ADVANCEMENT OF DISTRIBUTED CONTROL SYSTEM

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ADVANCEMENT OF DISTRIBUTED CONTROL SYSTEM ARTICLE · DECEMBER 2015

1 AUTHOR: Prem Baboo National Fertilizers Ltd.,India 17 PUBLICATIONS 0 CITATIONS SEE PROFILE

Available from: Prem Baboo Retrieved on: 17 December 2015

ADVANCEMENT OF DISTRIBUTED CONTROL SYSTEM

Author

Prem Baboo Sr. Manager (Prod) National fertilizers Ltd, India Sr. advisor, www.ureaknowhow.com Fellow of Institution of Engineers (India)

INTRODUCTION The field of instrumentation has witnessed a sea change during last ten years. While new plants are incorporating the DCS and small instrumentation. Old plants are also modernizing the instrumentation and control system. Digital systems in general are very flexible and very versatile for integrating into the total management system of other function as well. Today’s DCS systems (also known as Process Control Systems) are developed to allow you to quickly implement the entire system by integrating all of t hese databases into one which is designed, configured and operated from the same application. The table below explains how savings can be realized by using today’s DCS System over a PLC/HMI system. This information has been collected from decades of implementation expertise of ABB engineers, end user controls engineers, consultants, and multiple Systems Integrators who actively implement both types of control solutions based on application requirement and user preferences. Digital System Advantages; 1. They have less or fewer moving parts and hence chances of failure are very less. 2. The conventional 4~20mA system need individual pair for each transmitter, valves etc. On the contrary, digital systems technology use a truly two wire transmission. i.e., a single pair of wires handles almost all data and operates several valves, thus saving enormous field wiring cost and reduction in associated construction and maintenance costs. 3. The very powerful processing ability of the system has made computation such as mass flow computations, ratio control, biasing etc, easy and practical. 4. The system makes use of the standard pressures and temperature transmitters and performs the various calculations outlined above. One might recall the need for the difficult calibrations needed in terms of absolute pressure / temperature, individual analog computing relays and the extensive hard wiring and tubing necessary even for simple computation functions. 5. System changes are easy. Just do some reprogramming using hand held calibrators, or from the key board of a pc connected to the system. Time consuming re-wiring/re-tubing is not necessary. 6. Redundancy at all levels, field device management. 7. Alarm management each controller and it’s associated.

Distributed Control System Majority of control loops, especially of the past uses dedicated controller for each loop. Modern trend favors use of micro-processor based distributed control system (DCS) as in the figure1. Operators’ interface (Work station)

Process computer

Multi loop controller (Micro-processor) Field bus Digital I/O Interface (Field Bus Modules)

Analog transmitters

I/P transducers

Valves

Figure 1 Signal from Field Transmitters. Modern transmitters are known as two wires transmitter, as just two wires handle the 24 volts DC to power these, and the 4~20 m A output signals. Wires from each transmitter of a cluster of transmitter terminate in a nearby Field Junction Box (not shown in figure 1). Multi-core cables take off field JB and terminate in a set of terminals in a cubicle. The cubicle is located in a room adjoining the workstation room from plants with a few hundred loops. Plants having several hundred loops have these cubicles in Process Interface Buildings (PIB) in the field.

