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Coal
Proper Steam Bypass System Design Avoids Steam Turbine
Overheating
Combined-cycle power plants operate by integrating a combustion turbine generator
(CTG) and heat recovery steam generator (HRSG) with a steam turbine generator (STG).
Clarion Energy Content Directors
6.1.2003
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By: S. Zaheer Akhtar, P.E., Bechtel Power Corporation
Combined-cycle power plants operate by integrating a combustion turbine generator
(CTG) and heat recovery steam generator (HRSG) with a steam turbine generator (STG).
However, during some modes of plant operation – e.g., start-up, STG trip, simple-cycle
operation – it is desirable to isolate the STG from the CTG/HRSG. The isolation of the
CTG/HRSG from the STG is facilitated by means of the steam bypass system.
Without a bypass system, the steam generated in the HRSG has to be discharged to the
atmosphere until the STG is available to accept steam. Steam is discharged by means of
vent valves and/or atmospheric dump valves (sky valves) installed on the HRSG steam
headers. The dumping of steam to atmosphere is not desirable as it results in loss of
valuable condensate and also raises environmental concerns due to noise pollution.
On some combined-cycle plants, the isolation of the CTG/HRSG from the STG is provided
by the application of an HRSG bypass damper and bypass stack. In this case steam
generation in the HRSG can be controlled/eliminated by venting the CTG exhaust gas
through the bypass stack. This scheme facilitates simple-cycle operation and avoids
condensate loss, but it is capital intensive and presents the problem of noise pollution.
Modes of Operation
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The steam bypass system is generally used during the following modes ofo
operation:
start-up and shutdown, steam turbine trip, steam turbine no-load or low-load operation,
and simple-cycle operation.
On start-up, the isolation of the CTG/HRSG from the STG allows the CTG to be placed on
load without delay and well before the heat-up and roll-off of the STG. In addition, a faster
start-up of the STG is possible since the bypass system provides the capability of close
temperature matching between the steam inlet temperature and the steam turbine
metal temperature. This is achieved by continuous steam dumping to the condenser
until the optimum temperature, pressure and flow requirements are achieved for
starting and loading the steam turbine.
For a combined-cycle plant with multiple CTGs/HRSGs and a single steam turbine, the
bypass system allows for a sequential startup of the CTGs. If one or more CTGs are
already on-line and an additional CTG/HRSG needs to be brought on-line, the start-up
bypass system facilitates start-up by allowing the gradual warming of the lagging HRSG
and allowing for steam temperature matching between the leading and lagging HRSGs.
The steam bypass system is also used when the CTG/HRSG is up on load while the steam
turbine is off-line or under no-load/low load conditions. Because STG start-up takes
much longer than CTG start-up (due to larger mass of metal that needs to be gradually
heated prior to start-up), the STG could be at no-load or low-load conditions while the
CTG is at a significantly higher load. This results in excess steam generation from the
HRSG that is diverted to the bypass system.
During shutdown, the steam bypass system enables the steam turbine to be taken offline independent of the operation of the combustion turbine. The combustion
turbine/HRSG remain loaded while the steam generated in the HRSG is gradually diverted
to the condenser through the bypass system. In case of a steam trip, the bypass system
is placed in full service immediately. However, in a controlled shutdown, the STG load is
gradually reduced and excess steam generation is diverted to the bypass system. The
steam turbine can then be tripped at a reduced load and subsequently isolated. If the
steam turbine is isolated while the CTG/HRSG load is comparatively high, the steam
turbine metal temperatures remain high. This enables the steam turbine to be ready for a
hot start, which is preferable because it minimizes start-up time and start-up stresses in
the turbine metal.
The steam bypass system can also be used to operate a combined-cycle plant in simplecycle mode before the combined-cycle portion of the plant is completed. However, this
assumes that the CTG and HRSG are in place and a heat sink (condenser) is available for
condensing the steam passing through the bypass system.
Sky valves and electromatic relief valves (ERVs) dump steam to the atmosphere and can
be used to supplement the capacity of the steam bypass system. The response time of
the ERVs is faster than the conventional, pneumatically operated sky valves and are
o theSubscribe
typically used to prevent the boiler safety relief valves from lifting. However,
drawback with both the sky valves and the ERVs is that their use results in loss of
valuable condensate and generates noise concerns.
Steam Bypass Configurations
Two types of steam bypass systems are generally used in combined-cycle power plants:
Parallel bypass (also known as “direct” bypass or “dry” reheater bypass)
Cascade bypass (also known as “European” bypass or “wet” reheater bypass) with or
without the “Start-up Bypass”
The choice of the bypass system is based on economics and the characteristics of the
steam turbine. Most modern plants use the cascade bypass system.
