experiment 1- process control rig

Experiment 1-Process Control rigBEng year 2- Chemical Engineering

Anonymous

11Teesside University

School of Science and Engineering

Experiment 1-Process Control rig 2011

Summary

Nowadays, level control systems are really common within the petrochemical industry. The aim of this experiment is to investigate different control strategies which can be used to maintain the fluid (water) – level in the process vessel. As well as to find out how the process reacts when disturbances are introduced.

Introduction

The experiment consists of 7 exercises to carry out in order to find out the different control strategies used to set fluid (water) –level in the tank. Due to limited time, only 3 exercises were carrying out. As follows:

1. On/Off level switch;

2. Differential level switch;

3. Manual operation of normally closed solenoid valve.

During each exercise all the readings for every parameter (level, flow rate and time) were recorded and graphs of the fluid (water) –level against time were also plotted. The results from the graphs and readings table are also explained under the discussion session.

Carlos Almeida – K0156140 Page | 1


Process control

A process control system is used to monitor data and control equipment on the process plant. The objective of the process control system is to read values of different sensors and control valves switches to control the process. At the same time alarms, reports and other information within the plant are presented to the control room operator. (Devold, 2006)

The most common controlled and monitored parameters are:

Pressure;

Level;

Flow;

Temperature

The suitable way of monitoring and controlling the above parameters is to consider the appropriate control loop for the process control system.

A control loop is a system for controlling a single parameter within a process control system. For example:

Crude oil flow rate to storage tank;

Lubrication oil temperature in a gas turbine;

Pressure in wellhead riser;

Cooling water temperature;

Water injection pressure.

There are two main types of control loop:

Open loop;

Closed loop.



Open loop

The open loop is the type of controller that computes its input into a system applying only the current state and its model of the system. The open loop has no feedback. See fig 1

Fig 1- Open loop system (Devold, 2006)

The diagram above shows a typical open loop control system where the process fluid outlet flow rate is not controlled automatically, the level is not automatically controlled. Therefore the flow is being controlled by an outside operator.

There are some advantages and disadvantages for using this type of control.

Advantages:

Inexpensive

Disadvantages:

Poor control;

Subject to operator error;

Not safe as automatic(closed loop)

Applications:

The open loop control system is used on process whose variables does not change ;

The open loop control system is used for example in offloading road tanker operations as the storage tank has larger capacity than the tanker therefore there is no danger of overfilling. (Devold, 2006)



Closed loop

The closed loop system is the type of controller in which the feedback taken from the output and then this feedback feeds to the comparator for comparing with the set point and then the error is then applied to further circuit. See fig 2 (Kuo, 1991)

Fig 2 - Block diagram of the closed loop system

Set pointThe set point is the input that determines the desired operating point for the process. (Kuo, 1991)

Error AmplifierIt amplifies the error signal and give to the controller because the error signal is very weak in amplitude, it maybe not be directly operated by the controller, so this weak signal is amplified by the error amplifier. (Kuo, 1991)

ControllerIt produces the output signal for the process based on the input error signal. (Kuo, 1991)

Output attenuatorIt controls the output of the controller stage and input of the process state. (Kuo, 1991)

Sensor feedbackIt provides input from the process to set point or comparator which compares the set point and the feedback signal. (Kuo, 1991)

The feedback is the action of taking all or part of the output and feeding it back to alter the input.

Feedback is the part of the system that informs the comparator what the controlled condition actually is. For example: to “see” if the measured variable is at the desired value. The feedback can be provided by either a production operator (manual control) or a control device (automatic control). (Devold, 2006) Carlos Almeida – K0156140 Page | 4


DisturbanceEach process one or more disturbances tend to change the controlled variable. Therefore closed loop is used in order to regulate value of controlled variables, when disturbance changes it. (Kuo, 1991)

There are two classes of variables, when a process has been controlled.

1. Input variable, this variable shows the effect of the surrounding on the process. It normally refers to those factors that influence the process. There are effects of the surrounding that are controllable and some that are not. These are divided into two types of inputs.

Manipulated inputs: variable in the surrounding can be control by an operator or the control system in place. (Romagnoli, 2006)

Disturbances: inputs that cannot be controlled by an operator or control system. There exist measurable and immeasurable disturbances. (Romagnoli, 2006)

2. Output variable: sometime called as control variable, these are the variables that are process output that affects the surroundings. These variables may or not be measured.

As control problem has been considered therefore two major control structures can be looked at.

