Indian oil corporation limited

Bongaigaon refinery

Industrial training report

On instrumentation and control system

Submitted by : Hriday Nath (09-1-6-002)

Branch : Electronics & Instrumentation

College : NIT silchar

CONTENTS

1. ACKNOWLEDGEMENTS 2. INTRODUCTION 3. MEASURMENT OF THE PROCESS VARIABLES FLOW PRESSURE LEVEL TEMPERATURE4. CONTROLL OF OUTPUT VARIABLES DCS and PLC CONTRLLERS C 300 (HONYWELL) CS 3000 (YOKOGAWA) HART PROTOCOL CONTROLL ELEMENTS

Acknowledgement

Everything that happens in the world is an outcome of interaction of various factor, some of which are favourable while other not. Always for a desired result, the number of favourable factors is more. This work is NO

exception to this fact. We acknowledge that we’ve been fortunate enough to get the support, mentally and physically in everything that we do.

We would give our special thanks to Mr. Umesh Ch. Das (Manager, T&D), for giving his very kind permission to undergo the training programme under the guidance of IOCL engineers. We would thank Mr.G.K. Nag(ITM),Mr.H.A. Ahmed (DMIT)& Mr.J.K.Sharma(DMIT), Ms.Priyanka Baruah (SITE),Mr. G.Saharia(SITE), Mr. Poranjyoti Baruah(SITE) Mr. P.K.Bora(ITE),Mr.Rajesh Goyal(ITE) under whose able guidance we completed our training. All these people were of immense importance regarding the knowledge and supports for the well furnished equipments.

We greatly acknowledge the help and the mental strength provided by our entire family for encouraging us and providing us knowledge & guidance related with every deptt. of IOCL BGR

At last we conclude by thanking all the employees of IOCL BGR (both executives & workers) who helped us in making our training a boon for us.

The entire team of IOCL BGR for functioning of each department in a modernized and techno-commercial atmosphere to make the project touch such peaking performance.

Hriday Nath

B.Tech. 4th year

Electronics & Instrumentation Branch

(NIT Silchar)

INTRODUCTION

The Bongaigaon Refinery is the eighth largest refinery of IndianOil. Formed upon the amalgamation of Bongaigaon Refinery & Petrochemicals Limited (BRPL) with IndianOil on

March 25. 2009, Bongaigaon refinery is situated at Dhaligaon in Chirang district of Assam, 200 km west of Guwahati.

It two Crude Distillation Units (CDU), two Delayed Coker Units (DCU) and a Coke Calcination Unit (CCU) with a processing capacity of 2.35 MMTPA of crude oil. The first CDU with a capacity of 1 MMTPA was commissioned in the year 1979. The capacity was further increased to 1.35 MMTPA in 1986. An LPG Bottling Plant with a capacity of 44,000 MTPA was also commissioned in the year 2003.

Presently the refinery produces a wide range of petroleum products namely LPG, Naphtha, MS, SKO, HSD, LDO, LSHS, LVFO, RPC, CPC, Needle coke and solvents (Petrosol and Bonmex-II) by processing Assam Crude and Ravva Crude (from the Ravva oil fields of Krishna Godavari Basin). Bongaigaon refinery has also undertaken special endeavours towards environment protection and conservation. The refinery has developed an ecological park and a pond surrounding it containing 65,000 cubic meter of water, through which the storm water drains of the plant are routed for final discharge. Another natural pond with a capacity of 30,000 cubic meter of water has developed into an environment-friendly park-cum-pond for migratory birds. In addition, a rain water harvesting system has been installed in the Bongaigaon township complex and the installation of solar water heating systems (SWHS) and solar photovoltaic systems (streetlights) is underway.

In recognition of its green initiatives, Bongaigaon refinery has been a recipient of numerous prestigious awards, the latest being the National Award for "Prevention of Pollution" from the Ministry of Environment and Forests on September 16, 2010. The award acknowledges the Refinery’s outstanding contribution in environmental conservation and creating environmental awareness in the Bodo Territorial Autonomous District region of Assam, which is predominantly a tribal dominated area. Bongaigoan Refinery is the country’s first oil Refinery to have won this coveted honour. Last year the Refinery was honoured with the Indira Gandhi Paryavaran Puruskar by the Ministry of Environment, as well as the ‘Greentech Environment Excellence Gold Award 2008’ and the ‘Indira Gandhi Paryavarn Puraskar 2006’ for its outstanding environment performance.

FLOW MEASURMENT

Orifice meter Depending on the type of obstruction, we can have different types of flow meters. Most common among them is the orifice type flowmeter, where an orifice plate is placed in the pipe line, as

shown in fig.2. If d1 and d2 are the diameters of the pipe line and the orifice opening, then the

flow rate can be obtained using eq. by measuring the pressure difference (p1-p2).

