ELEC3320 Lab Report

ELEC3320 Lab Report, Semester 1, 2012

1 Tameem Mithaiwala, 10885946 Jai Kant, 20873374

Lab Report

Modeling the level and flow of water using a Basic Process Rig (BPR)

Tameem Mithaiwala, 10885946 Jai Kant, 20873374

School of Electrical, Electronic and Computer Engineering University of Western Australia

Unit Coordinator Dr. Tyrone Fernando

Lab Demonstrator Kianoush Emami

17/04/2012



Table of Contents 1 Introduction ................................................................................................................... 3

2 Assignments .................................................................................................................. 3

2.1 Assignment 1: Familiarisation ................................................................................. 3

2.1.1 Practical 1 ........................................................................................................ 3

2.1.2 Practical 2 ........................................................................................................ 5

2.1.3 Practical 3 ........................................................................................................ 6

2.1.4 Practical 4 ........................................................................................................ 7

2.2 Assignment 3: Interface Familiarisation................................................................... 8

2.2.1 Practical 1 ........................................................................................................ 8

2.2.2 Practical 2 ...................................................................................................... 10

2.3 Assignment 4......................................................................................................... 11

2.3.1 Practical 1 ...................................................................................................... 11

2.4 Assignment 7: Float Level Transmitter .................................................................. 12

2.4.1 Practical 1 ...................................................................................................... 12

2.4.2 Practical 2 ...................................................................................................... 12

2.5 Assignment 8: Pulse Flow Transmitter .................................................................. 12

2.5.1 Practical 1 ...................................................................................................... 12

2.5.2 Practical 2 ...................................................................................................... 12

2.6 Assignment 9: On/Off Level Control ..................................................................... 13

2.6.1 Practical 1 ...................................................................................................... 13

2.6.2 Practical 2 ...................................................................................................... 14

2.6.3 Practical 3 ...................................................................................................... 14

3 Conclusion:.................................................................................................................. 15

4 References ................................................................................................................... 16



1 Introduction

The aim of the laboratory is to provide an introduction to the various integral parts of the

Basic Process Rig (BPR) and become familiar with their operation. Through the experiments

performed, knowledge will be gained of:

• instrument characteristics

• core concepts in process control

• calibration

• devices used commonly in control processes

2 Assignments

2.1 Assignment 1: Familiarisation

2.1.1 Practical 1

Preparation and tasks:

• The two upper tanks are isolated with the orange rubber bung.

• All the water is released from the upper right tank.

• The servo valve is fully opened to allow the fluid to enter into the pump.

1) Special features and principle of operation of the centrifugal pump

The pump used in the rig is of the centrifugal type as opposed to the positive displacement type

that includes the reciprocating and rotary pumps. It is appropriate to use this type of pump in this

case as the operation in the rig involves high flows and low pressure heads.

The fluid flow is resulted due to the creation of an increase in the fluid pressure from the pump

inlet to its outlet. Fluid enters the pump at the axis of rotation, resulting in a pressure difference

which drives the fluid through the system. The centrifugal pump creates an increase in pressure by

transferring mechanical energy from the motor to the fluid through the rotating impeller. The fluid

flows from the inlet to the impeller centre and out along its blades. The centrifugal force hereby

increases the fluid velocity and consequently also the kinetic energy is transformed to pressure

(Grundfos, 2008). Figure 1 shows how the fluid flows through the pump.



Figure 1: Fluid Path through the centrifugal pump (Grundfos, 2008)

How the pump is used in this practical

In this practical, the centrifugal pump is used to pump water from the lower tank to the upper tank.

The overflow pipe is in place to prevent the tank from overfilling, it acts as a safety control.

Purpose of the overflow pipe

The pipe ensures that the water filling the upper tank does not exceed the maximum limit – in this

case, 100cm – thereby controlling the level of water in the tank.

2) Rate of flow of water

The servo valve was fully opened, and the water was allowed to fill the tank to the point of

overflow. The time taken was recorded as 39 seconds.

Volume of the tank is 3.07L (as given by the demonstrator).

Rate of �low of water = Volume of waterTime taken

= 3.07L39s

∗ 60 = 𝟒.𝟕𝟐𝟑𝐋/𝐦𝐢𝐧

The measured value is found to be 4.4L/min. This difference is probably because the flow gauge

used to measure the flow rate only ranges from 0.4 to 4.4L/min.

3) Flow through the piping network is less than the full flow capability of the pump

The flow through the pipe can only match up to the full flow capability under ideal conditions.

