Process-Control-Instrumentation narrative and hlom.pdf

Instrumentation process control

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INSTRUMENTATION & PROCESS
CONTROL

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CONTENTS
•Introduction to Process Control
•Definition of measurement and instrumentation
•Types of measurements
•Types of instruments in measurements
•Review in units of measurement
•Standard of measurement
•Calibration
•Application of measurement and instrumentation
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Introduction to Process Control
What is Process Control?
To maintain desired conditions in a physical system by adjusting
selected variables in the system at a desired set point.
Objectives of Process Control
• Safety
• Environmental Protection
• Equipment Protection
• Smooth Plant Operation and Production Rate
• Product Quality
• Profit Optimization
• Monitoring and Diagnosis
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Definition of measurement &
instrumentation
•Measurement:
A system which provides information of desired variable.
•Purpose of a measurement system
•Instrumentation
The measuring devices used in the measuring system.
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Generalised Measuring System
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•Stage 1: A detection-transducer or sensor-transducer, stage; e.g.
Bourdon tube
•Stage 2: A signal conditioning stage; e.g. gearing, filters, bridges
•Stage 3: A terminating or readout-recording stage; e.g. printers,
oscilloscope
•Stage 4: Data Presentation Element- Analogue or digital display
General Structure of Measuring System
Sensing
element
Signal
conditioning
element
Signal
processing
element
Data
presentation
element
Input Output
True
value
Measured
value

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Process Control Loop
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Block Diagram of Process control loop
Block Diagram of the elements in process control loop

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Process Control Loop Elements definition
ELEMENTS DEFINATION
Feedback loopIt is the signal path from the output back to the input to correct for any
variation between the output level from the set level.
Controlled
variable
It is the monitored output variable from a process.
Manipulated
variable
It is the input variable or parameter to aprocess that is varied by a
control signal from the processor to an actuator.
Set point It is the desired value of the output parameter or variable being
monitored by a sensor. Any deviation from thisvalue will generate an
error signal.
Instrument is the name of any of the various device types for indicating or measuring
physical quantities orconditions, performance, position, direction,etc.
Sensors are devices that can detect physical variables, such as temperature, light
intensity, or motion, and have the ability to give a measurable output that
varies in relation to the amplitude of the physical variable.
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ELEMENTS DEFINATION
Transducers are devices that can change one form of energy to another
Converters are devices that are used to change the format of a signal without
changing the energy form.
Actuators are devices that are used to control an input variable in response to a
signal from a controller.
Controllers are devices that monitor signals from transducers and take the necessary
action to keep the process within specified limits according to a
predefined program by activating and controlling the necessary actuators.
Error Signal is the difference between the set point and the amplitude of the
measured variable.
Correction Signal is the signal used to control power to the actuator to set the level of the
input variable.
Transmitters are devices used to amplify and format signals so that they are suitable for
transmission over long distances with zero or minimal loss of information.
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INSTRUMENT PARAMETERS
•Range: The range of a sensor is the lowest and highest
values it can measure within its specification.
•Span: The span of a sensor is the high end of the Range
minus the low end of the Range.
•Resolution: It is the smallest amount of input signal
change that the instrument can detect reliably.
•AccuracyIt is the difference between the indicated value
and the actual value of a instrument.
•PrecisionIt is the reproducibility with which repeated
measurements can be made under identical
conditions.
•Reproducibility:It is the ability of an instrument to repeatedly
read the same signal over time, and give the
same output under the same conditions.
•Linearity: It is a measure of the proportionality between the
actual value of a variable being measured and the
output of the instrument over its operating range.
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INSTRUMENT DYNAMICS
•Instrument:Gain in the process is defined as the Gain change
in input divided by the change in output. A
process with high gain will react more to the
controller output changing.
•Sensitivity:The sensitivity of a sensor is the ratio of
the output signal to the change in process
variable.
•Offset: is the reading of an instrument with zero
input.
•Drift:is the change in the reading of an
instrument of a fixed variable with time.
•Hysteresis:is the difference in readings obtained
when an instrument approaches a signal from
opposite directions
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INSTRUMENTS
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Temperature Measurement
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Glass stem Thermometer
-low cost, long life
-local readout, difficult to read, no transmitter
--200 to 600ºF, 0.1ºF accuracy
Bi-metallic Thermometer
- low cost
- -80 to 800ºF, 1ºF accuracy

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Temperature Measurement/ Control
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Fluid-filled Thermal Elements
-low cost, long life
--300 to 1000ºF, ±½ % of full scale
accuracy
-low accuracy, great for some
applications where tight control is
not req’d
-self-contained, self-powered
control (can use fluid expansion to
proportionally open control valve)
-dial read-out for indication, can be
remotely located

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Local or Remote Temperature
Measurement
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Thermocouples
- low cost sensor
- -440 to 5000ºF, typically 1
to 2ºF accuracy
- wide temperature range
for various types
- rugged, but degrades
over time
- many modern
transmitters can handle T/C
or RTD

