INDUSTRIAL TRAINING REPORT VENUE
Place: Power Plant
Duration th
st
24 June 2011 to 24 July 2011 Submitted to:
INVERTIS INSTITUTE OF ENGINEERING & MANAGEMENT BAREILLY BAREILLY
CONTENTS Preface Contents
Purpose About IFFCO AONLA Unit Introduction The Salient Features of Aonla Unit Plant Technology Achievements Employee Welfare Power Plant Instrumentation Section Various Activities of the Instrumentation Section What is instrument? What is Instrumentation? Measurement Output Control Transducers Pressure Transmitter Operating Principle Working Proof Circuit Description
Purpose About IFFCO AONLA Unit Introduction The Salient Features of Aonla Unit Plant Technology Achievements Employee Welfare Power Plant Instrumentation Section Various Activities of the Instrumentation Section What is instrument? What is Instrumentation? Measurement Output Control Transducers Pressure Transmitter Operating Principle Working Proof Circuit Description
Temperature Measurement in Process Industry Methods of Temperature Measurement Thermocouple Level Measurement Distributed Control System
Bibliography
ACKNOWLEDGEMENT It is my great pleasure to express my sincere gratitude to
Mr. D.Kalia , Manager Training Training and Development Section, IFFCO, Aonla Unit for his deep interest profile inspiration, valuable advice during the entire course of vocational training. I wish to express my gratitude to the DEPARTMENT (Power
INSTRUMENTATION
plant) of INDIAN FARMERS FERTILIZERS
COOPERATIVE LIMITED for allowing me to study various
functions of their department. It gives me an opportunity to understand the practical aspects
of
different
DEPARTMENT (Power
functions
of
INSTRUMENTATION
Plant) of INDIAN FARMERS FERTILIZERS
COOPERATIVE LIMITED . The present project bears the true
justification of their investment. I would like to add few heartfelt words for the people who were part of this project in numerous ways.
For all this I would like to convey my wholehearted thanks to those entire people who helped me directly & indirectly completing my report.
This project gives Rich insight about the various measurement & controlling techniques of IFFCO AONLA & also about various facilities provided by the company to its customers. Moreover this project will also help in learning Practical Aspects of different functions of Instrumentation Department which are very necessary to become a good instrument engineer.
Indian Farmers Fertilizers Cooperative Limited was established on 3rd November,1967 as a multi-unit cooperative organization with broad objectives of augmenting, fertilizer production, ensuring fertilizer availability at farmers door-step, strengthening cooperative fertilizer distribution system and education, training and guiding the farmers for improving agricultural productivity.
It is the federation of over 37,000 Societies, most of them being village cooperative, spread over in sixteen states and three union territories. The organization is distinct in the sense that the farmer owners represented through their village cooperatives also become its customers.
IFFCO presently own five giant fertilizer units at Kalol and Kandla in Gujarat, Phulpur and Aonla in Uttar Pradesh and Oman in Saudi Arab.
The production of IFFCO is NPK/DAP/UREA. All four fertilizer Units of IFFCO have displayed remarkable performance during 2002-2003 by producing 60.47 lakh tonnes fertilizer material comprising of 36.89 lakh tonnes of Urea and 23.62 lakh tonnes of NPK/DAP.
AONLA UNIT
INTRODUCTION
Towards increasing the fertilizer production under the over all national planning for utilization of natural gas available in Bombay High, a major programme for
setting up six new gas based fertilizer units was envisaged by Govt. of India along the H.B.J. gas pipe line.
IFFCO / Aonla complex is one of the giant cooperative fertilizer manufacturing industry in India.
• This complex consists of two Ammonia manufacturing units with capacities as 1350 MTPD using M/s Haldor-Topsoe, Denmark process technology.
• This complex consists of four Urea manufacturing units with capacities as 1100 MTPD each using M/s Snam-Progetti, Italy process technology.
• Apart from the above main processes we are having other Off-site and Utility
facilities to augment the main process units.
• Under offsite facilities we are having de-mineralized water treatment plants for
making boiler feed water, cooling towers for closed loop cooling water circulation for process medium cooling at various heat exchangers.
• Apart from the above facilities we are also having instrument air supply units, plant air supply units, inert gas supply units, effluent treatment units, ammonia storage units, naphtha storage units.
• Captive power generation plant with two units of gas turbines having capacities of 24 MW each supplied by M/s Hitachi, Japan, steam generation plant of 150 TPH high pressure steam at 105 kg/cm 2 pressure at 510° C.
• Two Bagging machines.
plants
with
automatic
bagging
• To run the total complex we are having detailed organization setup consisting of various agencies.
