POWER February 2013 - 21

ASSET MANAGEMENT
down can be initiated with equipment protection
in mind, even if the duration is uncertain.
Unplanned outages due to mechanical problems,
severe water chemistry excursions, loss
of connection to the grid, or other causes occur
with little or no advance notice and might
restrict the ability of the operator to control
the shutdown and provide little opportunity
to put layup plans into action, unless the
plant staff anticipated the need and planned
for such an outage in advance.
Advanced Planning Required
Planning for layups involves identifying the
roles, procedures, and practices that must
be addressed in advance of every shutdown,
including anticipated contingencies. Plants
with effective layup programs generally incorporate
the following elements:
■ Written procedures to delineate actions to be
taken and the person or organization responsible
for placing plant systems into layup.
■ Scheduling of layup for various plant systems
during planning for the next outage.
■ Development of plant-specific logic diagrams
to provide the basis for layup method
selection.
■ Designation of a plant layup coordinator
responsible for placing appropriate systems
and components into proper layup
conditions.
■ Assignment of a layup team to assist the
layup coordinator in accomplishing assigned
responsibilities.
One utility recently completed implementation
of a series of layup processes and procedures
based on these planning elements.
Plant staff report consistent and effective
protection of the steam generating equipment
in less time with better results, including improved
startup chemistry and reduced startup
time-a real win-win scenario.
Finally, the selection of layup methods
for affected plant systems and components
should begin when planning for the next outage
begins, not as an afterthought.
Layup Methods: Wet and Dry
The procedures for layup of idle equipment
generally fall into two categories: wet and
dry. The decision about which layup method
to use can be made for each piece of equipment;
for example, the boiler can be drained
and kept dry for repairs, while the deaerator
can contain water and a nitrogen blanket for
a quick restart.
Wet layup permits chemically treated and
deaerated water to remain in the boiler, deaerator,
condenser, and all associated piping.
The goal is to provide proper chemical treatment
and prevent air from entering any wet
area in the cycle where pitting and corrosion
can occur. As the unit cools, steam pressure
excludes oxygen from contact with equipment
as long as it remains pressurized and a
vacuum is maintained in the condenser. As the
unit approaches ambient conditions (temperature
and pressure), nitrogen is typically added
to prevent air from filling the steam space as
the steam condenses to water. Although the
operating chemistry may be satisfactory for a
short outage, some elevation of the chemical
dosing should be considered before the unit is
completely removed from service.
Dry layup is an alternative for protecting
closed-in equipment such as pressure vessels
and piping. It is an especially effective
method when equipment must be periodically
opened and inspected. Two options can be
used for dry layup: nitrogen gas blanketing
and dry air.
Nitrogen blanketing is not used for longterm
layup due to maintenance and operational
requirements to keep the gas in the systems,
but it is more effective when protecting an airtight
vessel for short periods. Nitrogen blanketing
cannot be used where entry into the
enclosed space is required due to the hazard of
personnel asphyxiation.
Dry or dehumidified air offers additional
advantages to nitrogen gas. The dry condition
can be maintained even while equipment is
opened for routine inspections. Dehumidified
air systems are also less costly to maintain over
the long term. The table provides examples of
steps followed in each layup practice.
Special Considerations for Layups
of Different Durations
One popular belief is that short-term layups are
less critical than longer-duration ones. The reality
is that all intermediate and long-term layups
go through a short-term lay-up phase. All outage
period durations are equally critical. (See sidePossible
Layup Practices Based on Return-to-Service Requirements
The following possible layup practices are based on how quickly
the unit needs to be returned to service.
Forced Outage for a Boiler Tube Failure-unit will be down
just long enough to repair the boiler tube-less than 24 hours:
■ Boiler: dry layup (the boiler will need to be drained for repairs)
■ Deaerator: wet layup
■ Condenser/turbine: wet layup (residual heat of turbine should
mitigate damage)
Scheduled Outage with Return to Service Within 8 to 24
Hours:
■ Boiler: wet layup (with nitrogen, if pressure decays)
■ Deaerator: wet layup (with nitrogen, if pressure decays)
■ Condenser/turbine: maintain steam seals and vacuum on condenser
Scheduled Outage with Uncertain Return to Service-needs
to be able to return to service quickly:
■ Boiler: wet layup with nitrogen
■ Deaerator: wet layup with nitrogen
■ Condenser/turbine: dry layup with dehumidified air or wet layup
with nitrogen
February 2013 | POWER
Seasonal Outage-may be called on to produce power with one
to two days' notice:
■ Boiler: dry with dehumidified air
■ Deaerator: dry with dehumidified air
■ Condenser/turbine: dry with dehumidified air
All Outage Conditions-a single methodology now being
evaluated:
■ Boiler: conditioned on shutdown with proprietary barrier-forming
amines
■ Deaerator: conditioned on shutdown with proprietary barrierforming
amines
■ Condenser/turbine: conditioned on shutdown with proprietary
barrier-forming amines
Note that conditioning with proper chemical dosages over a period
of 8 to 48 hours permits extended storage whether equipment
is full of treated water, drained in humid to semi-humid conditions,
or dry. Dry conditions have been demonstrated to actually improve
the corrosive inhibition properties when subsequent humid or wet
conditions are encountered.
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POWER February 2013

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