The major advantages of orifice plate are that it is low cost device, simple in construction and easy to install in the pipeline as shown in fig.3. The orifice plate is a circular plate with a hole in the center. Pressure tappings are normally taken distances D and 0.5D upstream and downstream the orifice respectively (D is the internal diameter of the pipe). But there are many more types of pressure tappings those are in use.

The major disadvantage of using orifice plate is the permanent pressure drop that is normally experienced in the orifice plate as shown in fig.3. The pressure drops significantly after the orifice and can be recovered only partially. The magnitude of the permanent pressure drop is around 40%, which is sometimes objectionable. It requires more pressure to pump the liquid. This problem can be overcome by improving the design of the restrictions. Venturimeters and flow nozzles are two such devices.

Rotameter The orificemeter, Venturimeter and flow nozzle work on the principle of constant area variable pressure drop. Here the area of obstruction is constant, and the pressure drop changes with flow rate. On the other hand Rotameter works as a constant pressure drop variable area meter. It can be only be used in a vertical pipeline. Its accuracy is also less (2%) compared to other types of flow meters. But the major advantages of rotameter are, it is simple in construction, ready to install and the flow rate can be directly seen on a calibrated scale, without the help of any other device, e.g. differential pressure sensor etc. Moreover, it is useful for a wide range of variation of flow rates (10:1).

The basic construction of a rotameter is shown in fig. 3. It consists of a vertical pipe, tapered downward. The flow passes from the bottom to the top. There is cylindrical type metallic float

inside the tube. The fluid flows upward through the gap between the tube and the float. As the float moves up or down there is a change in the gap, as a result changing the area of the orifice. In fact, the float settles down at a position, where the pressure drop across the orifice will create an upward thrust that will balance the downward force due to the gravity. The position of the float is calibrated with the flow rate.

Fig. 3 Basic construction of a rotameter.

Construction of the float The construction of the float decides heavily, the performance of the rotameter. In general, a float should be designed such that:

(a) it must be held vertical (b) it should create uniform turbulence so as to make it insensitive to viscosity (c) it should make the rotameter least sensitive to the variation of the fluid density.

.

TEMPERATURE MEASURMENT

Thermocouple Thomas Johan Seeback discovered in 1821 that thermal energy can produce electric current. When two conductors made from dissimilar metals are connected forming two common junctions and the two junctions are exposed to two different temperatures, a net thermal emf is produced, the actual value being dependent on the materials used and the temperature difference between

hot and cold junctions. The thermoelectric emf generated, in fact is due to the combination of two effects: Peltier effect and Thomson effect. A typical thermocouple junction is shown in fig. 5.

Fig. 5 A typical thermocouple

Thermocouples are extensively used for measurement of temperature in industrial situations. The major reasons behind their popularity are: (i) they are rugged and readings are consistent, (ii) they can measure over a wide range of temperature, and (iii) their characteristics are almost linear with an accuracy of about 0.05%. However, the major shortcoming of thermocouples is low sensitivity compared to other temperature measuring devices (e.g. RTD, Thermistor).

Thermocouple Materials Theoretically, any pair of dissimilar materials can be used as a thermocouple. But in practice, only few materials have found applications for temperature measurement. The choice of materials is influenced by several factors, namely, sensitivity, stability in calibration, inertness in the operating atmosphere and reproducibility (i.e. the thermocouple can be replaced by a similar one without any recalibration). Table-I shows the common types of thermocouples, their types, composition, range, sensitivity etc. The upper range of the thermocouple is normally dependent on the atmosphere whre it has been put. For example, the upper range of Chromel/ Alumel thermocouple can be increased in oxidizing atmosphere, while the upper range of Iron/ Constantan thermocouple can be increased in reducing atmosphere.

Table-1 Thermocouple materials and Characteristics

Type Positive lead

Negative lead

Temperature range

Temperature

coeff.variation

μv/oC

Most linear

range and sensitivity

in the range

R Platinum-Rhodium

Platinum 0-1500oC 5.25-14.1 1100-

(87% Pt, 13% Rh)

1500oC 13.6-14.1

μv/oC S Platinum-

Rhodium (90% Pt, 10% Rh)

Platinum 0-1500oC 5.4-12.2 1100-1500 oC

13.6-14.1

μv/oC K Chromel

(90%Ni, 10% Cr)

Alumel (Ni94Al2Mn3Si)

-200-

1300oC

15.2-42.6 0-1000 oC 38-42.9

μv/oC E Chromel Constantan

(57%Cu, 43%Ni)