There are a variety of factors that influence this change. They include:

• Orientation: The water was being pumped vertically, where as full flow might only occur

when it pumps horizontally

• Motor of pump likely not running at full capacity



2.1.2 Practical 2

In this practical, the BPR’s manual valves were operated to control:

• flow rate of the water supplied by the pump

• level of the water in the upper tank

By controlling the size of the valve orifice, i.e. by adjusting the valve knobs, it is possible to

control the amount (or volume) of water passing through the valve in a given time.


• Manual valve MV2 and servo valve were fully opened.

• Manual valve MV3 was first adjusted to maintain a constant water level.

• Next, MV2 was adjusted to reduce the rate of flow to an intermediate value in reference to the

visual flow meter.

Observations:

When MV3 was adjusted, the input flow rate was found to be the same as the output flow

rate. Adjustment of MV2 reduced the flow rate which implies that the output flow rate is

faster than the input flow rate.

1) Block Diagrams

Figure 2: Block diagram of level control

Figure 3: Block diagram of flow rate control

Type of Control System

These are manual systems with closed loops, where the valves are controlled manually through

visual inspection of the tank level and the flow meter.

2) Process Variables being controlled

In the first case the level of the water in the tank is being controlled, while in the second case the

flow rate is being controlled. The level and flow rates are interrelated; when the flow rate is

Visual Inspection Flow Meter

Desired Flow rate Flow Rate

Inspected Flow Rate

-

+ Adjustment of

MV 2

Visual Inspection Tank Level

Desired Level Level

Inspected Level

-

+ Adjustment of

MV 3



increase when all other elements are under constant conditions, the level of the water in the tank

increases as well. Both variables can be controlled simultaneously if the manual valves are

adjusted accordingly.

2.1.3 Practical 3


• MV2 was fully opened and MV3 was set to 50%.

• Servo gate was gradually opened in steps by changing the current signal.

Observations:

As the servo gate was opened, the flow rate increased thus filling the tank at a higher rate. The

flow rate was inspected using the visual flow meter. An abrupt change in current takes some

time to establish a constant flow due to the delayed response of the signal.

1) Servo valve

A servo valve is one which is used to direct fluid to an actuator in a closed-loop or servo system.

This is the reason it is given its name. It is controlled by electrical signals and consists of a torque

motor coupled directly to the output stage, usually a sliding spool (Thayer, 1962). These systems

rely on feedback to ensure they are operating at the right point.

The servo valve in the BPR uses a gate to block the path of the liquid through it. The position of

this gate is controlled by a 4-20 mA signal, where 4 mA corresponds to the gate being fully

lowered, thereby closing off the flow, while a 20 mA signal fully opens the valve. The gate

position takes time to change and hence it exhibits transient response.

2) Comparison of the servo valve with the manual valve

• Servo and manual valves both control fluid flow rate.

• Manual valves rely on human inspection and operation, which are likely to cause errors.

However, their advantage is that they can be changed at any time irrespective of how the

system is operating.

• Servo valves are automated systems using feedback loops to control flow rate. This

minimises the probability of errors and is more convenient as it does not require constant

monitoring. Valve automation brings significant advantages in the areas of process

quality, efficiency, safety, and productivity. The change in current signals does produce a

time lag which is a relatively minor disadvantage. They are usually programmed to

perform under certain conditions, which limits its ability to be changed during operation.

4) Before switching off the servo valve, it should be fully opened by setting the current to 20mA in

order to avoid any confusion in future labs.



2.1.4 Practical 4


• MV1 was fully opened with all others closed.

• Switched AC supply was turned on, followed by switching on solenoid valve SV1.

• MV1 was adjusted to maintain a constant water level in the upper right tank.

• The experiment was repeated by substituting SV2 with SV3.

Observations:

When the PI and switched AC supply were turned on, there was no water flow. After

switching on SVI the water started filling the tank, as expected.

When the water was maintained at a constant level by adjusting MV1 the values of flow rate

and level in the tank were 2L/min and 55mm respectively. Next, SV2 was switched with SV3

resulting in a rise in the water level. This MV1 had to be adjusted in order to bring the level

back to 55mm, which was accomplished by reducing the flow rate to 0.7L/min.

1) Solenoid valve

A solenoid valve is an electromechanical valve which can only be open or closed, unlike the

manual or servo valves which have full range of control. It is controlled by an electric

current through a solenoid.

The normal state of the valve is closed, and is controlled by passing an electric current through a

solenoid.

2) Comparison of the solenoid valve with the servo valve

• In comparison to a servo valve, a solenoid valve can be controlled remotely, and is therefore

suitable for automation.

• It has a simple mechanism and is relatively cheaper to install and maintain.

