Baku Supervisor: Manafaddin Namazov Contents Introduction. 2 Background. 3

Baku Higher Oil School

 

Course: Supervisory Control and Data
Acquisition systems

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Course Project

 

 

 

 

Project name:    Controlling
level of condensate in reflux drum of distillation column

 

 

Student
Name:   Elshan Mikayilov

 

Report submitted on:    18.01.2018

 

Supervisor: Manafaddin Namazov

 

 

 

 

 

 

 

 

 

 

Contents
Introduction. 2
Background. 3
Objectives. 4
Issues. 5
Piping and Instrumentation Diagram.. 6
SCADA system.. 8
SCADA components. 8
PLC system.. 9
Human Machine Interface. 13
Conclusion. 17
Appendix. 18
References. 19
                                                               

 

 

 

 

Introduction

In
this report I will explain how I designed SCADA system for the level control in
the reflux drum of the crude oil distillation unit. The main purpose is to
control the level of the light oil components in the reflux drum and keeping
the level at the desired value in order to reach the optimal separation and
reflux. Through this report I will firstly give problem statement and include
some details about the distillation process. Consequently, I will show some
possible solutions for the problem in question. Therefore I will show my
solution and how I build the PLC system and made HMI for this system in order
to effectively monitor, gather data about the process and control the
parameters. The whole system is built in the TIA Portal by choosing proper
controller and PC system for the process.

Background

In
this section I will give some brief information about the process itself since
it is crucial to understand the process before designing any process control
system. So, I need firstly determine the inputs and outputs of the system while
considering the conditions and properties that are required to reach the
desired output.

Generally,
distillation is a simple process to separate two or more substances due to the
difference in their boiling points. The same process happens in the crude oil
distillation unit. The crude oil distillation unit (CDU) is the first
processing unit in virtually all petroleum refineries. The CDU distills the
incoming crude oil into various fractions of different boiling ranges, each of
which are then processed further in the other refinery processing units. The
CDU is often referred to as the atmospheric distillation unit because
it operates at slightly above atmospheric pressure.

First
of all, crude oil stream (feed) is heated and pumped to the distillation column
with high pressure. When the crude oil enters the large volume of the
distillation column, the pressure suddenly falls and it makes the components of
the crude oil to separate. This separation happens because of the difference in
the boiling points of the components of crude. The heavy components such as
heavy fuel oils, wax, lubricating oils, asphalt are going to the bottom of the
distillation column while light distillates such as Liquid petroleum gas (LPG),
Gasoline (petrol), Heavy Naphtha are going to the top of the column where they
are condensed. There are also some middle distillates which can be kerosene, automotive
and rail-road diesel fuels, residential heating fuel and other light fuel oils
which leaves the column in various stages. There are sets of trays and packers
mounted inside the column in order to increase the contact between liquid and
vapor to make the separation better. Crude oil distillation column is a
continuous process and the bottom products and top products are not achieved in
one cycle. It means that bottom products are reboiled in the reboiler and sent
back to column while top products are condensed in the condenser and sent back
to the column through reflux drum. Some part of heavy components are taken out
of bottom which are bottom products and remaining are reboiled and sent back to
the column. In the same manner some amount of light components are taken out of
the reflux drum which are called top products and the remaining amount are
refluxed to the column.

My
task is to design control system for reflux drum in order to keep the level of
this tank desired to make the separation better. In this case, I will used PID
control which will efficiently keep the PV closest to the set point.
A proportional–integral–derivative controller (PID controller or three
term controller) is a control
loop feedback mechanism widely used in industrial control systems and a variety of other
applications requiring continuously modulated control. A PID controller
continuously calculates an error value e(t) as the difference between
a desired set point (SP) and a
measured process
variable (PV) and applies a correction based on proportional, integral, and derivative terms which give the
controller its name. The overall control function of PID can be expressed
mathematically as:

Objectives

Before
starting the design process we should consider the objectives that should be
met. These objectives may not lead to the optimal practical integrated
solution, it is just one part of design process and it is not possible to
design the whole control system for distillation process in one month or even
less. So, the current
objectives are:

 

·       
Start and stop the
process by using buttons in the operator room

·       
Pump the feed
through the heater

·       
Heat the feed

·       
Send the feed to
the distillation column

·       
Keep the level in
the reflux drum at the desired value

·       
Use PID for smooth
control

·       
Send some
excessive amount of reflux to the column by controlling valve

To
conclude up, my objective is to design the control system to automatically keep
the level of the light components in the reflux drum at the desired value while
pumping more feed into the column.

