Page 278 - Design of Simple and Robust Process Plants

264 Chapter 7 Optimization of an Integrated Complex of Process Plants
cooling. Electric power generation driven by steam turbines with extraction at differ-
ent levels is a generic solution. One disadvantage, however, is efficiency loss by con-
version to electrical power and re-conversion to mechanical power for the drive of
mechanical equipment. Before a selection can be made, the variability in steam con-
sumption has to be evaluated.
The variability of in the steam system is not a shortage problem, as more steam
can be generated and let-down systems are available. The concern is excess of steam
at the lower steam levels.
Sites are also subject to fluctuation over time. These are caused by:
. Variable capacity operation particular of large, low-level heat consumers.
. Lower efficiency operation of processes as catalyst aging, lower compressor
efficiencies or higher compressor load resulting in more lower pressure
steam, through turbine drivers.
. Higher environmental temperatures, like during a day and night tempera-
ture cycle at summer conditions, resulting in more duty on turbine drivers.
. Upgrading of the process efficiencies at the site may result in less heat con-
sumption particular at the lower levels.
The steam system should have the capability to cope with load variations.
The first rule always to be followed is: design the steam system with enough flex-
ibility to avoid excess at the lower steam levels. Any excess has a high energy penalty.
The options available for the steam system design are:

. Design with a structural shortage at the lower steam levels.
. Redundant mechanical equipment might respectively be driven by a steam
turbine and an electric motor. The advantages are that the drivers can avoid a
common cause failure. By this design the imbalance in the system can be
manipulated by switching between drivers.
. Install steam drivers on steam systems which might fluctuate in load. The
pronounced example is a electrical generator which easily can run on partial
load. This can be specified at the design. A selected turbine driver might later
on revamped by an other wheel configuration to better cope with a new con-
dition in the steam system.
. Install a flexible condensing stage at the turbine drive which can benefit
from any excess low-pressure steam by condensing operation.
. Design steam systems of plants rather independently, but at the selected site
steam levels. This makes the plant less vulnerable, and also avoids the trans-
fer of large steam flows over the site. This is also applicable for steam turbine
drivers, as these are often selected and are preferably closely situated to a
steam-generation facility.

The evaluation and selection of the turbine drivers alternatives must be completed.
Now, the site and process steam balances can be prepared, including all turbine dri-
vers.