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Local or Remote Temperature
Measurement
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RTD’s
- -300 to 1150ºF, typically
0.1ºF accuracy
- more fragile, expensive
than T/C
- better accuracy and reliability
than T/C
- very stable over time
- wide temperature range

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Flow Measurement
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Differential Pressure – Orifice Meter
- well-characterized and predictable
- causes permanent pressure (energy)
loss in piping system, typically 8’ W.C. loss (3 to
4 psi loss)
-5:1 range ability
-requires straight run of 20 pipe
diameters upstream, 5 downstream
-suitable for liquid, gas, and steam
-accuracy is 1 to 2% of upper range

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Flow Measurement
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Turbine Flow Meter
-accuracy is ±0.25% of rate
-good for clean liquids, gases
-5 to 10 pipe diameters
upstream/downstream
-10:1 turndown
-3 to 5 psig pressure drop

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Flow Measurement
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Magnetic Flow Meter (Mag Meter)
- 0.4 to 40 ft/s, bidirectional
- accurate to ±0.5% of rate
- fluid must meet minimum electrical
conductivity
- head losses are insignificant
- good for liquids and slurries
- upstream/downstream piping does
not effect reading
- linear over a 10:1 turndown

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Flow Measurement
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Vortex Flow Meter
‒suitable for liquids, steam, and gases
‒must meet min. velocity spec
‒0.5 to 20 ft/sec range for liquid
‒5 to 250 ft/sec for gases
‒non-clogging design
‒not suitable if cavitation is a problem
‒accuracy is ±½% of rate
‒up to 5 psig head loss
‒linear over flow ranges of 20:1

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Flow Measurement
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Coriolis Effect Mass Flow Meter
‒used for steam, liquids, gases
‒measure mass flow, density,
temperature, volumetric flow
‒expensive, but 0.2% of rate accuracy
‒very stable over time
‒100:1 turndown
‒negligible to up to 15 psig head loss

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Level Measurement
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Non-Contacting – Radar Level
‒suitable for liquids and solids
‒foaming, turbulence, vessel walls and
internals can effect signal if not installed
correctly
‒can use “stilling leg” if turbulence is
extreme
‒typically ±0.1 inch accuracy

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Level Measurement
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Contacting – dP Level
‒suitable for liquids only
‒foaming and turbulence will effect signal
‒can use “stilling leg” if turbulence is
extreme
‒typically ±0.05% range accuracy
‒100:1 turndown
‒uses same dP transmitter as in dP flow
measurement

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Level Transmitters
Different types of level transmitters are,
•Ultrasonic:
Ultrasonic level transmitters are used for non-contact level sensing
of highly viscous liquids, as well as bulk solids. They are also widely
used in water treatment applications for pump control and open
channel flow measurement.
•Conductive:
These use a low-voltage, current-limited power source applied
across separate electrodes. These are ideal for the point level
detection of a wide range of conductive liquids such as water, and is
especially well suited for highly corrosive liquids such as caustic
soda, hydrochloric acid, nitric acid, ferric chloride, and similar
liquids.
•Pneumatic:
These transmitters are intended to be used in hazardous
environments, where there is no electric power or its use is
restricted, and in applications involving heavy sludge or slurry.
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Pressure Measurement
•Gauges are a major group of pressure sensors that
measure pressure with respect to atmospheric pressure.
Gauge sensors are usually devices that change their shape when
pressure is applied. These devices include
diaphragms, capsules, bellows, and Bourdon tubes.
•Barometers are used for measuring atmospheric pressure. A simple barometer is the
mercury barometer
•A piezoelectric pressure gauge is shown in Fig. 5.12b. Piezoelectric crystals produce a
voltage between their opposite faces when a force or pressure is applied to the
crystal. This voltage can be amplified and the device used as a pressure sensor
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Bourdon tube for Pressure
measurement

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Pressure Measurement
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Pressure Transmitters
typically ±0.075% range accuracy
- Absolute pressure transmitter:
This transmitter measures the pressure
relative to perfect vacuum pressure.
- Gauge pressure transmitter:
This transmitter measures the pressure
relative to a given atmospheric pressure at a
given location.
- Differential Pressure transmitter:
This transmitter measures the difference
between two or more pressures introduced
as inputs to the sensing unit Differential
pressure is also used to measure flow or
level in pressurized vessels.

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Other types of Instruments
•Hygrometers-Devices that indirectly measure humidity by sensing
changes in physical or electrical properties in materials due to their
moisture content-Capacitive hygrometer, Piezoelectric hygrometers
•Psychrometers-It uses the latent heat of vaporization to determine
the relative humidity.
•Hydrometers-are the simplest device for measuring the specific weight
or density of a liquid-Thermohydrometer, Induction hydrometers
•Viscometers -are used to measure the resistance to motion of liquids
and gases-Rotating disc viscometer.
•pH sensor –Itconsists of a sensing electrode and a reference electrode
immersed in the test solution which forms an electrolytic cell. The
electrodes are connected to a differential amplifier, which amplifies
the voltage difference between the electrodes, giving an output
voltage that is proportional to the pH of the solution.
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