DEPARTMENT OF ENGINEERING

CE1.6 INTRODUCTION TO PROCESS CONTROL

TONY RICHARD AQUINO DE SOUZA

201300033

01/12/2013

2

Abstract

This report introduces the basic methods of process control, using final

control elements (valves and pumps), transmitters and controllers to control the

flow rate. Three experiments were conducted using a process control unit to

control pressure, heat and to find the time constant. The results were satisfactory,

being close to those found in theory.

3

Table of Contents

Abstract .............................................................................................................. 2

Table of Contents ............................................................................................... 3

Introduction......................................................................................................... 4

Theory Background ............................................................................................ 4

Flow control ..................................................................................................... 4

Level control .................................................................................................... 4

Pressure control .............................................................................................. 5

Time constant ................................................................................................. 5

Materials and Procedures .................................................................................. 6

Description of the experimental unit ................................................................ 6

Figure 1. Apparatus for the process control experiment. ............................. 6

Experiment 1 ................................................................................................... 7

Figure 2. Connections for Experiment 1. ...................................................... 7

Experiment 2 ................................................................................................... 7

Figure 3. Connections for experiment 2. ...................................................... 8

Experiment 3 ................................................................................................... 8

Figure 4. Connections for experiment 3. ...................................................... 9

Results and Discussion .................................................................................... 10

Experiment 1.6-1 Heat Loop Pump Flow Characteristic................................ 10

Figure 5. Relation between the flow rate (L/min) and the pump 1 voltage

(V). ............................................................................................................. 10

Experiment 1.6-2 Level Control of the Process Vessel ................................. 11

Figure 6. Calculation of the time constant using the software .................... 12

Experiment 1.6-3 Pressure Control of the Process Vessel ........................... 13

Figure 7. Calibration curve for the pressure transducer. ............................ 13

Figure 8. Pressure transducer (PT) against level transducer (LT) plotted by

the software. .............................................................................................. 14

Conclusion........................................................................................................ 15

References ....................................................................................................... 16

4

Introduction

Process control is a key subject in the industrial engineering. A good and

stablished controled process can provide a safety process plant, as well as

increase the efficiency, generating more profit to the company. Over the years,

the techniques of process control improved, such as automatic control valves,

pressure and temperature sensors, among others, allowing the industries to

make more and more products.

Theory Background

Flow control

The control of the flow rate can be achieved using two types of final control

elements: pumps or control valves. The pumps control the flow using propulsion,

while the valves control the flow retaining the fluid [1]. The flow in a process vessel

can be controled using a sensor or transmitter (FT), which will detect the flow rate

going into the tank and convert the data into eletrical signal, sending this out to

the flow controller (FC), Both methods allow the control to be done changing the

position, speed or the process loop/feedback, being possible to mix them

sometimes.

Level control

The level in a process vessel can be controled using a sensor or

transmitter (LT), which will detect the height of fluid in the tank and convert the

data into eletrical signal, sending this out to the level controller (LC), which will

convert the eletrical signal into mechanical reaction, adjusting the level [1][2]. It

is possible to adjust the level controlling the flow rate in different ways, such as,

5

adjusting the process loop valve position, adjusting the process loop pump speed,

adjusting the drain valve at the base of the tank or mixing these methods.

Pressure control

The pressure can be monitored through a pressure sensor or transmitter

(PT), which measures the pressure inside a process vessel when it is isolated

from the atmosphere. The signal is then sent to a pressure controller (LC) that

will regulate the pressure[1][2]. This adjustement can happen through different

ways, such as, opening vents to release the pressure, increasing the air flow to

raise the pressure and modifying the temperature to adjust the pressure.

Time constant

In engineering, it is important to characterise the response to a step input

in a system invariant in time. This parameter is known as time constant, which is

the time taken (in seconds), once the variation has started, to achieve 63.2% of

the total variation. In the process control, it can be read as the 63.2% of the time

required to go from one steady state to another [2][3]. The time constant is given

by Equation 1:

= (1)

Where A is the area of the process vessel (m) and R is the outflow

resistance, given by Equation 2:

=

(2)

Where is the difference of the steady state level (m) and the is the

difference of the flow rates (m/s).

6

Materials and Procedures

Process Control unit and control module

Digital computer with the CE2000 installed

Description of the experimental unit

The unit is composed of a process vessel with a heater exchanger

connected to an air vent (top) and a drain valve (bottom). The Pump 2 removes

water from the reservoir to the process vessel via the cooler. There is also a

bypass valve between the Pump 2 and the cooler, which allows the fluid to go

straight to the reservoir without passing through the cooler. The control systems

are a level transmitter at the left side of the process vessel; a flow transmitter at

the right side of the process vessel and at the top of Pump 1; a flow switch at the

left side of the tank to control the flow in the reservoir; a pressure transmitter at

the top of the module to measure the pressure inside the process vessel and a

thermal switch in the right side of the heater tank. The apparatus is shown in

Figure 1:

Figure 1. Apparatus for the process control experiment.

7

Experiment 1 In the computer, the CE2000 software was opened and the exp 1-1.ict

from folder CE2000/CE117 loaded.

The control module was connected to the unit, as shown in Figure 2. The

pump 1 switch was set to External.

On the software, the pump 1 voltage was set to 0 V and the experiment

started.

