Ratio Control (比值控制 )

Lei Xie

Institute of Industrial Control, Zhejiang University, Hangzhou, P. R.

China

2013/04/17

Contents

Concept of Ratio Control Design of Ratio Control Schemes Cross-limiting Control of Air/Fuel

Ratio in a Boiler or Furnace Summary

Blending of Two Liquid Streams

Problem: when the flow rate of NaOH stream varies, it is required to maintain the concentration of blended liquid at 6 ~ 8% by manipulating the water flow rate.

Schemes:

(1) Concentration control of the blended liquid------Feedback control

(2) Flow ratio control of input streams-----Feedforward control

Blending Tank

6~8% NaoH

30% NaoH H2O

QAQB

General Ratio Control Problem

Problem: to manipulate the flow rate of stream A to maintain the two streams in a constant ratio QA / QB = KAB, as the flow rate of stream B varies.

Stream B often referred to as wild flow.

DownstreamProcess

Stream BQB

QAStream A

max

max1

A

BAB Q

QKK

Ratio Control Scheme #1

K1

FC

QA

QB

IB

IA

Steady-state condition:

,max

1max B

B

A

A

Q

QK

Q

Q

B

AAB Q

QK

Suppose both of the flow transmitters are linear. Sometimes, they are nonlinear

mAIKII BA 4)4(10

Ratio Control Scheme #2

1max

max2

11

KQ

Q

KK

B

A

AB

K2 FC

QA

QB

IB

IA

Steady-state condition:

,maxmax

2B

B

A

A

Q

Q

Q

QK

B

AAB Q

QK

mAIKII AB 4)4(20

Ratio Control Scheme #3

Steady-state condition: IA = I0

max

max

max

max

A

BAB

A

B

B

A

Q

QK

Q

Q

Q

QK

For DDZ-III instruments, the output of the multiplier is

mAIKI B 4)4(0 I B K

QA

QB

IA

FC

×I0

Ratio Control Scheme #4

Steady-state: K4 = IA / IB

max

max

max

max4

A

BAB

A

B

B

A

Q

QK

Q

Q

Q

QK

RC

IA / IB

I B K4

QA

QB

IA

÷

There are strong nonlinearity in flow loop of stream A. Why?

mAI

II

B

A 44

40

For DDZ-III instruments, the output of the Divider is

Double Closed-loop Ratio Control Scheme

QA

QB

IB

IA

×

FC16

FC17

QB sp

QA spK1

Ratio control Simple control

QA

QB

IB

IA

FC16

FC17

QB sp

QA sp

Compare

Variable Ratio Control Scheme

T2

RV

FC

T2sp

RVsp

TC

Steam

Process Fluid

Condensate

RF

T1

Cascade Control Scheme

PV

Steady-state Equation is

Fp

VVRc

RHTT 12

F

V

pV

RRu

ucHTT

,/12

If main disturbances are from PV, RF; how can you improve control performances ?

T2

RVRF

FC

T2sp

RVsp

TC

× Steam

Process Fluid

Condensate

u

Cascade Ratio Control Scheme

T1

PV

Basic Boiler Control Problems

LT41

LC41

Steam

Boiler Feedwater

Stack Gases

Air

Fuel FC

FO

Pm

PSP

PT 22

PC 22

LSP

Design your control systems to satisfy

(1) Safety. Level & air/fuel ratio control

(2) Meet the need of users. Steam pressure control

(3) effective & economic burning. Air/fuel ratio & keep air-rich

Air/Fuel Ratio in a Boiler Control Scheme #1

Parallel control with manually adjusted air/fuel ratio

Background

Existing problem ?

LT LC

×

Steam

Boiler Feedwater

Stack Gases

Air

Fuel FC

FO

Pm

PSP

PT 22

PC 22

FC 23

RF

FT 23

FC 24

FT 24

RAKAF

RF, SP

RA, SP

Air/Fuel Ratio in a Boiler Control Scheme #2

Ratio control with manually adjusted air/fuel ratio

FC 23

FT 23

×

AirFuel FC FO

PSP

P

Stack

Steam

RF RAFC 24

FT 24

PT 22

PC 22

KAF

RF, SP

RA, SP

Air/Fuel Ratio in a Boiler Control Scheme #3

Ratio control with manually adjusted air/fuel ratio

Which difference from Scheme #2 ?

Existing problems ? FC 23

FT 23

AirFuel FC FO

PSP

P

Stack

Steam

RF

RA

FC 24

FT 24

PT 22

PC 22

×R4

KFA

RF, SP

Air/Fuel Ratio Control Scheme #4

FC 23

FT 23

AirFuel FC FO

PSP

P

Stack

Steam

RA

FC 24

FT 24

PT 22

PC 22

×

LS

HS

PC22.OP

FC23.SP

FC23.PVFC24.SP

FC24.PV

KFA

Cross-limiting control (双交叉控制 ) with manually adjusted air/fuel ratio

Explain its principle

Air/Fuel Ratio in a Boiler Control Scheme #5

Cross-limiting control with O2 trim

（带有 O2调节的双交叉控制）

FC 23

FT 23

AirFuel FC FO

PSP

P

Stack

Steam

RF RA

FC 24

FT 24

PT 22

PC 22

×

LS

HS

AT 25

AC 25

ASP

%O2

PC22.OP

FC23.SP

FC23.PV

FC24.SP

FC24.PV

KFA

Summary

Concept of Ratio Control Design of Ratio Control Schemes Cross-limiting Control of Air/Fuel

Ratio in a Boiler or Furnace

Exercise 6.1It is required to dilute NaOH liquid with water.

Assume that the transmitter span of NaOH liquid is 0 ~ 30 T/hr, the transmitter span of water flow is 0 ~ 120 T/hr, and both transmitters are linear DDZ-III instruments with 4 ~ 20 mA DC output. In addition, K is a ratio calculation unit which output satisfies

(1) Obtain the functional relation of K value and flow ratio F1/F2;

(2) Determine the value of K if the mass concentration of NaOH liquid is 20% and it is required to be diluted into 5% with water.

(3) Obtain the change range of I2 and I3 if F1 varies in 10 ~ 20T/hr.

3 24 ( 4)I K I mA K FC

F2

F1

I1

I2

NaOH liquid

Water

I3

Exercise 6.2

Problem 5-2 (p.112) in Automated Continuous Process Control. In detail,(1) Select the action of control valves (failed-open or failed-close) and determine the action of controllers (direct or reverse action); (2) Design your logical ratio control scheme and show it directly in process diagram; (3) Explain the automatic control process when we want to increase the methane required.

Next Topic

Override/Constraint Control Problem

Design of Constrain Control Systems

Reset Windup and Its Prevention in Constrain Control

Selective Control Schemes