POWER August 2013 - 55

PLANT DESIGN
Plant performance test results. Source: SME
Case
Output (MW)
Existing plant
Simple cycle
Combined cycle, no duct firing
60
85
125
Combined cycle, max. duct firing 150
psig. The exhaust flow was designed between
518,000 and 499,000 lb/hr at 3 inches HgA,
depending on the extraction flow to flash
evaporator. HP steam inlet flow decreased
by 3%; however, exhaust flow increased approximately
30% when the extraction ports
were secured and with the addition of the LP
steam admission.
Steam Turbine Bypass. The plant is
equipped with a 100% HP and LP steam turbine
bypass system, which directs steam to
the condenser rather than venting to atmosphere
during startup and trip conditions. The
bypass system required modification of the
existing condenser as well as the addition of
engineered HP and LP steam bypass valves.
During startup conditions, after condenser
vacuum is established, the bypass valves
control steam flow to the turbine based on
turbine shell temperature and bypass the remaining
steam flow to the condenser. During
trip conditions, the bypass valves direct
100% steam flow to the condenser and prevent
a steam system overpressure event that
can result in lifting of safety valves.
Control Components Inc. (CCI) provided
the bypass valves chosen for the steam byNA
42%
108%
150%
Increase
over
existing plant
(%)
Heat rate (Btu/
kWh)
11,700
10,500
7,000
7,400
Decrease
from existing
plant (%)
NA
10%
40%
36%
pass system. The HP bypass valve is configured
as a 10-inch inlet to 18-inch outlet
pressure-reducing valve with exterior steam
ring attemperator. The valve is designed to
reduce approximately 310,000 lb/hr of steam
at 931F/750 psig to steam at 417F/175 psig.
The LP bypass valve consists of a 10-inch inlet
to 10-inch outlet pressure-reducing valve
with integral steam attemperating nozzle.
The valve is designed to reduce approximately
64,000 lb/hr of steam at 521F/91 psig
to steam at 358F/70 psig. Both the LP and HP
bypass valves are located approximately 100
feet from their respective condenser inlets.
De Laval Turbine Inc. furnished the condensers
that were originally installed in 1968.
They are single-shell, two-pass, divided condensers
with an LP feedwater heater located
in the condenser neck. Thermal Engineering
International performed a mechanical design
evaluation and concluded that each of the
condensers could be retrofitted with a 100%
HP and LP steam bypass valve. An 18-inch
HP bypass line with 336 holes (½-inch) and
a 10-inch LP bypass line with 264 holes (3/8inch)
were designed to fit in the condenser
transition and pass 400,000 lb/hr and 70,000
lb/hr, respectively. A 3-inch spray water curtain
line was designed to fit in the condenser
above the bypass lines to keep temperatures
below 200F to protect the condenser expansion
joint and steam turbine.
Structural modifications were made to the
condenser transition to facilitate additional
loading from the bypass lines. Tube stakes
and steam impingement protection were
added to the tube bundles for reducing tube
vibration and for protection from increased
steam velocity from the bypass lines.
Performance Goals Met
Performance testing for the combustion turbines
in simple cycle operation was completed
in November 2011 and combined cycle
performance testing was completed in April
and October 2012, respectively. The table
shows the results of that testing for different
operating cases. The performance goals for
the project were met: Plant output increased
150% and the heat rate decreased 39%.
The additional operational flexibility provided
by the combustion gas bypass system
and the steam turbine bypass system now
permits stable operation from 55 MW to 150
MW for each of the two repowered units. In
addition, the capacity addition has increased
the SME's overall system efficiency so that
its members will enjoy reasonably price power
for decades to come. ■
-Joseph W. Mashek (jmashek@burnsmcd.
com) and Benjamin L. Frerichs (bfrerichs@
burnsmcd.com) are business development
managers for Burns & McDonnell. Chris K.
Rhodes (crhodes@smepa.coop) is generation
projects manager for South Mississippi
Electric Power Association.
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