After the voltage stabilised, the pump voltage was increased from 0 V to

10 V by 1 V steps, recording the data on the software and in the lab

notebook.

Using the software features, a graph was plotted.

Figure 2. Connections for Experiment 1.

Experiment 2 In the computer, the CE2000 software was opened and the exp 1-1.ict

fom folder CE2000/CE117 loaded.

The control module was connected to the unit, as shown in Figure 3. The

pump 2 switch was set to External.

The loop bypass valve was closed and the process vessel drain and air

vent fully opened.

8

On the software, the pump 2 voltage was set to 3.4 V (voltage which the

level was just above the top of the heat exchanger for 10 minutes) and

the valve voltage to 10 V.

After 10 minutes, the height of the water and the flow rate was recorded

as Level A.

The pump voltage was increased to 3.9 V and monitored until it

stabilised. The height of the water and the flow rate was recorded as

Level B.

Using the software features, a graph was plotted and the time constant

calculated.

Figure 3. Connections for experiment 2.

Experiment 3 In the computer, the CE2000 software was opened and the exp10-1.ict

from folder CE2000/CE117 loaded.

The control module was connected to the unit, as shown in Figure 4. The

cooler fan was set to minimum and the heater set to External. The pump

2 valve was switched to Manual.

The reservoir was filled, the process loop bypass valved and the drain

valve closed and the stirrer was switched off.

9

Using the manual control for pump 2, the level of water was increased

until it reached a height of 50 mm. The PT and LT values were recorded.

By 10 mm steps, the air level was increased and the values of PT and LT

were recorded.

Observation: the pump could not be turned off during the experiment,

preventing pressure loss.

Using the sotware features, a graph of pressure against level was

plotted.

Figure 4. Connections for experiment 3.

10

Results and Discussion

Experiment 1.6-1 Heat Loop Pump Flow Characteristic The Figure 5 shows the chart plotted by the sotfware of the flow transmitter

reading (L/min) against the pump 1 voltage (V).

Figure 5. Relation between the flow rate (L/min) and the pump 1 voltage (V).

Pump 1 voltage (V) 0 1 2 3 4 5 6 7 8 9 10

Flow transmitter reading (L/min) 0 0 1,6 2,7 3,5 4,2 4,7 5,1 5,4 5,7 6

Table 1. Measurements of flow rate with variations in the voltage.

11

The pump flow (blue line on graph) showed more a logarithmic relation

than a linear relation. While the R for the trend line considering it is logarithmic

is 0.9985, the R for a linear trend line is only 0.9137.

At voltage of 2 V is when the flow rate has the highest variation (from 0

L/min at 1 V to 1.6 L/min at 2 V). On the hand, it is at voltage of 6 V when the

variation of the flow rate starts to decrease (variation of 0.5 L/min), reaching the

lowest variation at 8 V, 9 V and 10 V (variation of 0.3 L/min). This happens

because the pression exerted by the pump is no longer as effective as it is at a

smaller voltage.

Experiment 1.6-2 Level Control of the Process Vessel

Internal radius of the process vessel= 150 mm

Table 2. Calculation of the time constant using the variables

The difference between the time constant found doing the calculations

(106 s) and the one using the software (130 s) is most because of a possible

misuse of the software feature, selecting incomplete levels of steady state.

Flow rate

(volt)

Flow rate

(L/minute)

Flow rate q

(m/sec)h (m)

Level A 3.4 3.4 0.0000567 0.114

Level B 3.9 3.9 0.000065 0.164

8.3E-06 0.05

0.017671459

6024.096386

(s) 106.4545703

Difference

= (m)

=

12

Figure 6. Calculation of the time constant using the software

13

Experiment 1.6-3 Pressure Control of the Process Vessel

Figure 7. Calibration curve for the pressure transducer.

The results showed that with the increase of the pressure (increase of

pressure voltage), the level of water in the process vessel increases (increase of

the level voltage). The relation between these variables seems to be linear, with

a correlation coefficient (R) of 0,9697, as shown in Figure 7. The graph plotted

by the computer proves the linear relation between PT and LT, as shown in Figure

8.

y = 1.0167x - 3.8704R = 0.9697

0

0.5

1

1.5

2

2.5

3

3.5

4

4 4.5 5 5.5 6 6.5 7 7.5

Pre

ssu

re t

ran

sdu

cer

volt

age

(V)

Level transducer voltage (V)

14

Figure 8. Pressure transducer (PT) against level transducer (LT) plotted by the

software.

15

Conclusion

The results showed that the relation between the pump flow and the pump

voltage is not linear (logarithmic trend line), while for the pressure transducer and

pressure controller, the relation is linear. The time constant can be calculated

manually or using a computer, but the manipulation of the data in the steady level

must be done precisely. Overall, the process control aims to enhance the

efficience of the processes by controlling the variables (pressure, flow rate) based

on the experimental data.

16

References

[1] Seborg D.E., Mellichamp D.A., Edgar T.F., Doyle F.J, Process

Dynamics and Control, John Wiley & Sons, 2010.

[2] Ogata K, Modern Control Engineering, Prentice Hall, 2010.

[3] Griffiths D.J., Introduction to Electrodynamics, Addison-Wesley

Professional, 1988.