Transcript 2002112695721 266

The following article was published in ASHRAE Journal, December 2002. © Copyright 2002 American Society
of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This
article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.
A Primer on
Protecting
Idle Boilers
By Howard Benisvy, Member ASHRAE
any boiler plants have excess boiler capacity. Typical boiler plants maintain
backup capacity that must be instantly ready to provide steam should a
mechanical problem occur on the primary boilers. Some boiler plants may have
had a greater steam demand at one time, and process changes have reduced
present steam requirements. The challenge to many managers is to protect the
integrity of this equipment while the boilers are not being used. A significant amount of damage that
occurs to boilers during their lifetime can be associated with idle periods and extended downtime. The
typical processes in place that provide protection for boilers from scale and corrosion during operational times may not be available when the boiler is not running.
M
This article focuses on boiler systems operating below 300
psig (2.07 MPa) and discusses: extended layup (typically greater
than 30 days idle time); short-term layup; implementation of
wet and dry layup options; and proper startup of idle boilers.
A typical boiler system is shown in Figure 1. Your system
may have some or all of the equipment shown.
Water quality guidelines for boilers are provided by the
American Society of Mechanical Engineers (ASME) (Table 1).
In addition, the ASME guidelines for feedwater and boiler
water quality should be followed for idle boilers.1 It is critical
to provide oxygen levels below 7 ppb to minimize the potential for oxygen-related corrosion. This is accomplished by mechanical deaeration of the feedwater plus proper chemical
treatment to eliminate oxygen ingress from various sources.
An understanding of the various types of corrosion that can
occur during idle periods helps ensure you develop a success30
ASHRAE Journal
ful plan. A combination of both chemical changes to your
water treatment program and mechanical changes to your system are necessary to provide proper system protection.
Waterside Corrosion
Two types of corrosion are typically seen in idle boilers:
acid attack and oxygen pitting.
Both high- and low-pressure systems can be adversely affected by a low pH-acid attack from acid contaminant sources.
These acid sources could include condensate system contaminants, a demineralizer system anion exchanger, and leaking
feedwater and boiler non-return valves.
By far, the most common corrosion in idle boilers is oxygen
pitting. If oxygen is present in the water, oxygen pitting can
About the Author
Howard Benisvy is a senior consultant for Ondeo Nalco Co., Naperville, Ill.
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December 2002
Boilers
Oxygen pitting in an idle steam boiler.
occur on carbon steel. The rate of corrosion is dependent on
the concentration of oxygen and temperature of the water.
An example of waterside corrosion on an idle boiler due to
oxygen pitting is shown at top right.2
• Can the boiler be sealed? Do the valves hold?
The two boiler layup methods used in the industry today are
dry and wet layups.3
Dry layup is recommended for boilers stored more than 30
days, and wet layup usually is used for boilers kept idle less
than one month. Under certain circumstances, it may be necessary to store a boiler wet more than 30 days. However, special
attention and planning must be used to protect these boilers.
Corrosion can begin within one week and can escalate to cause
serious damage within one month.
By far, the most effective way to protect the waterside of an
idle boiler involves instituting a dry layup procedure. Many
of the factors that cause severe corrosion, occur when the boiler
is wet. Both acid attack and oxygen corrosion occur in a wet
boiler. However, with a proper dry layup procedure, you can
easily prevent these types of corrosion.
Negligible corrosion occurs if the relative humidity remains
below 55%. A dry layup program must be controlled to below
55% relative humidity.
Fireside Corrosion
Fuels such as coal and oil can generate deposits on the fireside including iron, vanadium, sodium and sulfur. Condensation and humidity must be controlled in the furnace so these
compounds do not react to form corrosive, low pH deposits.
Idle Boiler Layup
When deciding on proper layup procedures for your boiler
plant, several factors must be considered, including:
• What is the duration of downtime?
• Does this boiler need to provide immediate backup steam
capacity?
• Does the boiler have a superheater?
• Is the superheater drainable?
• Can the boiler be drained and left dry without affecting
plant operations?
• What types of fireside deposits are present?
