Reference Manual

7−10
and pressure as well as the position of the valve.
Upstream and spraywater enthalpies are then
determined using an inherent steam table within
the DCS. The total spraywater required is
calculated from a heat balance using the final
enthalpy into the condenser. This method of
temperature control is a practical solution for
applications that do not have enough downstream
pipe distance for accurate measurement by a
temperature sensor.
Turbine Bypass Systems
The most severe and critical application of any
steam conditioning installation is that of the turbine
bypass.
The concept of the turbine bypass has been
around for a long time; however, its application
and importance has broadened significantly in
recent years. Steam turbine bypass systems
have become essential to today’s power plant
performance, availability, responsiveness, and
major component protection.
The following will concentrate on the general
application of bypass systems as used in fossil
fueled utility power plants. The closed
water/steam heat cycle of such typical units may
be comprised, but not limited to, sub- or
super-critical pressures, to single, double, or triple
reheat sections and to condensation at or near
ambient temperatures. The steam generating
principles where such bypass systems are
employed include natural or assisted circulation
drum boilers, combined circulation boilers, and
once-through boilers. The turbine may be of
single or double shaft design and operated either
at fixed inlet pressure or on sliding pressure.
Bypass System Benefits
Just how beneficial a bypass system proves to be
depends upon many factors (e.g., plant size,
mode of operation, age of existing components,
size of the condenser, main fuel type, control
philosophy, etc). However, the main benefits for
the application of a comprehensive bypass system
in the 25-100% size range are:
D The matching of steam and heavy turbine
metal component temperatures during the
startup and shutdown phase. This has proven
to be of major economic significance in terms of
fuel savings and the thermal protection of critical
heavy wall boiler and turbine components. By
limiting temperature differentials during turbine
admission the effects of thermal fatigue are
minimized and longevity of components
maximized. This is especially important for life
extension programs where the role and
justification of the bypass system may be centered
solely on this aspect.
D The ability to avoid a boiler trip following
a full load rejection. A boiler (HRSG) / turbine
unit with a bypass can withstand a complete
system load rejection and remain available for
rapid reloading after the disturbance has been
removed. This important advantage for system
flexibility and operating efficiency can make the
difference between a more costly and time
consuming warm start and a hot start.
D Reduction in solid-particle erosion of
turbine components. The loss of material from
the boiler tubing and internals is most prevalent
during commissioning startup and after the unit
has been shutdown for an expended period of
time. Thermal transients assist in the dislodging
of scale, oxides, and weldments within the boiler
circuit to form an abrasive steam flow that, over
time, could accelerate the wear of sensitive
turbine blades and seriously affect operating
efficiencies and maintenance costs. Damage can
be reduced or eliminated by routing the steam
through the bypass system.
D Independent operation of the boiler and
turbine set. The ability to operate the boiler
without the turbine, at any load up to the limit of
the bypass capacity, can be surprisingly useful for
operational or testing purposes. For example, all
boiler controls and firing systems can be tested
and fine-tuned independent of the turbine
operation. This significantly reduces both cost
and time relating to initial commissioning of the
plant, retrofitting and checking equipment
performance, and system troubleshooting.
General System Description
A complete and comprehensive turbine bypass
system can be comprised of many inter-linked and
coordinated components. These include the
bypass valves, spray water control valves, control
system, and the actuation and positioning system.
For this discussion, we will center our attention on
the bypass valves themselves.
The bypass system incorporates the dual
operating function of steam conditioning valves
(i.e., for the controlled reduction of both pressure
and temperature). The bypass valve incorporates