Single input-single output (SISO): for one control (output) variable there is one manipulate (input) variable that is used to affect the process.

Multiple input-multiple outputs (MIMO): there are several controls (output) that are affected by several manipulated (input) variables used in a given process. (Romagnoli, 2006)

A control system is really important within the process due to its task, which is to maintain the process at the operational conditions and set points and transitions the process from one operational condition to another.



Experiment procedures

Where:

PR- Pressure regulator; FI – Flow indicator; SOL1 – Solenoid valve 1; SOl2 – Solenoid valve 2; SOL3 – Solenoid valve 3; LCI – Level indicator control; PC – Pressure control

Safety always comes first in the whole petrochemical industry; every single person on site must be aware and follow all the site safety procedures so that nobody gets hurt and no damage in the environment. Therefore the appropriated PPE (personal protective equipment) was worn to carry out the experiment.



Exercise 1-On/off level switch procedure:

1. Firstly the mains power was switched on to the console;

2. Ensure the drain valve is opened to control the inlet pressure regulator;

3. The PCT40 software was run and selected section 1: Level control (inflow) then it was selected the view the mimic diagram screen.

4. The solenoid valve (SOL1) was opened to allow the water into the process vessel. However all the variables were checked if working properly by opened PID controller manual output to 100% the SOL1.

5. The drain valve was opened in the base of the vessel allow the water to drain from the vessel. The flow rate was checked if it was between 1350ml/min and 1450ml/min. otherwise it needed to be adjusted by regulating the pressure regulator.

6. The data was started to be logged or collected by selecting the icon (stop/go).and then stopped once the oscillations in the fluid(water) level was stabled for the magnitude and duration by selecting once again the icon(stop/go).

7. The disturbance was introduced in the outflow from the vessel by opening the normally closed solenoid valves SOL2 and SOL3. The SOL3 was opened to allow the fluid (water) level oscillations to settle to steady values. Then SOL3 was opened, so that SOL2 and SOL3 were both opened in order to get the process stabled.

8. Finally, SOL2 and SOL3 were closed, so that the fluid (water) returns to the set point. Then icon (stop/go) was selected to stop data been logged.

Exercise 2- Differential level switch

Although a differential level sensor has been coupled. But basically the equipment set up to carry out this task was the same used in previous exercise 1(On/Off level switch).



1. The differential level switch was checked and prepared before it can be used to control the flow level.

2. The water began to flow into the process vessel and reading of L1 started to varying with the rise of the fluid (water) levels.

3. The data was logged by selecting the icon (stop/go).Then stopped once the oscillations in the fluid (water) level were stabled for the magnitude and duration by selecting once again the icon (stop/go).

4. See procedure 7 and 8 from the previous exercise (On/Off level switch) for introducing disturbance in the outflow from the vessel.

Exercise 3- Manual operation of Normally Closed solenoid valve

1. The PCT40 software was run and selected section 1. Level control (inflow) afterwards the mimic diagram screen was selected.

2. The bottom drain valve of the vessel was partially closed in order to reduce the outlet flow rate slightly.

3. In the sample menu was selected “Configure”, and checked that the software is set to automatic sampling with a sample interval of 1 seconds and continuous duration.

4. The SOL1 was opened as follows: the “Controller” button radio was checked and selected in On/Off. The “Manual” operating mode was selected then “Manual Output” to 100%.

5. The fluid (water) was allowed to flow in the process vessel until the level reaches 200mm.

6. The solenoid valve SOL1 was closed via the PID controller and the fluid (water) stopped flowing into the process vessel.

7. The data was started to be logged /collected and stopped by selecting the icon (stop/go).

8. Although the task was performed manually. But for introducing disturbance in the outflow from the vessel the procedure is just the same as the exercise 1(On/Off level switch).See procedure 7 and 8.



Results

0 20 40 60 80 100 120 140 160 180-200

0

200

400

600

800

1000

1200

1400

1600On/Off Level switch

Level L1 [mm]Flowrate F1 [ml/min]

Time(min)

Leve

l(mm

) and

Flo

w ra

te(m

l/m

in)

Fig 3- on/off level switch graph

0 10 20 30 40 50 60 70 80 90 1000

200

400

600

800

1000

1200

1400

1600Differential level switch


Time(min)

Leve

l(mm

) and

Flo

w ra

te(m

l/m

in)

Fig 4- Differential level switch graph



0 50 100 150 200 250 300-200

0

200

400

600

800

1000

1200

1400

1600

Manual operating of normally closed solenoid valve


time(min)

Leve

l(mm

) and

Flo

w ra

te(m

l/m

in)

Fig 5- Manual operating of normally closed solenoid valve graph.

