ijetae_0113_83

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 1, January 2013)

516

An Intelligent Model Based Level Control of Boiler Drum K. Ghousiya Begum

1, D.Mercy

2, H. Kiren Vedi

3, M. Ramathilagam

4

1,4Assistant professor M.A.M College of Engineering, Siruganur, Trichy

2,3Associate Professor M.A.M College of Engineering, Siruganur, Trichy

Abstract—An Intelligent model is developed to control the

water level in Boiler Drum. There are three different types of

boiler such as Single element boiler drum level control, two

element boiler drum level control and three element boiler

drum level control. The parameters of boiler drum level

control system are determined using PID control tuning

methods such as Ziegler-Nichols method, Tyreus-Luyben

method and Internal Model Control (IMC), in which IMC

surpasses the performance of the conventional controller.

Keywords—Boiler Drum Level control, PID tuning, IMC,

Feed forward, MATLAB.

I. INTRODUCTION

Drum Level Control Systems are used extensively

throughout the process industries and the Utilities to

control the level of boiling water contained in boiler drums

on process plant and help provide a constant supply of

steam The purpose of the drum level controller is to bring

the drum up to level at boiler start-up and maintain the

level at constant steam load. A dramatic decrease in this

level may uncover boiler tubes, allowing them to become

overheated and damaged. An increase in this level may

interfere with the process of separating moisture from

steam within the drum, thus reducing boiler efficiency and

carrying moisture into the process or turbine. Boiler drum

water level control is critical to secure operation of the

boiler and the steam turbine. The functions of this control

module can be broken down into the following

Operator adjustment of the set point for drum

level.

Compensation for the shrink & swell effects.

Automatic control of drum level.

Manual control of the feed water valve.

Bumpless transfer between auto and manual

modes.

Indication of drum level and steam flow.

Indication of feed water valve position and feed

water flow.

Absolute/deviation alarms for drum level.

The most basic and pervasive control algorithm used in

the feedback control is the Proportional Integral and

Derivative (PID) control algorithm.

PID control is a widely used control strategy to control

most of the industrial automation processes. The 3 element

PID control system is introduced to regulate the drum level

with the fixed PID parameters. The control is not ideal

because the gains and time constants of the system

response change significantly with the change in steam

load and disturbances. Therefore, some other controller is

required to improve the performance of drum level control

system [1].

Internal model control is model based controller

structure that provides a suitable framework for satisfying

our objectives. The IMC structure which makes use of a

process model to infer the effect of immeasurable

disturbance on the process output and then counteracts that

effect. The controller consists of an inverse of the process

mode. Using the IMC design procedure, controller

complexity depends exclusively on two factors: the

complexity of the model and the performance

requirements. Furthermore, the proposed procedure

provides valuable insight regarding controller tuning

effects on both performance and robustness.

In process control industry, model-based control strategy

is used to track set point and reject load disturbances. The

proposed work illustrates how to design an IMC controller

for the boiler level control system using MATLAB.

The followings of the paper includes, Section II

describing the types and design of boiler. In Section III

development of virtual laboratory is explained. In Section

IV result and comparison between the boiler performances

is discussed. The paper concludes in Section V.

II. BOILER DRUM LEVEL CONTROL

In the process industries, boiling water to make steam is

a very important procedure. The control of water level is a

major function in this process and it is achieved through a

water steam interface established in a cylindrical vessel

called the drum which is usually lying on its side and

located near the top of the boiler. Maintaining the correct

water level in the drum is critical for many reasons. A

water level that is too high causes flooding of the steam

purification equipment; resulting in the carry over of water

and impurities into the steam system. A water level that is

too low results in a reduction in efficiency of the treatment

and recirculation function.



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It can even result in tube failure due to overheating from

lack of cooling water on the boiling surfaces. Normally

drum level is expected to be held within 2 to 5cm of the

set-point with some tolerance for temporary load changes.

There are several components affecting its operation.

Under boiling conditions, steam supporting field products

such as bubbles exist below the water/steam level interface.

These bubbles have volume and therefore displace water to

create a misrepresentation of the true water level in the

drum. Another effect upon drum level is pressure in the

drum. Because steam bubbles compress under pressure (if

the drum pressure changes due to load demands), the steam

bubbles expand or contract respective to these pressure

changes. A higher steam demand will cause the drum

pressure to drop, and the steam bubbles to expand to give

the appearance of a water level higher than it truly is. This

fictitious higher water level causes the feedwater input to

be shut down at a time when more water is really required.

