Ratio Control ( 比值控制 )

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

General Ratio Control Problem Problem: to manipulate the flow rate of stream A to maintain the two streams in a constant ratioQA/ QB= KAB, as the flow rate of stream B varies. Stream B often referred to as wild flow.

Ratio Control Scheme #1 Suppose both of the flow transmitters are linear. Sometimes, they are nonlinear Steady-state condition:

Ratio Control Scheme #2 Steady-state condition:

Ratio Control Scheme #3 For DDZ-III instruments, the output of the multiplier is Steady-state condition: IA = I0

Ratio Control Scheme #4 For DDZ-III instruments, the output of the Divider is Steady-state: K4 = IA / IB There are strong nonlinearity in flow loop of stream A. Why?

Double Closed-loop Ratio Control Scheme Compare Ratio control Simple control

Variable Ratio Control Scheme If main disturbances are from PV, RF; how can you improve control performances ? Steady-state Equation is

Basic Boiler Control Problems • Design your control systems to satisfy • Safety. Level & air/fuel ratio control • Meet the need of users. Steam pressure control • 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 ?

Air/Fuel Ratio in a Boiler Control Scheme #2 Ratio control with manually adjusted air/fuel ratio

Air/Fuel Ratio in a Boiler Control Scheme #3 Ratio control with manually adjusted air/fuel ratio Which difference from Scheme #2 ? Existing problems ?

Air/Fuel Ratio Control Scheme #4 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）

Summary • Concept of Ratio Control • Design of Ratio Control Schemes • Cross-limiting Control of Air/Fuel Ratio in a Boiler or Furnace

Exercise 6.1 It 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.

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

Ratio Control ( 比值控制 )