Start Up and Commissioning of Supercritical Power Stations: Risk Management Plan

Supercritical Units: Efficiency Advantage

Improvements in the efficiency of pulverized coal (P.C.) fired boiler/steam
turbine power plants have been pursued since the introduction of P.C. fired
boilers in the 1920’s.

This led eventually in the late 1960’s to the introduction of supercritical boilers
operating at about 570 Deg C (1060qF) superheat/reheat temperatures and 24
MPa (3400 psi) pressure for the steam.

Presently boilers with 300bar and 700 Deg C are being adopted for many of the
recent power plants.

Since at these pressures and temperatures there is no distinct differences
between the water and steam phases, no drums are required in the same.

Dr. Himadri Banerji:Conference on
Plant Commissioning Kuala
Lumpur 1st and 2nd Nov 2010 10/29/2010 1

The new
generation of
power plants
operate at
Conventional steam pressures higher
power plants operate at a than the critical
steam pressures in the pressure.
range of 170 bar and 565
Deg C.

These are Subcritical These are
power plants. Supercritical
power plants.

The operating
pressures are in
the range of 230
to 265 bar. And
565 Deg C
The next generation of
power plants will
operate with Steam
Pressures in the range
of 300 bar and
630-
Temperatures 630-
700 Deg Centigrade.

These are the Ultra
Super Critical Power
Plants.
Plants.


The lower part of the boiler furnace is arranged in a spiral
configuration such that the fluid path wraps around the boiler
as it travels up the furnace.


Comparison of Allowable Stresses between Conventional and Advanced
Stainless Steel Tubes, Advanced CrMo Steel Pipes


Advantages of Oxygenated Treatment OT of Supercritical Boilers

The benefits from a once-through supercritical boiler on OT include:

• Lowering overall corrosion rates by forming a protective, double-oxide layer
with a controlled amount of oxygen present in the condensate (This protective
layer is considered to be more stable than the oxide layer formed using AVT.)

• Decreasing boiler chemical cleaning frequency due to reduced amounts of
iron transport and deposition

• Allowing quicker, cleaner start ups and reduced corrosion product transport
rates during cold and hot start ups

• Allowing boiler operation at lower pH with overall objective of minimizing
chemical costs

• Eliminating feeding, handling, and storage of oxygen scavenger products

EPC Contractor’s Chemistry Challenges

Most once-through supercritical boilers have been converted from
previously predominant AVT to OT, with new facilities almost
exclusively using OT.

all-
This chemistry change requires all-ferrous metallurgy in the feed
copper-
water train, and precludes copper or copper-based alloy feed water
heat exchangers in system design and bronze impellers in
condensate pumps and valve trims in the condensate system. This is
primarily the challenge to the engineering group og the EPC
contractor to ensure that this is strictly adhered..all steels contain
traces of copper and zinc.


coal-
Shows the pressure drop history of a coal-fired supercritical plant in Japan that changed the water
chemistry from AVT to OWT in 1996.

After 7 years operation since the change, there has been no significant increase in pressure drop.

This plant has not required acid cleaning since the change, and no future cleaning
is planned.


RISK MANAGEMENT
Plant Commissioning

SUPERCRITICAL UNITS: EPC CONTRACTOR’S CHEMISTRY CONTROL PARADIGM



Some once-through supercritical boiler manufacturers have instituted penalties
against the allowable pressure drop during initial boiler performance testing, an
additional complication that may impact start up and commissioning activities.

These penalties are based on extended operation on all volatile treatment
reducing (AVT[R]) during start up and commissioning.

The reducing environment (negative oxidation reduction potential [ORP])
present when operating on AVT(R) may contribute to increased iron transport,
subsequently increasing the pressure drop through the boiler.

These pressure drop correction penalties will be fervently debated by the EPC
contractor during commissioning and challenged by both owners and plant
operators.


WARRANTY IMPLICATIONS

Steam turbine suppliers are also setting limits, in the
equipment contract, on the number of hours a turbine can be
operated with out-of-specification chemistry. These limits are
typically listed in an action-level These limits are typically
listed in an action-level format where minor chemistry
excursions are allowable for predetermined time
periods without violating the equipment warranty.
up—
Steam Purity Limits During Start up— EPC
Once-
Contractor Recommendation for Once-Through
Boilers...usually one week


During steam-side start up and commissioning, the EPC contractor is mostly
interested in main and reheat steam chemistry.

Table below lists EPRI recommendations for once-through boilers operating under
OT, including normal target value and action levels 1, 2, and 3.

Although recommendations listed in Table are
acceptable for targeted chemistry limits during
operation, EPC contractors would like to see the
table:
following two columns added to this table:

• Allowable chemistry excursions during
hot start up

• Allowable chemistry excursions during
cold start up

Feed Water Dr. Himadri Banerji:Conference on
Lumpur 1st and 2nd Nov 2010 10/29/2010
11

What Are Action Levels 1, 2, 3


EPC CONTRACTOR’S CHEMISTRY CONTROL PROGRAM

Start up chemistry guidelines should primarily focus on main steam chemistry
targets, including

cation conductivity: it warns of salts and acids that may cause turbine corrosion

silica : silicate scaling may contribute to turbine capacity and efficiency losses.

sodium: critical for avoiding corrosion because uncontrolled sodium hydroxide
concentrations are known to cause corrosion damage failures in boiler
tubes.

