235 Replacement Pressure Control at PG&E`s Moss Landing.indd

Replacement Pressure
Control and
Superheater Bypass
Valves Permit 93%
Cyclic Load Cutback at
PG&E’s 750 MW Units
at Moss Landing
...................................................
By Herbert L. Miller and Curtis G. Sterud
Presented at the American Power Conference,
Chicago, Illinois; April 24–26, 1989
22591 Avenida Empresa
Rancho Santa Margarita, CA 92688
949.858.1877 ! Fax 949.858.1878 ! ccivalve.com
235
|
06/00 ! ©2000 CCI ! DRAG is a registered trademark of CCI.
Replacement Pressure
Control and Superheater
Bypass Valves Permit 93%
Cyclic Load Cutback at
PG&E’s 750 MW Units at
Moss Landing
" By Herbert L. Miller, Vice President, Engineering,
and Curtis G. Sterud, Principal Engineer, CCI
Presented at the American Power Conference, Chicago, Illinois, in April, 1989.
Sponsored by the Illinois Institute of Technology Technology Center, Chicago, IL 60616
Introduction
A
mong the many system and equipment modifications
required to adapt Moss Landing, Units 6 and 7 for very
low-power cyclic operation were the complete replacement of the
pressure control valves and the superheater bypass valves. These
units were originally designed for base loading and were limited
to occasional cycling down to about a 30 percent load. Now they
are capable of daily cycling down as low as a 7 percent load.
Cycling frequency and the increased pressure drops involved
served to greatly magnify the service severity imposed on the
pressure control valves and superheater bypass valves.
Figure 1—Pacific Gas & Electric’s Moss Landing Generating
Station.
Cyclic Operation at Moss Landing
As originally installed for base-load service over twenty years
ago, Figure 1, these gas- and oil-fired, 750 MW, super-critical
units were capable of limited cyclic operation in the 240 MW to
particularly troublesome in that continuous leakage to the flash
tank through the old 202 valve in the original design, produced a
significant heat-energy loss to the system.
750 MW range. The increased availability of lower-cost, off-peak
Modified High-Cycle System Design
power from alternative sources made it economically imperative
Figure 2 compares the pertinent portions of the Moss Landing
to reduce minimum loads on these units, since shutting them
Units 6 and 7 cycle, both before and after modification for very
down at night and during weekends was not considered practical.
low, cyclic-load operation.
Many possible methods were considered. The modification
ultimately selected involved one that utilized the existing flash
tank, a modified control system, and the replacement of the
pressure control valves and the superheater bypass valves, among
other things. As a result of these modifications, Moss Landing,
Units 6 and 7 are now capable of operation from 50 MW to full
load, and a minimum load under manual control as low as 30
MW has been demonstrated (¹).
Figure 3 shows how turbine throttle pressure varies with load
from full load down to minimum operation at 30 MW. Note that
from 30 MW to about 165 MW (22 percent load), the flash tank
operates essentially like a boiler drum and its pressure at 1000 psi
is controlled by the 207 valve (the 200 and 201 valves are closed,
and the 205 stop-check valve is wide open). The pressure drop
between the steam generator and the flash tank (that is, across
the 207 valve) is approximately 2950 psi as it controls steam-
Prior Valve Design and Experience
generator pressure. Flash tank pressure is maintained by the 240
As originally installed, the superheater bypass valves (202) and
valve, and the flash tank level is controlled by the 241 valve.
the pressure control valves (201), were of the single-seated, single-
Turbine load is controlled by the turbine throttle valves.
stage type. Typically, this valve type encounters internal velocities
At about 165 MW, the turbine throttle valves are set up to 100
as high as critical or sonic speeds, depending upon whether
percent open, and the 201 corresponding to load demand. Steam-
the fluid is water or steam. Even under the relatively infrequent
generator pressure is maintained by the boiler feedpumps. This
operating conditions encountered before cycle modification,
operating mode continues up the ramp to 66 percent of full load.
satisfactory control was difficult and valve service life was severely
compromised by seat and plug erosion. This valve design was
2
It is very important that this 22 percent load transfer (from flash
Replacement Pressure Control and Superheater Bypass Valves Permit 93% Cyclic Load Cutback at PG&E’s Moss Landing | 235
©2000 CCI. All rights reserved.
