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B
F
Turbine-Bypass Systems
DRAG®
Valves n Desuperheaters n Actuation Systems n Controls
Over the years, turbine-bypass technology has advanced along with that of the power industry
and has grown in sophistication to enhance operational flexibility and to protect power plant
components during a variety of transient modes. With more than 1500 bypass valves supplied
over the last 40 years, the leadership of CCI, together with former Sulzer Thermtec, has been
proven time and time again.
Overview
Turbine-bypass systems are not only
essential for the flexible operation of
large coal-fired power plants but play
an equally important role in advanced
combined-cycle power plants.
Turbine-bypass systems permit
operation of the steam-generator
independently of the turbine during
start-up, shutdown and load disturbances. This enhances operational
flexibility during these transient operating
conditions. As a consequence, startup
and reloading time are reduced. In
addition, equipment life and overall
availability are increased.
To achieve the desired results, turbinebypass systems must be adequately
sized to meet the needs of these
transient operating modes. CCI, with
its long experience and know-how in
power plant design and operation, can
effectively support plant designers and
plant operators in selecting and
integrating a bypass system into an
overall plant design and operation. In
addition to providing originalequipment, CCI has extensive
experience in integrating bypass
systems into existing plants and in
retrofitting existing bypass valves that
have failed in service. CCI’s worldwide
sales and service organization and
local engineering centers are ready to
support customers in their evaluation
and engineering efforts.
CCI turbine-bypass systems meet the
requirements of all governing boiler and
valve codes, including ANSI, ASME,
TRD and many others.
Examples of CCI Bypass Systems:
1959
Philadelphia Electric, Eddystone 1 & 2, USA
(ultra-supercritical plant)
1967
RWE Frimmersdorf, Germany
(combined bypass and safety function)
1975
TEAS, Elbistan A, Turkey
(once through boiler)
1982
Chubu Electric, Kawagoe 1&2, Japan
(ultra-supercritical plant)
1988
HIPDC Shidongku, China
(supercritical power plant)
1988
Pacific Gas & Electric, Moss Landing, USA
(supercritical, cyclic operation)
1990
New England Power Service Co., Brayton Point, USA
(supercritical, cyclic operation)
1991
Korea Electric Power Corp., Yeongnam, Korea
(addition of a bypass system)
1991
Korea Electric Power Corp., Korea
(twenty supercritical units, Poryong, Taean, Hadong & others)
1993
China Light and Power, Black Point, China
(eight combined cycle plants)
1993
Kansai Electric, Himeji Units 1-5/1-6, Japan
(combined cycle units)
1995
Shell Australia Ltd., Geolong Refinery, Australia
(system redesign and valve replacement)
1996
VEAG, Schwarze Pumpe, Germany
(supercritical, combined bypass and safety function)
1965-98 Various plants, Japan
(over 40 bypass systems)
1971-98 Neyvilly Lignite 1-3, NTPC Rihand STPP, NTPC Talcher,
CESC Budge Budge and other plants, India
(over 100 bypass systems)
1972-98 Various plants, China
(over 130 bypass systems)
1981-98 EGAT, Mae Moh 4-13, Rayong, Ratchaburi, Thailand
(drum, combined cycle, and supercritical plants)
2
High pressure bypass for a coal fired 600 MW unit consisting of steam
control valve spraywater control valve and spraywater isolation valve
3
Turbine-bypass systems enable faster startup, can help avoid boiler trips, minimize
thermal stresses, prevent needless energy loss, lengthen trouble-free plant life, and
increase plant reliability.
Purpose
Faster Start-up and Minimized Thermal Stress
Integrating Turbine-Bypass Systems
The CCI turbine-bypass system reduces start-up time
under cold, warm, and hot conditions. Continuous flow
through superheater and reheater allow higher firing
rates resulting in quicker boiler warm-up. It also controls
superheater and reheater pressure during the entire
startup, keeping thermal transients in the boiler to a
minimum. Operating experience shows that power plants
equipped with bypass systems experience much less
solid particle erosion of the turbine blades, reducing
the need for expensive repair and replacement.
CCI supplies turbine-bypass systems for any type
of fossil-fired power plant. The schematics on page
5 show the integration of a turbine-bypass system
into a combined-cycle power plant (CCPPs) and a
500 MW fossil-fuel-fired supercritical plant. The
c h a r t b e l ow s h ow s a t y p i c a l h o t - s t a r t u p
characteristic for a supercritical, fossil-fuel-fired,
500 MW unit. Forty minutes after lighting off the
steam generator, the steam temperature is matched
to the turbine metal temperature. The bypass
system flow rate equals the difference between
steam-generator and turbine flows. In this case it
is 22 percent of full flow. The corresponding steamgenerator pressure is 30 percent of full-load
pressure (80 bar [ 1160 psig] at 40 minutes
compared to 260 bar [3800 psig] at full load). The
result is a required bypass-system capacity of
approximately 70 percent MCR at full load (percentsteam-flow divided by percent-pressure). Because
of the large bypass system designed into this
particular plant, coal firing can be initiated earlier,
thus reducing the amount and cost of oil necessary
in the start-up cycle.