-200-1000oC

25.1-80.8 300-800 oC 77.9-80.8

μv/oC T Copper Constantan -200-350oC 15.8-61.8 nonlinear

J Iron Constantan -150-750oC 21.8-64.6 100-500 oC 54.4-55.9

Reference Junction Compensation From above discussions, it is imminent that the thermocouple output voltage will vary if the reference junction temperature changes. So, for measurement of temperature, it is desirable that the cold junction of the thermocouple should be maintained at a constant temperature. Ice bath can be used for this purpose, but it is not practical solution for industrial situation. An alternative is to use a thermostatically controlled constant temperature oven. In this case, a fixed voltage must be added to the voltage generated by the thermocouple, to obtain the actual temperature. But the most common case is where the reference junction is placed at ambient temperature. For high temperature measurement, the error introduced due to variation of reference junction temperature is not appreciable.

Resistance Temperature Detector

Copper, Nickel and Platinum are mostly used as RTD materials. The range of temperature measurement is decided by the region, where the resistance-temperature characteristics are approximately linear. The resistance versus temperature characteristics of these materials is

shown in fig.1, with to as 0oC. Platinum has a linear range of operation upto 650oC, while the

useful range for Copper and Nickel are 120oC and 300oC respectively.

Construction For industrial use, bare metal wires cannot be used for temperature measurement. They must be protected from mechanical hazards such as material decomposition, tearing and other physical damages. The salient features of construction of an industrial RTD are as follows:

• The resistance wire is often put in a stainless steel well for protection against mechanical hazards. This is also useful from the point of view of maintenance, since a defective sensor can be replaced by a good one while the plant is in operation.

• Heat conducting but electrical insulating materials like mica is placed in between the well and the resistance material.

• The resistance wire should be carefully wound over mica sheet so that no strain is developed due to length expansion of the wire.

Fig. 2 shows the cut away view of an industrial RTD.

LEVEL MEASUREMENT

Hydrostatic Differential Pressure type The hydrostatic pressure developed at the bottom of a tank is given by: p= hρgwhere h is the height of the liquid level and ρis the density of the liquid. So by putting two pressure tapings, one at the bottom and the other at the top of the tank, we can measure the differential pressure, which can be calibrated in terms of the liquid level. Such a schematic arrangement is shown in Fig. 1 . The drum level of a boiler is normally measured using this basic principle. However proper care should be taken in the measurement compensate for variation of density of water with temperature and pressure.

Floats & Displacers Introduction

Floats and Displacers are simple level measurement devices. They are somewhat identical in their look but they work on different operating principles. 

Float level switches work upon the buoyancy Principle according to which “as liquid level changes a (predominately) sealed container will, providing its density is lower than that of the liquid, move correspondingly”. In other words, the buoyancy principle states that "the buoyancy force action on an object is equal to the mass of liquid displaced by the object.

Displacers operation is based upon the Archimedes Principle which says that “when a body is immersed in a fluid it loses weight equal to that of the fluid displaced. By detection of the apparent weight of the immersed displacer, a level measurement can be inferred.

Displacers and floats are strictly applied for level detection in case of moderately non-viscous and clean process liquids. They present their best operation in switching applications and over for small periods. One can achieve spans of up to 12m also, but in that case their use happens to be extremely costly.

Float Level Switches Float level switches are mainly employed for level measurement in narrow level differential fields, for example high level alarm or low level alarm applications. One of the significant types of float is a magnetrol float level switch which consists of a plain float and operates via a magnetic coupling action. The switch is designed in such a way that some part of float remains submerged in the liquid as it rides on the liquid surface. The float goes up and down on the surface depending upon the level of fluid in the tank. This causes a magnetic sleeve to travel in or out of the region of a magnetic

switch resulting in its activation. A non-magnetic tube is also provided in the design which acts as a barrier and helps in separating the switching arrangement from the controlled fluid.

These float based level switches include: a magnetic piston, a reed switch and a mercury switch. Among different float switch designs, the oldest and most precise one employed for continuous level detection is the tape level gage. Float level sensors are usually prepared from materials like stainless steel, PFA, Hastelloy, Monel, and several other plastic components. It is always required of floats to have their weights less than the minimum likely specific gravity of the liquid being measured. There are basically three kinds of Float level controls which are listed below:

1. Top mounting 2. Side mounting 3. External cage

An extensive choice of float level switches is accessible in the market which may include mercury, dry contact, hermetically sealed and pneumatic switching devices. The upper temperature and pressure limits of float level switches are +1000° F and 5000 psig respectively. They usually work with low specific gravities which can be around 0.32. They exist in variety of models such as single, dual and three switch models. Besides, for level detection of interfaces created between two fluids, customary float rides are available. Float operated control valves are also available which basically perform combined functions of level detection as well as level control via a single level controller. However, their use is limited to areas involving small flows with negligible pressure drops only.

