Chemical Engineering June 2012 - 47

Source
3
Source
1 piping
Source
1
Source
2
Source
2 piping
Source 2
piping out
Source 1
piping out
Common
header 1
Mix100
Combined
Pri
source
Source
3 pipe
User A
pipe
Source
3 out
Total production
Common
header
out 1
Mix-101
Common
header
pipe 2
FIGURE 2. This example is used to demonstrate a steadystate
model fl owsheet (Pri used in the fi gures stands for
primary)
Common
header
out 2
Master PC
Rate limiter 1
PV value
24.94 $
OP value 4.985e+004 lb/h
Rate limiter 1
Source
1 piping
Source
1
Source
2
Rate limiter 2
Rate limiter 2
PV value
24.94 $
OP value 4.985e+004 lb/h
Source
2 piping
Source 1
piping out
Common
header 1
Source 2
piping out
Mix100
Combined
Pri
source
Temperature
Pressure
Master
PC
SP 300.0 psig
PV 298.7 psig
OP 24.94 %
OP 24.94 %
OP 24.94 %
Source 3 FC
SP 50000 lb/h
PV 50000 lb/h
OP 50.00 %
Rate limiter 3
Source
3 FC
Source
3
Source
3 pipe
Source
3 out
Common
header
out 1
Combined Phi source
475.0 °F
298.7 psig
Mass flow 9.970e+004 lb/h
Common
header
out 2
FIGURE 3. A fl owsheet that is ready for dynamic mode is illustrated here
generally not able to risk a major
shutdown in order to test system responses
to the upsets of interest. The
next best option is to model the system
dynamically. The dynamic model
becomes a testing platform on which
control concepts can be proven and
adjusted if necessary [3].
Dynamic process simulations fill
the gap between different steadystate
operating cases, showing a
more complete picture of system behavior.
Using the knowledge gained
during the investigative process, a
model can be constructed that will be
useful for testing the system under
changing conditions.
Example. Suppose the system to be
modeled consisted of three sources and
two users of steam. The steady-state
model flowsheet may look like Figure
2. In this example system, two sources
of steam exist on one end of a main
steam header, while a third source sits
close to the process user areas.
This same simulation flowsheet can
be adapted for dynamic evaluation by
adding some basic controls as shown
in Figure 3.
Using this source-sink model of a
steam distribution header, some of
the aspects of the system behavior
can be explored. The system may have
two design cases that result in different
steam balances. The steady-state
model gives a snapshot of what is
happening when everything is stable.
Table 1 shows what these data may
look like.
Switching to a dynamic analysis
gives a more complete picture of the
system behavior in the time between
the two operating cases. At this point,
the previous consultation with operators
who understand the system
comes into play. Using the knowledge
To
users
B
Temperature
Pressure
Total production
Temperature
Pressure
475.0 °F
298.6 psig
Mass flow 1.497e-005 lb/h
Total production
Mix-101
Common
header
pipe 2
To users A
TEE-100
User B
pipe
User B pipe out
475.0 °F
298.5 psig
Mass flow 4.991e+004 lb/h
gained from the operators, the design
engineer must account for the time
factors involved in transitioning from
normal to alternate operation. For this
example, users in Area B are reducing
demand to reach the alternate operating
mode. Through consultation with
the operators, it becomes clear that
this demand reduction normally takes
place over a 2-min period. Figure 4 is
a graph of what this may look like in a
dynamic simulation.
Starting from a steady state corresponding
to normal operation, the
demand reduction begins at 120 seconds.
The User B demand is ramped
steadily downward for the prescribed
two minutes. The source-steam flow
controllers initiate a corresponding
reduction in steam production
to maintain the system balance.
This production decrease is typically
achieved through some type of master
CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2012 43
User A
pipe
User A
FC
User A
pipe out
User A
FV
User B FC
SP 5.000e+004 lb/h
PV 4.988e+004 lb/h
OP
24.84 %
User B
FC
User B
pipe out
User B
FV
Sink
B
Sink
A
User A pipe out
Temperature
Pressure
474.8 °F
297.6 psig
Mass flow 5.977e-004 lb/h
User A FC
SP 1.000e+005 lb/h
PV 9.973e+004 lb/h
OP
50.03 %
To users A
TEE-100
To
users
B
User B
pipe
User B
FC
User B
pipe out
User B
FV
Sink
B
User A
FC
User A
pipe out
User A
FV
Sink
A
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Chemical Engineering June 2012

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