THE STATEMENT FEATURES OF AONLA UNIT
(A)
Particular Aonla – II Capacity (per annum)
Aonla – I
Ammonia MT
4, 45,500 MT
4, 45,500
Urea 26,000 MT
7, 26,000 MT
7,
Project Zero Data Sep 1993
08 Jan 1985
30
Mechanical Completion Nov 1996
08 Jan 1988
30
Ammonia Production Started 15 May 1988 1996 Unit Urea Production Started Nov 1996 Unit
18 May 1988
Commercial Production Started Dec 1996
Feed Stock Gas With
13
26
16 July 1988
Natural Gas
Dec
25
Natural
Naphtha
Project cost (Rs. in Crores) Guaranteed specification with NG
666 8.03 Gcal
955 7.34 Gcal
Energy per MT Ammonia NG+Naphtha
7.43 Gcal
with
Urea
5.76 Gcal
5.4 Gcal
PLANT TECHNOLOGY
:Aonla – I
Aonla – II
Haldor Topsoe (Denmark)
Snamprogetti (Italy)
Dedicated to Nation 1996
Ammonia
Ammonia
Urea
Urea
17 May 1988
29
May
By Hon’ble P.M. of India I.K.Gujral
Late Sh. Rajeev Gandhi
Sh.
Instrumentation Section This section deals with various maintenance activities, upgradation of various instruments for process variable monitoring, indicating, controlling, recording, logging and reporting purposes used in ammonia, urea, offsites, power & PH plants
Various Section
Activities
of
the
Instrumentation
Preventative checking/ maintenance. Breakdown maintenance.
Shutdown maintenance. Calibration of instruments. General maintenance. Material, Planning & Procurement .
What is INSTRUMENT?
An instrument consist of a single unit which gives an output reading or signal according to the unknown variable(measurand) applied to it.
What is INSTRUMENTATION ?
Instrumentation is defined as “the art & science of measurement and control”. Instrumentation can
refer either to the field in which instrument technicians and engineers work, or to the available methods of measurement and control and the instruments which facilitate this.
Three phase of instrument..
1. Mechanical Instrument 2. Electrical instrument 3. Electronics Instrument
Measurement Instruments are devices which are used in measuring attributes of physical systems. The variable measured can include practically any measurable variable related to the physical sciences. These variables commonly include:
pressure flow temperature level
density current voltage frequency various physical properties, etc.
Instruments can often be viewed in terms of a simple inputoutput device. For example, if we "input" some temperature into a thermocouple, it "outputs" some sort of signal. (Which can later be translated into data.) In the case of this thermocouple, it will "output" a signal in mill volts.
Output Instruments communicate with some sort of signal, often adhering to a standard. This signal may be defined by standards associations, or it may be a proprietary standard. Some standards include:
Analog
Pneumatics (Signal lines/Supply lines)
3-15 PSI 1.5 – 4.5 kg/cm2
Voltage
1-5 V DC 0-5 V 0-10 V
4-20 mA
Current
Digital
HART Protocol SMART Protocol Field bus Modbus Profibus Industrial Ethernet
Control These devices are used to provide an input to a process controller, which may either take the form of a PID controller or Programmable Logic Controller. These devices perform a decision based upon their own configuration and the input, also known as the process variable, and output a desired response
PRESSURE MEASUREMENT: This module will examine the theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance and variable capacitance). It also covers the variables of an operating environment (pressure, temperature) and the possible modes of failure. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges. A vacuum gauge is used to measure the pressure in a vacuum --- which is further divided into two subcategories: high and low vacuum (and sometimes ultra-high vacuum). The applicable pressure range of many of the techniques used to measure vacuums have an overlap. Hence, by combining several different types of gauge, it is possible to measure -11 system pressure continuously from 10 mbar down to 10 mbar.
Absolute, gauge and differential pressures zero reference:
Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. Differential pressure is the difference in pressure between two points .
Bourdon tube: A Bourdon gauge uses a coiled tube, which, as it expands due to pressure increase causes a rotation of an arm connected to the tube.
In1849 the Bourdon tube pressure gauge was patented in France by
Eugene Bourdon
.
The construction of a bourdon tube gauge, construction elements are made of brass
The pressure sensing element is a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed. As the gauge pressure increases the tube will tend to uncoil, while a reduced gauge pressure will cause the tube to coil more tightly. This motion is transferred through a linkage to a gear train connected to an indicating needle. The needle is presented in front of a card face inscribed with the pressure indications associated with particular needle deflections. In a barometer, the Bourdon tube is sealed at both ends and the absolute pressure of the ambient atmosphere is sensed. Differential Bourdon gauges use two Bourdon tubes and a mechanical linkage that compares the readings. The transparent cover face of the pictured combination pressure and vacuum gauge has been removed and the mechanism removed from the case. This particular gauge is a combination vacuum and pressure gauge used for automotive diagnosis:
Indicator side with card and dial
Mechanical side with Bourdon tube
the left side of the face, used for measuring manifold vacuum , is calibrated in centimetres of mercury on its inner scale and inches of mercury on its outer scale. the right portion of the face is used to measure fuel pump pressure and is calibrated in fractions of 1 kgf / cm² on its inner scale and pounds per square inch on its outer scale.
Mechanical details:
Stationary Details.
Mechanical details
A: Receiver block. This joins the inlet pipe to the fixed end of the Bourdon tube (1) and secures the chassis plate (B). The two holes receive screws that secure the case. B: Chassis plate. The face card is attached to this. It contains bearing holes for the axles. C: Secondary chassis plate. It supports the outer ends of the axles. D: Posts to join and space the two chassis plates.
