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SPECIAL ISSUE: OUTAGE
HANDBOOK
TURBINE BYPASS SYSTEMS
How to verify proper operation of
bypass, spray-water valves
S
team-turbine bypass sys tems are used in combinedcycle plants during startup,
shutdown, and load rejection.
The most common arrangement,
shown in Fig 1, allows plant operators to route high-pressure (HP)
superheated steam through a pressure control valve/desuperheater station to the cold-reheat line,
thereby bypassing the steam turbine’s HP cylinder. This typically
is referred to as the “HP bypass.”
In addition to the HP bypass, an
IP/LP-turbine bypass allows operators to condition and dump steam
from the hot-reheat line directly to
the condenser via another pressure
control valve/desuperheater station.
This generally is called the “hotreheat bypass,” or more simply, the
“HRH bypass.”
The service life of a bypass system
can be shortened dramatically by poor
design and installation practices and
by ignoring control issues that cause
severe thermal stresses conducive to
metal fatigue. Cracking attributed to
thermal fatigue has occurred in valve
trim, bodies, desuperheaters, and
downstream piping, and is most acute
in plants cycling daily (Fig 2).
All plants with turbine bypass
systems should establish preventive maintenance and inspection
programs to determine if cracks are
HP bypass to cold reheat pressurecontrol valve/desuperheater
(details in Fig 3)
Spray-water
control valve
HP
IP
LP
Block valve
Cold-reheat line
Pressure control
valve/desuperheater
Hot-reheat or IP/LP
bypass to condenser
details in Fig 3)
HP superheater
HRSG
Reheater
HP evaporator
Condenser
Sparger
Hot-reheat line
Deaerator
Economizer
Feedwater
heater
Feedwater
pump
Spray-water valve
Condensate
pump
1. Steam-turbine bypass arrangement is typical for a combined-cycle plant.
HP bypass routes high-pressure superheated steam to the cold-reheat line; IP/
LP bypass reduces the pressure and temperature of hot-reheat steam
OD
2. Life of bypass system components can be shortened dramatically
by poor design and installation practices and by failure to confirm periodically the proper operation of critical
valves during startup, shutdown, and
normal operation. Photo A shows
crack originating in the heat-affected
zone between the bypass valve and
its desuperheating section; crack in B
is in the heat-affected zone of another
weld in the bypass system; cracking of valve trim caused by thermal
shock is in C; and a valve-body crack
caused by quenching of hot metal is
evident in D
COMBINED CYCLE JOURNAL, Third Quarter 2008
ID
A
B
C
D
27
TURBINE BYPASS SYSTEMS
SPECIAL ISSUE: OUTAGE
HOT-REHEAT BYPASS VALVE/DESUPERHEATER
HANDBOOK
Planning forensics
The case history that follows chronicles work undertaken by personnel at
InterGen’s La Rosita Power Project,
Mexicali, Mexico (sidebar), and CCI
~10 ft
engineers to determine the root cause
of cracks discovered in the HRH and
TC1
TC3
TC7
HP bypass systems for one of the
plant’s three gas turbine/heat-recovTC4
TC6
ery steam generator trains (so-called
Unit C). Details were provided by
La Rosita’s J Andres Felix, associate
TC5
TC8
TC2
Cross section 1 Cross section 2 Cross section 3
plant engineer, and CCI’s Dan Watson, development engineer.
Many plant-operations experts
recommend that the type of diagnostic study conducted at La Rosita
be done during the commissioning
of every plant and repeated about
TC9
~10 ft
six months before each hot-gas-path
HP-TURBINE BYPASS VALVE/DESUPERHEATER
or major inspection—depending on
service duty—to ensure needed parts
3. Bypass systems should be checked for proper operation about six months
are available for the outage.
before each hot-gas-path or major outage (depending on duty cycle) to allow
Felix said La Rosita contacted
sufficient time for ordering replacement parts that might be required. It’s relaCCI well in advance of an upcoming
tively simple to install the necessary thermocouples and collect data, tasks that
major inspection/overhaul and that
could be done by the plant staff with proper training
the vendor recommended the study
present or likely to develop. Normal lowing must be inspected, reviewed, profiled here. Reasoning was simple:
measures of plant cyclic life—such and evaluated periodically to obtain Knowing the root cause of the crackas number of starts, duration of a true assessment of condition:
ing, the plant could implement corstarts, number of trips, etc—are not n Temperature gradients in the rective during the outage. La Rosadequate for gauging the condition
bypass system.
ita runs base-load in summer and
of bypass systems. Rather, the fol- n Adjacent pipes and butt welds.
cycles (usually daily) during most
n Steam valves, spray valves, desuother months. It had operated about
perheaters, and dump devices.
