Page 81 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Power Equipment and Systems 59

and superheat the steam. On the other end of the process, the condenser produces as
much vacuum as possible, which is a function of the condensing temperature. Some very
large plants draw cold ocean or deep lake water. Still most of the steam energy is in the
phase change from vapor to liquid and is thus only partially available to a condensing
steam turbine.
Noise/Vibration
Steam turbines, like combustion turbines, experience high-frequency noise and rota-
tional vibrations. Noise from a steam turbine is generally around 85 dBA or less,
which requires hearing protection when spending an extended time near the equip-
ment. Noise from steam flow in pipes and the operation of pumps may actually be of
greater concern.
Controls
Controls for steam turbines are relatively simple and involve controlling the steam flow
to match the load, as well as protecting the STG from adverse operating conditions such
as loss of lube oil pressure. Starting systems for steam turbines can be as simple as an
actuator that is commanded or programmed to open a steam valve and to close the
valve to stop the turbine’s operation. Governor systems are used to control the rota-
tional speed of a steam turbine to ensure the consistent quality of the power generated
as well as to stop the steam in the event of loss of control. Most turbines have automatic
turbine trips and a manual emergency trip, for example, on loss of lube oil pressure.
Equipment Life, Operation, and Maintenance
The useful life expectancy for steam turbines depends on the number of run hours,
maintenance practices, and feedwater (steam) quality. Well-maintained steam turbines
operated with steam generated from good quality water can potentially have a life
expectancy exceeding 30 years.
While steam turbines, themselves have very high availability rates (typically
99 percent), steam turbines can be off-line for various other reasons including, but not
limited to, boiler or HRSG outages, CHP fuel delivery issues, boiler feed pump outages,
and piping leaks, for example. Steam turbines also must be off-line for regularly scheduled
maintenance. Overhauls and internal inspections may require turbines to be off-line for
between 150 and 350 hours every 18 to 36 months. As with reciprocating engines and
combustion turbine generators, good predictive maintenance can lead to reductions in
steam turbine generator outages.

CHP Prime Mover Comparisons
Reciprocating engines, gas combustion turbines, fuel cells, and steam turbines all have
various advantages and disadvantages when compared to one another. This section
compares the characteristics of the various prime movers available for installation in
CHP plants.

Electrical Output and Electric Efficiency
The electrical output of the various prime mover technologies ranges from a few
kilowatts with the microturbine to hundreds of megawatts that can be delivered by a
steam turbine.