Process Instrumentation Basic Definitions.pptx

Process Control & Instrumentation

Process Instrumentation • Many process variables have to be measured and manipulated to ensure a safe and efficient operation. • Instrumentation provides the means of monitoring, recording and controlling a process to maintain it at a desired state. • An instrument could indicate the process state at the location of its mounting. This form of data display is referred to as local or field indication. The instrument is referred to as a gauge. • Instead of indicating the process status locally, or transporting the actual process to the control room, it is often desirable to be able to transmit a representative signal corresponding to the process state, to the central control room for remote indication. The instrument used may be a switch indicating discrete states or a transmitter indicating continuously varying process states.

Characteristics of Instrumentation • Instrument is a device that transforms a physical variable of interest (the measurand) into a form that is suitable for transmission (the measurement). • It is common to employ a standard system of units by which the measurement from one instrument can be compared with the measurement of another. • The observable variable X need not necessarily be the measurand but simply related to the measurand in some known way.

Physical Measurement Variable Measurand

Physical Process

x

Signal Variable

Sensor

Simple Instrument Model

Measurement

M

s

Display

Functional Element of Instrument • Sensor has the function of converting the physical variable input into a signal variable output. • Signal variables have the property that they can be manipulated in a transmission system, such as an electrical or mechanical circuit. • In a basic instrument, the signal is transmitted to a display or recording device where the measurement can be read by a human observer. • The observed output is the measurement M. • There are many types of display devices, ranging from simple scales to sophisticated computer display systems. • The output signal can also be used directly by some larger system of which the instrument is a part, or may be used as the input signal of a closed loop control system. Physical Measurement Variable Measurand

Physical Process

x

Signal Variable

Sensor

Simple Instrument Model

Measurement

M

s

Display

Possible Measurement Variables Common Physical Variables: • Force • Length • Temperature • Acceleration • Velocity • Pressure • Flow • Frequency • Capacity • Resistance • Time • …

Typical Signal Variables: • Current • Voltage • Displacement • Force • Pressure • Light • Frequency

Basic concepts : Instrument Model: More General Input

Output Sensing element

Temperature Pressure Conductivity Velocity Force

Signal conditioning

Thermocouple Strain gauge Orifice plate

Bridges Amplifiers Filters

Signal processing

Data presentation

A/D, D/A converters F/V converter

Meters Recorders Computer

Sensing Element(Sensors/Transducer) ● ●

●

In contact with the process. Gives output which depends in someway on the variable to be measured (or converts the process variable into electrical parameter). Examples: Thermocouple – converts temperature into value of millivolts e.m.f Strain gauge – converts mechanical strain into resistance.

Signal Conditioning ● ●

●

Takes output of the sensing element. Converts the output into a form more suitable for further processing usually a d.c voltage, d.c current, or frequency signal. Example: Bridge – converts an impedance change into a voltage change. Amplifier – amplifies millivolts to volts. Oscillator – converts an impedance into a variable frequency voltage.

Signal Processing Takes output of the conditioning element and converts it into a form more suitable for presentation. ● Example: Analogue-to-Digital converter(ADC) - converts an analogue voltage into a digital form for input to computer Microcomputer - calculates the measured value of the variable from the incoming digital data. ●

Data Presentation ●

●

Presents the measured value in a form which can be easily recognised by the observer. Examples: A simple pointer – scale indicator Oscilloscope Chart recorder Alphanumeric display Visual display unit

Basic concepts : Measurement System - Transducers Monitoring function Input measurand

Transducer / Sensor

Signal conditioner Power supply

Regulatory function Protection function

Transducer Transducer converts one form of energy (measurand quantity) into another. Transducers can be classified as – 1. Active & Passive or 2. Analog & Digital or 3. Primary & Secondary. Active transducers do not need external excitation and operate as “energy conversion” devices. Examples – Thermoelectric, Piezoelectric, Photovoltaic, Electromagnetic transducers. Passive transducers require external excitation and operate as “energy controlling” devices. Examples – Resistive, Inductive, Capacitive, Thermoresistive, Photoconductive transducers. Signal conditioner may amplify, filter, linearise and convert the transducer output to analog or digital signal. From operational point of view transducers can be primary (sensor directly responding to process parameter) or secondary (device designed to measure the response of primary transducer and giving suitable signal output). This is a broader definition of transducers.

