INSTRUMENTATION & PROCESS CONTROL

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

• InstrumentationThe measuring devices used in the measuring system.

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Generalised Measuring System

• 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

Signalconditioning

element

Signalprocessingelement

Data presentation

element

Input OutputTruevalue

Measuredvalue

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Process Control Loop

Block Diagram of Process control loop

Block Diagram of the elements in process control loop

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ELEMENTS DEFINATIONFeedback loop

It 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 a process 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 this value will generate an error signal.

Instrument is the name of any of the various device types for indicating or measuring physical quantities or conditions, 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.

Process Control Loop Elements definition

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ELEMENTS DEFINATIONTransducers 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.• Accuracy It is the difference between the indicated value

and the actual value of a instrument.• Precision It 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 thesame 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

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

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

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

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 MeasurementDifferential 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

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

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 MeasurementVortex 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

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

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

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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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.

Level Transmitters

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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

Bourdon tube for Pressure measurement

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Pressure MeasurementPressure 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 – It consists 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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