Moving Parts: 1. Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block. 2. Moving end of Bourdon tube. This end is sealed. 3. Pivot and pivot pin. 4. Link joining pivot pin to lever (5) with pins to allow joint rotation. 5. Lever. This an extension of the sector gear (7). 6. Sector gear axle pin. 7. Sector gear. 8. Indicator needle axle. This has a spur gear that engages the sector gear (7) and extends through the face to drive the indicator needle. Due to the short distance between the lever arm link boss and the pivot pin and the difference between the effective radius of the sector gear and that of the spur gear, any motion of the Bourdon tube is greatly amplified. A small motion of the tube results in a large motion of the indicator needle. 9. Hair spring to preload the gear train to eliminate gear lash and hysteresis.
Regulater(peumatic) : opposite side output(hole) nob
Gauge hole N1
inside hollow
drainout
inlet
plug
spring
plug seat
inside
spring
filter
diaphram
Fig. Peumatic Regulater Constructional dig.of pneumatic regulater as shown
The diag. show the constructional detailed about the pneumatic regulater . It is contained spring ,diaphram, plug, plug sheat ,filter ,nob etc. from fig. . The working of regulater as when we have give the pressure in the inlet of regulater then it gose in the diaphragm and open the mouth of diaphragm and diaphram press the plug and plug sheat open it and it has gneo in the filter after that filter pressure in the pressure gauge and gauge show the how much pressure in the. The nob of regulater to
control the pressure and the drain is use to pass out the waste air and dust . As well as we varry pressure to the nob the gause varry according to the pressure.
Pnematic Valve: The constructional diag. of pneumatic valve which is use in flow as how much flow pass outside it is depend on the valve open and valve open due to pressure as from fig. below
It’s working as in which we have seen when we have pass the pressure to the inlet then we have seen pressure displaced the diaphragm as from fig when the diaphragm displaced then we have see the valve arrangement which is connected to the diaphragm also displaced with the displacement of diaphragm and the valve open. When the valve open the flow pass. The quantity of flow pass depend on the how much valve open. The manually regulater is regulate and set as the how much the valve open when the pressure is not flow in thr inlet.
Manually regulate
actuater
spring
Diapharm rubber
Pressure input
valve
inlet flow
outlet flow
Constructional Diag. of pneumatic valve on the principal diaphragm
Temperature measurement:In layman’n languge temperature defined as ―the degree of hotness or coldness of a body or an environment mesured on a definite scale‖ temperature is a fundamental quntity same as the mass, length and time. The law that is use in temp measurant in known as Zeroth Law of Thermodyanamics.
Devices for measuring temperature include:
Thermocouples Glass Thermometer Thermister
Resistance Temperature Detector (RTD) Pyrometers Langmuir probes (for electron temperature of a plasma) Infrared
Glass Thermometer: Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the glass thermometer. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid. Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer.
A medical/clinical thermometer showing the temperature of 38.7 °C(glass thermomeret)
Bimetallic thermometer :The operation of these thermometer is based on the principle of the coefficient of thermal expansion of different metals. Two different alloy having different physical characteristics are fused together and formed into spiral or helix, when heated caused to unbind the helix thus moving the pointer attached to it over a graduated temp. scale. Range of this instrument is – 300° F to +1000° F.
Bimetallic thermostat
Thermocouple:
A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor and can also be used to convert heat into electric power. Any circuit made of dissimilar metals will produce a temperature-related difference of voltage. Themocouples for practical measurement of temperature are made of specific alloys, which in combination have a predictable and repeatable relationship between temperature and voltage. Particular alloys are used for different temperature ranges.
Thermocouple plugged to a multimeter displaying room temperature in °C.
The three phenomena govern the behavior of thermocouple
1. The Seebeck Effect : In 1821, the German – Estonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature, and can typically be between 1 and 70 microvolts per degree Celsius (µV/°C) for the modern range of available metal combinations. This coupling of two metals gives the thermocouple its name.
Metal B junction 1 at temperature
junction 2 at temperature T2
metal A
metal A
A Thermocouple Circuit
SA and SB are the Seebeck coefficients (also called thermoelectric power or thermopower) of the metals A and B, and T 1 and T2 are the temperatures of the two junctions. The Seebeck coefficients are nonlinear, and depend on the conductors' absolute temperature, material, and molecular structure. If the Seebeck coefficients are effectively constant for the measured temperature range, the above formula can be approximated as:
V=(SB-SA).(T2-T1) 2. Peltier Effect :The Peltier effect is the reverse of the Seebeck effect; a creation of a heat difference from an electric voltage. It occurs when a current is passed through two dissimilar metals or semiconductors (n-type and p-type) that are connected to each other at two junctions (Peltier junctions). The current drives a transfer of heat from one junction to the other: one junction cools off while the other heats up; as a result, the effect is often used for thermoelectric cooling. This effect was observed in 1834 by Jean Peltier, 13 years after Seebeck's initial discovery. The flow of heat is necessary because the curre nt flowing through the thermocouple tends to cause the hot side to cool down and the cold side to heat up (the Peltier effect).