Targets for chlorides, sulphates, and organic compounds should be
deferred until the end of the commissioning cycle.

Degassed cation conductivity is the preferred conductivity to be
measured during commissioning since system air leaks are still
being discovered and sealed during the start up and commissioning
phase. The measurement of degassed cation conductivity will aid in
differentiating between air leaks and other contamination sources


The two most important parameters in Table are feed water cation
conductivity and pH.

Cation conductivity should be maintained below 0.15 µS/cm during
operation on OT. Normal pH range for feed water under OT is 8.0 to 8.5.

The EPC contractor is challenged with controlling
pH when feed water cation conductivity increases
to concentration levels listed in Table , action levels
1, 2, and 3 (≤0.3 µS/cm, ≤0.6 µS/cm, >0.6 µS/cm,
respectively).



The pH/conductivity relationship is crucial for once-through cycles on OT;
thus, the EPC contractor implements the chemistry control at its own risk.

Important issues to be addressed when implementing OT include:

• At what point during the start up and commissioning process should the
chemistry regime be switched from AVT to OT to prevent frequent switching back
and forth between a reducing and an oxidizing environment?

• What would be the “detrimental effects” of going from an oxidizing atmosphere to
a reducing (or close to reducing) atmosphere, for temporary periods?

• How can these “detrimental effects” be quantified and addressed during design
and equipment procurement?


Stringent steam quality limits implemented by steam turbine
suppliers, boiler manufacturers have tightened limits on feed water
chemistry.

Yet EPC contractor requires standards to be relaxed during
commissioning to permit timely unit start up.

The challenge also is in determining the appropriate time to switch
from AVT to OT
Once cation conductivity levels are stable below 0.15 µS/cm,
EPRI recommends operation on OT with oxygen injection in a
pH range of 8.0 to 8.5. EPRI guidelines also state that oxygen injection
into feed water may continue with pH controlled between 9.2 and 9.6
µS/cm µS/cm.
and cation conductivity between 0.15 µS/cm and 0.3 µS/cm. However,
µS/cm,
at cation conductivity levels greater than 0.3 µS/cm, EPRI recommends
that oxygen injection be terminated and AVT resumed. Upsets in
cation conductivity may lead to serious corrosion problems if oxygen
is continuously fed during upset conditions.


Comparison of Start-up Systems


The turbine bypass system was designed to minimize the start-up
time by controlling the main steam pressure and temperature before
turbine rolling, and enabling the steam to flow through the super
heater sections at a short time after light-off.

The low-load recirculation system was designed to recover residual
heat during start-up by circulation of the un-evaporated water from
the furnace back to the economizer inlet, which also can assist in
reducing start-up time.

As this system is automatically operated, the start-up process is as
simple as with a natural circulation (NC) boiler.


EPC Contractor Guidelines on Feed Water Chemistry

Startup

Start up Steam Chemistry Limits

Turbine Rolling Continued AVT(O)


Turbine Rolling Continued
Balance of Commissioning Period Chemistry Limits


Operation Guidelines with AVT R and AVT O

AVT(R) Feed water Chemistry Control Guidelines

AVT(O) Feed water Chemistry Control Guidelines


Suggested Start up Feed water Chemistry
Guidelines for Once-Through Cycles Without Deaerators
Once-

Practical start up chemistry guidelines should be established by
consensus among the turbine manufacturer, boiler manufacturer,
and EPC contractor early on in project development and outlined
in equipment contracts.


ROLE OF CONDENSATE POLISHERS
DURING COMMISSIONING


Once-through supercritical boilers are commonly installed with
full-flow
condensate polishers to control corrosive impurities concentration
in condensate and feed water systems.
The presence of impurities in feed water will
significantly affect feed water chemistry, potentially
exceeding boiler supplier feed water limits and
turbine supplier steam purity specifications.


Impurities shaken loose during start up may cause a chemistry hold, where
plant load increases are temporarily halted until these impurities are
removed from the system.

For a once-through supercritical boiler, impurities are removed exclusively
by condensate polishers subsequent to chemical cleaning and boiler flush.

Once impurities are removed, the chemistry hold is lifted and the plant is
allowed to continue to ramp up to full load without exceeding allowable
boiler or turbine chemistry limits.


Steam and Water Analysis System (SWAS) shall be furnished for continuous
monitoring and control of water and steam purity in the plant cycle


EPC Contractor Responsibility includes that Chemistry limits are before, during
and after once-through supercritical boilers commissioning

• Control system component cleanliness during shop fabrication
• Control system component cleanliness during construction
• Flush system components prior to start-up
• Implement stringent water quality requirements for hydro testing
• Perform boiler and feed water system chemical cleaning
• Flush system components thoroughly following chemical cleaning
• Perform steam blows to obtain steam cycle cleanliness
• Implement time-based, progressively improving feed water and steam
chemistry targets


Start Up and Commissioning of Supercritical Power Stations: Risk Management Plan

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