Basic concepts : Calibration, Standards and Traceability Calibration is the measurement of performance of an instrument, which ensures the continued accuracy of measurements made with it.

Calibration : Def.: A set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, and the corresponding values realized by standards. Calibration is done against a standard instrument of higher accuracy to detect, correlate, adjust, rectify and document the accuracy of the instrument being compared. Generally, calibration is regarded as the process of adjusting the output or indication on a measurement instrument to agree with value of the standard and includes adjusting the instrument to zero and setting the desired span. Calibration procedures involves a comparison of instrument with either a primary standard, or a secondary standard or a test standard.

Standards : Measurement standards, are physical measure, measuring instrument, reference material or measuring system intended to define, realize, conserve or reproduce a unit or one or more values of a quantity to serve as a reference. To be meaningful, the standard must satisfy the following requirements – 1.

the standard must be accurately known and internationally accepted,

2.

the standard must not change with time and place and

3.

the apparatus and experimental procedure adopted for comparison must be provable

On the basis of the accuracy of measurement the standards can be classified as primary standards and secondary standards. Primary Standard

Primary standard is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity. No pre-calibration is required for this instrument. It is used to calibrate the instruments having less accuracy. By comparing the readings of the two instruments, the accuracy of the second instrument can be determined.

Secondary Standard Secondary standards are standards whose value is assigned by comparison to a primary standard of the same quantity. Primary standards are usually used to calibrate secondary standards. For measurement of a quantity using secondary standard instrument, pre-calibration is required. Calibration of a secondary standard is made by comparing the results with a primary standard instrument or with an instrument having higher accuracy or with a known input source Working or Test standards: The primary /secondary standards are quite expensive and usually delicate which may be maintained in labs. The application of such standard is attractive in its promise of ultimate accuracy and repeatability, but it is difficult for practical use in the field. Therefore some working standards or test standards are used for field calibration of instruments. These are usually calibrated with reference to a secondary standard, and may be used to ensure that routine measurements are being carried out correctly.

Traceability : All calibrations should be traceable to a nationally or internationally recognized standard. Without documented traceability, the uncertainty of any measurement is unknown.

Traceability is defined as the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. Traceability is accomplished by ensuring the test standards we use are routinely calibrated by “higher level” reference standards. It is not necessary that a calibration lab/shop needs to have its standards calibrated with a primary standard. But calibrations should be traceable to national or international standards, no matter how many levels exist in between. BIPM Paris maintains international standards. National Institute of Standards and Technology (NIST) in United States, National Physical Lab in UK, Deutsches Institut für Normung (DIN) in Germany maintain the nationally recognized standards. . The National Physical Laboratory in New Delhi, under CSIR is the measurement standards laboratory of India.

Traceability Each calibration service provider must maintain an effective traceability chain. At the very least, the primary standard must be calibrated at an outside laboratory and then used for calibrations. In case the calibration service maintains working standards, all of them must be calibrated using primary standards including all supportive measurements.

The International System of Units The SI was developed in 1960 from the old meter-kilogramsecond (mks) system.

Base Units Quantity

Name

Length Mass Time Amount of substance Thermodynamic temperature Electric current Luminous intensity

meter kilogram second mole Kelvin amperes candela

Symbol m kg s mol K amps cd

The Official Standard Meter Historically, the meter was defined by the French Academy of Sciences as the length between two marks on a platinum-iridium bar, which was designed to represent 1⁄10,000,000 of the distance from the equator to the north pole through Paris. (From 1889 to 1960)

As 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. (From 1960 to 1983)

In 1983, the length standard, the meter, was redefined as the length of the path travelled by light in a vacuum during a time interval of duration 1/299 792 458 of a second.

The Official Standard Kilogram, Second The mass standard, the kilogram, is defined as being the mass of an alloy cylinder (90% platinum-10% iridium), held at the International Bureau of Weights and Measures at Sevres in France. Duplicates of this standard are held in other countries.