December 2002
Dry Layup Guidelines: More Than 30 days
Waterside
• Thoroughly rinse out all boiler internals with clean water.
• Dry the watersides with warm air (might take several days).
• Blank all valves to ensure water cannot leak into the boiler.
• Install a desiccant to maintain low humidity levels in boiler.
(Desiccants include commercial grade silica gel [see manufacturer guidelines for proper amount]; quick lime [not hydrated lime], use at least 7 lbs per 1,000 pph boiler steam
capacity and use at least 3.2 kg per 0.126 kg/s steam capacity.)
Instead of desiccants, a nitrogen blanket can be used to
prevent air contact on the waterside. The boiler must still be
dried completely with warm air and pressurized with nitrogen to prevent air intrusion. For this procedure to be effective, a constant nitrogen pressure of at least 5 psig (34.5 kPa)
must be maintained.
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Feedwater
Dissolved Oxygen ppm (mg/L) O2 (measured before
chemical oxygen scavenger addition)
Total Iron ppm (mg/L) Fe
Total Copper ppm (mg/L) Cu
Total Hardness ppm (mg/L) CaCO3
pH range at 25 C
Chemicals for Preboiler System Protection
Nonvolatile TOC ppm (mg/L) C
Oily Matter ppm (mg/L)
Boiler Water
Silica ppm (mg/L) SiO2
Total Alkalinity ppm (mg/L) CaCO3
Free Hydroxide ppm (mg/L) CaCO3
Unneutralized Conductivity mhos/cm ( S/cm) 25 C
Total Dissolved Solids in Steam
TDS (maximum) ppm (mg/L)
Economizer
< 0.007
< 0.1
< 0.05
< 0.3
8.3 – 10
NS
<1
<1
< 150
< 700
NS
5400 1100
1.0 – 0.2
Boiler Type: industrial watertube, high-duty, primary fuel-fired, drum-type.
Makeup Water Percentage: up to 100% of feedwater.
Conditions: includes superheater, turbine drives, or process restriction
on steam purity.
Steam: superheated and/or turbine.
Saturated Steam Purity Target: see tabulated values.
NS: not specified.
Feedwater
Heater
Condensate Treatment
Steam (Polisher, EMF, etc.)
Supply
Deaerator
Makeup
• Clean/remove deposits from firesides. If the fireside cannot be completely cleaned and sealed, apply a barrier coating
to provide a moisture barrier. Special precautions must be made
to ensure the fireside does not cool down and allow moisture
to form prior to application of barrier coating.
• Completely dry fireside by applying warm air throughout internals.
• Close all air dampers and blank stack to ensure no
air/moisture entry.
• Add desiccants on storage trays to absorb moisture.
• Install humidity indication cards in easily accessible locations.
Condensate
Receiver
Condensate
Pump
Boiler Feedwater Pump
Figure 1: Typical boiler system.
Any Suitable
Boiler
Connection
WaterLevel
Small Steel Drum
Valve
Boiler Drum
Table 1: ASME table of suggested water chemistry limits.1
Fireside
Process
Primary Secondary
Boiler Superheater
Figure 2: Filling boiler drum to the top and changing the
location of air water interface.
begin increasing chemical dosages to the boiler at least one
week prior to shutdown. Keep in mind that high alkalinity
levels can cause foaming and carryover during on-line operations on some boilers. To minimize the potential for foaming,
adjust alkalinity just prior to shutdown.
Once the boiler is shut down, it is difficult to achieve adequate mixing of chemicals unless the boiler is placed on-line
to increase circulation. Initially, maintain 400-ppm sulfite because air intrusion causes sulfite levels to drop. To prevent oxygen pitting, do not let sulfite levels drop below 200 ppm.
Inspection
ShortTerm W
et Layup
Short-T
Wet
Inspections of both the waterside and fireside should be
performed weekly. Placing humidity indicator cards near inspection ports allows easy evaluation. A thorough inspection
should be performed at least monthly to ensure there is no
condensation, water, leaks or active corrosion.
Once humidity increases above 55%, change out desiccants.