Temperature Matching
An adequately sized bypass system allows optimum
steam to metal temperature matching for all start-up
modes. The boiler load can be selected to reach the
desired superheater and reheater conditions for turbine
start. This results in reduced start-up time and
extented life for main turbine components.
Avoid Boiler Trip after Load Rejections
Fast-acting turbine-bypass systems allow boiler
operation to continue at an optimal standby load while
demand for turbine load is re-established after a load
rejection. The turbine can cover house load requirements. Pressure and temperature transients invariably
associated with boiler trip and restart are avoided.
Steam
temp press flow
bar %
˚C
Eliminate HP-Safety Valves
HP-bypass valves can serve as HP-safety valves,
when equipped with the necessary safe opening
devices. This eliminates the need for separate springloaded HP-Safety valves, associated piping and
silencers and can save millions of dollars in equipment
and maintenance costs. CCI’s engineering staff are
qualified to review applicable codes and system designs.
500
300
4.2
400
4.1
300
200
200
Preventing Energy and Feedwater Loss
100
Even when regulations require spring-loaded safety
valves, a large capacity bypass with fast acting actuators
can avoid lifting of the safety valves with resulting energy
and water losses under almost all upset conditions.
0
100
100
3.1
3.2
Sizing of the Bypass System
50
Turbine-bypass system sizing considerations must take
into account all plant operating conditions such as the
number of warm starts, hot starts and requirements for
house load operation. Later in plant life cyclic operation
may become common. Sizing of low pressure turbinebypass valves must take into account the desired reheater
pressure for turbine start and condenser capacity.
2.1
2.2
1.1
2.3
2.4
0
Light up
100 minutes
50
0
Synchr.
Full Load
Pulverizers
1.1
2.1
2.2
2.3
2.4
CCI turbine-bypass systems are custom designed to
meet the specific capacity requirements of the
individual plant. Capacity can range up to 100 percent
of the maximum continuous rating (MCR) boiler steam
flow for the HP-bypass as well as for the LP-bypass.
Firing Rate
Feedwater Flow
Waterwall Flow
Steam Flow (Superheater)
Steam Flow (Turbine)
3.1
3.2
4.1
4.2
Superheater Pressure
Reheater Pressure
Superheater Temperature
Reheater Temperature
Hot start of a supercritical 500 MW unit
4
Steam Turbine
HP
p
Safety
System
HP-Bypass
Controller
Superheater
IP
LP
G
p
Safety
System
HP-Bypass
T
p
T
Safety
System
Reheater
LP-Bypass
Controller
Condenser
RH-Safety Valve
LP-Bypass
Evaporator
Preheater
Economizer
Deaerator
Typical coal fired supercritical plant schematic
Heat Recovery
Steam Generator
Deaerator
HP-Bypass
Steam Turbine
Gas Turbine
HP
GT
IP
LP
LP-Bypass
HP-System
IP-System
LP-System
IP-Bypass
Typical combined cycle power plant schematic
5
G
CCI has developed a wide range of technologies for valves, desuperheating, actuators and
controls for turbine-bypass systems. This enables us to supply the proper solutions to suit the
needs of any type of plant.
Valves
Duty of a Bypass System
The primary job of any bypass system is steamconditioning i.e., high-pressure throttling combined with
desuperheating. Therefore, bypass valves must be able
to perform these functions without undue noise or
vibration and without destructive valve-trim wear. In
addition, bypass systems usually experience severe
temperature cycling.
In addition, the WING-type plug creates highly turbulent
zone immediately downstream of the valve seat. This
turbulent zone is ideally suited for the in-body injection
of desuperheating spraywater. Also, noise in attenuated
by water injection inside the valve. This valve design
ensures that this turbulent zone is removed from any
valve surface to eliminate a source of vibration.
Tight Shutoff
Depending upon plant design, bypass systems must
also perform additional functions, such as safe
HP-bypass opening and LP-bypass closing for
condenser protection during transient operating periods.
Lower Noise and Vibration
for Turbine-Bypass Valves
Excessive vibration has been known to break pipe
hangers and shake accessories off actuators, resulting
in high maintenance costs and unscheduled downtime.
Depending on the type of plant and its operating
conditions, CCI offers a variety of advanced valve
technologies to handle such severe-service conditions.