A surge in water level as a result of the drum pressure

decreasing is called 'swell'. A water level decrease due to

drum pressure increase is called 'shrink'. Providing tight

water level control in a drum is accomplished by utilizing

one of three types of drum level control: single-element,

two-element, or three-element [1].

A. Single element drum level control

Single-level element control uses only the level

measurement and the feed water valve. The controller

responds to a proportional signal from the drum level

transmitters by generating a proportional output to the

boiler feed water valve when needed. This approach is

often used when starting up a boiler and there is no steam

flow or when a flow meter has failed. The drawback of this

strategy is that the level is subject to uncontrolled

disturbances from the steam header and the feed water. For

example, if the feed water header pressure rises, the feed

water flow to the boiler also increases. Without a feed

water control loop, this situation would be uncorrected until

the level changes. In addition, the installed characteristics

of the feed water valve may compromise level control

performance over a large operating range.

B. Double element drum level control

The two-element level control adds the steam flow as a

feed forward element to the level controller output. A

steam mass flow rate signal is used to control the feed

water flow so that feed water demand can be adjusted

immediately in response to load changes. The level

controller is used to correct any imbalance between the

steam mass flow out of and the feed water mass flow into

the drum. This approach delivers more effective drum level

control than a single element.

It is well suited for use on a single boiler with a single

feed water pump using a constant feed water pressure. A

potential weakness is that the installed characteristics of the

feed water valve may compromise level control

performance over a large operating range. In addition,

steam feed forward may need to be characterized when

using this approach. .

C. Three element drum level control

Three-element level control as shown in Fig.1 is the

most common boiler drum level control strategy. A feed

water flow loop slave is added to the two-element strategy.

Three-element level control linearizes the feed water flow

with respect to the steam flow and the level controller

output. The control loop now requests volumetric flow

change, not just a change in the valve position. This

strategy attempts to compensate for changes or

disturbances in steam flow and feed water flow based on

the principle that flow in equals flow out. The installed

characteristics of the feed water valve are no longer an

issue because the flow controller can compensate. Using

this approach, the steam feed forward element can be a

simple gain without requiring characterization [1].

Fig. 1 Three element boiler drum level control.

III. CONTROL STRATEGIES AND SIMULATION

Control strategies are necessary for any system to

perform accurately. Some of these are given below.

D. PID Controller

A Proportional-Integral-Derivative (PID) controller is a

general feedback control loop mechanism widely used in

industrial process control systems. A PID controller

corrects the error between a measured process variable and

the desired set point by calculating the value of error. The

corrective action can adjust the process rapidly to keep the

error minimal.



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The PID controller separately calculate the three

parameters i.e. the proportional, the integral, the derivative

values. The proportional value determines the reaction to

the current error. The integral value determines the reaction

based on the sum of recent errors as past error. The

derivative value determines the reaction based on the rate at

which the error has been changing as a future error. By

tuning these three constants in the PID controller algorithm,

the controller can provide control action designed for

specific process control requirements.

Some applications may require only one or two

parameters of the PID controller to provide the appropriate

control on system. A PID controller will be called a PI, PD,

P or I controller in the absence of the respective control

actions. This is achieved by setting the gain of undesired

control outputs to zero. PI controllers are very common,

since derivative action is very sensitive to measurement

noise and the absence of an integral value may prevent the

system from reaching its target value due to control action

[2].

The simulation results are shown here for different

control strategies. The relationship between the feed water

flow rate and drum level for the boiler process are

expressed by the following equations [3]. The process

function, valve function and disturbance function is shown

below.

Gp(s)= [0.25(-s+1)] / [s(2s+1)] (1)

Gv(s) = 1/[0.15s+1] (2)

Gd(s)=[ -0.25(-s+1)]/ [s(s+1)(2s+1)] (3)

Following are the process used to determine the PID

gain parameter:

1) Ziegler–Nichols Method

This method is introduced by John G. Ziegler and

Nathaniel B. Nichols [8]. In this method, the Ki and Kd

gains are first set to zero. The Kp gain is increased until it

reaches the ultimate gain Ku, at which the output of the

loop starts to oscillate [4]. Ku is found to be 3.51, Pu is 9.8.

Ku and the oscillation period Pu are used to set the gains

as shown in Table 1.

TABLE.1

Z-N PARAMETERS

Ziegler-Nichol

Control Type

Kp Ki Kd

P 0.50 Ku -

PI 0.45 Ku 1.2 Kp/ Pu

PID 0.60 Ku 2 Kp/ Pu Kp Pu/8

Ziegler-Nichol

Control Type

Kp Ki Kd

PID 2.1 0.43 2.57

These gains apply to the ideal, parallel form of the PID

controller. When applied to the standard PID form, the

integral and derivative time parameters Ti and Td are only

dependent on the oscillation period Pu. The step response is

shown in Fig.2 and Fig.3.