Before
After
Figure 2—Original steam generator startup system vs. high-cycling system.
tank operation to 201 valve control) be accomplished without a
disk modulates flow by exposing more disks as required flow
steam-generator pressure swing to avoid a turbine trip. To have
increases.
such a bumpless transfer at this point, the rate of change of the
Valve Design Considerations
flow capacity between the opening 201 valves and the closing
207 valves must be the same. This is directly related to stroke
speed, trim capacity and capacity-change rate. Therefore, these
two valves must always be considered together.
An overriding design consideration in these pressure-control and
super-heater bypass services (201 and 207 valves) is to assure
absolute velocity control, repeatability, and resolution for the
high differential pressures at the variable flows encountered. This
At 66 percent load, the 201 valve is wide open, and control
is best accomplished in a stack of individual pressure-breakdown
transfers to the turbine throttle valves. At this point the 200 valve
disks that break the total flow into small flow channels. These
opens without impact on system operation.
small individual flow channels each incorporate the desired
While this describes the three-step procedure practiced by PG&E
number of abrupt, flow-direction changes, creating a tortuous
at Moss Landing, alternate methods of achieving the same end
path, to produce controlled pressure drops through shock loss
result have been developed by other utility companies operating
at each flow turn. In this way excellent control is achieved. The
super critical power systems (², ³, 4, 5). In these cases, the shape
number of flow turns shown in the typical disk in Figure 4 can
of the curve in Figure 3 is dictated by the utility’s efficiency
be varied widely to produce varying pressure drops. Disk stacks
requirements and load demand.
Replacement Valve Design
can therefore be characterized and made up of combinations
of several disk designs to provide varying pressure drops at any
point throughout the range of flow control. In this way the shape
Both the pressure control valves (201) and superheater bypass
of the percent stroke vs. percent flow curve can be altered to
valves (207) are of the “tortuous path” design. In the basic
match a specific need.
tortuous path valve design (Figure 4), eight disks per inch of
valve stroke make up a disk stack which dissipates pressure
through shock loss abrupt flow turns within the individual disks.
The 201 valve disk stack contains 80 disks, and the 207 valve disk
stack contains 50 disks. Since these flow paths are fixed in each
disk in the stack, individual disk flow is constant at any degree
Other important design considerations include such items as
valve seat design, which must ensure tight closure under highpressure differentials every time the valve closes. Over the years
there have been many materials and design approaches to this
critical area of valve designs (6, 7).
of modulation through the valve. Water velocities are thus limited
Pressure Control Valve Design Details
to less than 100 ft/sec. Valve plug position within the stack of
Figure 5 shows the construction of the 201 valves at Moss
©2000 CCI. All rights reserved.
235 | Replacement Pressure Control and Superheater Bypass Valves Permit 93% Cyclic Load Cutback at PG&E’s Moss Landing
3
Flash Tank
Operation
throughout the ramp from 886 psi to 3800 psi (full turbine
throttle pressure) and contributes significantly to a bumpless
Straight-Through Operation
transfer from 207 to 201 valve operation at 22 percent load.
25
Since this 201 valve operates in series with upstream and
downstream block valves, absolute tight closure is not a prime
3000
20
15
2000
design consideration for pressure-control valve applications.
mPa
Throttle Pressure PSIG
4000
However, this 201 valve could easily be designed as a pressurized
seat valve (as discussed later under the 207 valve), eliminating
the need for and expense of these upstream and downstream
10
1000
5
block valves.
Superheater Bypass Valve Design Details
Figure 7 shows the construction of the 207 valves, which are
10
20 30 40
50
60 70 80
90 100
Percent Load
closed at any load above 165 MW and must seal against the very
high differential pressure that exists during normal operation. An
exceptionally high closing pressure is needed here that would
Figure 3—Throttle Pressure Variation with Load
Landing. It is a tortuous-path, balanced plug design. It modulates
flow between 22 and 85 percent throttle flow and differential
pressures across the valve vary from about 2800 psid down to
near zero when the 200 valve opens.
