Studied Process Instrumentation Diagrams

Studied Process Instrumentation Diagrams

  1. Studied Process Instrumentation Diagrams, I have included my undergraduate project. The work was given to me as a Final Year Project on which I worked with a group of three students to undertake some research as well as some practical work. The project was titled “Automation of 25TH Boiler using SIEMENS SOMATIC PIC S7-300” completed with the help of SIEMENS Pakistan Engineering Company Limited. The period of the project lasted from January 2008 to November 2008.
  1. BACKGROUND Studied Process Instrumentation Diagrams
  1. My project was basically to automate a boiler using SIEMENS SOMATIC S 7-300. I created HIM using Win Cc flexible 2005 to present the graphical representation of the actual process and to enable the operator to set the alarm limits and change the set point. The boiler I worked on was a vertical water tube boiler with a steam production capacity of 25 TH (tones per hour).
  1. The setup I was assigned was of a 25TH boiler being used in a textile mill along with an economizer and a decorator. I had to automate three parameters:
  1. Level
  2. Pressure
  • Temperature
  1. Through consultation with my project advisers I learn that maintaining liquid level in the boiler steam drum is the highest priority. It is critical that the liquid level remain low enough to guarantee that there is adequate disengaging volume above the liquid, and high enough to assure that there is water present in every steam generating tube in the boiler. These requirements typically result in a narrow range in which the liquid level must be maintained. Boiler feed water controls are responsible for regulating the level of water in the boiler steam drum, by ensuring that the supply of feed water to the boiler keeps in step with the changes in boiler steam production. The interface level is subjected to several disturbances in the water/steam drum; not the least of which, are drum pressure and feed water temperature. As steam pressure rises or falls due to load demand there is a transient change in drum level due to the expansion or contraction of the steam bubbles in the drum water. When the steam pressure is lowered the water level rises as the steam bubbles expand (swell).
  1. Through internet research I found out that controlling water level of the boiler is normally addressed in one of three principal strategies:
  2. Single-element

Single element control is the simplest strategy. In this system drum level is measured using a single measurement device and provides a control signal to the feed water regulator in direct relation to the current operating drum level

  1. Two-element

This system uses the two variables, drum level and steam flow to mass balance the feed water demand. Since steam flow is very dynamic, the result of this strategy is that it will sense the rise or fall in load demand before the drum level begins to change. The strategy then adds or subtracts control output to stabilize the reaction of the drum level controller on the feed water control valve. And since steam flow is normally the larger variable it can easily over ride the trim effect of the drum level measurement on moderate load changes, ensuring a correct response to the demand change. The two-element drum level strategy is suitable for processes with moderate load swings and speeds, and it can be used on any size of boiler. Drum level is measured and the error between the desired set point and the actual control point is sent to a math summer as one of two process variables. Steam flow is measured and added (summed) to the math summer as the second process variable. The result of the math summer is the control output to the feed water control valve

iii.Three element control

To address the issues of phasing still present in the two- element control strategy, a third element, feed water flow is added to the drum level control strategy. In this system the math summer output of the two-element controller is cascaded down to a second feed water flow controller to act as a remote set point. By measuring the steam flow, the magnitude of demand changes can be used as a feed forward signal to the level control system. The feed forward signal can be added into the output of the level controller to adjust the flow control loop set point, or can be added into the output of the flow control loop to directly manipulate the boiler feed water control valve. The majority of boiler level control systems add the feed forward signal into the level controller output to the secondary (feed water flow) controller set point. This approach eliminates the need for characterizing the feed forward signal to match the control valve characteristic.

Studied Process Instrumentation Diagrams

The application of any one of these strategies depends on the specific boiler size (economics) and load variations (dynamics)

  1. In my project, I used the level control loop of boiler to control the water level of steam drum using 3 element control philosophy. It continuously monitored the level and gave alarm in case the pre set high or low limits are reached and the level control loop of decelerator controlled the water level of decelerator.
  1. I used the pressure control loop to control the boiler drum pressure by adjusting the firing rate according to increase or decrease of the steam demand. Pressure control loop of decelerator monitored and controlled the pressure of decelerator and gave alarm in case the pre set high or low limits reached.
  1. I assigned the temperature control loop to monitor the temperature of flue gases after economizer and also the temperature of flue gases before diverted damper.
  1. I used PLCSIM (PLC simulator) for simulation and debugging of the program coding.
  1. I used process and Instrumentation diagrams (P&ID) throughout which played a significant role in the maintenance and modification of the process. It was critical to demonstrate the physical sequence of equipment and systems, as well as how these systems were connected
  1. During the design stage, I also used the P&ID diagram for the development of system control schemes, allowing for further safety and operational investigations, such as the popular Hazards and Operability (HAZOP) study.
  1. Another important step was the HMI to enable the person operating and monitoring the PLC to be able to control and observe it remotely. Thus for the creation of Human Machine Interface (HMI) I used WinCC software to display a graphical presentation of all the elements being controlled. WinCC accomplished the following tasks:
  2. The process was visualized on the HMI device. The screen on the HMI device was dynamically updated. This was based on process transitions.
  3. The operator could control the process by means of the HMI. For example, the operator could preset reference values for different processes.
  • Critical process states automatically triggered an alarm, for example, when the set point value was exceeded.
  1. The HMI system could log alarms and process values. All process sequences were logged and it was possible to retrieve previous production data. 