40,000 equivalent hours by the time
n Desuperheater control
this study was conducted
La Rosita thumbnail
logic, and startup and
in March 2008.
shutdown data from the
Review of information
InterGen’s La Rosita Power Project,
plant historian.
gathered during annual
a 1065-MW, 501FD2-powered, 3 x
Perhaps the last thing
inspections and sent to
1 combined-cycle facility, designed
a roving operator wants to
CCI for the development
and built by Bechtel Power Corp,
see on rounds is steam (or
of a meaningful proposal
Frederick, Md, began commercial
water) leaking out from the
revealed the following peroperation in August 2003.
insulation at a bypass statinent details about Unit C:
Approximately 500 MW of the
tion. No way to tell at first
n HRH bypass. A crack
plant’s output is purchased by
sight if a catastrophic event
was discovered a few inches
Mexico’s Comision Federal de ElecWatson
is about to occur.
downstream of the first
tricidad (CFE) under a 25-yr BOO
Most plants have proweld after the spray nozzles
(build/own/operate) power purchase
grams in place for periodic
(Fig 3). The crack extended
agreement. Remaining capacity is
inspection of bypass syscompletely around the P22
available to meet energy needs in
tems, as outlined by Steve
(2¼ chrome) pipe, but only
the border region. Natural gas is
Freitas of CCI-Control Comextended through the pipe
supplied via a 126-mi dedicated,
ponents Inc, Rancho Santa
wall at the 6 o’clock posicross-border pipeline, owned by
Margarita, Calif, in the 2008
tion, as evidenced by a
Sempra Energy/PG&E Corp, that
Outage Handbook (access
water drip. Bulging of the
runs from Ehrenberg, Ariz, to the
www.combinedcyclejourpipe at that position was
plant.
nal.com/archives.html, click
noted by engineers.
Availability since startup is
3Q/2007, click “Key elements
n HP bypass. Cracks invisFelix
greater than 95%. La Rosita, which
of successful PM programs
ible to the naked eye were
ranks among the cleanest electric
for turbine bypass systems” on cover.
found on the (1) pressure-control
generating plants in Mexico, is parBut relatively few combined-cycle
valve’s body drain pipe after the
ticularly proud of its environmental
facilities run sophisticated diagfirst elbow, (2) first weld downrecord. NOx emissions are virtually
nostics on their bypass stations to
stream of the spray nozzles, (3) on
eliminated by the latest SCR emisensure they are operating as designthe weld connecting the bypass
sions control technology. Water use
ers intended and to verify that the
pipe to the cold-reheat header (P22
is minimized by a treatment plant
designers’ intentions are correct for
to P91 joint). Note that P91 is 9%
that processes municipal wastehow plants must operate today (for
chrome/1% molybdenum.
water into makeup for the coolingexample, cycling rather than baseIt took about a day for CCI engiwater system.
load as designed).
neers to instrument both bypass sta28
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Data recorded by CCI engineers on
the HRH bypass during one startup
are shown in Fig 4; information captured for the second startup essentially was the same. At numerous points
during the startup—those identified
by the number “1”—TCs 2, 7, and 8
registered sharp temperature drops.
Watson said that when pipe temperatures drop so quickly, the cause
almost certainly is water contacting
the pipe’s internal surface.
Next, look at points marked “2.”
They show that when the bypass
valve closes, the spray-water valve
still is at approximately 25% open
and proceeds to close completely over
the next 10-15 seconds. The drop in
temperature recorded by TC7 around
6 a.m. confirms that water is flowing
but steam is not. For this startup,
Watson said, the magnitude of the
temperature gradient was relatively
small. However, he added, during
full bypass operation on a steam-turbine shutdown, the gradient would
be much larger.