 

Issues

While
designing a control system there are some vital problems that should be
considered before design process. The first problem is related to the
instrumentation of the system. In my task distillation process happens under
high pressure and temperature conditions which means that proper transmitters,
final control elements and other equipment should be chosen with care to
correspond to explosion zone requirements (Ex0, Ex 1, Ex2) and SIL levels.  We can define SIL (Safety integrity level) as
relative level of reduction of risk delivered by a safety function or to identify aimed amount
of risk-reduction. Simply, SIL is the measurement of the performance
essential for safety instrumented function (SIF). Requirements of a
given Safety Integrity Level are not steady among the whole functional safety
standards. In the functional safety standards based on the IEC
61508 standard,
four SILs are defined, with SIL 4 the most dependable and SIL 1 the least. A
SIL is determined based on a number of quantitative factors in combination with
qualitative factors such as development process and safety life cycle
management.

Hazardous areas are classified
into zones based on an assessment of the frequency of the occurrence and
duration of an explosive gas atmosphere, as follows:

·        
Zone 0: An area in which an explosive gas atmosphere is present
continuously or for long periods;

 

 

·       
Zone 1: An area in which an
explosive gas atmosphere is likely to occur in normal operation;

·       
Zone 2: An area in which an
explosive gas atmosphere is not likely to occur in normal operation and, if it
occurs, will only exist for a short time.

Another main problem that
should be considered is selection of control strategy that will safely,
efficiently and fully implement the desires of the customer. There are many
types of control systems that have application in various fields. If your
process is simple, not hazardous and high quality is not required it means that
you can choose basic control algorithm which will be easy to monitor and
configure. Generally, there are two types of control: open loop control and
closed loop control. In open loop control, the controller take actions without
having a sense of output. In closed loop control there is feedback coming from
the output and the controller take proper actions according the error between
process variable and the set point. Of course, closed loop control is more
sophisticated and reliable. So, I will implement close loop control in my
control system.

Selection of proper control
algorithm 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 and
implement, but there will significant oscillation and overshoot from the set
point.

 

In my example, I will used PID
control which will efficiently keep the PV closest to the set point. A proportional–integral–derivative
controller (PID controller or three term controller) is a control loop feedback mechanism widely used in industrial control systems and a variety of other
applications requiring continuously modulated control. A PID controller
continuously calculates an error value e(t) as the difference between
a desired set point (SP) and a
measured process
variable (PV) and applies a correction based on proportional, integral, and derivative terms which give the
controller its name.

 

 

 

 

Piping and
Instrumentation Diagram

P&ID is a comprehensive detailed diagram in the process industry that shows the cabling, devices, piping and tanks in the process together with the instrumentation and controlling devices. A piping and instrumentation diagram
(P&ID) is defined by the Institute of Instrumentation and Control as
follows:

·       
A diagram which shows the interconnection of process
equipment and the instrumentation used to control the process. In the process
industry, a standard set of symbols is used to prepare drawings of processes.
The instrument symbols used in these drawings are generally based on International
Society of Automation (ISA) Standard S5.1

·       
The primary schematic drawing used for laying out a process
control installation.

Below, the P&ID of my task is
shown:

  Figure 1. Piping and Instrumentation Diagram of the
process

 

Here in this picture you can see
feed in which is pumped by a pump to the distillation tower. After the feed
enters the distillation column light and heavy components will be separated due
to their different boiling points. Heavy components will go bottom and reboiler
will re-boil them in order to feedback some part of this liquid while some
amount will be taken out as bottom product. We can see that flow rate of bottom
product is controlled with LC which will take the measurement from LT level
transmitter. In the re-boiler the heating is supplied by steam.