DRAG® technology effectively controls fluid velocities
using patented pressure-reducing disk stacks. This
technology employs a tortuous-flow path with multiple
right-angle turns. The result is lower fluid trim exit
velocities, longer lasting trim and elimination of
vibration. Noise generated by the valve can be kept
to below 85 decibels throughout the entire operating
range without the use of acoustical insulation.
The WING-type plug designs use wave-principle theory.
Unlike conventional plug designs the WING-type plug
has a specially contoured profile to produce wave
interference. This effectively “cancels out” some of the
aerodynamic effects as they impact valve parts. This
design incorporates specially engineered channels that
divide the steam flow into discrete paths and increase
generated noise frequency. This higher frequency noise
is more easily absorbed by the adjacent piping and
results in noise-level reduction of 10 dBA as compared
to conventional designs.
To prevent the high-energy steam heat loss that is
associated with internal valve leakage and the resulting
trim damage caused by steam cutting, CCI's turbinebypass-system valves are designed to provide longterm positive shutoff. Normally ANSI/FCI 70.2 (formerly
B16.104) Class V or tighter shutoff is recommended.
Use of CCI's pressurized-seat trim or it's unbalanced
plug design, both produce block-valve-type shutoff.
Prevention of Cavitation Erosion
in Spraywater Valves
DRAG® spraywater valves eliminate cavitation, trim
vibration and trim vibration. Thus trim life is considerably
increased and frequent repair and replacement is
eliminated. These valves incorporate a unique tortuouspath trim design made up of a stack of up 20 individual
disks, each having a series of right-angle turns that
produce multiple stages of pressure reduction.
A Modified Equal Percent characteristic is standard for
these spray valves. This permits fine temperature
control at steam flows for increased plant efficiency.
Several disk designs with varying Cv and pressure drop
are used to produce this characteristic. Class V shut
off is standard for CCI Spraywater Valves.
Isolation Valves
Specifications often call for steam-isolation valves for
the LP-bypass for condenser protection. CCI supplies
LP-bypass valves, which combine the control function
with a safe-closing function. However, if the specification
requires a bypass system with separate control and
isolation valves for reasons of plant safety, CCI can
also supply steam isolation valves.
6
DRAG® trim
WING-type trim
CCI turbine-bypass and spray valves using proven technology
7
The desuperheating function in a bypass must provide for excellent steam and spraywater
mixing and quick evaporation of the injected water without creating thermal stress that can
cause system damage.
Desuperheating
CCI has developed many different technologies for
desuperheating. Key requirements for any type of
desuperheater are complete evaporation of the injected
water and prevention of any water droplets hitting the
pressure boundary walls of the valve or downstream piping.
Essential here is, first the degree of atomization of the
injected water and the mixing with the steam, and second,
quick evaporation with proper location and direction of the
spray water jet. Complete evaporation must be attained
before the first pipe bend to prevent any erosion caused
by high-speed droplets contacting the pipe walls.
Ring-Type Desuperheater
Here, steam turbulence is created by the shape of the
steam-flow. The steam velocity is increased by a
contraction of the flow path, and the injection takes
place where the flow path abruptly expands. A concern
in the design of desuperheaters is the thermal stress
resulting from the high temperature differential between
the water and the steam. The design of CCI’s ring type
desuperheater takes into consideration the different
thermal expansions, thus avoiding cracking.
Spring-Loaded Injection Nozzles
The degree of spraywater atomization attained is
determined by the relative speed of steam flow to that
of injection water flow. Full atomization is therefore the
result of either high water injection speed or injection of
the desuperheating water into a zone of turbulent, highspeed steam flow. Many factors pertain here: control
accuracy, flow range, piping arrangement, and more.
The correct desuperheating technology varies with many
factors that include accuracy of controls, flow range,
piping arrangement etc. CCI’s experience and technology
ensures the optimal solution every time.
The atomizing principle of spring-loaded injection nozzles
is based on high-speed injection. Due to the design of the
spring-loaded nozzles, a sufficient injection pressure, and
therefore injection speed, already exists at minimum load.
The injection speed results not only in good atomization
but also in a sufficient penetration of the spraywater into
the steam flow. This insures good mixing of steam and
spraywater. Spring-loaded injection nozzles are preferably
used at the outlet of flow-to-close bypass valves where
the additional turbulence in the outlet enhances evaporation.
Steam-Assisted Desuperheaters
In-Body Desuperheating
Desuperheating inside the valve body makes the best
use of the principle of water injection into a zone of high
steam-flow turbulence. The spraywater is virtually
evaporated when the steam leaves the valve, thus providing
the shortest evaporation length. Proper design of in-body
desuperheating requires a detailed understanding of the
flow pattern inside the valve at all load conditions. CCI
has done extensive research into these flow patterns,
including spraywater injection and atomization with the
help of dynamic numerical calculations. Optimum
arrangement of the injection nozzles, material selection,
and shape and hole pattern of the cage around the injection
zone is the result of this extensive research.
