pid based flow loop control · is passes from tank to rotameter through pipe differential pressure...

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Research Paper

International Journal of Informative & Futuristic Research ISSN (Online): 2347-1697

Volume 2 Issue 7 March 2015

Abstract

In this paper we have proposed to design an PID controller which is used for controlling the flow rate and design PID controller using OPAMP. The closed loop flow control system is basically a feedback control system. Process loop control which used in chemical and petrochemical plants, oil refineries, steel plants, cement kilns, paper mills and pharmaceuticals, waste water treatment plants and the like. The early production process was natural scale up version of the traditional manual practices. In flow control loop various component are used which perform accurately according their function .In flow control loop flow which is to be controlled is passes from tank to rotameter through pipe differential pressure of this flow is sensed through venturimeter or orifice meter .

PID Based Flow Loop Control Paper ID IJIFR/ V2/ E7/ 026 Page No. 2166-2173 Subject Area

Instrumentation &

Control Engg.

Key Words PID Controller, Rotameter, I-P Convertor, Control Valve, Final Control element

Jadhav Vilas K. 1

Assistant Professor Department of Instrumentation & Control Engg. Pravara Rural Engineering College, Loni Ahmednagar - Maharashtra

Anap Tushar 2

B. E. Student Department of Instrumentation & Control Engg. Pravara Rural Engineering College, Loni Ahmednagar - Maharashtra

Gadakh Sachin 3


Dighe Sandip B. 4


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ISSN (Online): 2347-1697 International Journal of Informative & Futuristic Research (IJIFR)

Volume - 2, Issue - 7, March 2015 19th Edition, Page No: 2166-2173

Jadhav V.K., Anap Tushar, Gadakh Sachin, Dighe Sandip B:: PID Based Flow Loop Control

1. Introduction

The closed loop flow control system is basically a feedback control system. Process loop control

which used in chemical and petrochemical plants, oil refineries, steel plants, cement kilns, paper

mills and pharmaceuticals, waste water treatment plants and the like. The early production process

was natural scale up version of the traditional manual practices. In flow control loop various

component are used which perform accurately according their function .In flow control loop flow

which is to be controlled is passes from tank to rotameter through pipe differential pressure of this

flow is sensed through venturimeter or orifice meter .This differential pressure converted into

corresponding electrical signal by differential pressure transmitter, the output of transmitter is given

to through a square root extractor to the PID controller. The controller compares the input signal

with the desired flow signal and generates a corrective output signal in the process control industries

is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or

chemical compounds, to compensate for the load disturbance and keep the regulated process variable

as close as possible to the desired set point. The control valve assembly typically consists of the

valve body, the internal trim parts, an actuator to provide the motive power to operate the valve, and

a variety of additional valve accessories, which can include transducers, supply pressure regulators,

manual operators, Snubber, or limit switches. In our project we are going to use the control valve of

globe type to control the flow as a final control element. The project deals with the flow parameter

which is to be control with given set point, and the controller action by PID controller. The flow

transmitter sends the signal information of flow to PID controller. This is to be passing to control

valve acting as final control element.

To optimize control value, the instruments used in process should be properly installed, like Flow

transmitter used in process is DPT which generate nonlinear output and affects the performance of

system. To increase the performance of DPT we will introduce the square root extractor in the loop

which will generate the linear output. The PID block used as feedback controller which generates the

controlled output and minimizes the error between set point and actual flow from steam. In flow

control loop pneumatic control valve is two way, single seated type. Body of this valve is made of

pneumatic control loop; the output of the controller is given to a current to pressure converter which

gives a corresponding corrective signal to the pneumatic control valve which controls the flow. In

stainless steel, Plug of this valve is of Equal percentage type. In this electric loop, output of

controller is given to the auto manual station that generates 230v pulses to operate the electrical

control valve, which controls the flow. In flow control loop electrically operated control valve is two

way, single seated type. Body of this valve is made of stainless steel. Plug of this valve is Equal

percentage type. Actuator is electric motor. Signal for this is 230v ac. A solenoid valve is controlled

by electrical current, which is run through a coil. When the coil is energized, a magnetic field is

created, causing a plunger inside the coil to move. Depending on the design of the valve, the plunger

will either open or close the valve. When electrical current is removed from the coil, the valve will

return to its de-energized state which controls the flow stream..

