steam safety valves from operating frequently.
A main steam isolation
valve (MSIV) and a main
steam check valve (MSCV) are installed in a series in
each main steam line downstream from the main steam relief and
safety valves. For a failure of the main steam piping, these
isolation and check valves isolate the affected steam generator and
prevent the steam in other steam generators from being lost by back
flow.
In order to continue the plant operation during a rapid load
reduction exceeding the load following capability of the reactor
control system, part of the main steam flows directly to the
condenser through turbine bypass valves to mitigate the reactor
transient consequences, preventing the reactor from tripping. The
total capacity of the turbine bypass valves is determined based on
the maximum allowable design load rejection rate. In Japanese PWR
plants, the capacity of the main steam bypass system is designed to
have a capacity of either 40% or 70% of the full load main steam
flow rate. A plant design with the turbine bypass capacity of 70% is
intended to prevent the reactor from tripping in the event of a
complete loss of its external load, and to be ready to reconnect the
plant to the electrical grid after keeping the plant for a short
period in isolated operation conditions with the plant
in-house loads.
The part of the turbine system from the condenser to the deaerator
tank including the tank itself, is called the condensate
system, and the system from the outlet of the deaerator
tank to the steam generator inlets, the feedwater
system.
The condensed water in the condenser is collected in a
hot-well at the bottom of the condenser shell, and fed to the
condensate pumps.
The water discharged from the condensate pumps flows through the
gland steam condenser and the condensate demineralizer to the
condensate booster pumps. The condensate system pressure is raised
by these pumps as the water flows to the deaerator through the low
pressure feedwater heaters. The feedwater from the deaerator is
pumped to the steam generators through high pressure heaters by
feedwater booster pumps and main feedwater pumps. The feedwater
pressure at the outlet of the feedwater pumps is 7 to 9 MPa [gage].
Condenser
The condenser, not
only increases the turbine steam heat drops by maintaining a vacuum
in the turbine exhaust area, but also condenses the steam, removes
the non-condensable gases and returns the condensate to the
turbine-generator system.
In NPPs, high cooling water flow rates, about 250 kg/kWh, are
required to condense the turbine exhaust steam (cf. about 150 kg/kWh
in fossil fueled power plants). In Japan, the turbine cooling water
is always supplied from the sea. Cooling seawater flows through a
large number of condenser cooling tubes of 25-32 mm diameter and
15-17 m length, to condense the steam on the outside surfaces of the
tubes. The non- condensable gases are collected by a vent pipe
located at the center of a cooling tube bundle, and ejected from the
condenser by a mechanical vacuum pump to the atmosphere.
In the latest PWR plants, to prevent the seawater from leaking into
the condensate flow and to maintain the water quality in the
secondary side of the steam generators, whole-titanium type
condensers are employed instead of the conventional aluminum brass
tube condensers. The cooling tubes and the tube sheets of the
whole-titanium type condenser are both made of titanium which has
excellent corrosion resistance, and the tube sheet joints at both
ends of the tubes are seal-welded.
Seawater leakage into the condensate system is monitored by
measuring the after-cation- exchange conductivity of condensate
samples taken from the hot-well of the condenser (the conductivity
of acidified samples after treating with cation exchange resin can
be measured by the condensate salinometer with high sensitivity). If
the conductivity of the condensate increases, indicating a possible
seawater leakage into the condensate, the leakage point will be
identified
NSRA,
Japan
3~54
Main steam isolation and check valves
Turbine bypass valves
Condensate, Feedwater System and Other Related Systems
Condensate, feedwater system
Chapter
3 Systems
of
PWR Nuclear
Power
Plants
by switching over to each sample extraction points in the hot-well.
When the leakage point is identified, the affected section of the
condenser is isolated and temporarily repaired during the plant
operation. A cut-away view of a typical condenser is shown in Figure
3.5.5.