STEAMJET desuperheater is another CCI design,
which is based on a combination of high-speed water
injection into high-velocity steam flow. A "compound
swirl" nozzle provides high injection speed, and the
atomizing steam provides a velocity, which is almost
independent of the main steam flow.
Sparger Tubes
CCI also provides custom-engineered condenser sparger
(dump) tubes for the introduction of steam from the lowpressure bypass valves to the condenser. These optional
sparger tubes are custom designed to meet the space
constraints of the specific condenser and to protect the
internals of the condenser.
Spray pattern of a steam assisted desuperheater
8
Steam conditioning valve type DRE
using inbody desuperheating technology
DRAG® steam conditioning valve using
steam assisted desuperheating technology
Steam conditioning valve type NBSE
using spring loaded spray nozzles
Ring type desuperheater EK
9
Turbine-bypass valves and their actuators and controls must be matched for optimum
system performance.
Actuators and Controls
The key criterion for long and reliable system operation
is the proper selection and design of all of the system
components. CCI, with more than 1500 turbine-bypass
systems supplied worldwide over the last 40 years,
provides this expertise with every system installed.
Pneumatic Actuators
CCI’s pneumatic actuators feature a double-acting
pneumatic-piston actuator custom designed to meet the
application requirements for stroke, speed, and actuating
force. The actuator is equipped with quick-acting
components to achieve stroke speeds as fast as one
second.The positioner can accommodate the usual control
signals (e.g. 4-20 mA). Typically, trip modes are powered
by springs but air accumulators can also be used.
Power-Operated Reheater Safety Valves
Using the same technology as used for HP-bypass
valves incorporating a safety-valve function, CCI can
supply power-operated safety valves. These valves are
kept closed by hydraulic fluid force and, in the event of
a safety-valve trip, the valves are opened by flow-toopen steam force. Sufficient closing force is provided
to keep the safety valves completely tight shut.
Hydraulic Actuators
Hydraulic actuators are well suited for applications
requiring high force and high stroking speeds. Safe trip
devices can be easily mounted on hydraulic actuators.
CCI provides the complete system consisting of
hydraulic cylinders, control devices, and positioners as
well as hydraulic power units. The hydraulic power units
consist of a fluid tank, pumps, filters, accumulators,
and the necessary monitoring and controls.
Electro-mechanical Actuators
When fast load rejection is not required, electromechanical actuators are a good choice. CCI’s reliable
electro-mechanical actuators are easy to maintain,
with standard ac motors.
Safety Systems for High
Pressure Bypass valves
In countries where regulations allow the use of
HP-bypass valves as safety valves against superheater
overpressure, CCI can provide bypass valves,
actuators, and the necessary safety control equipment.
This system has been applied in many countries in
Europe, Asia, and Africa. The complete system has a
type approval (Bauteilkennzeichen) according to the
German TRD421 code.
Power operated reheater safety valves
Turbine-bypass Controller
A well-designed bypass controller is important for
smooth plant operation, especially during plant
start-up, shut down, and load disturbances. CCI has
more than 25 years’ experience in designing and
supplying turbine-bypass controllers. CCI’s latest AV6
series turbine-bypass controller uses advanced
control strategies i.e. state controller with observer
(SCO) and provides accurate control, thus producing
less thermal stress on valves and piping. The AV6
series controllers can easily interface to any boiler
and turbine control system.
10
Pneumatic actuator
Hydraulic actuator
AV6-bypass controller
Hydraulic power unit for a bypass system
11
CCI World Headquarters
CCI Japan
CCI Switzerland
CCI Korea
CCI World Headquarters
Telephone: (949) 858-1877
Fax: (949) 858-1878
22591 Avenida Empresa
Rancho Santa Margarita
California 92688 USA
Your Local CCI Contact
CCI Switzerland
formerly SULZER THERMTEC
Telephone: 41 52 262 11 66
Fax: 41 52 262 01 65
P.O. Box
Hegifeldstrasse 10
CH-8404 Winterthur, Switzerland
CCI Japan
Telephone: 81 726 41 7197
Fax: 81 726 41 7198
194-2, Shukunosho
Ibaraki-City, Osaka 567
Japan
CCI Korea
Telephone: 82 341 85 9430
Fax: 82 341 85 0552
26-17, Pungmu-Ri
Kimpo-Eup, Kimpo Gun
Kyunggi-Do, South Korea
MC-310-8/98 310
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