  1. After making a list of the tasks to be performed, I divided the project into two phases:
    Hardware Selection Phase (January-May)
    ii. Software and Programming Phase (June-November))
  1. First of all I studied the Process and Instrumentation diagrams (P&ID) of the boiler to study the pictorial representation of
  2. Key piping and instrument details
  3. Control and shutdown schemes
  • Basic start up and operational information
  1. Instrumentation and designations
  2. Mechanical equipment with names and numbers
  3. All valves and their identifications
  • Process piping, sizes and identification
  • Miscellanea – vents, drains, special fittings, sampling lines, reducers, increasers and swagers
  1. Permanent start-up and flush lines
  2. Flow directions
  3. Interconnections references
  • Control inputs and outputs, interlocks
  • Interfaces for class changes
  • Control system input
  1. Identification of components and subsystems delivered by others
  1. Next I decided whether to control the level using single element, two element or three element control. For this purpose, I used the internet to conduct an in depth research of all the level control strategies and discovered that the single element drum level strategy is only effective for smaller boilers with relatively steady demands and slow to very moderate load changes. This is because the shrink and swell effect causes an incorrect initial control reaction, which can lead to over/under filling of the drum. As steam demand increases, there is an initial lowering of the drum pressure resulting in an artificial rise in drum level as the steam bubbles expand and swell the drum water level. This phenomenon sends a false control signal to reduce feed water flow, when in fact the feed water flow should be increasing to maintain mass balance. Conversely, on a loss of steam demand, there is an initial rising of steam drum pressure which acts to lower the drum level by compressing the steam bubbles and shrinking the drum water level. This sends a false signal to increase feed water flow when in fact it should be decreasing to maintain mass balance. Processes experiencing sudden or large load changes, can result in ‘phasing’ of the shrink and swell effect causing the water level controller to lose control of the drum level and result in nuisance low water trips or high water priming and carry-over
  1. I discovered that the two element strategy had two drawbacks. First, like the single element strategy the two-element control cannot adjust for pressure or load disturbances in the feed water supply, as this is not a measured variable in this strategy. And second the two-element control cannot eliminate phasing interaction between feed water flow and drum level because only the relatively slow process of the drum level is controlled. This second issue can lead to sub-cooled drum water on a large increase in demand by allowing excessive feed water to enter the drum without consideration to the boilers thermal dynamic capabilities.
  1. Thus three element was chosen because in this type of control three process variables (PVs) are measured to effect control of the boiler feed water control valve. These measured PVs are:
  1. Liquid level in the boiler drum,
  2. Flow of feed water to the boiler drum
  • Flow of steam leaving the boiler drum.
  1. The next choice was whether to use open loop or closed loop. I finalized closed loop because a characteristic of the open-loop controller is that it does not use feedback to determine if its input has achieved the desired goal. This means that the system does not observe the output of the processes that it is controlling. Consequently, a true open-loop system cannot correct any errors that it could make. It also may not compensate for disturbances in the system.
  1. The third choice was selection of controllers. For this I read material on the internet and consulted previous Siemens automation projects to find standards and discussed the various options with Siemens Engineers. Finally, SIMATIC S7-300 was selected because it is an automation system that represents an optimal solution for applications in centralized and distributed configurations. SIMATIC S7-200 though inexpensive, did not have the desired processing power for a large automation project as it was designed to carry out less complex automation tasks. Moreover, SIMATIC S7-400 could control a large number of I/Os but it was expensive and our requirement was mainly of controlling three variables.
  1. I followed the same procedure as above was to finalize the HMI software. The advisor suggested to use protool but I preferred WinCC over others was because it is simple, flexible and easy to use. In industries, mostly WinCC is used because with its powerful process interfaces (to the SIMATIC range in particular) and secure data archiving, WinCC provides fault-tolerant solutions for instrumentation and control.SIMATIC WinCC fits into a wide range of applications since it is modular and can be expanded on a flexible basis.
  1. I made a list of the valves and sensors to be controlled:
  1. Pressure Sensor
  2. Flow Transmitter
  • Level Transmitter

I noted down their voltage and current ratings were noted to ensure their compatibility with the hardware connections of the controller.