30
900
100
Hot-reheat bypass to
condenser (CCI data)
TC1
800
90
80
Turbine-bypass valve stroke
700
70
TC2
600
60
3
Spray-water
valve stroke
1
500
400
40
300
30
TC7
200
2
20
100
0
50
10
4:27:53
5:17:53
Time
0
6:07:53
4. Sharp temperature drops recorded by CCI engineers almost certainly indicates that water is contacting the pipe’s internal surface. One cause was control
logic that allowed the spray-water valve to remain open for a few seconds after
the bypass valve had closed
600
(1112)
100
Hot-reheat bypass to condenser (DCS data)
500
(932)
2
Turbine-bypass
valve stroke
80
70
400
(752)
60
300
(572)
Spray-water
valve stroke
200
(392)
Spray-water flow
100
(212)
Steam temperature recorded
by permanent plant
thermocouple downstream
of turbine bypass valve near
cross section 3 in Fig 3
5:17:53
6:07:53
Time
0 (32)
90
4:27:53
50
40
1
30
20
10
Spray-water flow, tonnes/hr; Valve stroke, % open
HRH bypass
1000
Steam temperature, F
tions for diagnostic evaluation. Fig
3 shows this includes installation of
eight Type K thermocouples (TCs)
on both pressure control valves,
plus another on the body of the HP
bypass valve. Transducers also were
installed on both the HRH and HP
bypass and spray-water-control
valves to accurately measure valve
position.
The diagram shows where the TCs
were located to detect temperature
patterns indicative of abnormal or
improper valve and/or desuperheater
behavior. The transducers help identify errors between the valve demand
signals transmitted by the plant
control system (DCS) and the actual
positions of the valves.
Data were recorded continuously
over two-day periods by CCI engineers
on both the HRH and HP bypass systems. The plan was to capture data
from two startups and two shutdowns
of the gas and steam turbines. While
this goal was achieved for Unit C,
the steam turbine was in continuous
operation during the test period.
The analysis that follows focuses
on the startups. During shutdowns,
the HRH and HP bypasses did not
operate, or opened only a small
amount for a very brief period. However, the primary goal, to identify the
root cause of cracking, was achieved.
Finally, engineers noticed some
unusual behavior of the HP bypass
system during normal plant operation, as well as during startup, and
that is covered as well.
HANDBOOK
Valve stroke, % open
SPECIAL ISSUE: OUTAGE
Steam temperature, C (F)
TURBINE BYPASS SYSTEMS
0
5. Data on HRH bypass operation accessed from La Rosita’s DCS confirmed
CCI’s findings and helped identify the need for new valve trim as well as controllogic modifications
Another point to note is that
approximately a half hour after the
completion of startup, TC2 experiences a very sharp drop (follow
arrows from Point 3). It records a
temperature about 775 deg F lower
than the temperature indicated by
TC1. A significant temperature differential remains over the next 24
hours—although it slowly decreases.
What this suggested to Watson was
that water probably was leaking by
the “closed” spray-water valve.
HRH bypass, DCS data. Data on
HRH bypass operation captured
from the DCS, at the same time CCI
instruments were monitoring that
component, are presented in Fig
5. Note how closely the DCS data
for bypass and spray-water valve
strokes match the CCI curves in the
previous figure. Also, the temperature measured by the DCS control
thermocouple downstream of the
bypass valve (near cross section 3)
exhibits the same sharp temperature
drop as CCI’s nearby TC7.
InterGen’s Felix and CCI’s Watson
and Joe Polidan, and others in their
respective organizations, carefully
reviewed design data and compared
them to the information compiled
from the plant DCS and CCI field
instruments. Engineers concluded
COMBINED CYCLE JOURNAL, Third Quarter 2008
SPECIAL ISSUE: OUTAGE
1000
900
Steam temperature, F
800
HANDBOOK
100
TC2
HP-turbine bypass
to cold-reheat line
(CCI data)
90
TC6
Turbine-bypass
valve stroke
700
80
TC4
70
600
60
Spray-water
valve stroke
500
50
TC5
400
40
TC7
300
30
2
200
20
100
0
Valve stroke, % open
TURBINE BYPASS SYSTEMS
10
1
4:53:14
0
7:06:34
5:59:54
Time
6. Analysis of the HP bypass showed the spray-water valve remained open
after the bypass valve closed, as it did in the HRH bypass system
Steam temperature, C (F)
500
(932)
400
(752)
100
Turbine-bypass
valve stroke
HP-turbine bypass
to cold-reheat
line (DCS data)
90
80
Turbine-bypass
valve, steam
70
inlet pressure
3
Spray-water
valve, water
inlet pressure
300
(572)
Spray-water
valve stroke
200
(392)
Spray-water flow
1
0
(32)
50
40
30
20
2
100
(212)
60
10
4:53:14
Time
5:59:54
7:06:34
0
Pressure, bar (abs); Spray-water flow, tonnes/hr; Valve stroke, % open
600
(1112)
7. DCS data confirmed the behavior of bypass and spray-water valves characterized by the CCI test. Engineers learned that both the bypass and spraywater valves were leaking and required new trim
that approximately 20% more spray
water was being injected into the
steam flow than was indicated by
designers’ calculations.
