upgrade of coal fired plant startup · pdf filetemperature during a cold start and then the...
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22591 Avenida Empresa Rancho Santa Margarita, CA 92688
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Upgrade of Coal Fired Plant Startup Valves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
By Stan Miller, CCI; Curtis Sterud, Consultant; Herb Miller, Consultant
Electric Power Conference and Exhibition
May 2, 2007; Rosemont, Illinois
2 Upgrade of Coal Fired Plant Startup Valves | 890 ©2007 CCI. All rights reserved.
Upgrade of Coal Fired Plant Startup Valves By Stan Miller, CCI USA; Curtis Sterud, Consultant, USA;
Herb Miller, Consultant, USA; Electric Power Conference and
Exjibition, May 2, 2007; Rosemont, Illionis
Abstract
This paper focuses on the benefits of upgrades to the startup
system valves. The upgrades described allow quick load changes
from minimum boiler flow to 100 percent load. Thus the
system can be base loaded but still be responsive to requests
for load changes from the dispatcher without imposing undue
temperature transients to the turbine.
By upgrading the superheater bypass valves the system can be
brought up to pressure and temperature smoothly with valve
designs that can handle the wide variation in fluid density during
the startup process. These valves are designed to throttle the
flow with enough resolution to account for the sensitive boiler
pressure when the fluids is cool to the escalation of pressure
when steam is being produced for heat up of the superheaters
and turbine. Resolution is achieved by a unique characterization
of the multi-stage multi-path valve trim.
Upgrading the stop and control valves ahead of the finishing
superheater to designs that provide good control during high
pressure drop conditions permits load changes required during
startup and higher load to be optimized. These valves allow the
turbine to be operated at constant temperature, which in turn
permits quick load changes in response to grid demand. The
steam turbine valves are held at a constant position and the
control valve absorbs the pressure changes. Operation in this
manner minimizes transient impact on the boiler as well as the
turbine and allows other boiler systems to adapt to the load
change compatible with their response capabilities.
Actuators can be all pneumatic, which is most cost effective and
designs exist that meet all of the performance requirements.
Introduction
In this paper we are discussing changes to two different sets of
valves in the systems associated with sub and supercritical coal
fired once through boiler designs. The term “sliding pressure”
will be used in the paper however its meaning is restricted to
covering the existing load range of once thru operation for the
boiler. Operating at loads below the normal range would require
many more changes to the fluid cooling loop than the minimum
simplifications than are discussed in this paper.
The applicability of the discussion is directed to changes from
the original boiler fluid loops and valve configurations. The
older plants now need to be upgraded to extend their life and
to continue to supply their output. This can be done with fairly
minor changes and not only achieve increased life but have good
reliability and lower operational costs.
The following discussion covers changes to the valves before
the secondary superheaters that impact load changes over the
existing once-thru operating range and the valves around the
primary superheater used for initial startup and shutdown
prior to reaching the once-thru operation. It is changes in these
valves that can be made to increase plant reliability resulting
in minimal thermal transients on the turbine (and other
components) while allowing response to quick load changes.
Faster startup and shutdown times also result from the better
control valves in these two critical locations of the boiler circuit.
None of the changes discussed are experimental as they have
been successfully implemented either in whole or in part on
existing units, References 1 - 3. Valve operating conditions
are well known and the valve duty and special needs clearly
understood. The feedback on valve changes has indicated
dramatic improvements in system operation with the ability to
respond to load change demand effectively while minimizing the
wear and tear on the turbine and other boiler components.
The Boiler circuits
With an older coal fired boiler one of the more onerous tasks
is to have to be continually changing load. Although this is not
impossible it is difficult and likely involves a number of unique
steps by the operators to avoid undue transients on boiler
operations while trying to minimize temperature swings on the
turbine.
Once the boiler turbine is up and running at full load the
electrical output is dependent on the constant discharge steam
pressure entering the turbine. The turbine is equipped with
several valves, known as the turbine throttle valves, which
regulate the turbine inlet pressure. As load decreases, the valves
may close to reduce turbine inlet pressure, all valves may move
closed equal amounts in unison (Full Arc Admission) or they
may close sequentially (Partial Arc Admission). This is known as
constant pressure control operation.
