POWER February 2013 - 20

ASSET MANAGEMENT
■ Keep the chemical oxidation-reduction potential
of water in the cycle the same during
all operating conditions. This principle
refers not only to excluding air but also to
maintaining chemical residuals that exist
during operation. If reducing agents (such
as hydrazine) are used during normal operation,
they should be used during layup. If
they are not, they should not be introduced
just for layup (unless extenuating circumstances
exist that should be reviewed).
■ Keep water from becoming oxygenated by
the surrounding environment. Regardless
of the chemistry during operation, water in
the steam cycle should never become saturated
with oxygen by unrestricted contact
with air, because it will cause corrosion.
Stagnated oxygenated water (or moist surfaces)
have been demonstrated to cause
disruption of the passive oxide films on
metal surfaces and to initiate pitting as a
precursor to corrosion fatigue and stress
corrosion cracking.
■ Keep water and moisture out of steamtouched
components and any water-touched
surface to be maintained dry during the
shutdown period. Successful layup practices
require consideration of all water/steamtouched
equipment in the steam cycle and
should begin as the equipment is being
removed from service. Partial layup of the
system, or layup of only some of the equipment
(boiler), will not produce the desired
results or protect against corrosion. The simultaneous
presence of moisture and oxygen
should be avoided; dry surfaces ensure
that impurities deposited on the surface will
not generate highly concentrated electrolytic
(corrosive) solutions.
For plants without layup systems and procedures
in place, several preventative measures
can be taken. For example, consider
the equipment that is available and commonsense
operating procedures that can provide
significant benefits:
■ Maintaining vacuum on the condenser
during short outages.
■ Properly using existing steam spargers to
preheat and deaerate water in the condenser
or deaerator storage tank.
■ Promptly and completely draining all water-containing
equipment while it is hot at
the start of overhauls.
■ Using plant compressed air to facilitate
draining and drying of boiler and steam
tubing while metal is still warm, utilizing
residual heat to facilitate drying.
■ Filling the boiler from the bottom to minimize
aeration.
■ Using hot deaerated and treated water
from a sister unit to fill a dry boiler.
20
Duration of the Outage. The length of
shutdown is fundamental to the type of layup
procedure or technique selected. The rapidity
with which plant personnel need to return a
unit to service can place constraints on how it
is shut down or on the procedures and practices
used for layup. Although certain approaches
are considered more appropriate for
certain types of outages, more than one layup
approach can provide equipment protection.
EPRI defines shutdown periods as follows:
■ A " short-term shutdown " involves periods
extending overnight or through a weekend.
This shutdown period is typical of cycling
operation and utilizes a wet layup or hot
standby approach. Wet layup is generally
considered for shorter durations of a weekend
or one to two weeks, but it can be effective
for months, if properly implemented
and maintained. Dry layup is effective even
for the shorter durations, but it might be
desirable to avoid draining the equipment
if the unit could be returned to service on
short notice. Some components-especially
superheaters, reheaters, and, most
importantly, the steam turbine system-
require the avoidance of moisture or steam
condensation; maintaining temperatures
above saturation may be effective but is not
practical for extended periods.
■ An " intermediate shutdown " is longer than
a weekend and up to one week. This duration
typifies a shutdown for minor equipment
repairs. Either wet or dry approaches
can apply.
■ A " long-term shutdown " is one extending
from a few weeks to six months. Such outages
can involve major equipment repair, a
planned outage, or a long-term layup due
to system load requirements. It also could
include " mothballing " a unit. Both wet and
dry approaches can apply, but if return-toservice
timing is not an issue, totally dry
layup is preferred for extended outages.
Purpose of the Outage. A prime factor
affecting the choice of a layup procedure is
the type or purpose of the outage: economic
dispatch, forced outage, or scheduled outage
for maintenance. Also of importance in planning
is the actual outage length, which may
be subject to change. Factors include system
conditions or dispatch requirements, system
reserves, cost of generation, status of other
units, equipment inspection results during
outage, and other maintenance activities.
Return-to-Service Requirements. The
choice of layup practice will be driven by how
quickly the unit is expected to return to service.
For example, it takes longer to return a
unit to service when it has been in dry layup
than if it is full of water (generally due to acwww.powermag.com
tivities
such as disconnecting piping, removing
or installing blank flanges, realigning valves,
filling with treated water, and so on). A more
detailed discussion of these factors is found in
the concluding section of this article.
Maintenance Activities During Outage.
Strong consideration should be given
to the anticipated maintenance required
during the outage. This consideration may
affect whether the boiler is rapidly drained
while hot or allowed to cool with water in the
system. Maintenance scheduling may make
it preferable to take advantage of water in
the boiler to more rapidly cool thick-walled
headers and steam drums. Draining hot facilitates
the preferred dry storage of a boiler
for long outages; cooling with water requires
nitrogen-blanketing to prevent the introduction
of air to wet surfaces.
Environmental Conditions. The most
obvious environmental condition is the potential
for freezing water stored within tubes.
This condition can be mitigated by temporary
enclosures, heat tracing, or localized sources
of heat. Dust, salt spray (including cooling
tower drift), and high humidity also can
damage equipment. These conditions can be
mitigated by using temporary, environmentally
controlled structures (such as tents or
canopies) and controlling humidity and dust
with filtered air-handling systems. Changes
in ambient temperature (day-to-day and over
a single day) often reflect changes in the relative
humidity and cause the enclosed equipment
to " breathe. " These changes may require
pressurization with nitrogen or dry air or the
use of dehumidification systems.
Convenience. The layup method must be
easily implemented in a timely manner. For example,
if it takes a week or longer to implement
layup for a typical outage of six to seven weeks,
and another week or longer to prepare the system
or component for plant startup, the method
might be too complex and, in all probability, it
will not be accepted by plant personnel. Proper
outage planning and procedures plus equipment
engineering/design can mitigate some of the inconvenience
and setup/teardown time.
Plant Design. Plant design can affect the
selection of layup method for certain systems.
For example, main steam line pipe supports
are not generally designed to carry the additional
load of water in the lines. Therefore,
wet layup cannot be applied to plant steam
piping unless pipe supports are modified for
the additional load, or temporary supports are
installed. Staff experts must review the appropriateness
of the layup methods proposed
for each plant system.
Planned vs. Unplanned Outage. Many
shutdowns are elective. They typically occur
with some advance notice and usually offer
at least some flexibility in timing. The shutPOWER
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POWER February 2013

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