When a current I is made to flow through the circuit, heat is evolved at the upper junction (at T2), and absorbed at the lower junction (at T1). The Peltier heat absorbed by the lower junction per unit time, is equal to
3. Thomson Effect :-
The Thomson effect, named for William Thomson (Lord Kelvin), describes the heating or cooling of a current-carrying conductor with a temperature gradient. Any current-carrying conductor, with a temperature difference between two points, will either absorb or emit heat, depending on the material. If a current density J is passed through a homogeneous conductor, heat production per unit volume is Where
ρ is the resistivity of the material dT/dx is the temperature gradient along the wire
μ is the Thomson coeffi cient. The first term ρ J² is simply the Joule heating, which is not reversible. The second term is the Thomson heat, which changes sign when J changes direction. In metals such as zinc and copper, which have a hotter end at a higher potential and a cooler end at a lower potential, when current moves from the hotter end to the colder end, it is moving from a high to a low potential, so there is an evolution of energy. This is called the positive Thomson effect. In metals such as cobalt, nickel, and iron, which have a cooler end at a higher potential and a hotter end at a lower potential, when current moves from the hotter end to the colder end, it is moving from a low to a high potential, there is an absorption of energy. This is called the negative Thomson effect
Most common used thermocouple wires are the combinations of :Iron – Constantan Chromel-Alumel
0° to 1400° F. 500° to 2300° F.
Platinum / Rhodium-platinum 1000° to 2700° F. Copper – Constantan
-300° to +700° F.
Voltage – temperature relationship: The relationship between the temperature difference (ΔT) and the output voltage (v) of a thermocouple is nonlinear and is approximated by polynomial :
The coefficients an are given for n from zero to between five and nine To achieve accurate measurements the equation is usually implemented in a digital controller or stored in a look-up table.[3] Some older devices use analog filters .
Types Of Thermocouple: A variety of thermocouples are available for different measuring applications.
K Type K (chromel – alumel) is the most common general purpose thermocouple. It is inexpensive and available in a wide variety of probes. They are available in the −200 °C to +1350 °C range. The type K was specified at a time when metallurgy was less advanced than it is today and, consequently, characteristics vary considerably between examples. Another potential problem arises in some situations since one of the constituent metals, nickel, is magnetic. One characteristic of thermocouples made with magnetic material
is that they undergo a deviation in output when the material reaches its Curie point; this occurs for type K thermocouples at around 150 °C. Sensitivity is approximately 41 µV/°C.
K type thermocouple
J-type
E [3]
Type E (chromel – constantan) has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic.
J Type J (iron – constantan) is less popular than type K due to its limited range (−40 to +750 °C). The Curie point of the iron (770 °C) causes an abrupt change to the characteristic and it is this that provides the upper temperature limit. Type J thermocouples have a sensitivity of about 55 µV/°C
N Type N (nicrosil – nisil) thermocouples are suitable for use at high temperatures, exceeding 1200 °C, due to their stability and ability to resist high temperature oxidation. Sensitivity is about 39 µV/°C at 900 °C, slightly lower than type K. Designed to be an improved type K, it is becoming more popular.
B, R, S Types B, R, and S thermocouples use platinum or a platinum – rhodium alloy for each conductor. These are among the most stable
thermocouples, but have lower sensitivity, approximately 10 µV/°C, than other types. The high cost of these makes them unsuitable for general use. Generally, type B, R, and S thermocouples are used only for high temperature measurements. Type B thermocouples use a platinum – rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C. Type R thermocouples use a platinum – rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C. Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).
S and K type thermocouples, the S one is partially sheathed with an alundum tube.
T Type T (copper – constantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a
differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.
C Type C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures and must never be used in the presence of oxygen at temperatures above 260 °C
M Type M thermocouples use a nickel alloy for each wire. The positive wire contains 18% molybdenum while the negative wire [5] contains 0.8% cobalt. These thermocouples are used in the vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. Though it is a less common type of thermocouple, look-up tables to correlate temperature to EMF (milli-volt output) are available
Resistance thermometer: Resistance thermometers, also called resistance temperature detectors (RTDs), are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. As they are almost invariably made of platinum, they are often called platinum resistance thermometers (PRTs). They are slowly replacing the use of thermocouples in many industrial applications below 600 °C, due to higher accuracy and repeatability.
Resistance thermometer construction:
These elements nearly always require insulated leads attached. At low temperatures PVC, silicon rubber or PTFE insulators are common to 250°C. Above this, glass fibre or ceramic are used. The measuring point and usually most of the leads require a housing or protection sleeve. This is often a metal alloy which is inert to a particular process. Often more consideration goes in to selecting and designing protection sheaths than sensors as this is the layer that must withstand chemical or physical nts. attack and offer convenient process attachment poi
Tempature Sensor: spring
Front view
pointer
Bimetallic strip
fig. temp. sensor
The Constructional dig. Of the Temp. Sensor
The temp. sensor work on the principal of temp. diffrence . As from fig the tip of the temp. sensor is dip on where the temprature measured. The tip of the temp.sensor contained Bimetalic Element strip as from the property of the bimetal when the temp. increases the bimetal change it’s shape and then it has been pull out the wire which is join the bimetal then it pull the pointer which is join to the wire and pointer be move as from as the bimetal bend .