• One of its major disadvantages is the power needed for maintaining the solenoid valve in an

opened condition is high, resulting in high cost. In addition to that, the housing and the coil of

the solenoid valve gets heated up quickly thereby lowering its service life.

• Since it basically operates like an on-off switch, it is very limited in the range of control it can

provide. Servo valves on the other hand, can be controlled with a full range of motion.

3) Significance of the valve size

Difference in valve sizes causes changes in the flow rate. Smaller the valve size, slower is the flow

rate. It was observed that the flow rate was much slower when SV3 was open in comparison to

SV2 implying that SV3 has a smaller diameter. This is confirmed as the measured value of SV2

and SV3 are 5mm and 3mm respectively.



4) Comparing SV1 and SV2

Even though SV1 and SV2 are of the same diameter, when both valves were open, MV1 had to be

adjusted in order to maintain the water level constant at the desired intermediate level. This is

because of the following reasons:

• Orientation: one of them is horizontal while the other is vertically positioned.

• Gravity effects

• Pressure differences

2.2 Assignment 3: Interface Familiarisation

2.2.1 Practical 1


• Digital Display Module (DDM) was set to mA.

• Current in the loop was varied and values for current and percentage were recorded.

Observations:

Table 1 indicates the values of currents measured with the corresponding percentages.

Table 1: Measured values of currents and percentages

Current (mA) Percentage (%) 0.7 101.9 4.9 5.4 7.3 20.3

10.2 39 12.2 51.1 17 81

18.8 92.5 20.4 101.9

Industry standard current signals are 4-20mA and 0-20mA. In the BPR, the current signals

range from 4-20mA and hence the DDM is calibrated such that 4mA corresponds to 0% and

20mA to 100%. The plot of current against the percentage (Figure 4) is a linear graph

indicating accurate calibration. With no lead connected, there is no current in the loop.



Figure 4: Plot of current vs. percentage

1) Circuit breaker

A circuit breaker is an automatically operated electrical switch that provides protection

against:

• Overload currents: In times of fault, the circuit breaker automatically switches off the

supply thereby protecting the circuit against overload currents.

• Earth fault currents: If any difference between the currents flowing through the live and

neutral lines is detected, the circuit breaker opens the contact between the live and neutral

contacts.

The circuit can be tested by pressing the test button on the front of the device. Failure to

do so indicates either no supply to the circuit breaker or a faulty device.

2) Current loops

Current loops are used in process control to transmit information; it acts as a

communication interface which enables different elements within a system to

communicate with each other.

Current Source

The PI in the BPR uses a 4-20mA current loop for signaling using a two-wire connection

system.

0

20

40

60

80

100

120

0 5 10 15 20 25

Perc

enta

ge

Current

Current vs Percentage



Table 2: Comparison of 0-20mA and 4-20mA loops

0-20mA loop 4-20mA loop Resolution of 20mA. Resolution of only 16 mA. The transmitter must be provided with separate supply. This adds to the installation cost.

2-wire connection system, i.e. signaling and power supply in the same leads may be used.

It is not possible to provide a transmitter fail-safe system.

Simple to provide transmitter fail-safe system.

It is difficult to calibrate the zero. Simple to calibrate zero point because the lowest current can be reduced below 0%.

2.2.2 Practical 2


• Using a multimeter, voltages across terminals G and ground were measured for different

currents.

Observations:

Table 1 indicates the values of currents measured with the corresponding voltages.

Table 3: Measured values of currents and voltages

1) I-V converters

I-V converters are used to convert the current in the loop to a voltage across a 100Ω resistor. Their

uses are:

• The converted voltages can be used as inputs to the comparator or logic inputs in the on-off

section of the PI.

• They can also be used as inputs to various relays of the process controller, enabling different

modes of control.

2) As seen from Figure 5, the shape of the graph is linear. The gradients of the line is found to be

approximately 100, which is also the theoretical value i.e. the resistance of 100Ω.

This approximate slope is due to minor differences in the values of measured and theoretical

values. This could be due to inaccuracy of the resistance, as typical resistors can be off as much

as 10% from the "band" reading. The other possible source of error could be the multimeter used

to make the measurement

Current (mA) Voltage (V) 4.1 0.41 5.3 0.53 9.0 0.89

12.1 1.21 15 1.5



Figure 5: Plot of current vs. voltage

2.3 Assignment 4

2.3.1 Practical 1


• The current source knob was turned anti-clockwise fully, i.e. to the 4mA setting. Zero setting

was adjusted using a small screwdriver, so that the value of the current shown on the

DDM is exactly 4mA.

• Next, the current source knob was turned fully clockwise, i.e. to the 20 mA setting,

and the span setting was adjusted.