The bypass system consists of steam bypass piping, a steam conditioning station, and a
dump tube to the condenser. The steam conditioning station consists of a pressure
reducing valve and an attemperator supplied with spray water from the condensate
pump or the feedwater pump discharge. The dump tube (sparger) is installed
downstream of the steam conditioning station at the condenser inlet.
Parallel Bypass
Click here to enlarge image
In this arrangement (Figure 1), the steam generated at start-up in the HP drum and the IP
drum of the HRSG is sent directly to the condenser after being attemperated with spray
water from the condensate pump discharge. As such there is no flow through the
reheater, which operates “dry” when the bypass system is in service. The steam
generated in the LP drum can be similarly disposed of to the condenser but this is
generally not required for CTGs that do not use rotor air cooling for LP steam generation.
Instead the LP steam production is suppressed by bypassing the condensate pre-heater
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and introducing the cold condensate directly in to the LP evaporator or de-aerator
(if the
HRSG is designed with an integral de-aerator). Any excess steam that may be produced is
vented to the atmosphere through the vent/drain valves on the HRSG LP section and LP
steam piping.
With the reheater operating in the “dry” mode at start-up, the reheater tube metal tends
to overheat. As a result the metallurgy of the reheater tubes has to be enhanced, which
adds cost to the HRSG.
Another drawback of the parallel bypass system is that it requires the use of long lengths
of steam piping from the HRSG to the condenser. With the pressure letdown valve and
attemperating station located close to the condenser, most of the bypass piping for HP
steam is expensive alloy piping that adds to the capital cost of the plant.
In a combined-cycle plant with a multiple train configuration (2x2x1 or 3x3x1), the parallel
bypass pipe lengths increase with the number of HRSGs installed. To minimize capital
cost, the parallel bypass piping from multiple HRSGs is often combined into a single line
with a single steam conditioning station. With such a design, the overall capacity of the
bypass system is curtailed, introducing operational constraints during STG trip/start-up.
For example, it may not be possible to (i) keep all the CTGs on baseload following a STG
trip, or (ii) start-up the lagging CTG/HRSG without decreasing load on the leading
CTG/HRSG.
Cascade Bypass
Click here to enlarge image
In this arrangement (Figure 2), the HP steam generated at start-up is bypassed around
the HP section of the turbine to the cold reheat (CRH) line. The bypass line is equipped
with a pressure reducing valve and an attemperator using spray water from the feed
water pump discharge. The bypassed steam then mixes with the steam from the IP drum
and sent through the reheater. The hot reheat line (HRH) is provided with another
pressure reducing/attemperating station that directs the steam to the condenser.
The
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attemperating station on the HRH line uses spray water from the condensate pump
discharge. With this system, a continuous flow is maintained through the reheater (“wet”
reheater mode) that provides a cooling effect for the reheater tube metal. As such,
upgraded metallurgy is not required for the reheater tubes.
The cascade bypass system uses comparatively short lengths of steam piping. This is
because the HP to CRH bypass piping is located near the HRSG while the HRH to
condenser piping is located near the condenser. These pipe lengths are not dependent
on the distance between the HRSG and the condenser, which tends to increase
proportionally with multiple HRSGs.
Since the cascade bypass utilizes spray water from the feed water pump as well as the
condensate pump, some additional energy debits are incurred. Note that the parallel
bypass system uses only the condensate pump discharge for source of spray water and
hence the energy debit is less.
The main disadvantage of the cascade bypass is that the STG has to start up with a
pressurized reheater, which raises concerns about overheating of the HP turbine backend, especially during low flow conditions experienced during start-up.
Cascade Bypass fitted with a Start-up Bypass System
Click here to enlarge image
Several methods have been devised to alleviate turbine back-end overheating problems
during start-up. One is to keep the reheater pressure low by equipping the cascade
bypass with a start-up bypass system (Figure 3). The start-up bypass connects the HP
turbine exhaust to the condenser from upstream of the CRH line non-return valve. The
non-return valve in the CRH line isolates the HP turbine from the pressure in the reheat
section. This way even though the reheater is at a high pressure, the HP turbine exhaust
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pressure is low (at around the condenser pressure). The drawback of the o
start-up
bypass
is added capital cost.
HP Turbine Back-End Overheating Concerns
The HP turbine back-end and the cold reheat piping are generally made from carbon
steel, which experiences degradation due to graphitization at temperatures above 800 F.
If this temperature limit is expected to be exceeded, the construction material needs to
be upgraded to low alloy steel, thereby increasing the capital cost. To keep the
temperature below 800 F, the turbine steam inlet parameters parameters can be
adjusted, as discussed below under “Overheating Control Parameters.”