2. Flow Loop Control Flow control loop system is basically a feedback control system. Differential pressure is sensed

through a venture meter or orifice meter .This differential pressure is converted into a corresponding

electrical signal by a differential pressure transmitter.

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Figure 2.1: Process Flow Diagram

Elements of Flow control loop using PID controller:

A. Rotameter

B. Orifice

C. Venturi meter

D. Differential pressure transmitter

E. PID controller

F. I/P converter

G. Control valve

As shown in fig.2.1 flow control loop system is basically a feedback control system. Differential

pressure is sensed through a venture meter or orifice meter .This differential pressure is converted

into a corresponding electrical signal by a differential pressure transmitter. The output of the

transmitter is given through a square root extractor to the PID controller. The controller compares

the input with the desired flow and generates an corrective output signal. In the pneumatic loop, the

output of the controller is given to a current to pressure converter which gives a corresponding

corrective signal to the pneumatic control valve, which controls the flow. In the electrical loop, the

output of the controller is given to the auto manual station that generates 230 v pulses to operate the

electrical control valve, which control the flow.

In solenoid valve operation are also shown when the flow goes out of limits. The controller can be

tuned by varying the values of proportion band width, integral and derivation times. an I/P converter

is used to convert a 4-20 mA signal to 3-15 psi .This is mainly used for operating control valve by

using PID controller. I/P converter is force balance device in which a coil is suspended in the field of

magnet by flexure .In this flow control loop control valve is used is pneumatically operated

diaphragm valve ,type of this valve is two way single seated and equal percentage type. Electrically

operated control valve is also two way single seated and equal percentage types.

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3. Hardware Configuration:

Under this topic we discuss about the devices used by us in the loop, their construction details,

operational features and safety considerations.

A. Orifice Plate

The orifice plate is the most common form of restriction that is used in flow measurement. An

orifice plate is basically a thin metal plate with a hole bored in the centre. It has a tab on one side

where figure 3.1 shows a representative orifice plate. The specification of the plate is stamped. The

upstream side of the orifice plate usually has a sharp, edge when an orifice plate is installed in a flow

line (usually clamped between a pair of flanges), increase of fluid flow velocity through the reduced

area at the orifice develops a differential pressure across the orifice. With an orifice plate in the pipe

work, static pressure increases slightly upstream of the orifice (due to back pressure effect) and then

decreases sharply as the flow passes through the orifice, reaching a minimum at a point called the

vena contract where the velocity of the flow is at a maximum. Beyond this point, static pressure

starts to recover as the flow slows down.

Figure 3.1. Cross-Section of Orifice Plate

However, with an orifice plate, static pressure downstream is always considerably lower than the

upstream pressure.

B. Flow Transmitter

Figure 3.2. Flow Transmitters

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It helps to transmit the signal containing the necessary information of flow towards control

station. Differential pressure transmitter (DPT) is vastly used in process industries as flow

transmitter. DPT works on the principle, when fluid flow through pipe the orifice plates

generates pressure difference which bends diaphragm towards low pressure. This displacement

is measured with the help of LVDT and amplified to standard signal range of 4 to 20 mA using

conditioning circuit. ROSEMOUNT 3051 pressure transmitter is having time tested reliability,

the ROSEMOUNT 3051 is the industry standard.

C. Square Root Extractor

Figure 3.3. Connections for Square Root Extractor

Up to now, our flow measurement loop can be represented by the installation shown in Figure 9. The

high and low-pressure taps of the primary device (orifice type shown) are fed by sensing lines to a

differential pressure (D/P) cell. The output of the D/P cell acts on a pressure to mili-amp transducer,

Figure: Connections for Square Root Extractor which transmits a variable 4-20 ma signal. The D/P

cell and transmitter are shown together as a flow transmitter (FT).