Field Bus Modules and Controller Cubicle This cubicle contains the field bus modules (signal input) microprocessors (one redundant). Cluster of wires (two wires for each signal) from marshalling (arrange in logical order) terminals convey the signals to field bus modules (FBM) located in another cubicle nearby digital devices known as field bus modules convert the analog inputs into digital inputs and convey to the microprocessor (controller) by a single pair of wire known as field bus. The field bus can also accept and convey signals from programmable logic controllers (PLC) and others through appropriate signals conditioning cards. Intelligent Field Transmitte rs The advances in digital computer systems have led to the advent of intelligent field transmitters having the following advantages over conventional transmitters; 1. Outputs are digital signals, for directly feeding to downstream digital systems. Analog to digital cards are not necessary. 2. System has self-diagnostic features to indicate troubles of the transmitter. Thus maintenance is easy. 3. Re-calibration is easy as the following manual intensive and difficult steps are absent  Taking out of the process.  Cumbersome disconnection of transmitters located at difficult to reach points.  Depressurizing, draining.  Connection of signal feed- instruments and output instrument.  Locating the transmitter back and putting it on stream. 4. One just feeds the data from hand held calibrators or from PC connected to the data bus by punching proper buttons. The system calibrates itself. This is called programming or the instrument system. Microprocessor The microprocessor receives process data from several transmitters through FBM cards. These cards convert the analog data from field transmitters into digital signals acceptable to the microprocessor. The microprocessor compares the process values with that of the set point and sends the correction output to field mounted final control elements through output cards. These convert the digital output into analog output suitable for the field control valves. The very short processing time of the order of several nanoseconds (One billionth of a second; one thousandth of a microsecond) per data enables the microprocessor to handle several inputs, one at a time and control a process of over 1000 loops easily. The system periodically scans and updates the process variables and values

as in table-1 below. Control loops effect corrections based on the updated values of the process variables.



Time-(update time in seconds)

Signal



Open loop (O.P)

Close loop (C.P)

Flow, pressure, vibration, speed & others

10

1

Level, temperature

60

1

analyze r

60

1

Table 1- Update- time in seconds for different variables Dead band 2% of engineering values in all classes, i.e., updating takes place even before its scan time if there is change of 2% the existing values.

Redundant microprocessor Control systems usually have a redundant microprocessor, to take over automatically in case of failure, to ensure un- interrupted process control. Re dunda nc y ( A s ys te m de s ign tha t d up lic a te s co mpo ne nts to p ro vid e a lter na tive s in cas e o ne co mp o ne nt fa ils ) System access Each tag no. has a unique address. The microprocessor identifies each field instrument by this unique address to receive/ send out, signals. A control algorithm (PID) for each loop and various arithmetical functions like mass flow computation for pressure and temperature compensation of gas flow, ratio,3-element control etc, are programmable. Re-configuration of the system for changed process needs is also easy, by just reprogramming. The analog systems of the past require extensive substitution of instrument, and rewiring/re-tubing.

Workstation The operator interfaces with DCS by an operator console more technically called workstation. It consists of a set of pushbuttons, trackball and CRT (cathode-ray tube). Some systems have a touch screen in lieu of the trackball. The operator can call the required information by touching the relevant images on the screen. The work station communicates with control cubicle and other workstations by means of a fiber optic bus running across the complex. Workstation access Table 2 shows details of access to workstations. Access is usually by password.

Environment

Access level

Authorization

Field operator environment.

1

Can vie w displays, cannot change SP, output, auto/manual etc.

Control room operator environment.

2

As above, can change SP, output, auto/manual etc.

Supervisory environment.

3

As above, can change alarm sets, no tuning of controlle rs, no access to configuration/maintenance.

Maintenance engineer environme nt.

4

As above, tune controlle rs, access configuration/maintenance.

Table 2-Work station environme nt and access Graphics The graphics show process equipment, piping instrumentation, and control etc, as per P&I of the plant. Graphics has hierarchical levels as follows; Level 1 Plant wide; The following information is available.  Total complex over view,  Showing main plant area,  Status of various units and primary plant data.  No control or process information at this level.  This level is the first navigation step to other display area.

