Reference Manual

7−13
alternatives are also available for the TBX sparger
(figure 7-13).
Control of the HRH and LP bypass valves
normally is initiated via feedback input signals
from the hot reheat steam pressure and the
specified condenser inlet temperature/enthalpy.
The steam entering the condenser must be
controlled specifically to guard against excessive
thermal expansion of the tubing and shell. As in
the case of the HP bypass, the ratio of steam to
spray water is normally inversely proportional,
especially during startup and shutdown. In
addition, the dual role of the HRH and LP bypass
system in controlling the thermal admission
parameters to the condenser normally results in
the requirement for a prescribed amount of
over-spray.
This situation is compounded by the close
proximity of these valves to the condenser. This
makes any kind of feedback temperature control
almost impossible considering the quantity of
spray water to be vaporized and the short distance
available to measure the process. It is highly
recommended that feedforward control algorithms
be incorporated into the control system to provide
independent feedforward control for the
spraywater admission.
Spraywater for cooling is normally obtained from
the condensate boost pump discharge and is
regulated by a properly sized external spraywater
control valve.
Bypass Size
A comprehensive bypass system includes HP
bypass, HRH bypass and LP bypass valves.
However, they may or may not be sized for the
same capacity. There are many variables that can
influence the required size of each bypass system.
Bypasses for once-through boiler plants are
generally designed for 100% of full-load steam to
suit startup and part-load operation. If
conventional safety valves are omitted, 100%
bypass capacity is essential.
Bypass capacity for drum boiler plants involve
several different issues. Some argue that 100%
capacity bypasses are worthwhile, but experience
has proven that bypasses with capacities of
between 25 - 70% normally are sufficient to handle
most operating and trip conditions.
For temperature matching in a drum plant during
hot startup only, it may be possible to use a
bypass of only 30% when firing with oil and
40-50% for coal. Overall, these values are
considered the lowest practical load for the boiler
under automatic control.
On bypass applications requiring the control of a
full turbine trip, the values increase to 40% on gas
and oil-fired drum units and up to 70% for coal. In
selecting the bypass capacity, it is important to
consider all control systems and plant components
and their ability to turn down instantaneously from
full to auxiliary load.
Note also that if the high pressure bypass capacity
exceeds approximately 50%, and the low pressure
bypass passes all the steam to the condenser,
then condenser duty during bypass operation is
more severe than during normal, full-load turbine
operation. This fact may limit bypass capacity,
especially on systems being retrofit to existing
plants.
Starts, Trips, Load Rejection,
Two-Shift Operation
The worth of a turbine bypass and the flexibility,
added efficiency, and responsiveness are never
more apparent than during starts, trips, or load
rejections. Modern bypass systems operate
during:
D Cold starts
D Warm starts
D Hot starts
D Load rejection
D Quick turbine shutdown
D Two-shift operation
Bypass valves and systems that are designed
correctly have noteworthy advantages for these
individual modes. They are detailed as follows:
Cold Starts
A cold start typically occurs after the unit has been
down for over a week. Preheating of the system
is required as first stage and reheat temperatures
are normally below 200°F. The bypass system
permits involvement of the furnace, superheaters,
and reheater very early in the steam/water cycle.