Startup data from the technical
specifications indicated a fully open
bypass valve handling a steam flow
of 219 tonnes (T, a metric ton or 2205
lb)/hr and a matching spray-water
requirement of 72.4 T/hr. Test data
captured during the carefully monitored start last spring showed the
bypass valve was never more than
80% open. All other operating conditions either matched, or were very
close to, those calculated by designers.
When the bypass valve is at 80%
stroke, spray-water requirement
should be 57.9 T/hr, or 80% of the
32
72.4-T/hr design value. DCS data
show that the actual flow rate was
about 69 T/hr—nearly 20% more
than it should have been. The difference could be caused by instrumentation error or an error in control.
Engineers concluded that the HRH
bypass system had the following four
major problems, presented in the
order of their severity:
1. Spray-water valve was leaking, allowing water to flow into the
bypass piping. Evidence: The sharp
temperature drop recorded by TC2
(Fig 4) about a half hour after startup
was completed. The continual flow of
water cooled the bottom of the pipe to
near spray-water temperature while
the top of the pipe at the same cross
section (Fig 3) was 775 deg F hotter.
A temperature gradient of this magnitude creates enormous stresses on
the pipe and may be the primary reason that the pipe cracked.
Recommendation. Replace the plug,
seat, and trim in the spray-water
valve to eliminate the leak.
This work was done.
2. Poor synchronization of the
bypass and spray-water valves during closing. As mentioned earlier, the
spray-water valve closed about 15
seconds after the bypass valve closed.
Never allow water to enter the bypass
system when there is no steam flow.
This is one of the primary causes of
pipe quenching.
During the La Rosita startups analyzed by plant and CCI engineers,
the thermal gradients measured
on the pipe wall were relatively
small (not shown on the diagrams
to reduce clutter). But the effects
on the bypass-valve outlet diffuser
were almost certainly more severe.
Also, bypass operation during a
steam-turbine shutdown would likely
cause even more severe temperature quenching of the outlet piping
and the valve’s outlet diffuser. Such
sharp reductions in temperature create additional piping stress and are
conducive to cracking, especially during cyclic operation.
Recommendations. (A) Change control logic to ensure that the spraywater valve opens after—and closes
before—the bypass valve, so there
never is spray-water flow without
steam flow. (B) Modify control logic
to inject the proper amount of spray
water during startups. (C) Review
DCS data for other operating cases to
verify accuracy of spray-water flow.
Felix said the service firm La Rosita uses for its control-systems work
was assigned the task of troubleshooting spray-water valve control
logic. One issue identified was with
the enthalpy-calculational routine
and the equation governing that was
corrected. Also, the interlock that
prevents spray water from entering
the bypass system when steam flow
ceases was adjusted simply by tweaking the 4-20-mA signal setting that
controls the opening and closing of
the spray valve.
Tests conducted by the plant following these adjustments confirmed
proper valve operation because no
delta T was observed.
3. Too much spray water was
injected for the given startup steam
flow. This was corrected with changes to the control logic described in the
previous item.
4. Sharp temperature drops during startup are evident from the TC2
data shown in Fig 4. Engineers were
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not sure while these occurred and no
action was taken.
Other recommendations made
by CCI included the following: (1)
Inspect the inside of the pipe and
the valve’s outlet diffuser for damage that my not be visible from the
outside. (2) Service and inspect all
spray nozzles. (3) Calibrate spraywater flow meters. Plant completed
most of the work suggested and
900
Steam temperature, F
800
TC2
scheduled the remainder for future
outages.
HP bypass
Data recorded by CCI engineers on
the HP bypass during one startup
are shown in Fig 6; information
captured for the second startup was
similar. Lower two arrows originating at Point 1 in the diagram show
HP-turbine bypass to cold-reheat
line, “normal” plant operation
(CCI data)
100
TC2
90
TC4
TC4
80
700
70
600
60
500
400
300
200
100
0
50
TC5
1
Turbine-bypass
valve stroke
40
Turbine-bypass
valve stroke
30
2
20
3
10
Spray-water valve stroke
Day 1, 12:06:34
Day 1, 21:49:54
Time
Valve stroke, % open
1000
888-881-7118
Day 2, 7:33:14
0
8. Sharp temperature spikes recorded for the HP bypass during normal operation confirmed that water was entering the steam system via the spray-water line
34
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that when the bypass valve closes,
the spray-water valve is still open
approximately 8% and proceeds to
close over the next two minutes.