Under the International System of Units, the second is currently defined as The duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Derived Units Quantity Area Volume Force Pressure Energy Power Voltage Frequency Electric charge

Name square meter cubic meter newton pascal joule watt volt hertz coulomb

Symbol m2 m3 N Pa J W V Hz C

Basic concepts :Types of errors Error: It is the difference between the measured value and the true value. Error = Measured value – True value. Correction: True value – Measured value

Errors, in general, are classified as random and systematic errors.  Systematic error: Systematic errors have definite magnitude and direction. These are reproducible inaccuracy introduced by faulty equipment, calibration or technique. Systematic error is constant for the duration of a test or measurement.  Random error: As the name suggests, these errors are random in their occurrence and variable in magnitude, and usually follow a certain statistical law – a Gaussian (normal) distribution. Random errors are indefiniteness of result introduced by finite precision of measurement. Random errors cause scatter in the test result. Systematic errors may be corrected by calibration. Random errors are revealed by repeated measurements Random errors remain even after systematic errors are substantially reduced or accounted for. Random errors are real concern only in measurement requiring a high degree of accuracy.

Accuracy versus Precision (shooting at a target) Not accurate or Precise

Accurate and NOT Precise

Precise but NOT accurate

Accurate AND Precise

Accurate: relatively free from systematic errors. Accuracy determines how close the result comes to the true value. Defined as the difference between a measurement reading and the true value of that measurement. Precise: small random error. Precision is a measure of how closely the result is determined, without reference to true value. Defined as the ability to repeat the same accurate measurement over time.

Accuracy vs. Precision

Good accuracy Good precision

Poor accuracy Good precision

Random errors: reduce precision

Poor accuracy Poor precision

Systematic errors: reduce accuracy

Normal (Gaussian) Distributions

Chapter 21

Any summation of a large number of random variables results in a Gaussian distribution.

Probabilities associated with the normal distribution 8

Precision & Bias Error

Accuracy of an instrument can be improved by calibration, but not beyond precision. Accuracy therefore includes precision but the converse is not necessarily true.

Definitions Range: The region between the limits within which a quantity is measured, received or transmitted, expressed by stating the lower and upper range values. For example 0 to 150 deg C, -20 to 200 deg C. Span: The algebraic difference between the upper and lower values. For example for 0 to 150 deg C, span =150 deg C -20 to 200 deg C, span =220 deg C. Accuracy rating: Accuracy rating can be specified in a number of formsAs percentage of span : ±0.5% of span (or full scale (F.S.)) As percentage of upper range value : ±0.5% of upper range value As percentage of actual output reading : ±0.5% of reading

Definitions Sensitivity: is the change in the output per unit change in the input • A calibration curve is obtained by plotting the output vs. the input (y vs. x) • In the case of a linear calibration curve, the sensitivity is the slope of the (straight) line (also called Static Sensitivity) • Dynamic sensitivity : dy/dx Threshold The lowest value of a measured quantity that a given instrument or controller responds to effectively. Resolution The smallest increment of change that can be measured. Dead band The largest range through which an input signal may reverse direction without initiating observable change in output signal.

Definitions Repeatability The closeness of agreement among a number of consecutive measurements of the output for the same value of the input under the same operating conditions, approaching from the same direction, for full range traverses. It is usually measured as a nonrepeatability and expressed as repeatability in percent of span. It does not include hysteresis.

Definitions

Drift An undesired change in output over a period of time. Reproducibility The closeness of agreement among repeated measurements of the output for the same value of input made under the same operating conditions over a period of time, approaching from both directions. Reproducibility includes hysteresis, dead band, drift and repeatability. Hysteresis: That property of an element evidenced by the dependence of the value of the output, for a given excursion of the input, on the history of prior excursions and the direction of the traverse.

Definitions Linearity The closeness to which a curve approximates a straight line. It is usually measured as a nonlinearity and expressed as linearity; e.g., a maximum deviation between an average curve and a straight line. The average curve is determined after making two or more full range traverses in each direction. As a performance specification, linearity should be expressed as independent linearity, terminal based linearity, or zero-based linearity.

Types of Errors • • • • •

Hysteresis Error Linearity Error Sensitivity Error Zero Shift (null) Error Repeatability Error

Overall Instrument Error • An estimate of the overall instrument error is •

made based on all known errors For M known errors, the instrument error ec is given by ec = [e21 + e22 + e23 + . . . + e2M ]1/2

• For an instrument having known hysteresis, linearity, sensitivity and repeatability errors, the instrument error is estimated by ec = [e2h + e2L + e2K + e2R ]1/2

Analog and digital devices • Analog signal • Varies smoothly and continuously • Example: glass thermometer • Digital signal • Varies in a step-wise manner • Example: thermometer with digital display • 10011010=154

Analog Architectures Plant Syst 1

HSI

Logic

Instrumentation

Plant Syst 2

Plant Syst 3

Plant Syst n

Digital Architectures Plant Syst 1

HSI

Logic

Instrumentation

Plant Syst 2

Plant Syst 3

Plant Syst n

Sensors, Transducers and Transmitters Sensor is a generic name for a device that senses either the absolute value or change in a physical quantity such as temperature, pressure, flow rate and converts that change into a useful input signal for an information gathering system. A sensor is in contact with process and as such it is a primary transducer.

