upgrade of coal fired plant startup · pdf filetemperature during a cold start and then the...

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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)


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


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.


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.


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.


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.


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


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


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.


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.


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.

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