Constant pressure control has two adverse effects when large
load changes occur. First the turbine will experience temperature
fluctuations that will create fatigue and increase maintenance and
reduce life. The second effect is that the net thermal efficiency of
the turbine drops as the load is reduced.
©2007 CCI. All rights reserved. 890 | Upgrade of Coal Fired Plant Startup Valves 3
Sliding pressure operation is designed to mitigate the negative
influences of constant pressure operation. In the sliding pressure
operation the pressure to the turbine is allowed to reduce while
the steam temperature to the turbine is held constant. As the load
is reduced, the pressure to the turbine is maintained by control
valves located either between the superheaters as shown in
Figure 1a (B&W 200 and 201 valves) or before the superheaters
as shown in Figure 2b (CE BT and BTB valves and Foster Wheeler
W and Y valves). The turbine throttle valves are held at a near
full open position. The steam temperature is adjusted at the
superheaters by attemperation control so that the temperature to
the turbine is held constant at all loads.
The original steam boiler designs have valves in the locations
noted by Figure 1a and/or 1b. However, in the original units
these valves have been selected primarily to achieve good shut off
during boiler hydrostatic pressure testing and at the low loads on
the steam separator (flash tank). In most cases the large capacity
block/stop valves with a small control valve in parallel are jogged
to assist in getting the unit up to full load . The small control
valves do not have the capacity and are not equipped to provide
good control for the high pressure drop conditions over the full
once through operation range.
The once through boiler designs have valves either before the
superheaters or between the primary and secondary superheater
that bypass fluid around the secondary superheater to a separator
tank. This is necessary for system control while raising the fluid
temperature during a cold start and then the escalation of load
up to the minimum before once thru operation. The hot high
pressure water in the early stages of heat up must be letdown to a
separator (flash tank) to produce steam for controlled heating of
the superheaters and turbine. The control valves for this service
see a unique set of fluid conditions that were not fully accounted
for in the original designs. For an extended period in the heat up
the fluid density and temperature are changing without a change
in flow rate. This occurs when the pressure is held constant in
the furnace walls and heat is added in order to produce steam in
the separators to raise the superheater and turbine temperature.
Because the valves cannot control for small change in fluid
properties there is continuous oscillations in pressure both up
and down stream of the valves. In some cases these pressure
swings are excessive and can only be controlled by putting the
valves into manual control. The net result is a very uneven heat
up and load escalation that over time results in fatigue and
frequent maintenance of the equipment.
The labels for these start up valves are the 202 & 207, BE, and
W & P for the Babcock & Wilcox, Combustion Engineering and
Foster Wheeler designs, respectively. Typical flow loop schematics
and valves for these three systems are shown in Figures 2 through
4.
In the Combustion systems the circuit to the separator is initiated
between the furnace wall circuit and the primary superheater. For
the Babcock & Wilcox and Foster Wheeler systems the bypass to
the separator occurs between the superheaters. The Babcock &
Wilcox system also has valves (202’s) between the furnace wall
circuit and the primary superheater (for boiler pressure control
during startup)
4 Upgrade of Coal Fired Plant Startup Valves | 890 ©2007 CCI. All rights reserved.
Figure 2, Babcock & Wilcox Boiler flow loop schematic.
as possible in parallel to provide what was felt to be sufficient
control. With small control valves leakage would be minimized.
Actuation options included pneumatic, electric and hydraulic.
Figure 3, Combustion Engineering flow loop schematic.
The current valves
The original control valves in all of the locations discussed were
designed as ruggedly as possible at the time. The valves needed
to have some control for the high pressure drop conditions and
have minimum leakage through the valves during operation as
well as for hydrostatic pressure testing. Block valve designs were
the main high capacity conduits with as small a control valve
©2007 CCI. All rights reserved. 890 | Upgrade of Coal Fired Plant Startup Valves 5
Typical valves are shown in Figure 5. All are single stage pressure
drop devices, which are not very effective in providing the flow
control needed in these critical locations. Frequently sufficient
control can only be achieved by either putting the control of
these valves in manual or continually jogging the block valves as
flow is increased in the start up or shut down sequence.