The negative temperature gradient observed on the curves for TCs
2, 4, 5, and 6 at about 6:30 a.m.
confirmed spray water was flowing
when steam was not. As noted in the
section on the HRH bypass, such a
large gradient causes an enormous
amount of pipe stress. It could eventually lead to cracking if the root
cause is not corrected—especially
considering the unit’s daily-start/
shutdown regimen.
Point 2 reveals that TC7 was
at saturation temperature for the
duration of the startup. This suggests that the pipe was either in
constant contact with water or the
steam at that location actually was
at saturation temperature. Either
way, too much spray water was
being injected.
DCS data. Information on HP
bypass operation captured from the
DCS, at the same time CCI instruments were monitoring that component, are presented in Fig 7. Note
that DCS data on the stroke of bypass
and spray-water valves exhibit the
same behavior as that identified by
CCI in Fig 6.
Comparison of design and operating data for the HP bypass, as was
done for the HRH bypass, revealed
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that approximately 82% more spray
water was being injected during
startup than specified. Specifically,
a wide-open bypass valve handles
166 T/hr of main steam and requires
18.8 T/hr of spray water. During the
startup evaluated, the bypass valve
reached a maximum stroke of 97%
and should have been supplied 18.2
T/hr of spray water. But actual flow
was 33.2 T/hr. As mentioned earlier,
this could be caused by instrument
or control error.
The spray-water inlet pressure
during the startup observed was 55
bar (abs). According to the design
specifications, the inlet pressure
should have been 42.2 bar (abs) for
all operating conditions. Thus the
spray-valve inlet pressure was about
30% higher than specified.
Conclusions. Engineers concluded
that the HP bypass system had the
following three major problems, presented in the order of their severity:
1. Poor synchronization of the
bypass and spray-water valves during closing. During the startups
evaluated, injection of spray water
after steam flow had ceased resulted
in the rapid reduction in pipe wall
temperature as measured by TCs 2,
4, 5, and 6 in Fig 6. The 500-deg-F
temperature drop experienced qualifies as “thermal shock” and one of
the main reasons for the cracks at
the desuperheater outlet.
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2. Bypass and spray-water valves
were leaking.
3. Too much spray water was
injected for the given startup steam
flow.
Recommendations a n d a c t i o n
taken by the plant to correct the
root causes of the HP bypass problems experienced were the same as
those for the HRH bypass described
earlier, with one exception: Plug,
seat, and trim were replaced for
the bypass valve, in addition to the
spray-water valve.
Normal plant
operation
HP bypass data also were recorded
between the two startups analyzed—that is, during normal plant
operation. However, the TC curves
in Fig 8 were not what engineers
considered “normal,” exhibiting
sharp temperature spikes while the
unit was in service (region between
the startup peaks for the first and
second days at the left- and righthand sides of the chart). During this
time there was no movement of the
bypass or spray-water valves.
From the detailed information collected during the first startup (Figs 6
and 7), engineers knew that the HP
bypass valve was leaking and constantly passing steam to the downstream piping. This would cause high-
COMBINED CYCLE JOURNAL, Third Quarter 2008
er downstream temperatures, ones
that should be relatively constant.
However, curves TC2 and TC4
reveal sharp drops in the downstream temperature, as the arrows
from Point 1 highlight. Only injection of water could make this occur.
Engineers concluded that either of
the following was happening: The
spray-water valve was slightly open
and constantly passing water, or it
was closed and leaking.
More evidence is provided by curve
TC5, which remained at or very near
saturation temperature and exhibited only small temperature variations
(Point 2). Data from TC3 (not shown
to minimize graphics “clutter”), collected at the top of the same pipe
cross section as TC5, was constantly
300 to 400 deg F higher than the TC5
readings, proving water was flowing
along the bottom of the pipe.
Finally, position-feedback data
from the spray-water valve indicated that it might have been from
2% to 4% open during operation (follow arrows from Point 3). But based
on DCS data, engineers determined
that the zero point for the spray-water valve’s position-feedback measurement may have been skewed
by movement of the feedback transducer. Conclusion: The spray-water
valve actually was shut off during
normal operation, reinforcing the
belief that it was leaking. ccj
35