Wet layup, regardless of length of time, needs to maintain the
same chemical parameters to minimize the waterside corrosion.
Plants that have several boilers sometimes choose to cycle all
their boilers on a regular basis. This routine sometimes causes
short two to three day run times. The number of startups and
shutdowns influences boiler life. The heating and cooling of
the boilers due to short run times increases mechanical stress
on the boiler. Ultimately, frequent expansion and contraction
along various joints and welds can lead to leaks.
These guidelines do not refer to boilers kept in hot standby.
However, plants that have several boilers may choose to keep
them in a hot standby mode at normal operating chemistry
levels. Boilers operating under these conditions are very susceptible to significant corrosion. It is important to evaluate
how many boilers need to be kept in hot standby vs. using
proper layup procedures. When possible minimize short run
times. Instead consider the following wet layup options:
Wet Layups: Less than 30 days3
Chemistry guidelines for wet layups of less than 30 days are
sulfite: 200 to 400 ppm; Hydroxide alkalinity: 600 to 800
ppm; and scale inhibitor that is in normal operating range
(avoid a precipitating program to minimize sludge formation).
This chemistry can vary, but the goal of the program is to
maintain a boiler pH of 11 and an oxygen-free environment.
Also, this chemistry cannot be used for layup of superheaters.
Demineralized water and all volatile chemistry must be used.
To achieve the proper wet layup chemical levels in the boiler,
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Boilers
Nitrogen Blanket
A nitrogen blanket is formed by filling the boiler completely
with water and pressurizing with nitrogen gas. The nitrogen gas
forms an inert barrier, and minimizes corrosive oxygen intrusion into the water phase. Generally, nitrogen blankets are difficult to maintain and only are used for short intervals. The
nitrogen blanket can prevent air introduction into the idle boiler.
It only can be effective on boilers that are capable of being
completely sealed.
Add treated water to the boiler in the recommended layup
ranges. All valves should be blanked to avoid leakage. Fill the
water level of the boiler to the top of the drum. Apply nitrogen
at 5 psig (34.5 kPa) to the steam drum. Slowly lower drum level
while maintaining at least 5 psig (34.5 kPa). Nitrogen must be
continuously maintained with a minimum of 5 psig (34.5 kPa)
to maintain the blanket.
A nitrogen atmosphere is fatal, and oxygen testing is required to ensure safe breathable air is available prior to any
boiler inspection.
been done very successfully. However, meticulous implementation of guidelines and continuous evaluation and monitoring were critical to the success of the layup program. Plant
personnel must evaluate idle boiler conditions as much as
those of on-line boilers to minimize the potential for failures
and extend equipment longevity. I have seen many more failures than successes with the extended wet layup option. By
far, dry layup provides the best boiler protection for extended
periods of downtime.
Water Sampling & Inspection
Water sampling should be performed at least once per week.
Since the water is stagnant, it is difficult to obtain accurate
samples. Test from several locations. If the chemistry falls below
any of the recommended levels, add more chemicals. Inspect the
boiler prior to bringing back on-line.
Drainable and Non-Drainable Superheaters
Protecting an idle superheater is extremely challenging. If
you have drainable superheaters, dry layup is the best option.
Flooded Wet Layup
Ensure the superheater is completely dry. A nitrogen cap in
Flood the boiler with treated water and maintain a 55-gal- combination with draining can provide the best protection for
lon drum (208 L)(with treated water) above the height of boiler a drainable superheater.
with a sight glass (Figure 2). Install a cover. The drum allows
Superheaters require the highest purity water and all-volafor expansion and minimizes air intile layup chemistry. Demineralized
Surface Blowdown
trusion into boiler.
water quality or better is recomTo
Blowdown
mended.
Steam
Flash Tank
Cascading Blowdown
Steam
Back filling the superheater with
Drum
Drum
treated water with an all-volatile
Cascading blowdown involves
chemistry will allow the introducfeeding blowdown water from the
on-line boiler to the idle boiler. The
tion of treatment. Fill the superMud
Mud
Drum
Drum
heater completely to minimize air.
idle boiler receives treated water,
A nitrogen blanket also can be apremaining heated and pressurized.
