Reference Manual

7−14
This is important in the production of steam purity
before the turbine start. Steam flow through the
superheater and reheater enhances the tube
cooling effect, thereby allowing greater latitude in
gas and steam temperatures. During the startup,
thermal stresses are controlled while achieving the
fastest possible loading rate. Depending on the
size of the bypass system, the unit can typically
be brought on line in 4.5 - 9 hours.
Warm Starts
A warm start is indicative of a weekend shutdown.
In this case, the HP turbine casing is usually
above 450°F. As with the cold start, the steam
temperature can be controlled to permit the
matching of steam and metal temperatures under
all operating conditions. Expected startup time is
between 2.5 - 5 hours.
Hot Starts
A hot start is usually associated with a minor
disturbance that created a unit trip. The bypass
allows the boiler to remain on line until the
disturbance is cleared and the unit can be
reloaded in the shortest possible time, which is
usually between 1 - 2 hours.
Load Rejection/Quick Restart
During load rejection, the bypass system provides
the necessary control and flow path for unit
runback to minimum load and for the
establishment of a definitive course of action (i.e.,
complete shutdown or quick restart). All systems
are protected, and a minimum of condensate is
lost.
Two-Shift Operation
Two-shift operation may become necessary if a
utility grid has a number of large base-loaded
units, which are not as maneuverable as the
smaller fossil fueled units used for peaking
purposes. This would require that the smaller
units be shutdown every night and restarted every
morning, which is a very material-life consuming
means of operating. Once again, the bypass
system provides a means for the efficient and
timely matching of steam and metal temperatures.
This allows the efficient startup of the units every
morning without thermally stressing the
components, yet it increases unit efficiency and
availability.
Chapter 7 — Steam Conditioning
Summary
The implementation of a properly designed turbine
bypass system can be beneficial and instrumental
in the pursuit of increased efficiency, flexibility,
and responsiveness in the utility power plant.
Component life can be extended as the ability to
regulate temperatures between the steam and
turbine metal is enhanced. Commissioning time
and cost can be reduced through independent
boiler and turbine operation. The magnitude of
return on investment hinges on the specific
application mode, style or service of plant, and
equipment supplied. While not discussed here,
this logic applies as well to combined cycle plants,
cogeneration facilities, and industrial power
facilities.
Short Notes:
D A desuperheater is a device that sprays a
precisely controlled amount of water into a steam
line to modify steam temperature.
D System parameters and required turndown
are the most influential parameters in
desuperheater selection.
D Desuperheating is done primarily to improve
efficiency of thermal transfer devices and to
provide temperature protection for process,
product and equipment.
D Another reason to desuperheat is to control
the “unintentional superheat” created by pressure
reduction valves.
D Proper installation is key to best
performance. Guidelines for piping geometry and
placement of downstream temperature sensors
are available.
D Steam conditioning is the process of
combining pressure reduction and desuperheating
into a single control element.
D Turbine bypass systems are beneficial and
instrumental for achieving high efficiency,
flexibility, and responsiveness in today’s power
plants.