Displacer Switches

In a typical displacer switch design, a spring is provided which is burdened with weighted displacers. The displacers having weights greater than the process fluid gets submerged in the liquid resulting in a buoyancy force change. This will cause a variation in the net force operating on the spring. In general, the spring will compress with the raise in buoyancy force. Just like the float level switches, a magnetic sleeve and a non-magnetic barrier tube is also incorporated in displacer switches. The magnetic sleeve is attached to the spring and it moves according to the spring movement resulting in activation of switching mechanism. An in-built limit switch is provided in the design which proves useful in level surge conditions since it keeps a check on the over stroking of the spring. The operating principle of a typical Displacer switch is illustrated in the figure below.

Displacer switches are most commonly employed in oil and petrochemical fields as level transmitters and local level controllers. These switches offer extremely correct and consistent measurement results in applications where clean liquids having stable densities are concerned. They are particularly not appropriate for slurry or sludge type applications since coating of the displacer causes a change in its volume and a resulting change in its buoyancy force. Temperature adjustments should also be done for these switches, specifically in areas where changes in process temperature can significantly affect the density of the process liquid.

The performance of displacers can be influenced by non-stability in process density in view of the fact that the displacement i.e. the weight loss of the material is equivalent to the weight of the liquid dislocated. As soon as the specific gravity of the process varies, the weight of the displaced material also varies accordingly, resulting in a change in the calibration. Due to this, one can specifically face problems in cases of interface level detection between two liquids having different densities, where the relative signal depends upon the difference between two densities. An important requirement while working with displacers is that even after commissioning, the liquid being detected must retain its density for getting good repeatability.

Advantages

Following are the major advantages associated with the use of floats and displacers:

They perform extremely well with clean fluids. Use of these level sensors proves to be very accurate. They are flexible to extensive changes in density of the medium.

Floats v/s Displacers

Following are the major points of distinction between floats and displacers:

“Float Switches are available with a glandless design and are capable of fail safe operation in extreme process conditions, unlike displacers, which if the torque tube fails can provide a leak path.”

A float generally rides above the surface of liquid whereas a displacer remains either partly or totally immersed in process liquid.

Displacer switches are considered to be additionally stable and dependable as compared to standard float level switches in case of turbulent, surging, frothy and foamy services. However in case of refineries, the use of displacers is decreasing owing to their high installation cost and inaccurate performance due to process density changes. In these applications, float level switches have been found to be reliable and useful.

Settings of displacers can be changed very easily since they can be shifted at any place along the length of the suspension cable. Moreover, these level devices have the provision of interchangeability between tanks. This is due to the fact that the differences in process density can be endured by varying the tension of the spring attached to the displacers.

Testing the appropriate working of a displacer switch is much easier than a customary float level switch since the former requires just lifting of a suspension whereas the latter necessitates filling of liquid in the tank upto the actuation mark.

PRESSURE MEASUREMENT

Bourdon gaugeThe Bourdon gauge (see Fig. 2a) consists of a bent tube with an elliptic cross section closed at one endand connected at the other open end to the chamber in which the pressure is to be measured. Pressure differences between the environment of the gauge and the interior cause forces to act on the two walls ofthe tube (Fig. 2b) so that it is bent by an amount that depends on the pressure difference between the

environment and the interior. The bending is transformed by a lever to a pointer whose position can becalibrated. The importance of this type of gauge is that it is very robust and that it covers a range ofpressure measurement from pressures higher than atmospheric pressure down to rough vacuum (about10 mbar). The accuracy and reproducibility are relatively poor, so that it is not suitable for precisionmeasurements, and its usefulness for vacuum measurements is limited.

Fig. 2 Bourdon gauge, a) principle, b) distribution of forces

Diaphragm gaugesIf a diaphragm or a bellows separates two regions with different pressures (p1, p2,) the difference p(p = p1 – p2) of these two pressures causes a force that deforms the diaphragm or bellows. There aremany possibilities for measuring this deformation, e.g. mechanically by a lever and a pointer, opticallyby a mirror and a light pointer, or electrically by changes of the capacity of a capacitor formed by thediaphragm and an additional electrode which is usually placed in a region of very low pressure (seeFigs. 3a ... 3c). For precision measurements one side of the diaphragm is evacuated to very low pressure.This is called a reference vacuum. The other side is exposed to the pressure to be measured. Thedeformation of the diaphragm depends on, but is not proportional to, the pressure difference . Thesedays linearization of the pressure vs. deformation reading is mostly performed by electronic circuits.Thus it is possible to make pressure measurements in a range between some hundred mbar and 10-4