Initially the 201-valve, 80-disk stack incorporated two disk
designs, but since flow and differential pressures both vary,
linearity in terms of percent flow vs. percent valve stroke was
less than optimal. This led to higher-than-desired valve position
change in response to varying demand and produced a degree
of “hunting” in turbine-throttle pressure, which affected boiler
feedpump speed control. To overcome this problem, the disk
stacks have been re-characterized to include two additional disk
designs in this 80-disk stack. Linearity has been considerably
improved (Figure 6). This has stabilized turbine throttle pressures
Figure 5—Sectional drawing of 201 valve.
require an over-sized actuator. Therefore, this valve employs a
pressurized seat design which uses an internal pilot valve to
load and unload the closing force on the plug. When the valve
is closed, full upstream pressure behind the plug exerts about
100,000 lb. of closing force on the plug. When the actuator is
called upon to open the valve, the internal pilot opens first,
bleeding this high pressure downstream through ports within the
plug itself. This immediately balances the forces acting on the
plug, permitting normal actuator loadings to control valve flow
modulation.
This 207 valve also functions as a dump valve on turbine trip to
conserve condensate and minimize safety-relief valve operation.
Therefore, it must be capable of absorbing an internal thermal
shock during very rapid heat-up, to insure valve integrity under
Figure 4—Typical torturous disk design.
4
these conditions. Belleville springs between the disk stack and the
Replacement Pressure Control and Superheater Bypass Valves Permit 93% Cyclic Load Cutback at PG&E’s Moss Landing | 235
©2000 CCI. All rights reserved.
pressure valves in power plant service, has dropped dramatically.
In fact, it is necessary to look at valve-stem motion to determine
whether or not valves are in operation. Troublesome valve
and piping vibration has been eliminated. Previous energy
losses through the 207 valve have been stopped through the
exceptionally high seating forces produced by this pilot-operated
plug design.
Conclusions
At Moss Landing, Units 6 and 7 have successfully operated in this
low-load mode for over two years. The application of these valves
has been an important part of this achievement.
References
Figure 6—Enhanced linearity of 201 valves achieved through disk
recharacterization.
valve body allow rapid, disk stack expansion independently of
the slower valve body temperature increase.
Both the 201 and 207 valves at Moss Landing are electrically
actuated. However, this valve design is not sensitive to actuator
design, electrical, pneumatic, or hydraulic, as long as rate of
change in capacity can be closely controlled by actuator stroke.
Since the installation of these new tortuous-trim valves, operating
noise, frequently an undesirable characteristic of high differential-
1. Laszlo, J. and Chan, B. K., “PG&E Experience in Cycling
Conversion of Gas Fired Supercritical Power Plants.” Paper
presented at the Electric Power Research Institute Seminar on
Fossil Plant Cycling, Princeton, N.J., October 20–22, 1987.
2. Baer, R. H. and Turbiville, D., “Modifying Supercritical Pressure
Boilers for Sliding Pressure Control.” Power Engineering, pp. 58-60,
January 1985.
3. Schaeper, W. and Vera, R. and L. G., “Modifying Supercritical
Pressure Units for Cycling Operation.” Power Engineering, pp.
54–57, June 1984.
4. Martinez, O. and Makruch, J. A., “Variable-Pressure Operation
Both Flexible and Efficient.” Power, pp. 62–63, January, 1982.
5. Laney, B. E. and Vera, R. L., “Adaptation of a Supercritical
Unit to a Present and Future Generation Management Plan.”
Paper presented at the 25th Power Instrumentation Symposium,
Instrument Society of America, Phoenix, Arizona, May 24–26,
1982.
6. Miller, H. L. and Sterud, C. G, “A High-Pressure Pump
Recirculation Valve.” Paper presented at the Electric Power
Research Institute Power Plant Valve Symposium, Kansas City, Mo.,
August 1987.
7. Miller, H. L., and Sterud, C. G., “How Feedpump Recirc Valves
Withstand Severe Duty.” Power, pp. 79–88, August, 1987.
Figure 7—Sectional drawing of 207 valve.
©2000 CCI. All rights reserved.
235 | Replacement Pressure Control and Superheater Bypass Valves Permit 93% Cyclic Load Cutback at PG&E’s Moss Landing
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