Rosemount OCX 8800, Instruction Manual

Page: 147 / 266

Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-47
3
8
7
4
0
0
9
8
Figure 7-18. PID Function Block Diagram for Feedfoward Control
Outlet
Temperature
Input
BKCAL_IN
BKCAL_OUT
TT100
Cascade Control with Master and Slave Loops
Situation
A slave loop is added to a basic PID control configuration to measure and
control steam flow to the steam heater. Variations in the steam pressure
cause the temperature in the heat exchanger to change. The temperature
variation will later be sensed by TT101. The temperature controller will
modify the valve position to compensate for the steam pressure change.
The process is slow and causes variations in the product temperature.
Figure 7-19 illustrates the process instrumentation diagram.
Figure 7-19. PID Function Block
Cascade Control Example
AI
Function
Block
OUTOUT IN
FF_VAL
PID
Function
Block
CAS_IN
AO
Function
Block
OUT
TT101
TC101
TCV101
Inlet
Temperature
Input
AI
Function
Block OUT
FC
101
TC
101
FT
101
TCV
101
Steam
Supply
TT
100
TT
101
Steam Heater
Condensate
3
8
7
4
0
0
9
9