Baku Higher Oil School Course: Supervisory Control and DataAcquisition systemsCourse Project Project name: Controllinglevel of condensate in reflux drum of distillation column StudentName: Elshan Mikayilov Report submitted on: 18.01.2018 Supervisor: Manafaddin Namazov Contents Introduction. 3 Background. 3 Objectives. 4 Issues.
5 Piping and Instrumentation Diagram.. 7 SCADA system.. 8 SCADA components. 9 PLC system.
. 9 Human Machine Interface. 13 Conclusion.
17 Appendix. 18 IntroductionInthis report I will explain how I designed SCADA system for the level control inthe reflux drum of the crude oil distillation unit. The main purpose is tocontrol the level of the light oil components in the reflux drum and keepingthe level at the desired value in order to reach the optimal separation andreflux. Through this report I will firstly give problem statement and includesome details about the distillation process. Consequently, I will show somepossible solutions for the problem in question. Therefore I will show mysolution and how I build the PLC system and made HMI for this system in orderto effectively monitor, gather data about the process and control theparameters. The whole system is built in the TIA Portal by choosing propercontroller and PC system for the process. BackgroundInthis section I will give some brief information about the process itself sinceit is crucial to understand the process before designing any process controlsystem.
So, I need firstly determine the inputs and outputs of the system whileconsidering the conditions and properties that are required to reach thedesired output. Generally,distillation is a simple process to separate two or more substances due to thedifference in their boiling points. The same process happens in the crude oildistillation unit. The crude oil distillation unit (CDU) is the firstprocessing unit in virtually all petroleum refineries.
The CDU distills theincoming crude oil into various fractions of different boiling ranges, each ofwhich are then processed further in the other refinery processing units. TheCDU is often referred to as the atmospheric distillation unit becauseit operates at slightly above atmospheric pressure.Firstof all, crude oil stream (feed) is heated and pumped to the distillation columnwith high pressure.
When the crude oil enters the large volume of thedistillation column, the pressure suddenly falls and it makes the components ofthe crude oil to separate. This separation happens because of the difference inthe boiling points of the components of crude. The heavy components such asheavy fuel oils, wax, lubricating oils, asphalt are going to the bottom of thedistillation column while light distillates such as Liquid petroleum gas (LPG),Gasoline (petrol), Heavy Naphtha are going to the top of the column where theyare condensed.
There are also some middle distillates which can be kerosene, automotiveand rail-road diesel fuels, residential heating fuel and other light fuel oilswhich leaves the column in various stages. There are sets of trays and packersmounted inside the column in order to increase the contact between liquid andvapor to make the separation better. Crude oil distillation column is acontinuous process and the bottom products and top products are not achieved inone cycle. It means that bottom products are reboiled in the reboiler and sentback to column while top products are condensed in the condenser and sent backto the column through reflux drum. Some part of heavy components are taken outof bottom which are bottom products and remaining are reboiled and sent back tothe column. In the same manner some amount of light components are taken out ofthe reflux drum which are called top products and the remaining amount arerefluxed to the column.
Mytask is to design control system for reflux drum in order to keep the level ofthis tank desired to make the separation better. In this case, I will used PIDcontrol which will efficiently keep the PV closest to the set point.A proportional–integral–derivative controller (PID controller or threeterm controller) is a controlloop feedback mechanism widely used in industrial control systems and a variety of otherapplications requiring continuously modulated control. A PID controllercontinuously calculates an error value e(t) as the difference betweena desired set point (SP) and ameasured processvariable (PV) and applies a correction based on proportional, integral, and derivative terms which give thecontroller its name. The overall control function of PID can be expressedmathematically as: ObjectivesBeforestarting the design process we should consider the objectives that should bemet. These objectives may not lead to the optimal practical integratedsolution, it is just one part of design process and it is not possible todesign the whole control system for distillation process in one month or evenless. So, the currentobjectives are: · Start and stopthe process by using buttons in the operator room· Pump the feedthrough the heater· Heat the feed· Send the feed tothe distillation column· Keep the levelin the reflux drum at the desired value· Use PID forsmooth control· Send someexcessive amount of reflux to the column by controlling valveToconclude up, my objective is to design the control system to automatically keepthe level of the light components in the reflux drum at the desired value whilepumping more feed into the column.
IssuesWhiledesigning a control system there are some vital problems that should beconsidered before design process. The first problem is related to theinstrumentation of the system. In my task distillation process happens underhigh pressure and temperature conditions which means that proper transmitters,final control elements and other equipment should be chosen with care tocorrespond to explosion zone requirements (Ex0, Ex 1, Ex2) and SIL levels. Safety integritylevel (SIL) is defined as a relative level of risk-reduction provided bya safety function, or to specify atarget level of risk reduction.In simple terms, SIL is a measurement of performance required for a safety instrumented function (SIF). Therequirements for a given SIL are not consistent among all of the functionalsafety standards. In the functional safety standards based on the IEC 61508 standard, four SILs are defined, withSIL 4 the most dependable and SIL 1 the least.