Fig. 2 Step input response

Fig. 3 Step response for Ku=Kp=3.51



519

2) Tyreus-Luyben Method

This method is introduced by Tyreus-Luyen. In this

method, the Ki and Kd gains are first set to zero. The P

gain is increased until it reaches the ultimate gain Ku, at

which the output of the loop starts to oscillate [4]. Ku and

the oscillation period Pu are used to set the gains as shown

in Table 2. The Z-L and T-L Matlab Simulink Model and

the response of the two conventional PID controllers is

shown in Fig.4 and Fig.5.

TABLE.2.

T-L PARAMETERS

Tyreus –Luyben

Control Type

Kp Ki Kd

PI 0.3125 Ku Kp/ 2.2Pu -

PID 0.4545 Ku Kp/ 2.2Pu KpPu/ 6.3

Tyreus –Luyben

Control Type

Kp Ki Kd

PID 1.59 0.073 2.47

Fig. 4 Z-L and T-L Matlab Simulink Model

Fig. 5 Response of 2 Conventional PID

3) Internal Model Control (IMC)

The IMC based PID structure uses the process model as

in IMC design. In the IMC procedure, the controller Qc(s)

is directly based on the invertible part of the process

transfer function. The IMC results in only one tuning

parameter which is filter tuning factor but the IMC based

PID tuning parameters are the functions of this tuning

factor. The selection of the filter parameter is directly

related to the robustness [5]. The process model is

considered

Gp(s)= [0.25(-s+1)]/ [s(2s+1)(0.15s+1)] (4)

and correspondingly the controller output is calculated as

Gc(s) = [1.2s2+8.6s+4]/ [s2+3s+4] (5)

The IMC MATLAB Simulink Model and its output

response is shown in Fig.6 and Fig.7. Load disturbance

given and IMC with feed forward Simulink Model and its

output response is shown in Fig.8 and Fig.9.

Fig. 6 IMC MATLAB Simulink Model



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Fig. 7 Output response using IMC

Fig. 8 Load disturbance and IMC with feed forward Simulink Model

Fig. 9 Output response with the load disturbance and of IMC with

Feedforward

Fig. 10 Simulink model of all the controllers

Fig. 11 Comparison of their output responses



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IV. RESULTS

TABLE.3

COMPARING OF VARIOUS TIME DOMAIN SPECIFICATIONS

Controller Time Domain Specifications

Tr Ts %Mp

ZLPID 4 30 75%

TLPID 4.5 40 20%

IMC 4.2 10 0%

IMCFF 4.1 9 0%

V. CONCLUSION

This paper presents a novel design method by

introducing an intelligent model to achieve the expected

output. The comparison between the methods are shown.

Through the simulation all the controllers perform an

efficient search to obtain an optimal solution that achieve

better performance criterion with respect to rise time,

settling time, percentage of overshoot. The use of IMC with

Feed forward controller improves the performance to great

extent than both of these Zeigler-Nichol and Tyreus-

Luyben PID tuning techniques.

REFERENCES

[1 ] Roopal Agrawal, Umesh C. Pati, ―Design and Data Logging of Three Element Boiler Level Control Using LabVIEW‖, National

Conference on Recent Advances in Chemical and Environmental

Engineering (RACEE), Rourkela, Jan 2012 [2 ] Xiang fei, ZOU Li hua, ‖Optimization design of PID controller and

its application”, 2011 Third International Conference on Measuring

Technology and Mechatronics Automation, vol.2, pp. 803-806, Jan 2011.

[3 ] B. Wayne Bequette, Process Control Modeling Design & Simulation, Pearson Education Inc 2003

[4 ] Liu Jinkun, ―MATLAB Simulation of Advanced PID Control[M],‖

Electronic Industry Press, Beijing, 2006, pp. 102-129.

[5 ] I.L.Chien,. and P.S Fruehauf,, ― Consider IMC tuning to improve

controller performance‖, Chemical Engineering Progress, pp. 33 - 41. 1990 .

[6 ] A.M.D. Poar, M. O’Malley, Controllers of Ziegler-Nichols type for unstable processes, Int. J. Control 49 (1989) 1273–1284.

[7 ] W. Tan, Y. Q. Yuan, Y. G. Niu, Tuning of PID controller for

unstable process, in: Proc. of the IEEE International Conf. on Control Applications (CCA), Vol. 1, Hawaii, USA, 1999, pp.121–

124.

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