With these single seated valves in high pressure drop applications
the service life is limited and the violent flow through these
valves cause vibration that damages actuators and results in poor
control. The valves are very noisy. Velocities up to sonic levels
lead to rapid seat and plug erosion with subsequent leakage
when the valves are closed.
Multiple valves of the Figure 5 valve type are used in the start
up systems. Multiples range to as high as seven to ten valves
in parallel on a single unit. This results in added complexity
in the control systems, extensive maintenance on each shut
down and many different leak paths when the valve trim is just
slightly worn. Figures 6 and 7 show examples of the reduction
in the number of valves. In Figure 6, nine valves were used in
the original design and control is now achieved with 5. Figure
7, shows the combination of the 200 and 201 valves with an
elimination of the 202 valves and flow circuit.
Studies of just the impact of leakage in the original number and
designs in these valve locations has indicated losses on the order
of 3 to 5 percent of the unit output with absolute values as high
as 35 Megawatts.
Figure 5, Typical single stage start up valve designs.
Figure 4, Foster Wheeler flow loop schematic.
6 Upgrade of Coal Fired Plant Startup Valves | 890 ©2007 CCI. All rights reserved.
The new circuits and valves
The benefits of being able to control steam pressure with control
valves upstream of the secondary superheater is achieved by
using control valve designs that:
Provide block valve shut off performance
Can control the flow with high pressure drop
These two changes result in fewer valves and increased reliability
using proven experience in the applications.
The use of fewer valves with higher capacity such as illustrated
in Figure 6 and 7 have many benefits. The control system is
greatly simplified just because there are fewer loops. The smaller
number of valves also improves the control function because of
fewer introductions or shutdowns of extra equipment into the
system as load is changed. The number of potential leakage paths
is also reduced in proportion to the valve quantity change. Using
reliable designs also reduces the maintenance cost and time.
There are likely many other benefits in reducing the number of
loops that could range from more space on the control panel to
just a cleaner plant with more space for maintenance.
To achieve the control system benefits it is essential that the
proper valve design is selected so that one is assured of good
control and tight shut off when needed. A design that meets
these criteria is illustrated in Figure 8. The generic description of
the valve design is a “pressurized seat with characterized tortuous
path trims and cage design.” Figure 8 shows the valve in three
different positions; fully closed, internal pilot valve open and
modulating. The flow direction is “over the plug”, which is also
referred to as “flow to close.”
Figure 6, Showing reduction of number of valves on a CE unit.
©2007 CCI. All rights reserved. 890 | Upgrade of Coal Fired Plant Startup Valves 7
Figure 7, Showing a reduction of the number of valves on a B&W unit.
When the valve is closed, View A Figure 8, the inlet fluid leaks by
the piston ring balance seal and pressurizes the volume above
the plug. This pressure times the plug area results in a very large
force holding the plug against the seat ring and assuring a tight
shutoff. Frequently this force is a ton of load for every inch (25
mm) of seat ring circumference. The wedging angles of the seat
amplify this force and a block valve seal is achieved. The actuator
force adds to this pressure load and also causes a very tight seal
between the pilot seat and the stem. Similar forces per unit
length of seal circumference can be achieved on the pilot seat.
When the stem is pulled up, View B, it opens the pilot valve
inside the plug so that the high pressure above the plug
can be relieved. The actuator can then lift the plug off the
main seat and start the flow control function by modulating
the plug. A spring, not shown, inside of the plug helps to
keep the stem and plug separated during the modulating
mode. A differential area between the maximum diameter
of the plug, X dimension, and the top part of the plug, Y
dimension, also provides a significant separation force
between the stem and the plug.
In View C, the plug is now being easily modulated by the
actuator as pressure forces on the top and bottom of the
plug are essentially balanced.
The second feature of the Pressurized Seat control Valve is
the trim design that is used. It consists of a characterized
trim that is made up of multi-path multi-stage components
that are selected to control the fluid velocity as the pressure
is let down.
