Most utility systems have common elements, including similar “handles” or variables for
optimization. In particular, many steam system elements are common from site to site.
Steam venting and steam letdown from a higher to a lower pressure header are
typical and are often useful optimization handles that can provide significant savings.
Since letdowns and vents are not typically under the direct control of operators, changes
to other equipment under the operators’ direct control are required in order to affect them.
A common way to change the letdowns and vents is through switching between
pairs or groups of steam-driven and electric motor-driven pumps in shared services. By
selecting the most appropriate of these machines to run at any given time, the operator
can do much to minimize vents and letdowns. For instance, starting a steam turbinedriven
pump to replace a motor-driven pump can reduce an open steam letdown, if the
inlet and exhaust headers are the same for both the turbine and letdown. The benefit is
seen in reduced electric power consumption. This concept is discussed in more detail in
Chapter 18.
Figure 19.3 illustrates the reduction in letdown and vent provided by site-wide
optimization. The numbers next to the bold gray lines indicate the difference in flow
between the starting point and the optimized solution recommendations. The turbine-driven BFW Pump 4 replaces motor-driven Pump 3. Net 600 psig steam export goes down by
20 klb/h, and 600-150 psig steam letdown is reduced by 29 klb/h.
Other common optimization “handles” include steam producers, fuel sources, steam
turbines, and a number of cogeneration plant variables.
On-demand steam producers, such as auxiliary boilers and duct firing on heat
recovery steam generators, can provide excellent handles for reducing cost site-wide
through boiler-to-boiler rebalancing for maximum system efficiency. When multiple fuel
options such as refinery fuel gas and natural gas are available, the optimization can
negotiate the complexities of differential pricing and constraints to find real-time
operational savings. Even complex constraints are respected, such as the requirement
to consume all refinery fuel gas (discussed in Chapter 20) to avoid flaring.
Steam turbines can provide excellent handles for optimization if their operation can
be adjusted. For example, a steam turbine generator with a manual set point provides the
optimizer with an opportunity to react to changes in the incremental fuel versus
electricity pricing. So, if the price of electricity increases in comparison to the price
of fuel, the steam turbine may be a cost-effective option for either reducing site-wide
power purchases or increasing power sales.
Even extraction/condensing turbines with a constant horsepower requirement to
the process can provide savings by rebalancing the horsepower between the high- and
low-pressure sections of the turbine with the adjustment of other steam equipment in
manual control.
Cogeneration systems with gas turbines and heat recovery steam generators can
react to changes in fuel versus electricity purchase and sales pricing if the power
generation is in manual control. Excess steam or water injection for power augmentation,
inlet air chilling, and combustion air recirculation all provide additional flexibility in
cogeneration operation and handles the optimization system can use to increase the
savings it provides through real-time recommendations