Observations:

Before calibration, the values of the current shown on the DDM at the fully anti-clockwise

and clockwise positions of the knob were found to be 4.1mA and 20.5mA respectively.

1) Controls associated with the current source calibration

The current source calibration is achieved by controlling the span and zero controls; the zero

control is used to set the zero of the current source, which corresponds to 4mA or 0% on the

DDM, and the span control sets the maximum current, which in our case is 20mA or 100%. Both

are screws that are adjusted using a small screwdriver.

2) Calibration for high accuracy

In order to ensure high accuracy, the current source should be calibrated before every use. Relying

on the last use of the current source is considered bad practice and high attention to detail is

required to maintain reliability. Further, accuracy would most likely require automated or

computerised calibration, instead of human calibration where errors are more likely to occur.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2 4 6 8 10 12 14 16

Vol

tage

(V)

Current (mA)

Current vs Voltage



2.4 Assignment 7: Float Level Transmitter

2.4.1 Practical 1

1) The signal produced by the potentiometer is likely to be very small. Also, being a voltage signal, it

is very susceptible to resistances. These two properties make it difficult to transfer accurate

information over long distances. Whilst converting this to a current signal, accurate transmission

can be done over long distance.

2) The float level transmitter provides information to the controller about the water level in the tank.

This enable feedback control can be carried out if controller uses this information to change the

input or output flow rates to the tank.

3) If the pulse flow sensor and transmitter combination is not producing a 4 to 20mA signal

proportional to the rate of flow, then it indicates that the combination has not been calibrated.

2.4.2 Practical 2 1) Calibration process does the comparison between already known values and observed values

during experiment. As we have used all of the listed instruments for the practical and each

instrument has connection with another so each of these instruments has to be calibrated to rely on

its absolute readings. If they are not calibrated, there would be no guarantee that the correct values

would be generated by these instruments.

2.5 Assignment 8: Pulse Flow Transmitter

2.5.1 Practical 1

1) The pulse flow sensor produces a pulse train which converts to a standard 4-20mA current signal

by using a transmitter and makes it suitable for transmission and compatible with other devices in

the system.

2) If the pulse flow sensor and transmitter combination is not producing a 4 to 20mA signal

proportional to the rate of flow, then it indicates that the combination has not been calibrated.

2.5.2 Practical 2 1) To ensure the accuracy of the measurements we should always do the recalibration before

performing practical every time.

2) We do calibration so all of the instruments i.e. pulse flow sensor and transmitter can provide

accurate information about the rate of flow. This can be used in feedback loop, which would

enable the rate of flow to be controlled, and hence the level in the tank to be controlled.



3) We use different transmitters for different purposes. The pulse flow transmitter is specifically

designed to convert a pulse train signal, whereas the float level transmitter is designed to convert

the potentiometer voltage signal. As both transmitters have different application and output so we

cannot use same transmitter for two similar flow sensors without adjustment for reliable outputs.

Calibration would be required for optimum usage.

2.6 Assignment 9: On/Off Level Control

2.6.1 Practical 1

1) What is on/off control and why is it sometimes preferable to other types of control? What are the

disadvantages of on/off control?

On/off control can maintain the water level automatically. It can control and maintain the output

accurately. It always performs at extreme state as fully open and fully closed, additionally, this

control will be always subject to the unnecessary switching.

2) How is on/off control used in this practical to control the level of the water in the upper tank?

Which pieces of equipment are involved?

In this practical, On/Off control the water level through comparing the feedback signal with the

reference point.

The equipment includes a comparator and Schmitt trigger. The output of the comparator is fed into

the Schmitt trigger, which produces 0V or 5V depending on the deviation between the reference

and measured values. It is these values that turn the pump on or off.

3) What are the meanings of the following terms: reference value, measured value, and deviation?

What do they represent in this practical, how are they produced and how are they used?

• Reference value: The reference value is the desired tank water level. The water level will

be steady when current is steady at this value.

• Measured value: The practical current value or the actual tank water level. This is

determined by the float level sensor, which measures the tank water level and sends a

signal to the transmitter. The transmitter then converts the signal into a standard current

signal, and after being converted into a voltage, becomes the measured input to the

comparator.

• Deviation: The difference between above two values. This is determined by the hysteresis

curve of the comparator, and can be adjusted through a control on the PI.

The desired water level is reference point, and the measured water level are the measured points.

4) What is the significance of manual valve MV3 connected to the upper tank (with reference to the

operation of the on/off level control and how should it be set for correct operation)?



MV3 should be set as open enough for the upper tank that the water level can rise up with pump

on and drop down as pump off. This enables the system to operate around the reference value.