Level Measurement: With the wide variety of approaches to level measurement and as many as 163 suppliers offering one or more types of level-measuring instrument, identifying the right one for your application can be very difficult. In recent years, technologies that capitalized on microprocessor developments have stood out from the pack. For example, the tried-andtrue technique of measuring the head of a liquid has gained new life thanks to ―smart‖ differential pressure (DP) transmitters. Today’s local level-measuring instruments can include diagnostics as well as configuration and process data that can be communicated over a network to remote monitoring and control instrumentation. One model even provides local PID control. Some of the most commonly used liquidlevel measurement methods are:
• RF capacitance • Conductance (conductivity) • Hydrostatic head/tank gauging • Radar • Ultrasonic Before you can decide which one is right for your application, however, you need to understand how each works and the theory behind it. (Each method has its own abbreviations, so you may find the sidebar, ―Abbreviations for Common Flow Sensing Terminology,‖, a use ful reference during the discussions that follow.)
Photo 1. This view of a typical RF capacitance probe shows the electronic chassis enlarged to twice the size of its housing.
RF Capacitance:
RF (radio frequency) technology uses the electrical characteristics of a capacitor, in several different configurations, for level measurement. Commonly referred to as RF capacitance or simply RF, the method is suited for detecting the level of liquids, slurries, granulars, or interfaces contained in a vessel. Designs are available for measuring process level at a specific point, at multiple points, or continuously over the entire vessel height. Radio frequencies for all types range from 30 kHz to 1 MHz. An electrical capacitance (the ability to store an electrical charge) exists between two conductors separated by a distance, d, as shown in Figure 1. The first conductor can be the vessel wall (plate 1), and the second can be a measurement probe or electrode (plate 2). The two conductors have an effective area, A, normal to each other. Between the conductors is an insulating medium — the nonconducting material involved in the level measurement. The amount of capacitance here is determined not only by the spacing and area of the conductors, but also by the electrical characteristic (relative dielectric constant, K) of the insulating material. The value of K affects the charge storage capacity of the system: The higher the K, the more charge it can build up. Dry air has a K of 1.0. Liquids and solids have considerably higher values, as shown in Table 1. Abbreviations for Common Flow Sensing Terminology Abbreviations Term
Related Technology
A AM C FMCW
RF capacitance Radar or microwave RF capacitance Radar or microwave
FM GWR H HTG I RF K RF LT P DP PT R RF
Admittance Amplitude modulated Capacitance Frequency-modulated continuous wave Frequency modulated Guided wave radar Head or hydrostatic head Hydrostatic tank gauging Impedance Relative dielectric constant Level transmitter Pressure Differential pressure Pressure transmitter
Radar or microwave Radar or microwave Hydrostatic head gauging Hydrostatic head gauging capacitance capacitance Hydrostatic head gauging Hydrostatic head
RF RF TT TDR
Resistance Radio frequency Temperature transmitter Time-domain reflectometer
gauging Hydrostatic head gauging Hydrostatic head gauging capacitance capacitance Hydrostatic head gauging Radar or microwave
The capacitance for the basic capacitor arrangement shown in Figure 1 can be computed from the equation: C = E (K A/d) (1) where: C = capacitance in picofarads (pF) E = a constant known as the absolute permittivity of free space K = relative dielectric constant of the insulating material A = effective area of the conductors d = distance between the conductors To apply this formula to a levelmeasuring system, you must assume that the process material is insulating, which, of course, is not always true. A bare, conductive, sensing electrode (probe) is Figure 1. Basic capacitors all share the inserted down into a tank (see Figure 2,) same principle of operation. to act as one conductor of the capacitor. The metal wall of the tank acts as the other. If the tank is nonmetallic, a conductive ground reference must be inserted into the tank to act as the other capacitor conductor.
With the tank empty, the insulating medium between the two conductors is air. With the tank full, the insulating material is the process liquid or solid. As the level rises in the tank to start covering the probe, some of the insulating effect from air changes into that from the process material, producing a change in capacitance between the sensing probe and ground. This capacitance is meas ured to provide a direct, linear measurement of mass tank level. As shown in Figure 2, the TABLE 1 electrode sensor, or probe, connects Dielectric Constants of Sample directly to an RF level transmitter, which is Substances mounted outside the tank. In one design, Substance Value Isopropyl alcohol 18.3 with the probe mounted vertically, the Kerosene 1.8 system can be used for both continuous Kynar 8.0 level measurement and simultaneous Mineral oil 2.1 80 multipoint level control. Alternatively, for Pure water Sand 4.0 point level measurement, one or more Sugar 3.0 probes can be installed horizontally Teflon 2.0 through the side of the tank; Figure 2 shows this type being used as a high-level alarm. Photo 1 shows a typical probe assembly with an enlarged view of the microprocessorbased transmitter that fits in the housing; in use, its digital indicator faces up. Trans mission of the level-measurement signal can take several forms, as can the in strument that receives the signal at either a local or a remote location. Referring to Figure 2, the transmitter output is 4 – 20 mA DC plus optional HART Protocol for remote diagnostics, range change, dry calibration, and so on. The instrument receiving the signal can be a distributed control system (DCS), a programmable logic controller (PLC), a Pentium III PC, or a strip or circular chart recorder.
