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Controlling Level of Liquid in The Horizontal Tank

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This report contains information about my course project called “Controlling level of liquid in the horizontal tank”. This control system is designed in the TIA portal which integrates Simatic Step 7 for PLC programming and WinCC for Human Machine Interface. Firstly, I will explain the problem statement and the main background for this problem. Then I will introduce the Piping and Instrumentation Diagram of this control system showing the main devices that help to implement my solution. Then I will also explain the ladder logic diagram, SCADA system and graphical interface that I built in the TIA portal. In the end, I will give my conclusion about my solution and how it can be developed and improved further. In this task, I need to build a control system that effectively controls the level of the liquid in the horizontal tank by opening and closing the draining valve.

A control system manages, commands direct, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large Industrial control systems which are used for controlling processes or machines. For continuously modulated control, a feedback controller is used to automatically control a processor operation. My control system continuously compares the value or status of the process variable (level of the liquid in the tank) being controlled with the desired value or setpoint (this can be varied according to needs), and applies the difference as a control signal to bring the process variable output of the plant to the same value as the set point. For sequential and combinational logic, software logic, such as in a programmable logic controller, is used.

There are two common classes of control action:

  • Open loop and closed loop.
  • In an open-loop control system, the control action from the controller is independent of the process variable.
  • An example of this is a central heating boiler controlled only by a timer.

The control action is the switching on or off of the boiler. The process variable is the building temperature. This controller applies heat for a constant time regardless of the temperature of the building. However, in a closed-loop control system, the control action from the controller is dependent on the desired and actual process variable. In my case, for example, the output of the control system (level of the liquid) is continuously measured and is sent as a feedback to the PLC to compare the value with the set point and make corrective actions according to the error. In this task proper selection of the level sensor, valves, pumps, PLC and communication system is important and will be discussed further.

Aims/Goals Main objectives of this task are listed here:

  • Pumping the liquid into the horizontal tank continuously with a pump
  • Measure the level of the liquid in the tank by using a proper radar level sensor
  • Creating a start and stop buttons
  • Connecting the drainage (output) of the tank to a valve with an actuator
  • Opening and closing the valve automatically according to the set point with PLC
  • Showing high and low alarms to increase safety There are some main points that should be considered before designing the control system. When designing the control system instrumentation of the system should be carefully chosen in order to make it reliable and easy to use. Also, proper control strategy should be selected to meet the requirements of the task stated.

A control strategy can be different types:

  • Logic control (with relays and timers),
  • Linear control (using linear negative feedback),
  • On-off control (bang-bang control),
  • Proportional control,
  • PI control,
  • PID control and so on.

However, we should choose our control strategy with care in order to make the system work properly while keeping cost and complexity of the system minimal. It means that if our application is not hazardous and does not contain any valuable substances we can minimize the cost by applying simple techniques. In my task, we simply need the controlling level where there are very little potential hazards and we can confidently use the on-off control in order to make thing simple and easy to configure while working properly. Although the PV will have some slight oscillation around set-point when it suddenly changes but it will be the intolerable amount and on-off control matches our requirement. In my task, there is a direct relationship between the level of liquid and flow rate in the drainage valve. So, there should be direct control of this control system. It means that if the level increases above the set-point the flow rate in the drainage valve also should increase and this shows direct control. Piping and Instrumentation Diagram A piping and instrumentation diagram (P&ID) is a detailed diagram of the process industry which shows the piping and vessels in the process flow, together with the instrumentation and control devices. So P&ID 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.

P&ID of my task is given below: Here we can see a horizontal vessel having one input pipe and one output pipe. The input of this vessel is connected to a pump which will send the liquid into the tank when the start button is pressed. In this diagram, LG stands for level gauge which directly shows the level of the liquid in the task in the field. LT 135 is a radar type level transmitter which measures the level of the liquid by sending pulses into the tank and according to their reflection, this transmitter can send 0-10 V or 4-20 mA signal which can be scaled in PLC and processed to control the level. LI 135 is a level indicator of the LT 135 sensor. LIC135 is the level controller which has auxiliary location accessible to the operator. We can set the set-point (SP) on this controller. LY is the level relay which converts an electrical signal into 20 PSI pneumatic signal which will control the final control element- actuator to open and close the valve. LV 135 is the controlled valve which will change the flow of draining, thus altering the level of the liquid in the tank. PLC systems To implement a control algorithm for this control system Programmable Logic Controller will be used.

A PLC is an industrial digital computer which has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, or robotic devices, or any activity that requires high-reliability control and ease of programming and process fault diagnosis. PLCs are used in the shop floor of the manufacturing process as a part of process control. For larger and high-reliability systems DCSs are used and they are effective for many loops. For our application PLC system is suitable since our process is not complex and contains one loop. In my control system, I used Siemens S-300 CPU 315-2 PN/DP which has its own input and output modules so I do not need to add I/O modules. This PLC is connected to a PC system SIMATIC PC Station WinCC RT Advanced through HMI connection. So, the device configuration is finished and the PLC should be programmed to implement the required logical operations. In this picture below, the device configuration of the system is shown: After proper device configuration, I started programming the PLC in LAD language and the code is shown below: In this ladder logic diagram, network 1 is a start-stop scheme.

The start is normally open contact when it is closed the Run coil is energized and it closes the auxiliary contact Run below the Start contact. The stop button is normally closed contact. So, when we need to stop the process we should open this normally closed contact and the system will stop operation. In network 2, after the Start is pressed and the Run coil is energized the Q0.1 pump starts working whose PLC tag is named as “Filling”. Here closed contact of draining valve (Q0.2) is used to turn off the filling pump when the set-point is exceeded and the draining valve is working. So, this pump serves for pumping the liquid into the vessel, but it will stop working when draining happens. In network 3, the control action is shown. Here Run contact is firstly added to operate only when the Start button is pressed. Then comes a comparator which compares the Level of the liquid (MW2) with the set-point (MW8). If the level is higher than the desired set-point the Draining valve Q0.2 will be energized the valve will be opened to reduce the level of the liquid in the tank. The value of the level MW2 is simulated by a slider in HMI which will be discussed later.

So, continuously, if we increase the level of the liquid above the set-point, drainage valve will try to decrease the level until it reaches the set-point. After reaching the set-point the valve will be closed automatically and the filling will start. In network 4 and 5 working of low and high alarms is shown. When the level of the tank is too low below 10 cm the low alarm will turn to red showing the operator that the level is too low. When the level is too high above 90 cm the high alarm will turn to red to make the operator aware of the hazardous case. The graphical interface is made simplistic in order to make it easy to monitor the process. The input from the level sensor is simulated with a vertical slider which changes from 0 to 100 and this corresponds to the level of the tank from 0 cm to 100 cm. Start and Stop button is used to run and stop the process. These buttons have events connected to them with PLC tags. When the start button is pressed the filling pump will turn to green showing that this pump is running. So, firstly, the low alarm will turn to red because the level will be too low. After some time level should increase and the low alarm will turn to yellow.

After the PV goes above-set point (50 cm is set with right side slider) the draining valve will turn to green from red showing that this valve is open. While the drainage valve is open the filling pump should be stopped in order to reach the set-point easily. When the level goes to high alarm level the High alarm will turn to red showing that the PV is at a dangerous level.

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Controlling Level of Liquid in the Horizontal Tank. (2018, July 19). GradesFixer. Retrieved May 27, 2022, from
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