Layup Practices for Fossil Plants

Courtesy: TVA

Improper layup practices are a major contributor to boiler tube failures and to steam turbine pitting and cracking in U.S. fossil plants. EPRI’s research into identifying damage mechanisms, utility best practices, and innovative new methods to protect plant equipment during outages will aid plant operators in achieving a successful layup.

For several decades, the Electric Power Research Institute (EPRI) has conducted ongoing research into the causes of damage mechanisms, utility best practices, and innovative new methods for protecting boiler and turbine equipment. This research has helped to identify the most effective planning strategies, evaluate protection techniques, and outline key principles for successful layups.

Our research confirms that damage to plant equipment from improper layup procedures continues to be a problem for U.S. fossil plants. There are many underlying factors to be considered, particularly at plants where equipment is aging and more vulnerable to damage. Our research confirms that improper layup practices are a major contributor to boiler tube failures and to steam turbine pitting and cracking, a major cause of reduced plant reliability and availability.

The causes of improper layup procedures are many. At some plants, the duration of the required layup is not always clear at the beginning of an outage; short-term outages can quickly become long-term outages, with different unplanned-for layup requirements or preservation techniques required. For example, some layup techniques, such as nitrogen blankets and dehumidified air, require capital expense for equipment and may take time to procure and assemble.

At other plants, correct layup procedures are well known but not always followed. An EPRI study found that only 37% of utilities surveyed routinely nitrogen-blanket the boiler, and only 6% protect the turbine.

Additionally, with historically low prices of natural gas, an increasing number of coal-fired plants designed for baseload service are now experiencing short-term outages due to age and cycling, as well as in response to reduced or seasonal dispatch demands. For these plants, layup practices may not have been a significant concern in the past, but now they must perform reliably when experiencing many starts per year or extended downtimes between periods of operation.

Choosing a Layup Procedure

Proper layup practices must consider the entire unit. Protection strategies should take into account site-specific factors, operational requirements, and unit design. A seamless transition from service through shutdown, into the out-of-service or layup period, and through the subsequent unit startup and return-to-service status must be factored into the strategy. Finally, proper storage of all major components or systems should be incorporated into a comprehensive layup procedure for the unit (see sidebar “Damage Mechanisms from Improper Layup Practices”).

Damage Mechanisms from Improper Layup Practices

Inadequate layup practices allow metal to remain in contact with oxygen and water for extended periods of time. The presence of salts, particularly chloride salts, can accelerate the corrosive effects of water and oxygen on metal surfaces.

Corrosion fatigue is a major failure mechanism in boiler tubes. The “fatigue” portion of the corrosion fatigue mechanism depends on mechanical and thermal stresses developed during the cycle. At a greater than 2% strain, the protective magnetite oxide of the stressed component cracks, exposing unprotected surfaces to the boiler water. Pitting, the “corrosion” portion of the mechanism, can significantly increase the propagation rate through the material (Figure 2). Pitting corrosion is the most prevalent layup-initiated corrosion mechanism. Other corrosion mechanisms, such as stress corrosion cracking and various under-deposit corrosion mechanisms, are initiated or exacerbated by pitting corrosion.

2. Permanent damage. This image shows oxygen pitting in a boiler drum that was given inadequate layup protection. Courtesy: EPRI

Poor layup practices also affect steam turbines. Left open to the atmosphere and sitting over a full and warm hotwell, the steam turbine is bathed in warm and humid conditions that are conducive to pitting, particularly on the low-pressure turbine blade/disk surfaces in the phase transition zone, which is a precursor to corrosion fatigue on the turbine (Figure 3). In cases where the turbine has been contaminated (for example, following a condenser tube leak), corrosion can be rapid and severe.

3. Pitiful problem. Deposits that contain chloride aggressively pit the turbine when it is allowed to sit in moist conditions. These pits become sites for corrosion fatigue. Courtesy: EPRI

The corrosion potential and damage risk during a unit outage are nearly independent of the outage duration. Mitigation of corrosion and preservation of the asset need to be implemented during the unit shutdown process. Two key points should be emphasized:

• Thermal cyclic stresses and rapid startup/shutdown operations mean that short-term outages can actually be more detrimental than longer, planned outages with controlled shutdowns.
• Corrosion activity is most aggressive in the minutes and hours following shutdown when the moist, aerated environment is at its highest temperature. A rule of thumb is that the rate of corrosion doubles for each 10 degrees Celsius of temperature elevation.

Frequent, short-term outages (from unit cycling) are more problematic and damaging than traditional, but infrequent, long-term outages, because the percentage of operating life and annual hours that the components are strained or imperfectly protected is increased by nearly an order of magnitude. The more time cumulatively spent in these operating conditions and the higher the frequency of cycling, the greater the chance of damage.

As a simple example, consider the number of times you can bend a paper clip before it breaks. Regardless of how slowly you repeat the cycle, the paper clip will eventually fail. But more rapid and frequent cycling decreases the time before it breaks. Unlike a paper clip, steam-generating equipment protection must also consider correct water chemistry (corrosive damage) and operating temperatures (thermal stresses).

The shutdown and layup periods should be viewed as a continuation of the good water chemistry practices used during operation. The primary purpose of the cycle chemistry is to provide protective oxide surfaces on all components throughout the steam and water circuits in order to minimize corrosion and reduce concentrating and performance-robbing deposits on heat transfer and aerodynamic surfaces. The primary purpose of the shutdown and layup practice should be to preserve those protective oxide surfaces and prevent damaging corrosion (Figure 1).