A SIL is determined based on anumber of quantitative factors in combination with qualitative factors such asdevelopment process and safety life cycle management. Hazardous areasare classified into zones based on an assessment of the frequency of theoccurrence and duration of an explosive gas atmosphere, as follows:· Zone 0: An area in which an explosive gas atmosphereis present continuously or for long periods; · Zone 1: An area in which an explosive gas atmosphereis likely to occur in normal operation;· Zone 2: An area in which an explosive gas atmosphereis not likely to occur in normal operation and, if it occurs, will only existfor a short time. Another main problem thatshould be considered is selection of control strategy that will safely,efficiently and fully implement the desires of the customer. There are manytypes of control systems that have application in various fields. If your processis simple, not hazardous and high quality is not required it means that you canchoose basic control algorithm which will be easy to monitor and configure.
Generally, there are two types of control: open loop control and closed loopcontrol. In open loop control, the controller take actions without having asense of output. In closed loop control there is feedback coming from theoutput and the controller take proper actions according the error betweenprocess variable and the set point. Of course, closed loop control is moresophisticated and reliable.
So, I will implement close loop control in mycontrol system. Selection of proper controlalgorithm is also essential which can include on-off control, batch control,linear control, relay control, proportional control, PI control, PD control,PID control and so on. For example, on-off control is easy to design andimplement, but there will significant oscillation and overshoot from the setpoint. In my example, I will usedPID control which will efficiently keep the PV closest to the set point.
A proportional–integral–derivative controller (PIDcontroller or three term controller) is a controlloop feedback mechanism widely used in industrial control systems and a variety of otherapplications requiring continuously modulated control. A PID controllercontinuously calculates an error value e(t) as the difference betweena desired set point (SP) and ameasured processvariable (PV) and applies a correction based on proportional, integral, and derivative terms which give thecontroller its name. Piping andInstrumentation DiagramA pipingand instrumentation diagram (P&ID) is a detailed diagram in the process industry which shows the piping and vessels in the process flow, together with the instrumentation and control devices.
A piping andinstrumentation diagram (P&ID) is defined by the Institute ofInstrumentation and Control as follows:· A diagram which shows the interconnection ofprocess equipment and the instrumentation used to control the process. In theprocess industry, a standard set of symbols is used to prepare drawings ofprocesses. The instrument symbols used in these drawings are generally basedon International Society ofAutomation (ISA) Standard S5.1· The primary schematic drawing used for laying outa processcontrol installation.Below, the P&ID of my task isshown: Here in this picture you can seefeed in which is pumped by a pump to the distillation tower. After the feedenters the distillation column light and heavy components will be separated dueto their different boiling points. Heavy components will go bottom and reboilerwill re-boil them in order to feedback some part of this liquid while someamount will be taken out as bottom product. We can see that flow rate of bottomproduct is controlled with LC which will take the measurement from LT leveltransmitter.
In the re-boiler the heating is supplied by steam. On the top of the distillationcolumn light components are taken out. First of all, heated vapor goes throughcondenser to condense down and convert to liquid phase. Therefore, this liquidis directed to a horizontally located tank called reflux drum. In reflux drumsome amount of light components are taken out as top distillate products andsome amount again is sent back to distillation column to separate.
Level of thereflux drum is controlled by LC level controller in order to make theseparation quality better. Another pump will pump the liquid and LV level valvewill control the flow rate according to the set point of the level in the tank.Overhead gases are also taken from reflux drum when the pressure inside thetank is so high.
Relief valve can be used for this purpose.SCADA systemSupervisory control anddata acquisition (SCADA) is a control system architecture that uses computers, networked datacommunications and graphical user interfaces forhigh-level process supervisory management, but uses other peripheral devicessuch as programmable logic controllers and discrete PID controllers to interface to the process plant or machinery. Theoperator interfaces which enable monitoring and the issuing of processcommands, such as controller set point changes, are handled through the SCADAsupervisory computer system. However, the real-time control logic or controllercalculations are performed by networked modules which connect to the fieldsensors and actuators.In my case, I need to build a simpleSCADA system to control distillation process and monitor the level of thereflux drum.
SCADA systems have some crucial components and they are discussedbelow. SCADA componentsMy SCADA system is hierarchal andthere are process control level and supervisory level. In my project, theprocess control level consists of one PLC, two level sensors, two valves withactuators, pumps, heaters, condensers and so on. These final control elementsand transmitters are connected to PLC and then they get commands from PLCaccording to the written program or commands of the operator from the operatorroom. The supervisory level consists of PC system which has centralizedcomputer getting data from PLCs and monitors to show human machine interface,input devices (such as mouse, keyboards) to enter some command to the SCADAsystem. The general device configuration ofthe system is shown below: Here we can see that PLC is selectedas CPU 315-2-PN/DP which has I/O module inside. This PLC is connected to our PCsystem which is SIMATIC PC Station through PN/IE line. The HMI is programmedwith WinCC RT Advanced and IE general is used as communication module.
PLC systemAfter device selection andconfiguration is finished I need to program the PLC in order to make it serve forour purpose. LAD language will be used to program this CPU. This PLC will havesome input from push buttons (Start and Stop), measurements from sensor comingto Input modules and some output commands to final control elements. All thesedata variables will be handled through PLC tags and proper naming is importantin order to make easy to use and modify. The main OB block of my PLC is shownbelow, it contains 7 networks: Network 1 is built to implementbasic start and stop function.
