a letter code inside the symbol.a– 1. The first letter represents the process variable

(for example. L=Level)– Next letters denote a device or a function

(for example. C=Control)– Identification letters are located above the middle, alapuolelle

bellow the middle a tag is located)– Letter code and the tag together become a position-label.(for

example. FI-12)


• The following parts belong to the instrumentation chart:– Graphical symbols of instrumentation can describe

either the device or operation– Midline: a control room instrument– Without a line: a locally installed instrument


• http://knowpap.hut.fi/KnowPap

EnglishStart

• KnowPap (http://knowpap.hut.fi)Measurement and actuators

Instrumentation Graphical Symbols Graphical symbols in instrumentation

Example 1

Temperature and flow cascade control

Example 2

A feedback flow control, in which the valve closeswhen flow rate is higher than aset point

Documents

CHEM-E7140 - Process Automation Lecture 2: Basic measurements of process industry Jukka Kortela