2.6.2 Practical 2 1) Why is on/off control of the pump avoided and a solenoid valve used instead?

This is because On/Off control decreases the pump's working life. A better way of controlling the

tank level is to use a different two-state device known as a solenoid valve. The solenoid valve can

be repeated switching in mind. So, it is suitable for on/off control.

2) Describe the different actions that can be produced from the comparator and Schmitt trigger

arrangement, depending where the reference voltage is connected.

There are two different actions:

• Inverting: The SV1 must be open while the water level is less than desired level, and

closed when more than desired level. The reference signal is connected to the inverting

input and the measured signal to the non-inverting input. When the measured value is

greater than the reference value, the deviation is negative, and will become positive when

the measured value is less than the reference. As the deviation moves from negative to

positive, the output will changes from positive to negative.

• Non-inverting: This action uses solenoid valve SV2, connected to the upper tank. Now,

when the actual tank level is above the desired level the valve should be switched on, so

that more water can flow out. When the actual level is less than the desired level it should

be switched off, so that the water level can rise. The reference signal is connected to the

non-inverting terminal and the measured signal is fed into other input. So, the deviation is

positive when the measured value is less than the reference value, and will become

negative when the measured value is greater than the reference. As the deviation changes

from negative to positive, the output changes from negative to positive.

3) What is being imitated when the water is swished around in the upper tank? How does the

hysteresis level affect this? How does it control disturbance rejection?

Water being rotated in the upper tank, imitates the FLT’s floating disk spinning due to changes in

water level. The hysteresis level controls the width of variation in the water level, increasing the

hysteresis would increase its immunity to disturbances. The disturbances can be rejected by

setting up a period time delay response.

2.6.3 Practical 3 1) Why was it necessary to recalibrate the FLT before beginning this practical?

To ensure the accuracy of the measurements we should always do the recalibration before

performing practical every time.



2) Description of whole process during practical:

• We switch on the PI and keep the water level less than the desired level. Conversion of

signal (4mA) through transmitter ensures that SV1 is open. Water flows into the tank and

level increases.

• The water keeps flowing into the tank and measured level passes the desired level. Barrel

keeps rising from bottom of the stem and system makes no noticeable changes.

• The barrel reaches the top of the stem, closing the relay. Transmitter converts this state to

a 20mA signal. This change closes SV1 and water no longer flows into the tank.

• The water level is greater than the desired level and SV1 is closed, with no input flow,

water level decreases with the barrel restoring its previous position.

• The process goes into the reverse phase. i.e. the measured level becomes lesser than the

desired level. Water continuous to flow out and barrel continuous to going down to touch

the bottom of stem and system shows no noticeable change.

• Finally barrel touches the bottom of the stem and opens the relay. Conversion of signal

(4mA) through transmitter ensures that SV1 is open. Water flows into the tank and level

increases.

• The deviations are bounded by the lower and upper limits of the float switch, and we did

not observe any changes as a result of changing the hysteresis level. If the hysteresis is

greater than the maximum deviation, the system should continue to complete the

hysteresis loop.

3) How does varying the current source (and so the reference voltage) affect this process?

By varying current source we noticed the change in the reference water level.

4) What is happening when the upper tank is shaken?

The system operates as normal when we shake the upper tank unless the shake push the barrel to

the triggering point to process the conversion of signal through transmitter.

3 Conclusion:

In this laboratory, knowledge of the basic processes of a control rig was gained through

hands-on experience with the BPR. This included the working of a valve-pump system and

methods of controlling various parameters. It provided a practical example for a range of

control methods and the application of core concepts in process control.



4 References

• Fernando, T 2012, ELEC3320 Laboratory One, laboratory notes distributed in Process Instrumentation and Control ELEC3320 at The University of Western Australia.

• Grundfos Research and Technology 2005, The Centrifugal Pump. [ONLINE] Available at: http://dk.grundfos.com/content/dam/Global%20Site/Industries%20%26%20solutions/Industry/pdf/The_Centrifugal_Pump.pdf [Last accessed 14 April 2012].

• Thayer, W 1962, Specification Standards for Electrohydraulic flow Control Servovalves. [ONLINE] Available at: http://www.servovalve.co.uk/technical/new_tb117.pdf [Last accessed 15th Apr 2012].

http://dk.grundfos.com/content/dam/Global%20Site/Industries%20%26%20solutions/Industry/pdf/The_Centrifugal_Pump.pdf

http://dk.grundfos.com/content/dam/Global%20Site/Industries%20%26%20solutions/Industry/pdf/The_Centrifugal_Pump.pdf

http://www.servovalve.co.uk/technical/new_tb117.pdf

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