D. PID Controller:

The P stands for proportional control, I for integral control and D for derivative control. This is also

what is called a three term controller. The basic function of a controller is to execute an algorithm

(electronic controller) based on the control engineer's input (tuning constants), the operators desired

operating value (set point) and the current plant process value. In most cases, the requirement is for

the controller to act so that the process value is as close to the setpoint as possible. In a basic process

control loop, the control engineer utilizes the PID algorithms to achieve this. The PID control

algorithm is used for the control of almost all loops in the process industries, and is also the basis for

many advanced control algorithms and strategies. In order for control loops to work properly, the

PID loop must be properly tuned. Standard methods for tuning loops and criteria for judging the loop

tuning have been used for many years, but should be reevaluated for use on modern digital control

systems.

While the basic algorithm has been unchanged for many years and is used in all distributed control

systems, the actual digital implementation of the algorithm has changed and differs from one system

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to another. Controllers are designed to eliminate the need for continuous operator attention. Cruise

control in a car and a house thermostat are common examples Error is defined as the difference

between set-point and measurement. (error) = (set-point) - (measurement).The output of PID

controller will change in response to the error

Figure 3.4: Working Diagram of PID Controller

With proportional band, the controller output is proportional to the error or a change in

measurement. Drawbacks -with a proportional controller offset is present. Increasing the controller

gain will make the loop go unstable. Integral action was included in controllers to eliminate this

offset. With integral action, the controller output is proportional to the amount of time the error is

present. Integral action eliminates offset. Integral action gives the controller a large gain at low

frequencies,

4. Tunning Of Controller

Setting of control parameters to Ziegler/Nichols Determine the operating point Set the controller to

‘Manual’ and determine the possible control range by means of changing the correcting variable.

Select the operating point so that the controller has sufficient "reserve" in both modulation

directions, (e. g. in the Centre of the control range Configure the controller as a P controller: To do

this, set Tn at the highest possible value (9999.) and TV at 0.Determine the critical amplification Kr

(stability limit, closed control loop is in the process of carrying out continuous oscillation), by

analyzing small set point step-changes around the operating point after each newly set amplification.

This determines the critical amplification factor KKR and the period of oscillation Tk of this

continuous oscillation.

There are three method of tuning:

a) Ziegler-Nichols Method

b) Frequency Response Method

c) Open loop transient Method

5. I To P Convertor:

In this project the pneumatically operated control valve is connected to the current to pneumatic

convertor. This Ito P convertor connect the control valve to the process the main advantage of this

instrument is to operate the valve with current signal which is generated by the controller i.e. PLC

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and it separates the hazardous and non-hazardous area. The basic construction detail of I-P convertor

is, it has two types of connection one from electronic controller and other from storage pressure tank

with constant pressure 20psi.

6. Final Control Element:

Final control element used in the flow loop control is control valve .Most of the time control

designer prefers use of control valve rather than the variable speed pump. Because in a liquid

system, inertia is significant; flow cannot be started or stopped without accelerating or decelerating.

To decompensate the dynamic character of inertia i.e. by reducing the time constant therefore, flow

controlled by speed variation will always have a higher time lag than the same flow controlled by a

valve.

Figure 6.1: Final Control Valve

7. Implementation

Figure 7.1:Connection Diagram of Flow Loop Control

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An orifice plate is basically a thin metal plate with a hole bored in the center. It has a tab on one side

where the specification of the plate is stamped. The upstream side of the orifice plate usually has a

sharp, edge. An orifice plate generates pressure difference. Differential pressure transmitter (DPT) is

vastly used in process industries as flow transmitter.

8. Conclusion

We have studied the working of differential flow meter, DPT, PID controller, control valve etc. We

have understood the tuning of PID controller. Learn the electrical wiring and connection of control

panel. Thus we had controlled the flow loop system by using PID Controller.

References

[1] Liptak, Bela. Instrument Engineers' Handbook: Process Control. Radnor, Pennsylvania: Chilton

Book Company. pp. 20–29. ISBN 0-8019-8242-1, 1995

[2] Curtis D. Johnson‘Process control and instrumentation technology’ PHI publication

[3] K. Astrom and T HagglundPID Controllers: ‘Theory, Design, and Tuning’, 2nd Edition Publisher:

Instrument Society of America ISBN 1-55617-516-7

[4] Douglas M. Considine ‘Process / Industrial Instruments & Controls Handbook’

Fourth Edition Publisher: McGraw-Hill, Inc.ISBN 0--07--012445--0

pid based flow loop control · is passes from tank to rotameter through pipe differential pressure...

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