Typical multi-path multi-stage disk and trim for these valves
is shown in Figure 9. The right angles in the flow path cause
the resistance to the flow and lower the fluid velocity. With
lower velocities the fluid energy is reduced, as the square of
the velocity, and this allows good flow control for the high
pressure drop. A number of these disks are brazed together
to form the valve trim and since each disk can be difference
the design can be customized to fit the plant conditions of
pressure drop versus flow. Capacity of the trim is achieved
simply by adding enough disks to the stack to satisfy the
flow needed.
8 Upgrade of Coal Fired Plant Startup Valves | 890 ©2007 CCI. All rights reserved.
Figure 8, Pressurized Seat Design.
With the unlimited ability to characterize the flow versus valve
stroke a unique design to fit the application requirements can be
achieved. The equal percentage form is the ideal characterization
for the pressure control valve between the superheaters. A unique
characterization is needed for the superheater bypass valves
because at some point in the heat up the valves are stroked to a
point where boiler pressure is held constant while heat is added
to the water. As the water is heated the density is changed in the
fluid through the bypass valve. In order to maintain constant
pressure the valve must move to adjust for this small flow rate
change that is altered only by the density change. With a single
stage valve trim the pressure swings can be extreme with changes
as much as plus and minus 200 psi (1.4 MPa). To avoid this
with a single stage valve the operator usually puts the valve
in manual control. However with the unique characterization
permissible with the multi-path multi-stage designs the valves
continue the heat up transient in automatic control with small
perturbations in the boiler pressure. Actual results for such a
unique characterization of a disk stack are shown in Figure 10 in
comparison to a linear characterization. The pressure swings are
reduced by a factor of 8 to 10 with the characterization.
The longer valve stroke resulting from adding of disks to achieve
capacity has an additional benefit of providing better control
because less fluid change results from the minimum change in
plug position in comparison to a single stage cage design. The
minimum change in plug position is driven by the actuator
resolution and a longer stroke enhances the control function.
Linear StackLinear Stack
•• All Disks Have the Same All Disks Have the Same
Number of Passages and Number of Passages and
Turns, the Same Flow AreaTurns, the Same Flow Area
•• Flow is Directly Proportional to Flow is Directly Proportional to
the Valvethe Valve’’s Stroke at Constant s Stroke at Constant
Differential PressureDifferential Pressure
Characterized StackCharacterized Stack
•• All Disks are Not the SameAll Disks are Not the Same
•• Provides Precise Variable Flow Provides Precise Variable Flow
Versus Pressure Drop Over the Versus Pressure Drop Over the
Full Range of the ValveFull Range of the Valve
Disk Stack FlowDisk Stack Flow22
TurnsTurns
1818
TurnsTurns
88
TurnsTurns
00
1010
2020
3030
4040
5050
6060
7070
8080
9090
100100
00 1010 2020 3030 4040 5050 6060 7070 8080 9090
% Flow% Flow
% Stroke% Stroke
Modified LinearModified Linear
LinearLinear
Modified EquaModified Equa
Figure 9, Typical Multi-path Multi-stage Disks and Trim.
The trim characterization can include a single stage cage design
with large flow windows on top of the disks. The cage would
be used for the portion of operation when pressure drops are
minimal and good control is easily achieved with a single stage
design.
©2007 CCI. All rights reserved. 890 | Upgrade of Coal Fired Plant Startup Valves 9
With the valve designs described above control is achieved under
the most severe pressure drop conditions. This allows the boiler
to be started from a cold condition under automatic control and
then when the load is sufficient and the superheater is no longer
bypassed, control of constant turbine temperature. Both of these
benefits have significant payback in that the boiler and turbine
upset transients during the load changes are avoided. The result
is minimum wear and tear on all of the equipment involved.
With the pressurized seat design for the valves block valve
performance is achieved with the control valves and output is not
loss by leakage around the turbine.
Figures 11 present typical valve cross sections for the superheater
bypass and pressure control valve designs. Both designs are
pressurized seat designs with multi-path multi-stage trim. The
valve on the right, globe configuration, also includes a cage on
top of the disks and is used in the Pressure Control valve location
either before or between superheaters. Its function is to control
flow through the superheater for once through operation at
reduced loads. The left valve in Figure 11, has only the multi-
path multi-stage disk sets and is used in the superheater bypass
application during the start up and shut down transients. For the
startup and shutdown transients the superheater bypass pressure
drop is always high and the extra fluid velocity control in the
valve is needed for good flow control.
