Page 327 - Design of Simple and Robust Process Plants

8.3 Automation of Operation 313
. Differentiate between alert and alarming levels
. Prevent alarm ªshowersº by:
± assigning alert/alarms to each operational state and transient operation;
± using an early warning for preventive actions;
± analyzing control programs for potential situations of alarm showers, specifi-
cally during trips and ESD; and
± keeping operators alert by asking them to carry out certain activities.
The alarm levels are shown in Figure 8.14, where the different alerting/alarming
levels ± request, alert, pre-alarm, and alarm ± are shown in relation to the control
levels.
Requests are primarily implemented to involve operation; typical requests are a
permission to start with a certain transient operation (not described here):

. Recipe change for a batch process
. Switch from one operational set-up to another; for example, lining up adsorp-
tion beds from regeneration to adsorption
. Start of a transient operation, including a shut-down sequence
. Start of a critical unit, such as a reactor or a furnace
Other requests can be formulated to enforce operation to certain actions, such as
checking the performance of a certain unit, or analyzing the frequency signal of a
measurement to diagnose for pump or valve cavitation.
Alerting is applied to notify operation of deviations between a measurement and a
set-point, while the process is still in its normal operational range. This is often
applied during a run state to warn the operators of a deviation. The alerting alarms
are typically step-dependent. These are also applied during a transient deviation, but
with a wider range to prevent over-alerting for what is normal in that transition.

Physical protection
Interlocking

Alarm
Alarming
Max. Oper. range
Pre alarm
Normal Oper. range
Alert
Alerting
Request
Set point

Control Alerting/alarming

Fig. 8.14. Alerting/Alarming levels in relation to control levels.