Transcript CHEM-E7140_basic_measurementx

CHEM-E7140 - Process Automation
Lecture 2:
Basic measurements of process industry
Jukka Kortela
Contents
4.1 Basic measurements
4.1.1 Temperature
4.1.2 Pressure
4.1.3 Level
4.1.4 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:
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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 Basic measurements
4.1.1 Temperature
4.1.2 Pressure
4.1.3 Level
4.1.4 Flow
4.2 Instrumentation symbols
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 sensors
1/2
• 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 sensors
2/2
• 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 temperaturedependent resistance changes
of the material
• Probe:
– Metal (RTD) tai semiconductor
(thermistor)
– For probe, a material is selected whose
resistance is highly temperaturedependent 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 Basic measurements
4.1.1 Temperature
4.1.2 Pressure
4.1.3 Level
4.1.4 Flow
4.2 Instrumentation symbols
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
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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 Basic measurements
4.1.1 Temperature
4.1.2 Pressure
4.1.3 Level
4.1.4 Flow
4.2 Instrumentation symbols
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 is
suitable for clean
fluids
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
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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 tefloncoated 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, highviscosity 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
Level measurement, Methods based on
hydrostatic pressure 2/4
• 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
Level measurement, Methods based on
hydrostatic pressure 3/4
• 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
Level measurement, Methods based on
hydrostatic pressure 4/4
• 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 Basic measurements
4.1.1 Temperature
4.1.2 Pressure
4.1.3 Level
4.1.4 Flow
4.2 Instrumentation symbols
4.1.4 Flow measurement,
sensors
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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
1 2
p
v  gh   vakio
2

– 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),
 2 2
p1  p2  ( v2  v1 )
2
– Flow rate will remain the same in each cross-section
A1v1  A2v2
Flow measurement,
differential pressure methods 2/2
• The pressure difference can
be measured and from it
the flow rate can be
calculated
Flow measurement,
Differential pressure methods
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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 than
measurement 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 measuring
high-pressure vapor and abrasive materials
– Lower pressure drop than in
measurement flange
– dirt and wear affect it significantly
less
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
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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 bubbles
contained 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
4.1 Basic measurements
4.1.1 Temperature
4.1.2 Pressure
4.1.3 Level
4.1.4 Flow
4.2 Instrumentation symbols
4.2 Instrumentation symbols
• Standards SFS-ISO 14617,
SFS-ISO 14617-5, SFS-ISO 14617-6
4.2 Instrumentation symbols
• 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)
Letter Codes in Instrumentation
• 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
Letter Codes in Instrumentation
• http://knowpap.hut.fi/
KnowPap
English
 Start
• 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 closes
when flow rate is higher than aset point