Figure 10, Multi-path Multi-stage Valve Trim Characterization.
Figure 11, Cross Sections of the Superheater Bypass and Pressure Control Valves.
A Retrofit Solution.
In some cases it may be possible to retrofit an existing valve in
the field to achieve better control and allow sliding pressure
control for a major part of the load range. Reference 4 discusses
the retrofit of 500 valves that were problems in the field. The
retrofits discussed in the reference replaced the original trim of a
valve in the field with the multi-path multi-stage trim type shown
in Figure 9. There are other methods of retrofitting trim to bring
about a better control valve application. An example is shown
in Figure 12, for the BTB valves in which the single stage design
is converted to a multi-stage design. A Drilled Hole cage has
been added upstream and downstream of the main valve orifice.
The seat ring diameter also has been increased to maintain
full capacity. For the BT valve the Stem is retrofit with an equal
percentage trim, Figure 13, also with an increase in seat ring
10 Upgrade of Coal Fired Plant Startup Valves | 890 ©2007 CCI. All rights reserved.
diameter. These changes allow the valves to provide good control
for the high pressure drop conditions. Control is achieved with
out the usual noise, vibration, and erosion associated with the
single stage trim designs.
Figure 12, Retrofit of BTB Single Stage Valve Trim.
Figure 13, Equal % Retrofit of BT valves.
Stem Gland Improvement
Frequently the stem packing in the valves for these applications
have a short life. The life is shortened because of the very high
pressure drop and the extensive modulation needed during the
transient. A solution to this is to consider the use of a packing
free stem penetration with leak off connections that productively
use the blow by steam. This stem packing box is illustrated
in Figure 14. The steam “leak off” connections are directed to
heaters, gland seals and the Deaerator so that there is no loss in
unit efficiency. The temperature at the top of the packing box is
low enough so that a long life PTFE material may be used instead
of the more friable Graphite packing.
This stem seal significantly reduces the friction and improves
control valve resolution. It assures a long term seal for the high
duty valves and is especially beneficial in the case of a steam
supply unit that is frequently required to change load.
Figure 14, Valve Bonnet with Packing Free Stem Seal and Steam Leak Off Connections.
Actuator Improvements
In the past all types of actuators have been used for these control
valves. The Pressure Control valve frequently uses a hydraulic
actuator because of the need for a high seat load and the stiffness
needed for control under the high pressure drop conditions. For
the single stage, unbalanced control valves the higher strength
capabilities dictated hydraulic actuators. If preferred any type of
actuator can be used for these valve applications.
A piston pneumatic actuator is well suited for the pressurized
seat designs used on the Superheater Bypass and Pressure
©2007 CCI. All rights reserved. 890 | Upgrade of Coal Fired Plant Startup Valves 11
Control valves. Boosters and lockup valves are likely needed
to meet speed and failure conditions. The piston actuator can
provide the capabilities that are needed due to the longer stroke
requirements on the multi-path multi-stage designs and the high
thrust dictated by the application.
An advanced new piston actuator, Reference 5, that emulates the
features of a hydraulic actuator, is available and preferred because
of the higher reliability and lower maintenance of pneumatic
designs. This actuator, illustrated on Figure 15, achieves the
high speed needed without boosters using a spool valve with a
capacity about 50 times larger than normal positioning systems.
High seating loads are achieved in the piston actuator utilizing
100 psi (0.7 MPa) air instead of the much lower air pressure
used in conventional pneumatic actuators. This actuator has
the capability to achieve full stroke without overshoot in less
than 2 seconds and provide up to 20,000 pounds (90 kN) of
thrust. The standard unit uses a magneto restrictive device to
obtain an absolute position feedback from the control valve. It
is completely contained inside the actuator and has no moving
parts or linkages. Resolution of the actuator can be less than
0.25 percent for low friction applications and is never more
Figure 15, Advance Pneumatic Actuator.
than 1 percent for the higher friction applications. Calibration of
the actuator system is all automatic and done by the controller
within seconds. All of the features of the advanced pneumatic
actuator are well suited for the Superheater Bypass and Pressure
Control valve applications.
Boiler Performance.