A transducer converts the physical phenomenon to an electrical signal. Generally transducer produces a low level electrical signal which cannot be "transmitted" any significant distance without degradation. A transmitter provides a conditioned output signal which is suitable for transmission. A pressure transmitter always has a transducer internally, whether it's peizoelectric or capacitive or inductive. A temperature transmitter has thermocouple or RTD and converts the low EMF of a thermocouple or the resistance of an RTD to a conditioned output signal suitable for transmission. Typical analog transmitter outputs are 4-20 mA or 0-10 VDC(electrical transmitters), 3-15 psi (pneumatic transmitters). Digital transmitters (fieldbus/wireless) transmit digital signals based on specific protocol.

FLOW SHEET/PFD AND P&ID The flow sheet or process flow diagram (PFD) is a simplified or conceptual design schematic of the process/ system. The flow diagram uses symbols to represent equipment and interconnecting piping of the entire system. Individual instruments are rarely represented in a PFD, because the focus of the diagram is the process itself. P&ID (Process and Instrumentation Diagrams (or sometimes called Piping & Instrumentation Diagram) is like a flow diagram as it uses symbols but the P&ID contains more detailed information about the equipment; such as pipe fittings and all instrumentation including sensors or actuators. Pipe sizes are shown with text alongside the line. In P&ID, unlike in Flow Sheet, each pipe including minor piping such as overflow, drain lines, bypasses etc. are identified.

Instrumentation in P&ID In addition to the mechanical components, P&ID include instruments, signal modifiers, controllers, and their interrelationships. The P&ID should show the sensor, the type of input or output, the final indications or selections necessary to be seen on the control room operator’s panel, and the presence of an interlock if applicable. A P&ID shows the process with instruments superimposed on the diagram showing what gets measured and what gets controlled. Here, one can view the flow of the process as well as the “flow” of information between instruments measuring and controlling the process. P&ID gives a name ("tag") to each sensor and actuator, along with additional parameters. This tag identifies a "point" not only on the screens and controllers, but also on the objects in the field.

P&ID The P&ID mixes pneumatic / hydraulic elements, electrical elements and instruments on the same diagram It uses a set of standard symbols.

Examples of symbols: pipe 350 kW

heater

valve

one-way valve (diode)

vessel / reactor

binary (or solenoid) valve (on/off)

analog valve (continuous) heat exchanger pump

Loop diagrams, hook-ups diagrams, Control Schemes Instrument personnel is interested in the interconnections of individual instruments, including the wire numbers, terminal numbers, cable types, instrument calibration ranges, etc. The proper form of diagram for these detail is called a loop diagram. Here, the process vessels and piping are rarely represented, because the focus of the diagram is the instruments themselves. For showing instrument connection to process, hook-up diagrams are used which show instruments to process connections using impulse lines and connectors/fittings but no wiring details as in loop diagrams. Control Schemes are used to document the control logic or strategy of a control system to control final control elements. Here emphasis is placed on the logic or algorithms used to control a process, as opposed to piping, wiring, or instrument

SYMBOLS It is necessary to have a uniform system for designating the equipment and measures in P&ID and all plant documents. For PFBR there is a document “Symbols, Abbreviation and Coding Procedure for Equipment and Measured Variables” listing all the symbols. This procedure lays down symbols that are to be used while preparing process flow diagrams as well as piping and instrumentation diagrams in order to represent the major requirements of plant. Symbols are classified into 3 groups –

Mechanical (IS 3232 : 1999 Recommendations on Graphical Symbols for Process Flow Diagrams, Piping and Instrumentation Diagrams) Electrical (IS 12032 : Part 1 to 13 : 1987/IEC 617-1 : 1985 Graphical symbols for diagrams in the field of electrotechnology)

Instrumentation (ISA-5.1-1983 (Revised 1992) Graphic Symbols for Distributed Control/Shared Display Instrumentation, Logic, and Computer Systems) Remaining symbols are adopted as in FBTR.