mbar with such a precision that this type of gauge can be used as a secondary standard gauge. The lowerpressure limit is caused by the thermal dilatation that has the same order of magnitude as the deformationat very low pressures. Some special alloys like stainless steel or special ceramics such as Al2O3 withhigh density, are used as materials for the diaphragms. Generally the low pressure in the region of thereference vacuum is maintained by the use of getters. Frequently the electrodes and the circuits for thepressure reading are placed in the region of the reference vacuum. Figure 4 shows a diaphragm gaugewith electrical reading. The pressure reading is independent of the gas composition.

CONTROL SYSTEMS

DISTRIBUTED CONTROL SYSTEMS

Introduction

Generally, the concept of automatic control includes accomplishing two major operations; the transmission of signals (information flow) back and forth and the calculation of control actions (decision making). Carrying out these operations in real plant requires a set of hardware and instrumentation that serve as the platform for these tasks. Distributed control system (DCS) is the most modern control platform. It stands as the infrastructure not only for all advanced control strategies but also for the lowliest control system. The idea of control infrastructure is old. The next section discusses how the control platform progressed through time to follow the advancement in control algorithms and instrumentation technologies.

Figure 2: PC network

Programmable Logic Controllers

Programmable logic controller (PLC) is another type of digital technology used in process control. It is exclusively specialized for non-continuous systems such as batch processes or that contains equipment or control elements that operate discontinuously. It can also be used for many instants where interlocks are required; for example, a flow control loop cannot be actuated unless a pump has been turned on. Similarly, during startup or shutdown of continuous processes many elements

must be correctly sequenced; that is, upstream flows and levels must be established before downstream pumps can be turned on.

The PLC concept is based on designing a sequence of logical decisions to implement the control for the above mentioned cases. Such a system uses a special purpose computer called programmable logic controllers because the computer is programmed to execute the desired Boolean logic and to implement the desired sequencing. In this case, the inputs to the computer are a set of relay contacts representing the state of various process elements. Various operator inputs are also provided. The outputs from the computer are a set of relays energized (activated) by the computer that can turn a pump on or off, activate lights on a display panel, operate solenoid valve, and so on.

PLCs can handle thousands of digital I/O and hundreds of analog I/O and continuous PID control. PLC has many features besides the digital system capabilities. However, PLC lacks the flexibility for expansion and reconfiguration. The operator interface in PLC systems is also limited. Moreover, programming PLC by a higher-level languages and/or capability of implementing advanced control algorithms is also limited.

PLCs are not typical in a traditional process plant, but there some operations, such as sequencing, and interlock operations, that can use the powerful capabilities of a PLC. They are also quite frequently a cost-effective alternative to DCSs (discussed next) where sophisticated process control strategies are not needed. Nevertheless, PLCs and DCSs can be combined in a hybrid system where PLC connected through link to a controller, or connected directly to network.

Commercial Distributed Control Systems

In more complex pilot plants and full-scale plants, the control loops are of the order of hundreds. For such large processes, the commercial distributed control system is more appropriate. There are many vendors who provide these DCS systems to IOCL BGR such as Honeywell, Rosemont, Yokogawa, etc. In the following only an overview of the role of DCS is outlined.

Conceptually, the DCS is similar to the simple PC network. However, there are some differences. First, the hardware and software of the DCS is made more flexible, i.e. easy to modify and configure, and to be able to handle a large number of loops. Secondly, the modern DCS are equipped with optimization, high-performance model-building and control software as options. Therefore, an imaginative engineer who has theoretical background on modern control systems can quickly configure the DCS network to implement high performance controllers.

A schematic of the DCS network is shown in figure 3. Basically, various parts of the plant processes and several parts of the DCS network elements are connected to

each others via the data highway (fieldbus). Although figure 3 shows one data highway, in practice there could be several levels of data highways. A large number of local data acquisition, video display and computers can be found distributed around the plant. They all communicate to each others through the data highway. These distributed elements may vary in their responsibilities. For example, those closest to the process handle high raw data traffic to the local computers while those farther away from the process deal only with processed data but for a wider audience.

The data highway is thus the backbone for the DCS system. It provides information to the multi-displays on various operator control panels sends new data and retrieve historical data from archival storage, and serves as a data link between the main control computer and other parts of the network.