Discussion

On/off level switch –fig 3The controlled variable (level) of the process vessel is measured in the output. This value is then compared to the desired value (200mm) of the set point. The difference between them is called an error or deviation, which is the input of the feedback controller. Afterwards it calculates a signal to adjust the flow rate (manipulated variable). Then the output of the feedback controller sent a signal to a VC (control valve) to close or open depending on the level in the process vessel.

The graph shows that when SOL1 is open the fluid (water) started to fill the vessel and once the fluid level in the process vessel reaches the LS (level switch) it automatically sends a signal to the controller in order to maintain the desired fluid level. Therefore the solenoid valve SOL1 is closed automatically. As the time response was really fast therefore the disturbance was introduced in the process by opening SOL2. The process reacted by adjusting the flow rate (manipulated variable) of the process vessel. Maintaining SOL2



opened the controller increases the flow rate automatically to maintain the desired fluid level in the vessel. When SOL3 is opened at this stage the time response is slower to increase the flow rate than reducing the fluid level in the process vessel. When SOL1, SOL2 and SOL3 are opened, the process variable (level) responds by increasing the fluid flow rate to maintain the set point. The period of oscillation also increases.

Differential level switch- fig 4

When SOL1 is opened the process controller increases the flow rate until the fluid (water) level in the process vessel reached the set point. Then the controller automatically closed SOL1 to maintain the fluid (water) level in the process vessel stabled. At the stage the period/time for the process to respond is very short due to disturbance within the process.

When disturbance was introduced (SOL2 opened), the fluid (water) level in the process vessel were decreasing. Therefore the controller automatically increased the flow rate to maintain the fluid (water) at the desired level in the vessel. The period for the process to respond was longer due to disturbance within the process.

When SOL3 was opened, by this time both SOL2 and SOL3 opened, the controller increased the flow rate even higher to maintain the fluid level as desired. So the period for the process to respond was even longer due to both valves been opened.

When both valve, SOL2 and SOL3 were closed, the controller automatically reduced the flow rate so that the fluid (water) level in the process vessel reaches only the set point then the controller automatically closes SOL1 and the process gets back to normal.

Manual operating of normally closed solenoid valve- fig 4

When SOL1 was opened manually, the fluid (water) level in the process vessel increased around 200 mm as the desired value. But in the graph (fig4) the level was little bit higher than 200 mm due to control operator been delayed on closing the valve SOL1 manually. At this point the period for the process to respond any internal disturbance was really short due to it not been introduced in the process yet.

When SOL2 was opened, the control operator manually opened/closed SOL1 and the controller automatically increased the flow rate to maintain the desired fluid (water) level in the process vessel. The period of responding was longer due to disturbance been



introduced. The fluid (water) level was monitored very as the graph (fig4) shows a horizontal line which represents the fluid level in the vessel.

When both valves, SOL2 and SOL, the control operator open/closed SOL1 manually and the controller once again increased the flow rate in order to maintain the fluid level in the vessel. At this stage the period for the process to respond was even greater due to SOL2 and SOL3 been opened.

At some stage the fluid level in the process vessel dropped to 200 mm as it is shown in the graph(fig 4) it might happened due to control operator delayed on opening SOL1 so that the control could increases the flow rate. But afterwards it was correct and the fluid level was monitored successfully.

Conclusion

The process control responds really fast when there is disturbance within the process in order to reduce or get it of itl;

The process control responds slowly when there is not disturbance;

The manual operating of normally closed solenoid valve is less safe and accurate than the others(automatics) due to operator error;

The on/off level switch offers more stability in the fluid level than the others.

The solenoid valves SOL2 and SOL3 are normally closed valve and are opened to introduce disturbance within the process.



Reference

Devold, H. (2006). Oil and Gas Production Handbook. Oslo: ABB ATPA Oil and Gas.

Kuo, B. (1991). Automatic Control System. New jersey: Prentice Hall.

Romagnoli, J. A. (2006). Introduction to Process Control. New york: CRC Press.

Stephanopoulos, G. (1984). Chemical Process Control. Eaglewood Cliffs: Prentice Hall,Inc.


experiment 1- process control rig

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