Deciphering instrumentation symbols: Different types of graphical elements

are defined—discrete instruments, shared control/display, computer function, and programmable logic controller—and grouped into three location categories (primary, auxiliary, and field mounted). Discrete instruments are indicated by circular elements. Shared control/display elements are circles surrounded by a square. Logic functions are are indicated by a rotated square(diamond) inside a square. Adding a single horizontal bar indicates the function resides in the primary location. A double line indicates an auxiliary location, and no line places the device or function in the field. Devices located behind a panel-board are shown with a dashed horizontal line. Abbreviations of the user's choice may be used when necessary to specify location. Primary location accessible to operator Discrete instruments Shared display, shared control Logic control

Field mounted

Auxiliary location accessible to operator

Common signal lines Connection to process, or instrument supply Pneumatic signal Electric signal Capillary tubing (filled system) Hydraulic signal Electromagnetic or sonic signal (guided) Internal system link (software or digital data link) Source: Control Engineering with data from ISA S5.1 standard

Signal lines

P&ID example Piping and Instrumentation Diagram for MTG100FC Engine Tests

TI TA22C

TE

TE

7, Heat exchanger

IC IGNITC1

TI TA22A TI TA22B

TI TC1M1 - M10

TE

Ingnitor Box C1

BS FLAMDETC1

PI PT22

TI TA21C

BE

PT

TI TA21B 10 x TE

Chimney

TI TA21A

TE

TE

PI PT21 TE

TI TA62

TE

TE

TI TW72

PT 6, Recuperator

2, Air Heater C1

IC VMPWMC1 Atmosphere

IC VPPWMC1

S IC SVGAS3

FO

S IC SVGAS1

IC SVGAS2 Fuel flow C1

S

MFM

Emission Analysis

S

Fuel Supply

Fuel flow C2

Regulator Valve

I TY

E EMIO2

AIT

E EMINOX

IC VPPWMC2

AIT

E EMICO

AIT

E EMIUHC

AIT

Blow Off Valve IC TBVDEP

IC TBVCOOL

AIT

MFM

Process Air Exhaust IC VMPWMC2

E EMICO2

I

P

TY

TE

3, SOFC Outlet

TE

TI TA12

TE

PT

PI PT52

TE

TI TA52

SI SPEED

PI LOP

ST

PT

Latchable Check Valve IC V12

S

R

1, C

G

PCS

S

TI TA32C

TE

S

5, T

Ingnitor Box

TI TA32B

PT

AC Grid

IC IGNITC2

TI TA32A

PI PT12

S

S

FO P

IC V52

TI TC2M1 - M10 PI PT32

BS FLAMDETC2

PT

BE

10 x TE

4, Combustor C2

TI TA51A PI PT51 PT

TI TA51C

TI TA51B TE

TE

TE

PI PT02

PT

TI TA02

TE

Modulatable Load

0, Air Inlet

S

Rotary block valve

S

From sample probe at C1 exit

3, SOFC Inlet

Instrumentation identification The first letter defines the measured or initiating variables such as Analysis (A), Flow (F), Temperature (T), etc. with succeeding letters defining readout, passive, or output functions such as Indicator (I), Record (R), Transmit (T), and so forth

FIC V1528 tag name of the corresponding variable

mover (here: solenoid) S

function (here: valve)

Letter and number combinations appear inside each graphical element and letter combinations are defined by the ISA standard. Numbers are user assigned and schemes use either sequential numbering, or instrument number tied to the process line number, or adopt unique numbering systems.

The output of FIC 101 is an electrical signal to TY 101 located in an inaccessible or behind-the-panel-board location.

Square root extraction of the input signal is part of FIC 101’s functionality. FT101 is a field-mounted flow transmitter connected via electrical signals (dotted line) to flow indicating controller FIC 101 located in a shared control/display device The output signal from TY 101 is a pneumatic signal (line with double forward slash marks) making TY 101 an I/P (current to pneumatic transducer)

TIC 101’s output is connected via an internal software or data link (line with bubbles) to the setpoint (SP) of FIC 101 to form a cascade control strategy

TT 101 and TIC 101 are similar to FT 101 and FIC 101 but are measuring, indicating, and controlling temperature

The ISA code for instrument type

The ISA code for instrument type