When the process material is conductive, the sensing probe is covered with an insulating sheath such as Teflon or Kynar. The insulated probe acts as one plate of the capacitor, and the conductive process material acts as the other. The latter, being conductive, connects electrically to the grounded metallic tank. The insulating medium or dielectric for this application is the probe’s sheath. As the level of conductive process material changes, a Figure 2. In the RF capacitance proportional change in capacitance method of liquid level measurement, the electrode sensor connects directly occurs. Note that this measurement is unaffected by changes in the temperature to an RF transmitter outside the tank. or exact composition of the process material. RF Impedance or RF Admittance. When another electrical characteristic, impe dance, enters the picture, the result is further refinements in RF level measurement. Offering improved reliability and a wider range of uses, these variations of the basic RF system are called RF admittance or RF impedance. In RF or AC circuits, impe dance, Z, is defined as the total opposition to current flow: Z = R + 1/ j 2 p f C (2) where: R = resistance in ohms j = square root of minus 1 ( – 1) p = the constant 3.1416 f = measurement frequency (radio frequency for RF measurement) C = capacitance in picofarads An RF impedance level-sensing instrument measures this total impedance rather than just the capacitance. Some level-meas uring systems are referred to as RF admittance types. Admittance, A, is
defined as a measure of how readily RF or AC current will flow in a circuit and is therefore the reciprocal of impedance (A = 1/Z). Thus, there is no basic difference between the RF impedance and RF admittance as a level-measurement technology. In some cases, the process material tends to build up a coating on the level-sensing probe. In such cases, which are not uncommon in level applications, a significant meas urement error can occur because the instrument measures extra capacitance and resistance from the coating buildup. As a result, the sensor reports a higher, and incorrect, level instead of the actual tank level. Note that the equation for impedance includes resistance, R. The RF impedance method can be provided with specific circuitry capable of measuring the resistance and capacitance components from the coating and the capacitive component due to the actual process material level. The circuitry is designed to Figure 3. In the conductive type of level measurement, two dual-tip solve a mathematical relationship probes detect the maximum and electronically, thereby producing a 4 – 20 minimum levels in a tank. mA current output that is proportional only to the actual level of the proc ess material. It is virtually unaffected by any buildup of coating on the sensing probe, enabling an RF system to continue functioning reliably and accurately.
Conductance
The conductance method of liquid level measurement is based on the electrical conductance of the measured material, which is usually a liquid that can conduct a current with a low-voltage source (normally <20 V). Hence the method is also referred to as a conductivity system. Conductance is a relatively low-cost, simple method to detect and control level in a vessel.
One common way to set up an electrical circuit is to use a dual-tip probe that eliminates the need for grounding a metal tank. Such probes are generally used for point level detection, and the detected point can be the interface between a conductive and nonconductive liquid. Figure 3 shows an arrangement with two dual-tip probes that detect maximum and minimum levels. When the level reaches the upper probe, a switch closes to start the discharge pump; when the level reaches the lower probe, the switch opens to stop the pump. Hydrostatic Head
One of the oldest and most common methods of measuring liquid level is to measure the pressure exerted by a column (or head) of liquid in the vessel. The basic relationships are: P = mHd or: H = mP/d (3) where, in consistent units: P = pressure m = a constant
Figure 4. The hydrostatic head, or differential pressure, method can add measurements (at left) for hydrostatic tank gauging (HTG).
H = head d = density P is commonly expressed in pounds per square inch; H, in feet; and d, in pounds per cubic feet; but any combination of units can be used, so long as the m factor is suitably adjusted. The density of a liquid varies with temperature. For the highest precision in level measurement, the density must therefore be compensated for or
expressed with relation to the actual temperature of the measured liquid. This is the case with hydrostatic tank gauging (HTG) described below. For decades, DP-type instruments — long before the DP cell — were used to measure liquid level. Orifice meters, originally designed to measure differential pressure across an orifice in a pipeline, readily adapted to level measurement. Today’s smart DP transmitters adapt equally well to level measurements and use the same basic principles as their precursors. With open vessels (those not under pressure or a vacuum), a pipe at or near the bottom of the vessel connects only to the highpressure side of the meter body and the low-pressure side is open to the atmosphere. If the vessel is pressurized or under vacuum, the low side of the meter has a pipe connection near the top of the vessel, so that the instrument responds only to changes in the head of liquid (see Figure 4). DP transmitters are used extensively in the process industries today. In fact, newer smart transmitters and conventional 4 – 20 mA signals for communications to remote DCSs, PLCs, or other systems have actually resulted in a ―revival‖ of this technology. Problems with dirty liquids and the expense of piping on new installations, however, have opened the door for yet newer, alternative methods. Hydrostatic Tank Gauging. One growing, specialized application for systems that involve hydrostatic measurements is hydrostatic tank gauging (HTG). It is an emerging standard way to accurately gauge liquid inventory and to monitor transfers in tank farms and similar multiple-tank storage facilities. HTG systems can provide accurate information on tank level, mass, density, and volume of the contents in every tank. These values can also be networked digitally for multiple remote access by computer from a safe area. Figure 4 shows a simplified system that incorporates only one pressure transmitter (PT) with a temperature transmitter (TT) and makes novel use of a level transmitter (LT) to detect accumulation of water at the bottom of a tank. Mass (weight) of the tank’s contents can be calculated from the hydrostatic head (measured by PT) multiplied by the tank area (obtained from a lookup table). The liquid’s temperature-density relationship can be used to calculate the volume and level, provided the
tank is not under pressure. Data fed into a computer system make it possible for all calculations to be automatic, with results continuously available for monitoring and accounting purposes. The level transmitter, with its probe installed at an angle into the bottom portion of the tank, is an innovative way to detect accumulation of water, separated from oil, and to control withdrawal of product only. Moreover, by measuring the water-oil interface level, the LT provides a means of correcting precisely for the water level, which would incorrectly be measured as product. Though the DP transmitter is most commonly used to measure hydrostatic pressure for level measurement, other methods should be mentioned. One newer system uses a pressure transmitter in the form of a stainless steel probe that looks much like a thermometer bulb. The probe is simply lowered into the tank toward the bottom, supported by plastic tubing or cable that carries wiring to a meter mounted externally on or near the tank. The meter displays the level data and can transmit the information to another receiver for remote monitoring, recording, and control. Another newer hydrostatic measuring device is a dry-cell transducer that is said to prevent the pressure cell oils from contaminating the process fluid. It incorporates special ceramic and stainless steel diaphragms and is apparently used in much the same way as a DP transmitter.