Start (M0.0) contact is normally open contactwhich will start energizing the Coil (Q0.0) when pressed. Stop (M0.1) isnormally closed contact which will be open when it is pressed and the wholesystem will stop running.
Auxiliary contact Q0.0 is used here also to keep thesystem running even if the Start is de-energized. In network 2, after start is pressedand coil is energized Pump_feed (Q0.1) will start running and the heater (Q0.2)will also start heating the feed. At the same time re-boiler and pump at thebottom of the column will start running. In network 3, after the start, thescaling will be done.
The feedback coming from the level sensor will besimulated by a slider in the HMI screen. I considered this input as a voltagelevel between 0 V and 10 V. Of course, this voltage level doesn’t show the reallevel of the tank. It means we need to scale this input in order to make itsuitable for processing. I have done scaling by using math operators. My sensorsends signal from 0-10 V and level range of the tank is between 0-150 cm.
Fromthis relation real value of level can be calculated with this equation: reallevel=(sensor_reading*150)/10. This equation will make the MD10(level) variablebetween 0 and 150 cm. In network 3, you can see that firstly, MUL operator isused to multiply sensor reading to 150 and DIV operator is used to divide thisvalue to 10 to get the actual level. However, math operation can be done foronly double integer variables.
So, my MD10 is Dint variable. However, PID workswith only real variables. So, I have used CONV block in network 4 to convertfrom integer to real. So, MD20 is real actual level in the tank.In network 5, you can see implementationof PID algorithm. This is continuous controller and there are many pins. Firstof all, manual on pin should be set to false in order to run automatically. So,I connect M0.
5 boolean variable to MAN_ON to make it false. MD20 is the PV (processvariable) which is connected to PV_IN pin and set point is set as 70 cm. Outputof this PID is taken from LMN tag which is connected to MD100 variable. In network 6, SUB block is used tomake the output of PID suitable for our application. As I explained earlierthere is inverse relationship between level of the tank and the flow rate ofdrainage valve. This PID gives manipulated variable for direct control.
So, Ineed to subtract this MV from 100 in order to make it suitable for ourapplication. Output of PID (MD100) is subtracted from 100 and MD110 is realvalve opening percentage which will be sent to actuator of valve. In network 7, comparator is used tosimulate the drainage valve. If the valve opening percentage is greater than 0it means that some proportion of the valve is open and we will turn the valve togreen in the HMI. If the percentage is 0 the valve will stay red showing that itis not running. Human MachineInterfaceHMI is a part of SCADA systems andit is physically screens of PC systems which are available to operators.
Operatorscan monitor the process via HMI and also they can give control commands toPLCs. I designed graphical interface in a way that it is easy to understand andmonitor the main points. The overall designof the screen before running is like this: HMI screen after running: HMI screen after pressing Startbutton: Increasing the level with slider: PVis below the SP (70 cm) Tank level above set point: PID ischanging the valve position (59 %) Higher tank level: In this HMI design the slider on theright side is used to simulate the signal coming from the level sensor. Startand Stop buttons are used to start and stop the process respectively. As we cansee from the screenshots after the start button is pressed the feed pump,heater of feed, re-boiler and bottom pump starts running and they turn to greenfrom red. When the level of the tank is below the set-point (chosen as 70 cm)the pump of reflux drum and drainage valve is red showing that they are notenergized.
When the level is going above the set-point the PID block willautomatically calculate the proper opening percentage for the valve to decreasethe level. When there is some amount of valve opening it will be shown in greenand the pump will start pump the liquid out of the reflux drum. When we run thesimulation we can see that how the valve position changes when the PV goesabove SP and in comparison with on-off control the PV will settle down to SPsmoother and faster in PID control. Scaled level of the liquid is shown on thetank and the percentage of valve opening is shown below the drainage valve. There is events created underbuttons and animations behind some elements that are connected to proper PLCtags. Here I will show how to setup an animation for a final control element tochange its appearance according to the change of bit: In this picture you can see that aPLC tag named Valve having address of Q0.4 will change its color from red togreen when its bit changes from 0 to 1. In design if buttons we will createevents with PLC tags.
ConclusionIn this project, I simulated thesensor input by using a slider and scaled the values of slider to a real levelindication and used this parameter in PID setup to keep the level of the refluxtank in the desired value (70 cm) by implementing PID algorithm. By working onthis project I learned how to scale sensors in S7 1200 and 300 while I also usedanother solution which involves math operators. I also learned how to setup PIDin S7 1200 and 300.
PID control worked properly for my solution and it is veryfast and reliable. If want to change the settling time of the PID I play withTi (integral action) to make the process smoother. I also learned how to workwith different types of variables like double integer, integer, word, real andso on.
Therefore, I learned how to convert from one variable type to another. Inmy solution I used CONV block to convert from double integer to real and Ifound out that conversion from integer to real is not possible. In HMI design Imade some events under buttons and created animations to show some change ofparameters in the screen. Appendix PLC tags: Simulator: