Page 40 - Industrial Plant 2015

As a result, the general recommendation for plant In order to control efficiency and hence maximize
operators is, whenever possible, to increase the load of
the HP turbine at the expense of the MP turbine so to
increase overall efficiency.
Load distribution between turbines, although important
for the overall efficiency, must be operated with
caution: it is known from practical experience that the
turbine of HP loses frequency control ability when its
actuator position is over 92%. Similarly, scheme to automatically generate up/down pulses to
Equipment efficiency plant operators should avoid bringing
is quite different for MP turbine to its low limit as this
the two boilers and, condition may also affect overall ability that shifting production between the two turbines has
especially, for the to control frequency.
two turbines. The In addition to that, there is a general
HP turbine is quite target of reducing the average opening
newer compared to of Pressure Reduction Valves (PRVs)
the MP turbine and its between the headers. Although this
specific production, objective is obvious from the energy
expressed as power point of view, in practice the demand
produced divided by for process steam is on average higher
the steam flow inlet, is than steam needed to balance the
significantly higher electric load, so PRVs needs to be
partially open in most cases. Therefore,
if a complete closure of these valves is
not possible, valve-opening minimization is extremely
important and should be pursued at all times.
Combining these practical limitations with the
continuous changes in plant loads and in plant steam
demand renders powerhouse optimization a
demanding task. In practice, Operators tend to
optimize powerhouses only to a certain extent, always
retaining significant safety margins. With no APC
system in place, operators do not update setpoints as
process conditions change continuously, thanks to
these safety margins, but that happens at the expenses
of additional production costs.
Implementing an APC system for supervision and
control of powerhouse along with process to streamline
utility management looked to be the ideal solution to
optimize production. Still, introducing APC in a
Powerhouse is not a trivial task due to several reasons
e.g. fast dynamics and use of complex equipment.
Table 1 APC main CVs Process constraints
variables for
Powerhouse PRVs valve openings
HP turbine actuator
MVs Methane usage
FFs HP steam pressure
MP steam pressure
LP steam pressure
MP boiler output
Attemperators HP / MP / LP
Turbine load distribution
Sugar production load
Industrial Plants - May 2015
38
profits, the APC system should vary HP and MP
turbines generation. However, considering that the
plant operates as an electrical island, it is not
appropriate to move separately HP and MP turbine, as
the total generation must continuously equal the total
electric consumption in the plant.
It is obvious, therefore, that any decrease in MP turbine
generation must counterbalance a similar increase in
HP turbine generation, so that the action of APC
system is neutral to the frequency control.
However, this specific Powerhouse did not allow direct
control of turbine generation due to equipment age.
Turbine generation trim was possible only by means of
up/down signals, adding further complexity to the APC
implemented. The project team implemented a DCS
the two turbines as needed distribute the load in a
balanced way and following APC software requests.
A further hurdle to the power management efficiency is
an effect on the overall power factor for the generators,
which in turn has an impact on the internal grid
electrical losses. So, in order to keep the two generators
with a balanced power factor and optimize the internal
grid efficiency, ABB implemented an automatic trim of
generators output voltage to rebalance power factors
as the APC system acts on the generators active
power output.
The table 1 shows a list of the main controlled and
constrained variables (CVs) and manipulated variables
(MVs).
Project execution
One of the key points in the execution of successful
APC projects is the need for a continuous and
proactive cooperation between the APC supplier and
the customer. In fact, the very nature of this technology
requires that the APC implementation team members
have a good understanding of the process, including
its bottlenecks and basic economics. Although it is
minimize
keep close to a maximum
minimize
strong link to product quality
strong link to product quality