Ultrasonic and Sonic:
Figure 6. In continuous ultrasonic level measurement, a transducer mounted at the top of the tank sends bursts of waves downward onto a material to determine its level
Both ultrasonic and sonic level instruments operate on the basic principle of using sound waves to determine fluid level. The frequency range for ultrasonic methods is ~20 – 200 kHz, and sonic types use a frequency of 10 kHz. As shown in Figure 6, a top-of-tank mounted transducer directs waves downward in bursts onto the surface of the material whose level is to be measured. Echoes of these waves return to the transducer, which performs calculations to convert the distance of wave travel into a measure of level in the tank. A piezoelectric crystal inside the transducer converts electrical pulses into sound energy that travels in the form of a wave at the established frequency and at a constant speed in a given medium. The medium is normally air over the material’s surface but it could be a blanket of nitrogen or some other vapor. The sound waves are emitted in bursts and received back at the transducer as echoes. The instrument measures the time for the bursts to travel down to the reflecting surface and return. This time will be proportional to the distance from the transducer to the surface and can be used to determine the level of fluid in the tank. For practical applications of this method, you must consider a number of factors. A few key points are:
• The speed of sound through the medium (usually air) varies with the medium’s temperature. The transdu cer may contain a temperature sensor
to compensate for changes in operating temperature that would alter the speed of sound and hence the distance calculation that determines an accurate level measurement.
• The presence of heavy foam on the surface of the material can act as a sound absorbent. In some cases, the absorption may be sufficient to preclude use of the ultrasonic technique. • Extreme turbulence of the liquid can cause fluctuating readings. Use of a damping adjustment in the instrument or a response delay may help overcome this problem. To enhance performance where foam or other factors affect the wave travel to and from the liquid surface, some models can have a beam guide attached to the transducer. Ultrasonic or sonic methods can also be used for point level measurement, although it is a relatively expensive solution.. The signal from the receive crystal is analyzed for the presence or absence of tank contents in the meas urement gap. These noncontact devices are available in models that can convert readings into 4 – 20 mA outputs to DCSs, PLCs, or other remote controls.
.
Soleinoid:
Soleinoid a instrument which is act as valve. It is consist a magnetic coil, plunger,capacitor bridge regulater as from fig. etc..
Construction: output
Outer cap
Input
drain out wire
capacitor
inside chamber plunger
magnetic coil
nut
bridge A.C. supply
Construction of soleinoid
Working of soleinoid: Firstly we give the A.C. (110V-230V) then it gose in bridge rectifier it is convert AC in the DC from then it’s output join to the head cap as from fig. after that it’s output gose in the magnetic coil and it is magnetized due to magnetization it attract the plunger towards magnetic field then the valve open and pressure comes outside.If we disconnect the the power supply the magnetic coil loss their magnetic property and it leaves the plunger and sut down the plunger
Current to pneumatic converter(I/PAC):
gauge 4-20mA
Current to
Source
Penumatic
transmitter
Positioner
Control valve
converter
24 V DC power supply
Regulater
Current to pneumatic converter convert the electrical signal in to pneumatic from. In which we has give the 24-V power supply then it will go in the 4-20mA transmitter which is transmit the current in the I/P converter then it convert current into pneumatic from . When we increase the current slitly the pressure control with the current to pneumatic converter as well as when we increase current the pressure is also increases and when decrease current the pressure also decrease. After that the controlled pressure move in the positioner and positioned is as well as open the control valve as the pressure increases and decreases. The regulater regulate the pressure as well as it will increases and decreases pressure as per requirement.