1. Trouble spots. Areas of the steam cycle affected by layup and startup practices. Source: EPRI

Based on EPRI research and personal experience, three guiding principles should govern all layup decisions and practices:

• 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 steam-touched components and any water-touched surface to be maintained dry during the shutdown period. Successful layup practices require consideration of all water/steam-touched 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 common-sense 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.

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-to-service 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 activities 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 shutdown 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 long-term 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 sidebar “Possible Layup Practices Based on Return-to-Service Requirements.”)

The approach to a shutdown and subsequent layup may be different depending on the outage duration. Planned, long-term outages are simpler to prepare for. Addressing short-term outages that become longer is trickier. For example, for a short-term layup, the turbine set will be hot (above saturation temperature) in many parts, and warm, dry air can be used to prevent condensation on the surface. But after a time, the metal will cool, and a transition to dehumidification is required to stay below a relative humidity of 40% and prevent hygroscopic absorption of moisture and oxygen absorption on the dry salt deposits. When and how to do this without proper preplanning becomes guesswork, so planning for these eventualities should be part of every shutdown routine. The same applies to a boiler that is left full, hot, and pressurized to prevent air intrusion; nitrogen capping must be done before losing the steam blanket, which happens when the unit cools.

Short-Term Layups. Short-term layup presumes that the unit will be required to operate within a relatively short timeframe. In general, no major changes are required from normal operations except that the unit must be protected from air ingress. Short-term outages are often scheduled outages, as opposed to maintenance outages. The equipment is to be maintained in a standby condition for restart either on a planned schedule or upon request. Typically, unit availability is maintained, and the return-to-service period can be as short as 12 to 24 hours or up to 72 hours.

These are some key considerations for short-term layups:

• Plant operators should take advantage of residual heat in heavy-walled components to facilitate drying of wetted components and to purge vapor spaces, by blowing dry air through the hot components to the cooler, wet regions.
• If a short-term layup is known to be less than a few days, the optimum protection is to keep the unit pressurized (to exclude air) and hot (to eliminate condensation and eliminate oxygen in wet areas). Flooded components should be kept flooded and vapor spaces kept hot (non-vented); the condenser and feedwater heater should be kept under vacuum (keep vacuum pumps running).
• Although corrosion fatigue in boilers is dependent on the strain induced by rapid startup and shutdown and disproportionate thermal expansion of joined components, maintaining proper water pH and oxygen below 100 ppb is a controllable factor that is known to markedly reduce corrosion fatigue damage in steam generation equipment.
• Turbines, superheaters, and reheaters should be kept dry. The Point Defect Model, which was developed in the 1990s to describe pit nucleation, found that aerated moisture is the key precursor to pit formation.
• For shutdown of drum-type units or heat-recovery steam generators, slowly dropping the steam pressure and temperature generally results in a wet steam with very low impurities. This moisture will effectively “wash” the turbine blades, removing water-soluble impurities. The process should allow liquid films of high-purity water to form on the material that is “rinsed” away by the subsequent water. The turbine should still be dried by purging with hot, dry air.

Follow instructions. Examples of steps used in each of the layup practices discussed. Source: EPRI

Intermediate Layups. Typically, during intermediate layups, the pressure within equipment will eventually decay to atmospheric pressure and approach ambient temperature. The length of an intermediate-term shutdown allows some additional flexibility for layup techniques; however, the selected technique should be determined not by the length of the outage alone but with consideration of other factors such as return-to-service needs and maintenance work to be performed.

Wet layup conditions should be used when it might be necessary to return the unit to service on short notice or when makeup water capacity is limited. Dry storage is the preferred method as the term of the outage increases or is of an indeterminate length.

Long-Term Layups. For long-term layups, many of the intermediate-term procedures can effectively protect equipment for a six-month period if conditions are properly monitored and maintained. During the outage, plant equipment must be “stored” in a condition that prevents both corrosion of idle components through moist, aerated conditions and deterioration of idle components through oxidation of elastomerics and drying of materials requiring moisture.

Wet layup techniques can be used for normally water-filled or wetted components such as feedwater heaters, condensate/feedwater piping, deaerators, and boilers. The main disadvantage of the wet layup method, particularly for long-term conditions, is that it requires staff time to ensure that the nitrogen or inert atmosphere and chemical conditioning are appropriately maintained.

Dry layup using dehumidified air is typically chosen for long shutdown periods.

Ongoing Research

EPRI is conducting research on barrier coatings or inhibitors that protect exposed metal surfaces by preventing interaction of air or moisture with the metal surface. These alternative preservation techniques can be applied when convenient, do not require capital expenditures and complicated operating procedures, and are flexible to changes in outage duration period. Current research has demonstrated that the development of a protective barrier on the metal surface established by the hydrophobic formation of polyamine products inhibits corrosion and pitting activity in the presence of aggressive chemical species and moisture.

New products and application methods are currently under test. Showing good promise are various proprietary products used in predetermined dosages in the hours or days preceding shutdown. Application during operation of the unit ensures complete distribution to all steam- and water-touched surfaces. It also provides the needed time and conditions to establish the impervious barrier on the metal surfaces. The flexibility of this technique, requiring only turning on a chemical injection pump prior to shutdown, would provide an economic and simple way to preserve equipment in a wet unit for a period of up to 30 days or in a dry storage condition for many months. Expect introduction of these new products within a year or two.

— James Mathews ([email protected]) is the manager of EPRI’s Boiler and Turbine Steam and Cycle Chemistry Program in Charlotte, N.C. He served as the consulting chemist for the Fossil Generation Division for Duke Energy, where he was employed for 36 years prior to joining EPRI.