On the top of the hierarchy, a supervisory (host) computer is set. The host computer is responsible for performing many higher level functions. These could include optimization of the process operation over varying time horizons (days, weeks, or months), carrying out special control procedure such as plant start up or product grade transition, and providing feedback on economic performance.

Figure 3: The elements of a commercial distributed control system network

A DCS is then a powerful tool for any large commercial plant. The engineer or operator can immediately utilize such a system to:

•Access a large amount of current information from the data highway.

•See trends of past process conditions by calling archival data storage.

•Readily install new on-line measurements together with local computers for data acquisition and then use the new data immediately for controlling all loops of the process.

•Alternate quickly among standard control strategies and readjust controller parameters in software.

•A sight full engineer can use the flexibility of the framework to implement his latest controller design ideas on the host computer or on the main control computer.

In the common DCS architecture, the microcomputer attached to the process are known as front-end computers and are usually less sophisticated equipment employed for low level functions. Typically such equipment would acquire process data from the measuring devices and convert them to standard engineering units. The results at this level are passed upward to the larger computers that are responsible for more complex operations. These upper-level computers can be programmed to perform more advanced calculations.

5. Description of the DCS elements

The typical DCS system shown in Figure 3 can consists of one or more of the following elements:

•Local Control Unit (LCU). This is denoted as local computer in Figure 3. This unit can handle 8 to 16 individual PID loops, with 16 to 32 analog input lines, 8 to 16 analog output signals and some a limited number of digital inputs and outputs.

•Data Acquisition Unit. This unit may contain 2 to 16 times as many analog input/output channels as the LCU. Digital (discrete) and analog I/O can be handled. Typically, no control functions are available.

•Batch Sequencing Unit. Typically, this unit contains a number of external events, timing counters, arbitrary function generators, and internal logic.

•Local Display. This device usually provides analog display stations, analog trend recorder, and sometime video display for readout.

•Bulk Memory Unit. This unit is used to store and recall process data. Usually mass storage disks or magnetic tape are used.

•General Purpose Computer. This unit is programmed by a customer or third party to perform sophisticated functions such as optimization, advance control, expert system, etc.

•Central Operator Display. This unit typically will contain one or more consoles for operator communication with the system, and multiple video color graphics display units.

•Data Highway. A serial digital data transmission link connecting all other components in the system may consist of coaxial cable. Most commercial DCS allow for redundant data highway to reduce the risk of data loss.

•Local area Network (LAN). Many manufacturers supply a port device to allow connection to remote devices through a standard local area network.

6. The advantages of DCS systems

The major advantages of functional hardware distribution are flexibility in system design, ease of expansion, reliability, and ease of maintenance. A big advantage compared to a single-computer system is that the user can start out at a low level of investment. Another obvious advantage of this type of distributed architecture is that complete loss of the data highway will not cause complete loss of system capability. Often local units can continue operation with no significant loss of function over moderate or extended periods of time.

Moreover, the DCS network allows different modes of control implementation such as manual/auto/supervisory/computer operation for each local control loop. In the manual mode, the operator manipulates the final control element directly. In the auto mode, the final control element is manipulated automatically through a low-level controller usually a PID. The set point for this control loop is entered by the operator. In the supervisory mode, an advanced digital controller is placed on the top of the low-level controller (Figure 1). The advanced controller sets the set point for the low-level controller. The set point for the advanced controller can be set either by the operator or a steady state optimization. In the computer mode, the control system operates in the direct digital mode shown in Figure 1.

One of the main goals of using DCS system is allowing the implementation of digital control algorithms. The benefit of digital control application can include:

•Digital systems are more precise.

•Digital systems are more flexible. This means that control algorithms can be changed and control configuration can be modified without having rewiring the system.

•Digital system cost less to install and maintain.

•Digital data in electronic files are easier to deal with. Operating results can be printed out, displayed on color terminals, stored in highly compressed form.

7. Important consideration regarding DCS systems.

7.1 The control loop

The control loop remains the same as the conventional feedback control loop, but with the addition of some digital components. Figure 4 shows a typical single direct digital control-loop. Digital computer is used to take care of all control calculations. Since the computer is a digital (binary) machine and the information coming out of the process in an analog for, they had to be digitized before entering the computer. Similarly the commands issued by the computer are in binary, they should be converted to analog (continuous) signals before implemented on the final control element. This is the philosophy behind installing the A/D and D/A converter on the control loop. Signal conditioning is used to remove noise and smooth transmitted data. Amplifier can also be used to scale the transmitted data if the signals gain is small. Signal generators (transducer) are used to convert the process measurements into analog signals. The most common analog signals used are 0-5 Volts and 4-20mA. Some of the process variables are represented in millivolts such as those form thermocouples, strain gauges, pH meters, etc. Multiplexers are often used to switch selectively a

number of analog signals.