Distributed control system : History Electronic digital based DCS goes back to circa 1974 with Honeywell (and others) launching digital DCS eg TDC200. BBC (Brown Boveri et Cie now part of ABB) also had a digital DCS. By the 1990's most large industrial plants had replaced the majority of pneumatic and hydraulic control systems. In the early 2000's most Industrial DCS's were ported to Windows (R) OS platforms after presure from cost cutting and emmbedded IT infrastructure.Not every distributed control system is part of a manufacturing systemA distributed control system (DCS) refers to a control system usually of a manufacturing system, process or any kind of dynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. The entire system of controllers is connected by networks for communication and monitoringDCS is a very broad term used in a variety of industries, to monitor and control distributed equipment.
Electrical power grids and electrical generation plants
Environmental control systems Traffic signals Water management systems Oil refining plants Chemical plants Pharmaceutical manufacturing Sensor networks Dry cargo and bulk oil carrier ships
Elements
A DCS typically uses custom designed processors as controllers and uses both proprietary interconnections and Communications protocol for communication. Input & output modules form component parts of the DCS. The processor receives information from input modules and sends information to output modules. The input modules receive information from input instruments in the process (a.k.a. field) and transmit instructions to the output instruments in the field. Computer buses or electrical buses connect the processor and modules through multiplexer or demultiplexers. Buses also connect the distributed controllers with the central controller and finally to the Human-Machine Interface (HMI) or control consoles.
DCS CLOSE LOOP DIAGRAM: I/P signal conditioner
Marselling Cabinet I/P cord
control Section Coral
pressure I/P 1-5V I/P
Set point 1-5V
A/D PID D/A
Isolatar
bus 8 I/p
I/P
8 o/p
Multipoint Analog cord for Digital Signal
Sequense Bus
Terminal Boared
Relay Boared
eops
16 I/P 16 I/P Sequense Bus
I/P nest Signal conditioner nest terminal Boared
Station Control nest
EFCD(Duplex field control Station)
DCS Working: Working of DCS as from privious fig. signal from transmitter(pressure, temp,level transmitter) comes in JB(junction Box) and then from junction box it’s came in Marselling Room which is exist in control room then it’s input in isolater(8 I/P) as from fig then in the system cabinet which is based on PLC in which analog signal converted in digital(A/D) as shown above fig. after that it comes in set point cabinet where where a PID controller which is set the value as from requird after that it is gose in D/A converter then it is in the isolater then positioner Then control valve as from fig. We have Yokogawa make Centum – XL DCS in Aonla-II unit.
Close Loop Diagram of Control System:
Orifice
positioner Control valve
flow inlet
flow outlet pneumatic line transmitter
I/P converter I/P
Indicater
I/C
controller
Working of close loop control system is as from fig. when the flow coming in the flow pipe then we have seen the orifice which is give the pressure difference . Pressure difference is feedback in the transmitter and the transmitter give the response to indicater which is show the reading how much pressure and the flow in the pipe. Transmitter give the detailed to controller how much the flow pass. After that the controller the signal gose in the current to pneumatic converter and it convert signal in the from of pneumatic then through the pneumatic line it will go in the positioner and then it have shift the position of control valve how much the output we have need.
It’s process reapeat as well as.
Legends for different components of DCS: ENGS
: Engineering station
EOPC
: Operator console
EOPS
: Operator station
EFCD
: Duplex field control station
AC
: Auxiliary console
PC
: Personal computer
A/R/C/D/R/C : Amplifying relay cabinet
PDS
: Power Distribution Cabinet
FIC
: Free issue item cabinet
GPMC
: General-Purpose marshalling
P
: HF Bus Terminator
J
: HF Bus Terminator
MV
: Manipulated variable output
SV
: Set value
PV
: Process value
EPRT2
: Serial printer
EPCH2
: Color Hard copy unit
Applications Distributed Control Systems (DCSs) are dedicated systems used to control manufacturing processes that are continuous or batch-oriented, such as oil refining, petrochemicals, central station power generation, pharmaceuticals, food & beverage manufacturing, cement production, steelmaking, and papermaking. DCSs are connected to sensors and actuators and use setpoint control to control the flow of material through the plant. The most common example is a setpoint control loop consisting of a pressure sensor, controller, and control valve. Pressure or flow measurements are transmitted to the controller, usually through the aid of a signal conditioning Input/Output (I/O) device. When the measured variable reaches a certain point, the controller instructs a valve or actuation device to open or close until the fluidic flow process reaches the desired setpoint. Large oil refineries have many thousands of I/O points and employ very large DCSs. Processes are not limited to fluidic flow through pipes, however, and can also include things like paper machines and their associated variable speed drives and motor control
centers, cement kilns, mining operations, ore processing facilities, and many others. A typical DCS consists of functionally and/or geographically distributed digital controllers capable of executing from 1 to 256 or more regulatory control loops in one control box. The input/output devices (I/O) can be integral with the controller or located remotely via a field network. Today’s controllers have extensive computational capabilities and, in addition to proportional, integral, and derivative (PID) control, can generally perform logic and sequential control. DCSs may employ one or several workstations and can be configured at the workstation or by an off-line personal computer. Local communication is handled by a control network with transmission over twisted pair, coaxial, or fiber optic cable. A server and/or applications processor may be included in the system for extra computational, data collection, and reporting capability.