Feedwater heaters
Feedwater heaters,
constituting heat regenerative cycles, increase the
thermal efficiency of the turbine system by heating the condensate
or the feedwater using extracted steam from the turbine-generator
unit. The number of the feedwater heating stages is between 5 and 7.
One of these stages is a high pressure stage and the rest are low
pressure stages. In a PWR plant, one of the low pressure heaters is
usually a direct contact deaerator type and the others are shell and
tube types and are arranged to get two or three cascade trains
(depending on the plant output). The heat transfer tubes of the
feedwater heaters are usually made of copper alloys (aluminum brass
for low pressure heaters and copper-nickel alloy for high pressure
heaters), and the tube sheets,
shells, tube supporting plates and channel heads of the heaters are
made of carbon steel. Stainless steel tubes with good anti-aging
characteristics are used instead of copper alloy tubes, for heat
transfer tubes in some of the recent PWR plants.
Deaerator
The deaerator is
designed to discharge noncondensable gases contained in the
feedwater to the atmosphere, together with a small amount of steam.
The water storage tank beneath the deareator holds a volume of water
equivalent to that required for a 4 to 7 minute operation of the
turbine-generator unit at its rated power for the purpose of
accommodating emergency shutdown operations or load transients.
The shell of the deaerator is made of carbon steel plate the same as
used for boilers, and the materials of the trays and the spray
nozzles are stainless steel. The configuration of a typical
deaerator is shown in Figure 3.5.6.
Moisture separator and reheater
The moisture separator and
reheater remove moisture contained in the high pressure
turbine exhaust steam and then reheat the nearly-dried
Water chamber
Cooling water inlet nozzle
Cooling water outlet nozzle
Water
curtain spray piping
Turbine
bypass piping
Middle
shell
Shell
Tube
plate
Air
ejecting tube
Low
pressure feed water heater
Stiffening
pipe structure
Air
ejecting tube
Hot
well
(Source]
Mitsubishi Heavy Industries Catalog
Figure
3.5.5 Condenser (example)
Tray
Cooling
tube
Tray drain tube
Tube support plate
3-55
NSRA,
Japan
Vent
nozzle
Tray
bypass tube
[Source]
Mitsubishi Heavy Industries Catalog
Figure
3.5.6 Cut-away view of deaerator (example)
steam with part of the main steam to superheat the steam before it
flows into the low pressure turbine. These processes improve the
plant efficiency, as well as reduce the erosion of the low pressure
turbine blades, exhaust steam piping and low pressure feedwater
heaters. Recent PWR plants employ a two-stage reheating cycle which
utilizes steam flow taken from the main steam line and steam flow
extracted from the high pressure turbine for reheating. The
two-stage reheating cycle improves the plant thermal efficiency as
compared to that of the usual reheating cycle.
The pressure vessel of the moisture separator and reheater is
constructed of the same carbon steel plate as used for boilers, and
the moisture separator and the heat transfer tubes are made of
stainless steel and copper-nickel alloy, respectively.
The configuration of a typical
moisture separator and reheater is shown in Figure 3.5.7.
Gland steam condenser
The gland steam condenser
prevents steam leakage from the turbine gland into the
atmosphere by maintaining the turbine
gland exhaust pressure slightly lower than the atmospheric pressure.
The gland steam condenser is designed to utilize the condensate from
the turbine condenser as its cooling water to improve the overall
heat regeneration rate of the turbine system.
Condensate pumps
The condensate is pumped from the condenser to the low pressure
feedwater heaters and subsequently to the feedwater booster pump by
two 50% capacity condensate pumps. (A third 50% capacity pump is in
a standby condition as their backup). In a plant having a condensate
demineralizer unit in its condensate system, the discharge pressure
of the condensate pumps is usually designed to stay at low level for
the purpose not to raise the design pressure of the demineralizer
vessels, and instead, three booster pumps (one of them is in a
standby condition) are installed downstream from the demineralizer
unit
Feedwater pumps
The feedwater is pumped from the deaerator to the high pressure
feedwater heaters, and subsequently to the steam-generators by
NSRA,
Japan
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