Figure 4: The component of a digital control loop

All instrumentation hardware (1-9) is designed, selected, installed and maintained by an instrumentation engineer. The computer is responsible for making decisions (control actions). It can host a simple control algorithm or a more advanced one. The latter can either purchased from a commercial vendor or developed in-house by a process/control engineer . The terminal is the main operator interface with the control system. The operator can use the terminal to monitor the control performance, adjust the set points and tune the controller parameters.

C300 Controller (Refinery I)

Honeywell’s C300 Controller provides powerful and robust process control for the Experion® platform. Based on the unique and space-saving Series C form factor, the C300 joins the C200, C200E, and the Application Control Environment (ACE) node in operating Honeywell’s field-proven and deterministic Control Execution Environment (CEE) software.

Ideal for implementation across all industries, the C300 controller offers best-in-class process control. It supports a wide variety of process control situations, including continuous and batch processes and integration with smart field devices. Continuous process control is achieved through an array of standard functions that are built into control strategies. The C300 controller supports the ISA S88.01 batch control standard and integrates sequences with field devices, including valves, pumps, sensors, and analyzers. These field devices track the state of the sequences to perform pre-configured actions. This tight integration leads to quicker transitions between sequences, increasing the throughput.

The controller also supports advanced process control with Honeywell’s patented Profit® Loop algorithm as well as custom algorithm blocks, which let users create custom code to run in the C300 controller.

Like C200/C200E and the ACE node, the C300 operates Honeywell’s deterministic Control Execution Environment (CEE) software which executes control strategies on a constant and predictable schedule. The CEE is loaded into the C300 memory providing the execution platform for the comprehensive set of automatic control, logic, data acquisition and calculation function blocks. Each function block contains a set of pre-defined features such as alarm settings and maintenance statistics. This embedded functionality guarantees consistent process control strategy execution.

The controller supports many input/ output (I/O) families, including Series C I/O and Process Manager I/O, and other protocols such as FOUNDATION Fieldbus, Profibus, DeviceNet, Modbus, and HART.

C300 allows engineers to address their most demanding process control requirements from integration with complicated batch systems to controlling devices on a variety of networks such as FOUNDATION Fieldbus, Profibus, or Modbus. It also supports advanced control with Profit Loop, which puts model-based predictive control directly in the controller to minimize valve wear and maintenance.

CENTUM CS 3000 R3 (Refinery II)

Yokogawa released CENTUM CS 3000 R3 in 1998 as the first Windows-based production control system under our brand. For over 10 years of continuous developments and enhancements, CENTUM CS 3000 R3 is equipped with functions to make it a matured system. With over 7600 systems sold worldwide, it is a field-proven system with 99.99999% of availability.

CENTUM CS 3000 R3 features an open architecture, flexibility, and compatibility with existing systems. It can be seamlessly connected with the previous CENTUM systems, as well as it is easily upgraded into CENTUM VP system which was released in 2008.

Yokogawa guarantees the long-term support and supply of CENTUM CS 3000 R3 systems so that our customers will enjoy the optimized use of the asset.

With Windows Remote Desktop capability, plant operation, monitoring, and engineering can be performed from a personal computer in your office or at a remote field location without any additional software.

The same HIS displays in the control room can be shown on the PC in your office. For production facilities in remote locations around the world, remote operation and monitoring can be simply structured.

The engineering work for modification can be performed remotely via a network, eliminating the need for dispatching engineers and reducing both maintenance and engineering cost.

HART PROTOCOLIn today’s competitive environment, all companies seek to reduce operationcosts, deliver products rapidly, and improve product quality. The HART®(highway addressable remote transducer) protocol directly contributes tothese business goals by providing cost savings in: Commissioning and installation Plant operations and improved quality MaintenanceThe HART Application Guide has been created by the HARTCommunication Foundation (HCF) to provide users of HART productswith the information necessary to obtain the full benefits of HART digitalinstrumentation. The HART communication protocol is an open standardowned by the more than 100 member companies in the HCF. Products thatuse the HART protocol to provide both analog 4–20 mA and digital signalsprovide flexibility not available with any other communication technology.The following four sections provide you with an understanding of how theHART technology works, insight on how to apply various features of thetechnology, and specific examples of applications implemented by HARTprotocol users around the world: Theory of Operation Benefits of HART Communication Getting the Most from HART Systems Industry Applications

Communication ModesMASTER-SLAVE

MODEHART is a master-slave communication protocol, which means that duringnormal operation, each slave (field device) communication is initiated by amaster communication device. Two masters can connect to each HARTloop. The primary master is generally a distributed control system (DCS),programmable logic controller (PLC), or a personal computer (PC). Thesecondary master can be a handheld terminal or another PC. Slave devicesinclude transmitters, actuators, and controllers that respond to commandsfrom the primary or secondary master.