When high-temperature conditions exist at the HP exhaust, the turbine is unable to
extract sufficient energy from the steam and convert it to work. As a result, the HP
turbine exhaust temperature remains high, resulting in overheating at the back-end.
High exhaust pressures at the HP turbine back-end also result in higher windage heating
during periods of low steam flow (as experienced during start-up).
To start the steam turbine with the cascading bypass configuration, the reheater
pressure must be maintained at a low value as recommended by the STG vendor.
Typically, the maximum reheater pressure should be about 20 percent of rated pressure
during a cold start and at 35 percent of rated pressure during a hot start. Somewhat
higher reheat pressures may be possible by lowering the main steam temperatures, but
this should be based on STG vendor recommendations.
If the maximum reheater pressure cannot be limited to the recommended value during
start-up, and HP turbine back-end temperatures are expected to exceed the 800 F limit,
then other control parameters (discussed in next section) should be evaluated and
adjusted as required. If the overheating problem still persists, then a start-up bypass can
be considered along with the cascade bypass system.
Some steam turbines that operate with the cascade bypass configuration utilize a startup procedure by rolling off on the IP turbine. In this process, the steam turbine is rolled
and synchronized with the intercept valves. The HP turbine remains isolated and is
cooled by back-flow of cooling steam through the CRH line. The cooling steam is then
vented through a bleed line/valve to the condenser. Reverse flow through the HP turbine
continues until the turbine is placed on minimum load. The turbine is then switched over
to forward flow through the HP section. Even with this arrangement, low reheat pressure
is important to avoid overheating due to abbreviated steam expansion and windage
heating of the HP turbine back-end during the transition to forward flow.
Overheating Control Parameters
As discussed earlier, the main control parameter for controlling HP turbine exhaust
temperature at start-up is the HP back-end pressure. Other parameters that affect the
HP turbine exhaust temperature include HP turbine inlet steam enthalpy, steam flow
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through the HP turbine, and rate of ramp-up allowed on the HP turbine. For
reheat
oa given
system pressure and a fixed inlet steam flow to the HP turbine, the higher the inlet steam
enthalpy, the higher the HP turbine exhaust temperature. Also, the higher the steam flow
through the HP turbine, the lower the HP turbine exhaust temperature.
At a fixed inlet steam temperature, the enthalpy increases as the steam pressure is
reduced. High enthalpy of inlet steam to HP turbine is experienced during various modes
of operation, such as 1) single train operation of a 2x2x1 plant or 3x3x1 plant and 2) unfired
HRSG operation with STG designed for fired HRSG operation. In these scenarios, the HP
steam inlet is at a relatively lower pressure with normal operating temperature resulting
in high enthalpy steam. Based on this, the start-up conditions require that the steam
pressure should be above the minimum “floor” pressure.
As mentioned above, the inlet steam enthalpy is determined by the inlet steam
temperature and pressure. The inlet steam temperature is usually based on the turbine
rotor metal temperature, which determines the type of start procedure (hot start, warm
start or a cold start). A cold start requires lower steam inlet temperatures to the steam
turbine as compared to a hot start. As a result the problem of HP back-end overheating
during a cold start is not as severe as in the case of a warm start or a hot start. Generally
a warm start is more critical than a hot start from the viewpoint of HP turbine
overheating. A hot start allows fast ramp-up rates, enabling the steam flow through the
turbine to increase rapidly, negating the overheating effect at the HP turbine back-end.
In cases where the steam turbines roll-off on the HP turbine, the HP control valve
regulates the flow through the HP turbine to control the exhaust temperatures. During a
hot start, when inlet steam temperature is high, a high flow through the HP turbine is
maintained. On a cold start, when inlet steam temperature is low, a lower flow through
the HP can be sustained based on the HP exhaust temperature.
Steam Bypass System Design Considerations
At a minimum, the steam bypass system should be designed for the start-up cases (cold
start, warm start, hot start) and the load rejection (STG trip) case.
In the basic design, the bypass system should be designed to handle the steam
generated in the HRSG during start-up and prior to STG roll-up/synchronization. At this
point the CTG will be operating at a stable load point with HRSG steam conditions
suitable for admission to the steam turbine. Generally it is expected that the STG will be
rolled-up/synchronized with CTG load not in excess of 40 percent for any type of start
(hot, warm or cold). At the same time the reheater pressure will be at around 20-35
percent of rated pressure. The bypass system should be checked to ensure that it can
handle the steam flow associated with the low reheater pressure and the start-up load on
the CTG.
In the load rejection (STG trip) case with no reduction of CTG load, the reheat system will
operate at rated pressure and the bypass system will be required to passo
the normal
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operating steam flow. However, since the steam flow will be attemperated, the total
mass flow rate through the bypass system will be somewhat higher due to the spray
water flow. Under these conditions, the condenser pressure could be higher than normal
since the STG is not in service. However, on STG re-start, the condenser pressure will
have to be reduced by cutting back on the CTG load.