Figures 16 and 17 show the before and after load transients for
the retrofitted valve trim of Figures 12 and 13. With the retrofit
the initial BT valve is opened at 13 percent load and with a
pressure drop of 2500 psi ( KPa) instead of 18 percent load and
1500 psi (10 MPa). Also the remaining BT valves are opened
at higher loads so that the pressure control to the turbine is
maintained up to 70 percent load with the turbine control valve
held constant. This allows significantly more load range in which
the temperature to the turbine can be held constant. As noted
also there are significantly fewer oscillations on the BTB valve
during the transfer and operation of the first BT valve because
the BT valves are opened earlier and under much higher pressure
drop conditions. The smooth ramp up of the turbine throttle
pressure reduces the stresses on the boiler as well as the turbine.
12 Upgrade of Coal Fired Plant Startup Valves | 890 ©2007 CCI. All rights reserved.
Another example showing the pressure control from the cold
conditions up to full load is shown on Figure 18. As shown in
the plot the boiler pressure is brought up to 3600 psi (24.8 MPa)
and held while the fluid temperature is raised to more than 500
F (260 C). The transfer to the Pressure Control valve from the
Superheater bypass then occurs at less than 20 percent load and
the Pressure Control Valve then maintains the turbine throttle
pressure all the way to full load while the boiler pressure is
escalated to 3900 psi (26.9 MPa). The pressure drop across the
Pressure Control Valve at full load is approximately 40 psi (275
kPa) as the bulk of the steam flow is through the cage section of
the trim.
Figure 16, Transients Prior to Valve Retrofits.
Figure 17, Transients After Valve Retrofits.
The Figure 17 BT valves could have controlled pressure to higher
loads than 70%. The top value of this pressure ramp is dependent
upon the owner’s use and need for the boiler-turbine set.
Conclusion
Upgrading the Pressure Control valves between the superheaters
and the Superheater Bypass valves provides significant advantages
for the once through coal fired boiler designs. The general
upgrade to these valves is to change them to pressurized seat
designs and to incorporate a multi-path multi-stage trim design.
These changes assure tight shut off when required and provide
excellent turbine pressure control for load changes up to near
or full load which ever is preferred. The changes also allow
pneumatic actuation of all of these valves so that electric and
hydraulic actuators can be eliminated if desired. The advanced
pneumatic actuation systems provide reliability, good resolution
and speed as well as reduced maintenances and complexity.
This upgrade has been shown to pay back quickly with the
increase in megawatt output with minimum to zero leakage. In
some cases this gain has been as high as 35 megawatt. The grid
demands for load change are easily accommodated because the
valves can now control pressure to the turbine reliably and allow
the boiler attemperation to provide a constant temperature to the
turbine. This in turn reduces the temperature transients on both
the boiler and the turbine resulting in lower maintenance and
repair costs. New life is gained with the existing boiler turbine
equipment keeping the older units productive and efficient.
Figure 18, Example of Pressure Control up to Full Load.
©2007 CCI. All rights reserved. 890 | Upgrade of Coal Fired Plant Startup Valves 13
References
1. Sterud, C. G. and Miller, H. L., “Replacement Pressure
Control and Superheater Bypass Valves Permit 93 % Cyclic
Load Cutback at PG&E’ Moss Landing,” American Power
Conference, Chicago, April 1989.
2. Brailey Jr., Edwin J., Miller H. L. and Sterud, C. G., “Control
Valves Limit Turbine Temperature Swings,” Power
Engineering, April 1991.
3. Miller, H. L., “Heavy Duty Control Valves,” 20th Japan
Electric Measuring Instruments Manufacturing Association
International Exhibition, Tokyo, October 18-21, 1983.
4. Miller, H. L., Stratton, L. R., and Hollerbach, M. A., “Fluid Jet
Energy Criterion Eliminates Control Valve Problems,” Valve
Magazine Spring 2006, Vol. 18, No. 2, Valve Manufacturers
Association of America, Washington D. C., April 2006.
4. Miller, S. F., “Electronic Valve Controller Replaces
Conventional Pneumatic Systems,” Instrumentation,
Systems, and Automation Society (ISA) Houston, October
1-7, 2004.