BURST MODE Some HART devices support the optional burst communication mode.Burst mode enables faster communication (3–4 data updates per second). Inburst mode, the master instructs the slave device to continuously broadcasta standard HART reply message (e.g., the value of the process variable).The master receives the message at the higher rate until it instructs the slaveto stop bursting.

HART Networks

HART devices can operate in one of two network configurations—point-topointor multidrop.

POINT-TO-POINT In point-to-point mode, the traditional 4–20 mA signal is used tocommunicate one process variable, while additional process variables,configuration parameters, and other device data are transferred digitallyusing the HART protocol (Figure 2). The 4–20 mA analog signal is notaffected by the HART signal and can be used for control in the normal way.The HART communication digital signal gives access to secondaryvariables and other data that can be used for operations, commissioning,maintenance, and diagnostic purposes.

MULTIDROP The multidrop mode of operation requires only a single pair of wires and, ifapplicable, safety barriers and an auxiliary power supply for up to 15 fielddevices (Figure 3). All process values are transmitted digitally. Inmultidrop mode, all field device polling addresses are >0, and the currentthrough each device is fixed to a minimum value (typically 4 mA).

CONTROL VALVE

Introduction The control action in any control loop system, is executed by the final control element. The most common type of final control element used in chemical and other process control is the control valve. A control valve is normally driven by a diaphragm type pneumatic actuator that throttles the flow of the manipulating variable for obtaining the desired control action. A control valve essentially consists of a plug and a stem. The stem can be raised or lowered by air pressure and the plug changes the effective area of an orifice in the flow path. A typical control valve action can be explained using Fig. 1. When the air pressure increases, the downward force of the diaphragm moves the stem downward against the spring. Classifications Control valves are available in different types and shapes. They can be classified in different ways; based on: (a) action, (b) number of plugs, and (c) flow characteristics.

(a) Action: Control valves operated through pneumatic actuators can be either (i) air to open, or (ii) air to close. They are designed such that if the air supply fails, the control valve will be either fully open, or fully closed, depending upon the safety requirement of the process. For example, if the valve is used to control steam or fuel flow, the valve should be shut off completely in case of air failure. On the other hand, if the valve is handling cooling water to a reactor, the flow should be maximum in case of emergency. The schematic arrangements of

these two actions are shown in Fig. 2. Valve A are air to close type, indicating, if the air fails, the valve will be fully open. Opposite is the case for valve B.

(b) Number of plugs: Control valves can also be characterized in terms of the number of plugs present, as single-seated valve and double-seated valve. The difference in construction between a single seated and double-seated valve are illustrated in Fig. 3.

Referring Fig.1 (and also Fig. 3(a)), only one plug is present in the control valve, so it is single seated valve. The advantage of this type of valve is that, it can be fully closed and flow variation from 0 to 100% can be achieved. But looking at its construction, due to the pressure drop across the orifice a large upward force is present in the orifice area, and as a result, the force required to move the valve against this upward thrust is also large. Thus this type of valves is more suitable for small flow rates. On the other hand, there are two plugs in a double-seated valve; flow moves upward in one orifice

area, and downward in the other orifice. The resultant upward or downward thrust is almost zero. As a result, the force required to move a double-seated valve is comparatively much less.

But the double-seated valve suffers from one disadvantage. The flow cannot be shut off completely, because of the differential temperature expansion of the stem and the valve seat. If one plug is tightly closed, there is usually a small gap between the other plug and its seat. Thus, single-seated valves are recommended for when the valves are required to be shut off completely. But there are many processes, where the valve used is not expected to operate near shut off position. For this condition, double-seated valves are recommended.

(c) Flow Characteristics: It describes how the flow rate changes with the movement or lift of the stem. The shape of the plug primarily decides the flow characteristics. However, the design of the shape of a control valve and its shape requires further discussions. The flow characteristic of a valve is normally defined in terms of (a) inherent characteristics and (b) effective characteristics.

Ideal Characteristics

The control valve acts like an orifice and the position of the plug decides the area of opening of the orifice.

the control valves can be classified in terms of their m vs. x characteristics, and three types of control valves are normally in use. They are:

(a) Quick opening

(b) Linear

(c) Equal Percentage.

The characteristics of these control valves are shown in Fig. 4. It has to be kept in mind that all the characteristics are to be determined after maintaining constant pressure difference across the valve as shown in Fig.4.