If plant operation requires the bypass system to cope with other plant specific operating
scenarios, the bypass system and condenser should be designed accordingly. Specific
scenarios include: 1) bringing a second train in operation with the first one already
operating at a high load, 2) operating the CTG/HRSG at high loads while the steam turbine
is down or at no-load/low load operation, or 3) placing the STG in service after a trip
without reducing load on the CTG(s).
In summary, there are several parameters that can affect the HP turbine exhaust
temperature. These parameters should all be evaluated and adjusted to avoid high
exhaust temperatures at HP turbine outlet. The use of low alloy steel construction at the
HP exhaust or installing a start-up bypass in addition to the cascade bypass incur higher
capital cost and should be used only after determining that adjustments to the inlet
steam conditions and reheater pressure will not solve the problem. Also, the bypass
system design basis should be clearly defined and take into consideration all operating
scenarios expected to be included in the plant design.
Steam Bypass in the field
The Intergen North America Standard Plants used at the Cottonwood (Texas), Redbud
(Oklahoma) and Magnolia (Mississippi) sites feature 1x1x1 configurations utilizing GE-7FA
combustion turbine generators followed by triple-pressure-level HRSGs and axial
exhaust steam turbine generators. All units are equipped with a start-up bypass line
(from the HP turbine exhaust to the condenser) to control HP turbine exhaust
temperatures at start-up. The HRSGs are duct fired, and in the unfired condition, the HP
steam inlet enthalpies are high, resulting in HP turbine exhaust temperatures exceeding
750 F during normal operation. This temperature tends to be even higher during start-up
when flow rate is low, approaching the 775-800 F limits for carbon steel. The start-up
bypass valve and line (14-inch NPS) maintain the start-up temperatures below the carbon
steel limits by reducing the HP exhaust pressure at start-up to less than 50 psig. A desuperheating station in the start-up bypass line reduces the steam enthalpy to around
1200 Btu/lb (which includes some superheat) at the condenser inlet.
The closing logic of the start-up bypass valve is based on estimating the temperature
after the valve is closed. If the estimated temperature is less than 800 F, the valve will
close. Also, the lower the reheater pressure, the lower the minimum flow requirement
through the HP turbine, which means that the start-up valve can close sooner.
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The LaRosita Power Plant in Mexico is a 3x3x1 configuration utilizing Siemens
o
Westinghouse W501F combustion turbine generators followed by triple-pressure-level
HRSG and a steam turbine generator. The plant experienced problems with overheating
of the exhaust during hot start only. Instead of using a start-up bypass line, the problem
was mitigated by maintaining low reheater pressure and low steam enthalpy at the HP
turbine inlet during hot start-up. To keep the steam enthalpy at low levels, the plant
increased the start-up steam pressure to around 1450 psig, while maintaining the
reheater pressure below 160 psig. In addition, LaRosita relied on flow trimming control,
which adjusts the flow ratio between the HP control valve position and the intercept
control valve position, to increase the HP turbine flow and limit the HP exhaust
temperature.
The GE design for the 3x3x1 power island at the Hsin Tao Power Plant in Taiwan and
several other steam turbine generator vendor designs have overcome the start-up
problem of HP exhaust overheating by starting the STG with steam admission to the IP
turbine instead of the HP turbine. Forward flow steam admission to the HP turbine is
introduced only after the STG is synchronized and placed on load with steam to the IP
turbine. The heat generated in the HP turbine due to windage during a hot start is
removed by bleeding a small quantity of reverse-flow cooling steam. The cooling steam
enters the HP turbine exhaust (through the bypass line to the CRH line check valve) when
a set speed is achieved and back-flows through the HP turbine. The cooling steam is then
discharged to the condenser through a 4-inch NPS ventilating line. The back flow of
cooling steam is no longer required once the forward-flow steam is admitted to the HP
turbine. This design with an IP turbine start thus precludes the problem of overheating
HP turbine exhaust during start-up.
Author —
S. Zaheer Akhtar, P.E., is a Senior Engineering Specialist with Bechtel Power
Corporation, Frederick, Md. He has more than 25 years’ experience in power generation
and process plants, including positions with SCECO’s Ghazlan Power Plant in Saudi
Arabia, Exxon Chemical Co., General Physics Corp. and BE&K Inc. Akhtar received a
bachelor’s degree in chemical engineering from the University of Engineering and
Technology in Pakistan and a master’s degree in chemical engineering from the
University of Manchester Institute of Science and Technology in the U.K. Akhtar is a
member of the ASME PTC-4.3 Code Committee on Air Heaters.
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