Level 2

Complex level graphics; This graphics displays an overview of each of the major complex of the plant. Here again no process control is possible. However, this graphics displays that the unit has some faulty conditions. This initiates further navigation to level 3 and level 4 to locate the problems. Level 3 Graphics with details of process equipments piping etc; Level 3 displays unit level graphics with details of;  Process equipments  Piping  Control loops  Emergency shutdown systems (ESD).  Alarms etc. One can see all followings;  Controllers.  Important process variables.  General monitoring parameters  ESD trips. Operators can do followings;  Acknowledge alarms.  Manipulate control loops.  Operate valves.  Pumps. Unit level graphics should be in several pages to give details of the unit. These are according to the P&I of the units. Obviously this most often used graphics. Level 4 Detailed level graphics; This is detailed level graphics. One can access the following;  All loops.  Monitoring functions.  Equipment status.  Trips and ESD.  Alarm acknowledgements.  Face plates.  Groups and trends. This level is useful for detailed analysis and trouble shootings. Touch Screen; In computer science, a computer screen designed or modified to recognize the location of a touch on its surface. By touching the screen, the user can make a selection or move a cursor. 1. The simplest type of touch screen is made up of a grid of sensing lines, which determine the location of a touch by matching vertical and horizontal contacts. 2. Another, more accurate, type uses an electrically charged surface and sensors around the outer edges of the screen to detect the amount of electrical disruption and pinpoint exactly where contact has been made.

3. A third type embeds infrared light-emitting diodes (LED) and sensors around the outer edges of the screen. These LED and sensors create an invisible infrared grid, which the user's finger interrupts, in front of the screen. Infrared touch screens are often used in “dirty” environments where contaminants could interfere with the operation of other types of touch screens. The touch screen's popularity with personal-computer users has been limited because users must hold their hands in midair to point at the screen, which is prohibitively tiring over extended periods. Also, touch screens do not offer high resolution—the user is not able to touch only a specific point on the screen. Touch screens are, however, immensely popular in applications such as information kiosks and automatic teller machines because they offer pointing control without requiring any movable hardware and because touching the screen is intuitive. FIELD BUS AFVANTAGES 1. Technological Advantages i. Reduces Cost ii. Errection & Installation iii. Engineering iv. Commissioning 2. Easy Adaptability i. Implementation ii. Operational iii. Maintenance 3. Leveraging Technology i. Predictive Analysis ii. Control- in Field for faster loop execution iii. Advance diagnosis. DIGITALSIGNAL INTEGRITY BETTER ACCURACY 1. HARDWIRED Accuracy lost in signal conversations digital to analog & analog to digital 2. Accuracy lost in current calibration differences. 3. FIELD BUS Measurements are not distorted particularly important (i) tank gauging(ii) batching PROJECT STAGE BENEFITS 1. Reduced wiring & associated hardware 2. Reduced control room space.

3. Reduced Engineering Cable wiring has reduced the size MULTIPLE MONITOR SUPPORT The HIS supports the use of multiple monitors, meaning that more windows can be displayed at the same time and enabling process variations and even alarms to be handled more promptly. This ability to display increased amounts of data at the operator console improves operation efficiency. CENTUM CS 3000 R3 supports up to four monitors which can be stacked vertically or positioned side by side and a mouse can move over those four screens. A large-size monitor can also be use as shown in the figure-2

Fig.-2 OPERATION WINDOWS Inheriting the same operation window designs from the legacy CENTUM systems, operators may feel familiar to the CENTUM CS 3000 operation windows from the first day they start using the system, as shown in the figure.-3

Fig.-3

HUMAN INTERFACE STATION (HIS)

Operators access production control system via CENTUM CS 3000's human interface station (HIS) to make fast and intelligent decisions that maximize performance and minimize risks. Based on the ergonomic design concept, the HIS can be selectable from desk top, open display style console, and enclosed display style console types. Multiple monitors can be set, and each one of the monitor displays multiple operation windows.

CONCLUSION If you are using PLC’s and HMI to control your process or batch applications, your application is a great candidate to reduce costs and gain better control. Your savings would be significant and will continue to lower your costs over the life of your system. We look forward to helping you identify these savings and realize them in your next implementation. In the past, DCS Systems were large, expensive and very complex. This drove many control engineers to use Pro grammable Logic Controllers (PLCs) and Human Machine Interface (HMI) in order to lower cost. Today, these implementations are consistently more expensive than DCS systems for the same process or batch application. The Yokogawa is the efficient control of q uality, costs and schedule is vital for a successful project. It is recommended that an activity on the user requirements for the DCS range be carried out on regional and global level. This activity should include also requirements on the infrastructure and financing. *******************************************************************