  1. The next task was connection. I gave analog inputs on analog lines and digital inputs on digital lines. I showed the proper connections in a drawing made using AutoCAD.
  1. Then came the task of programming. I used SIMATIC STEP 7 software for programming the PLC. An analog value of voltage or current (depending upon the type of transducer) was sent through the PIW (Peripheral input word) to the scaling block (FC 105). The input coming through PIW depended upon the current value of the parameter being controlled. The scaling block converted the process variable which was in the form of integer values to real values as the PID block could only work with real values.
  1. The output of the scaling block became the input to the PID controller. I sent the process variable to the PV_IN input the set point at the SP_INT input and took the output at the LMN output.
  1. For boiler level control I used two PIDs as I added the level output of PID1 to the feed water flow and sent it to the PV_IN input of PID2 and made the saturated steam flow the SP_INT input of PID2.
  1. I then gave the outputs of the scaling block to the input to the PID controller, sent the process variable to the PV_IN input, the set point at the SP_INT input and took the output at the LMN output
  1. This output of the PID became the input of unscaling block and I sent the final unscaled output through PQW (Peripheral output word) to the valve, the position of which was then controlled in accordance with the value of voltage coming at the output.
  1. The digital portion of the program dealt with alarms. I implemented them using comparators. For the high limit a greater than or equal to comparator was used. I gave the PV output from the PID as one input to the comparator whereas I made the high limit of the parameter being controlled. the second input to the comparator.
  1. For the low limit I used a less than or equal to comparator. I gave the PV output of the PID as one input to this comparator whereas I made the low limit of the parameter being controlled the second input to the comparator
  1. For pressure and limit switches, I programmed digital inputs. As soon as the switches were closed by any parameter exceeding or decreasing beyond its preset limits, the digital inputs will go high and turn on the alarms.
  1. Next task was WinCC software I connected each parameter with the corresponding tag on the graphical user interface of the WinCC so that the parameter would be displayed graphically and the user could see each parameter.
  1. For the achievement of the project, I worked actively with the team of three students. I, myself, took the leadership of the team and assigned tasks to every individual to perform at individual level. I, also, arranged some brain storming sessions through out the project to carry out the discussion in order to get everybody’s feedback to its full extent.
  1. I made great use of MS-Project software to develop the project schedule and timelines. It helped me to carry out the daily tasks and to evaluate the performance in the project which, in turns, led me to the success of the project. I have used my wide project management knowledge and techniques to understand and balance the project resources to address various limitations / constraints with in the project scope.
  1. I undertook study work well with in the university’s library as well as made extensive use of my excellence in internet surfing. In this regard, I worked with great accord with my colleagues, project members and others including Laboratory personnel, teaching faculty, project supervisors (Mr. Jawed, Assistant Lecturer NED and Mr. Alman Anwer Khan, Design Engineer-SIEMENS Pakistan). Whenever required, I held meetings with these members to apprise them of the progress and take in their valuable feedback.
  1. I have turned up with excellent data collection abilities and by using them, I gathered an in-depth data about techniques from IEEE Journals and university Library resources. This data then helped me in developing a full understanding of automation practices by keeping in mind the end user requirements. This enhanced my design work which was awarded a sign of appreciation at the time of its judgment.
  1. As a group leader, I always discussed and presented the technical issues to my team members, supervisor and to the laboratory personnel to find out the optimal solution. I carried out an in depth study of already functioning automation scenarios and also consulted the PLC programming department of SIEMENS Pakistan to get an overview of how the design can have an influence on the manufacturing and also, about the merits and demerits of the design work and to get an idea about existing industry standards. 
  1. I made use of my excellence in AutoCAD for developing the drafts for my designs to visualize the best optimum design. The CAD drawings added great knowledge to me as well as to other team members.
  1. With every step of the project, I established a documented report comprised of each and every step of the work being done and finally, after the completion of the project, I added all my efforts to bring the project bounded dissertation from our group to our supervisor which was ranked as a First class piece of work by the authorities. I also made a presentation using MS POWER POINT to summarize the working of my project and show the supervisors the final outcome of my project.
  1. After the accomplishment of all my targets, with in the project group, I took part in comprising the project bounded dissertation in which I used my excellence of MS-OFFICE suite. I prepared tables, mathematical equations representations and presented them in a suitable format which was rated as a first class piece of work. I included all my project group results achieved throughout the project.
  1. I have completed a challenging project without compromising on quality work and any rules/regulations, meeting the high expectations keeping in view the social norms and environmental responsibilities. I consider myself very fortunate who got the chance to utilize engineering knowledge which mainly includes, use of software like PLCSIM, STEP 7, WinCC & MS Project, use of engineering knowledge pertaining to Industrial Automation in true letter and spirit for the successful completion of the project. The project successfully accomplished its target in specified time and the dissertation was submitted to the board of supervisors. I also submitted this project to DICE (Digital Innovation Competition and Exhibition) 2008, an annual competition held every year in Pakistan in which students from engineering universities all over Pakistan participate with their projects and are judged by leading Pakistani engineers and researchers. I performed the necessary calculations to prove that this automation project if implemented properly can result in immense reduction in labor and equipments costs for any industry. The judges were very impressed by this and my project received first prize in DICE 2008 and I got a shield and twenty thousand rupees for presenting such an innovative project.

David Marks

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