Rosemount OCX 8800, Instruction Manual

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Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-45
•
Local Override is a non-target mode that instructs the block to revert to
Local Override when the tracking or fail-safe control options are
activated.
•
Out of Service mode disables the block for maintenance.
Abrupt changes in the quality of the input signal can result in unexpected loop
behavior. To prevent the output from changing abruptly and upsetting the
process, select the
SP-PV Track in Man I/O option. This option automatically
sets the loop to Manual if a Bad input status is detected. While in manual
mode, the operator can manage control manually until a Good input status is
reestablished.
Application Examples
Basic PID Block for Steam Heater Control
Situation
A PID block is used with an AI block and an AO block to control the flow
steam used to heat a process fluid in a heat exchanger. Figure 7-15
illustrates the process instrumentation diagram.
Figure 7-15. PID Function Block
Steam Heater Control
Solution
The PID loop uses TT101 as an input and provides a signal to the analog
output TCV101. The BKCAL_OUT of the AO block and the BKCAL_IN of
the PID block communicate the status and quality of information being
passed between the blocks. The status indication shows that
communications is functioning and the I/O is working properly. Figure 7-16
illustrates the correct function block configuration.
TCV
101
TC
101
Steam Supply
TT
100
TT
101
Steam Heater
Condensate
3
8
7
4
0
0
9
5