On the top of the distillation
column light components are taken out. First of all, heated vapor goes through
condenser to condense down and convert to liquid phase. Therefore, this liquid
is directed to a horizontally located tank called reflux drum. In reflux drum
some amount of light components are taken out as top distillate products and
some amount again is sent back to distillation column to separate. Level of the
reflux drum is controlled by LC level controller in order to make the
separation quality better. Another pump will pump the liquid and LV level valve
will 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 the
tank is so high. Relief valve can be used for this purpose.

SCADA system

Supervisory control and data
acquisition (SCADA) is a control system architecture that uses computers, networked data communications
and graphical user interfaces for
high-level process supervisory management, but uses other peripheral devices
such as programmable logic controllers and
discrete PID controllers to interface to the process plant or machinery. The operator
interfaces which enable monitoring and the issuing of process commands, such as
controller set point changes, are handled through the SCADA supervisory
computer system. However, the real-time control logic or controller
calculations are performed by networked modules which connect to the field
sensors and actuators.

In my case, I need to build a simple
SCADA system to control distillation process and monitor the level of the
reflux drum. SCADA systems have some crucial components and they are discussed
below.

 

SCADA
components

My SCADA system is hierarchal and
there are process control level and supervisory level. In my project, the
process control level consists of one PLC, two level sensors, two valves with
actuators, pumps, heaters, condensers and so on. These final control elements
and transmitters are connected to PLC and then they get commands from PLC
according to the written program or commands of the operator from the operator
room. The supervisory level consists of PC system which has centralized
computer getting data from PLCs and monitors to show human machine interface,
input devices (such as mouse, keyboards) to enter some command to the SCADA
system.

The general device configuration of
the system is shown below:

Figure
2. Network view

Here we can see that PLC is selected
as CPU 315-2-PN/DP which has I/O module inside. This PLC is connected to our PC
system which is SIMATIC PC Station through PN/IE line. The HMI is programmed
with WinCC RT Advanced and IE general is used as communication module.

PLC system

After device selection and
configuration is finished I need to program the PLC in order to make it serve
for our purpose. LAD language will be used to program this CPU. This PLC will have
some input from push buttons (Start and Stop), measurements from sensor coming
to Input modules and some output commands to final control elements. All these
data variables will be handled through PLC tags and proper naming is important
in order to make easy to use and modify. The main OB block of my PLC is shown
below, it contains 7 networks:

 

 

 

                             Figure 3. Ladder diagram of the PLC

Network 1 is built to implement
basic start and stop function. Start (M0.0) contact is normally open contact
which will start energizing the Coil (Q0.0) when pressed. Stop (M0.1) is
normally closed contact which will be open when it is pressed and the whole
system will stop running. Auxiliary contact Q0.0 is used here also to keep the
system running even if the Start is de-energized.

In network 2, after start is pressed
and 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 the
bottom of the column will start running.

In network 3, after the start, the
scaling will be done. The feedback coming from the level sensor will be
simulated by a slider in the HMI screen. I considered this input as a voltage
level between 0 V and 10 V. Of course, this voltage level doesn’t show the real
level of the tank. It means we need to scale this input in order to make it
suitable for processing. I have done scaling by using math operators. My sensor
sends signal from 0-10 V and level range of the tank is between 0-150 cm. From
this relation real value of level can be calculated with this equation: real
level=(sensor_reading*150)/10. This equation will make the MD10(level) variable
between 0 and 150 cm. In network 3, you can see that firstly, MUL operator is
used to multiply sensor reading to 150 and DIV operator is used to divide this
value to 10 to get the actual level. However, math operation can be done for
only double integer variables. So, my MD10 is Dint variable. However, PID works
with only real variables. So, I have used CONV block in network 4 to convert
from integer to real. So, MD20 is real actual level in the tank.