Standby Boiler
Operating Boiler
Cascading blowdown has been
plied to minimize air intrusion.
successful where the boiler is on a Figure 3: Cascade system with operating and
Chemistry for idle superheaters
low suspended solids treatment standby boilers.
includes a morpholine-type chemprogram such as one provided by
istry to raise the pH to 10 and a
all-polymer chemistry. This is not recommended for super- volatile oxygen scavenger. Test weekly to ensure chemistry is
heaters.
in range and that residual oxygen scavenger is present.
To successfully implement a cascade system, you must have
sufficient continuous blowdown to keep the standby boiler Proper Startup of Idle Boilers
treated and hot. Basically, surface blowdown from the on-line
Proper startup of boilers is critical to minimize scale and
boiler is supplied to the mud drum of the standby boiler. The corrosion, and to ensure good steam quality. It is not unusual
surface blowdown of the standby boiler is wasted to a blowdown for iron levels to be slightly elevated from layup. Every effort
flash tank (Figure 3).
needs to be made to ensure the boiler water quality is returned
If there are corrosive fireside deposits, it is critical that the to ASME guidelines as soon as possible. Prior to returning an
boiler be maintained at a pressure and temperature to prevent idle boiler to operation, a full inspection should be performed
condensation and potential corrosion in the furnace. All damp- to evaluate and record the condition of the waterside and fireers on the furnace should be closed to minimize any heat loss. side. The following procedure for startup is recommended:
1. Perform a boiler inspection and record condition (strongly
recommended).
Extended Wet Layup: More than 30 days
Wet layup periods exceeding 30 days have the greatest po2. If boiler is in dry layup, remove all desiccants and hutential for significant corrosion. There are cases where it has midity cards.
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3. Open and clear all valves on the waterside.
4. Open all dampers to the f ireside. Remove any
desiccants/humidity cards.
5. Fill boiler with high quality feedwater per ASME guidelines.
6. Increase surface blowdown to reduce chemistry to normal
operating ranges (if boiler was not drained and inspected prior
to startup).
7. Increase frequency of bottom blowdown for first week of
operation to remove suspended solids: maintain short 3 to 5
second blowdowns.
8. Monitor chemistry and adjust to get in range.
9. Monitor iron levels in feedwater and boiler to ensure they
are within ASME guidelines.
In my experience, I have seen each of these guidelines work
well in different boiler plants. Where these techniques have
failed, it has been due to poor implementation practices. Dry
layup has the best success rate, providing you follow the guidelines and ensure the boiler is kept completely dry. You must
ensure the fireside is free of corrosive deposits.
Some of the issues that prevent a successful layup include:
• Valves not completely sealed or blanked, allowing corrosive untreated water to enter the boiler.
• Inadequate sampling locations.
• Insufficient logging and evaluation of data.
• Lack of weekly testing and adjustment of chemistry.
• Inadequate cleaning of the fireside.
• Infrequent inspection.
Conclusion
As much as practical, this article provides guidance for protecting idle boilers. ASME is working on a detailed document
on this subject titled, “Consensus for the Layup of Boilers,
Turbines, Turbine Condensers, and Auxiliary Equipment.” It
should be available by the end of 2002.
Hopefully, these guidelines have provided a good foundation
for you to develop a successful layup system for your boilers.
Evaluating the mechanical limitations of your equipment is important to selecting the best layup approach for your facility. By
selecting the approach that can best be followed properly, you
help ensure good results and extended boiler equipment life.
References
1. Holloway, R.T., et al. 1994. “Consensus on operating practices for
the control of feedwater and boiler water chemistry in modern industrial
boilers.” American Society of Mechanical Engineers.
2. Port, R.D. and H.M. Herro. 1991. “The NALCO guide to boiler
failure analysis.” McGraw-Hill, New York, pp. 109 – 117.
3. Kemmer, F.N. 1988. The NALCO Water Handbook. Second Edition. McGraw-Hill, New York, pp. 39.44 – 39.45.
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