Page 340 - Design of Simple and Robust Process Plants

326 Chapter 8 Instrumentation, Automation of Operation and Control
. MVs are preferred which have a direct effect on the CV. A direct effect on a
CV results in fast responses, which are preferred. The removal of inert from
a condenser works rapidly and more effectively than an increase in the cool-
ing flow.
. Avoidance of recycling disturbances. Disturbances are at best prevented, but
when they are introduced they should leave the process through its exit
streams. When disturbances are recycled or feed streams are manipulated,
they have a larger impact (see disturbance variables).
Measured variables are to be selected by the following guidelines:
. Measured variables must reflect the CVs. Preferably, do not measure the volu-
metric flow when you want to control the mass flow. Temperature measure-
ments representing dual composition in a distillation column require pres-
sure correction.
. Measurement should be reliable, robust, and accurate, and have a sufficient
operational range.
. Location of the measurement should be sensitive to a disturbance. Pressure
measurements should be at the compressor, and not at downstream or
upstream units. Temperature measurements should be installed where varia-
tions are observed at first, such as in a steep temperature transient in a dis-
tillation column, or at the front end of a plug-flow reactor.
. Measurements should minimize time delays and time constants. This is
determined by the type of instrument, the location, and any sampling sys-
tem.
Disturbance variables (DVs) are selected for testing the robustness of the designed
control configuration, but they must be minimized by appropriate action. Distur-
bances can be split in internal disturbances and external disturbances:
External disturbances are:

. Weather conditions, as day and night temperature cycle, humidity variations
for cooling water systems, rain showers. Atmospheric conditions changes
over a longer time period (daily or weekly or even seasonally are not experi-
enced as a disturbance) are not experienced as a disturbance for control of
the process system which mostly have hourly or shorter response times.
. Feed conditions and compositions.
. Utility conditions.
Internal disturbances are:
. Economical disturbances which determine capacity, product distribution,
internal quality set points, equipment scheduling. These are grouped under
internal disturbances as the implementation of modified conditions is under
operational control.