In network 5, you can see
implementation of PID algorithm. This is continuous controller and there are
many pins. First of 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 (process variable) which is connected to PV_IN pin and set point is
set as 70 cm. Output of this PID is taken from LMN tag which is connected to
MD100 variable.

In network 6, SUB block is used to
make the output of PID suitable for our application. As I explained earlier
there is inverse relationship between level of the tank and the flow rate of
drainage valve. This PID gives manipulated variable for direct control. So, I
need to subtract this MV from 100 in order to make it suitable for our
application. Output of PID (MD100) is subtracted from 100 and MD110 is real
valve opening percentage which will be sent to actuator of valve.

In network 7, comparator is used to
simulate the drainage valve. If the valve opening percentage is greater than 0
it means that some proportion of the valve is open and we will turn the valve
to green in the HMI. If the percentage is 0 the valve will stay red showing
that it is not running.

Human Machine
Interface

HMI is a part of SCADA systems and
it is physically screens of PC systems which are available to operators.
Operators can monitor the process via HMI and also they can give control
commands to PLCs. I designed graphical interface in a way that it is easy to
understand and monitor the main points. 
The overall design of the screen before running is like this:

Figure 4. Graphical user interface

 

 

 

 

HMI screen after running:

                                                      
Figure 5. Running HMI screen

HMI screen after pressing Start
button:

                                                        Figure 6. HMI screen after Start

 

Increasing the level with slider: PV
is below the SP (70 cm)

                                          Figure
7. HMI when PV  is below SP

Tank level above set point: PID is
changing the valve position (59 %)

                                          Figure 8. HMI when PV is above SP

 

Higher tank level:

                                                     Figure
9. HMI for higher level of tank

In this HMI design the slider on the
right side is used to simulate the signal coming from the level sensor. Start
and Stop buttons are used to start and stop the process respectively. As we can
see 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 green
from 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 not
energized. When the level is going above the set-point the PID block will
automatically calculate the proper opening percentage for the valve to decrease
the level. When there is some amount of valve opening it will be shown in green
and the pump will start pump the liquid out of the reflux drum. When we run the
simulation we can see that how the valve position changes when the PV goes
above SP and in comparison with on-off control the PV will settle down to SP
smoother and faster in PID control. Scaled level of the liquid is shown on the
tank and the percentage of valve opening is shown below the drainage valve.

There is events created under
buttons and animations behind some elements that are connected to proper PLC
tags. Here I will show how to setup an animation for a final control element to
change its appearance according to the change of bit:

Figure
10. Making animation for valve

In this picture you can see that a
PLC tag named Valve having address of Q0.4 will change its color from red to
green when its bit changes from 0 to 1. In design if buttons we will create
events with PLC tags.

Conclusion

In this project, I simulated the
sensor input by using a slider and scaled the values of slider to a real level
indication and used this parameter in PID setup to keep the level of the reflux
tank in the desired value (70 cm) by implementing PID algorithm. By working on
this project I learned how to scale sensors in S7 1200 and 300 while I also
used another solution which involves math operators. I also learned how to
setup PID in S7 1200 and 300. PID control worked properly for my solution and
it is very fast and reliable. If want to change the settling time of the PID I
play with Ti (integral action) to make the process smoother. I also learned how
to work with different types of variables like double integer, integer, word,
real and so on. Therefore, I learned how to convert from one variable type to
another. In my solution I used CONV block to convert from double integer to
real and I found out that conversion from integer to real is not possible. In
HMI design I made some events under buttons and created animations to show some
change of parameters in the screen.

 

 

Appendix

 

PLC tags:

 

Simulator:

 

References

 

https://en.wikipedia.org/wiki/Programmable_logic_controller

https://en.wikipedia.org/wiki/PID_controller

https://en.wikipedia.org/wiki/Continuous_distillation

https://support.industry.siemens.com/cs/document/48711306/in-step-7-(tia-portal)-how-do-you-convert-the-data-types-for-the-s7-1200-s7-1500?lc=en-WW=0

https://support.industry.siemens.com/tf/WW/en/posts/simulate-hmi-in-tia-portal/144121?page=0=10

 

 

 

 

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