The first evaluation task in the unit FAC program is to identify all piping systems, or portions of systems, and vessels that could be susceptible to FAC. FAC is known to occur in piping systems made of carbon and low-alloy steel with flowing water (single-phase) or wet steam (two-phase). All such systems should be considered susceptible to FAC. The unit line list, if available, and the Piping & Instrumentation Diagrams (P&IDs) can be used to ensure that all potentially
sections in their FAC program. Note that some lines supplied by an equipment vendor are often not on the unit line list or shown on the P&IDs. Care should be taken to ensure that such
susceptible lines are included in the FAC program. Additionally, this evaluation should be periodically reviewed to ensure that it is kept current with unit design changes and ways that systems are being operated. Clearly, each conventional fossil and HRSG unit will have a large number of systems and components that will be susceptible to FAC damage. The critical next step is to prioritize efforts such that any serious damage is detected and addressed as early as possible.
4.2.1 Exclusion of Systems From Evaluation
Some susceptible systems, or portions of systems, can be excluded from further evaluation due to their relatively low level of susceptibility. Based on both laboratory and industry experience, the following systems can be safely excluded from further evaluation:
• Systems of stainless steel or low alloy (nominal chromium content equal to or greater than 1.25%) steel piping. This exclusion pertains only to complete piping lines manufactured of FAC-resistant alloy. If some components in a high alloy line are carbon steel (e.g., the valves), then the line should not be excluded. Also, in lines where only certain components or sections of piping have been replaced with an FAC-resistant alloy, the entire line,
including the replaced components, should be identified as susceptible and analyzed. Note that high chromium materials do not necessarily protect against other damage mechanisms, especially cavitation and liquid impingement erosion. Thus, if the wear mechanism has not been identified it is not prudent to exclude the replaced components from the inspection program.
• Superheated steam systems with zero moisture content, regardless of temperature or pressure levels. Drains, however, from superheated steam systems should not be excluded
automatically. Experience has shown that some systems designed to operate under superheated conditions may actually be operating with some moisture in off-normal or reduced power level conditions. Care must be exercised not to exclude such systems. • Raw water systems, such as service water (high dissolved oxygen content).
• Single-phase systems with a temperature below 200ºF (93°C) (low temperature). A note of caution is made that, if measurable wall loss is identified in nearby piping operating slightly above 200ºF (93°C), then it is recommended that the system’s exclusion be reconsidered. Importantly, there is no temperature exclusion limit that can be recommended for two-phase systems. Note that other damage mechanisms, such as cavitation, are predominant below 200°F (93°C) and need to be taken into account. This document, however, does not address these other damage mechanisms.
• Systems with no flow, or those that operate less than 2% of plant operating time (low operating time); or single-phase systems that operate with temperature > 200ºF (93°C) less than 2% of the plant operating time. Note that if the actual operating conditions of the
system cannot be confirmed (e.g., potential leaking valve, time of system operation cannot be confirmed), or if the service is especially severe (e.g., flashing flow), that system should not be excluded from evaluation based on operating time alone. A further caution—some lines that operate less than 2% of the time have experienced damage caused by FAC. These lines
include feedwater recirculation, and steam line drains downstream of traps. Such lines should be excluded only if no wear has been observed and continued operation under existing parameters is assured.
It is recommended that a list be developed of the systems excluded from the FAC program. The list should note the basis for the system exclusion. This list should be appropriately documented and periodically reviewed. It has proven useful to have plant operating personnel review the list of excluded systems.
Systems should not be excluded from evaluation based on low pressure. Pressure does not affect the level of FAC wear. Pressure only affects the level of consequence should a failure occur. A failure in a low pressure system could have significant consequences (e.g., failure in a low pressure extraction line). Also, arbitrary ranges of velocity or other operating conditions should not be used to exclude a system from evaluation.
The systems excluded by these criteria will not experience significant FAC damage over the life of the plant. However, it should be noted that such systems could be susceptible to damage from other corrosion or degradation mechanisms. These include cavitation erosion, liquid
impingement erosion, intergranular stress corrosion cracking (IGSCC), microbiologically-
influenced corrosion (MIC) and solid particle erosion. These mechanisms are not part of an FAC program and should be evaluated separately.
4.2.2 Prioritize Units and Systems For Evaluation
Importantly, each conventional fossil or HRSG unit contains a significant number of components susceptible to FAC damage. Expanding the unit-specific level to a fleet of units within an
organization further increases the potential scope of work to be performed under an FAC program. It is, therefore, essential to put in place a prioritization plan that focuses resources on the most at-risk systems. The subsections below will provide initial guidance in prioritizing units both from a conventional fossil unit perspective and from an HRSG perspective. Industry experience, as reviewed previously, clearly offers an effective tool for the initial prioritization efforts.
4.2.2.1 Conventional Fossil Units
Analysis (summarized in Section 2.2.5) of the serious incidents of FAC (major bursts of pipes and tubes, and fatalities) in conventional fossil power plants and industrial steam plants provides a very clear priority or ranking order of units that are susceptible to FAC damage and should be inspected to establish baseline condition:
• Units with all stainless steel tubing in feedwater heaters, HP and LP.
• Units with stainless steel tubing in feedwater heaters in either HP or LP trains. • Units with carbon steel tubing in feedwater heaters.
The feedwater heater drain lines are susceptible locations on all units and currently account for the most frequent occurrences of FAC damage (Table 1-2). This situation is especially true in units with all-ferrous feedwater heaters, if the heaters operate with the vents open then this releases much of the dissolved oxygen content, thus increasing the probability of FAC.
The next level of prioritization is to determine the large list of susceptible systems on each unit. All of these systems must be evaluated. However, if time or resources are limited, it may be necessary to prioritize the scheduling of evaluations. The following is a reasonable, first-order listing of priorities:
1. Large-bore piping.
2. Susceptible small-bore piping and tubing with the most significant consequences of failure. (essentially economizer inlet tubing from the economizer inlet header and heater drain lines) 3. The remaining small-bore piping.
A failure in a large-bore piping system has potentially more significant consequences to personnel safety and plant availability, and thus these should be given first priority. Analysis and inspection of all susceptible large-bore piping systems is recommended. At a minimum, initial inspections of large-bore systems should be conducted at the next scheduled outage, if they have not yet been performed. For purposes of FAC evaluations, large-bore piping is defined as piping with a nominal diameter of greater than 2.5 inches (63.5mm).
Recommendations are provided in Section 5 for inspecting large bore piping.
Although the consequences of failure may be less, problems with small-bore piping and tubing in general, and socket-welded fittings in particular, have been experienced. FAC-related leaks and ruptures, some resulting in plant shutdowns, have been reported in small-bore lines. For the purpose of FAC evaluation, small-bore piping and tubing is defined as both butt-welded and socket welded piping and tubing with a nominal diameter of less than or equal to 2.5 inches (63.5mm).
The number of inspections performed for small bore piping and tubing is plant dependent. Economics could determine the extent of inspections performed versus wholesale replacement with FAC-resistant materials.
Use of EPRI CHECUP software to help prioritize actual inspections by component in FAC susceptible systems will be discussed in Section 5. The objective of this second tier
prioritization is to improve the cost-effectiveness of the inspection process and is not intended to replace this unit-by-unit prioritization.
4.2.2.2 HRSG Units
HRSGs do not typically have feedwater heaters, and the prioritization of units for FAC actions does not focus on consideration of feedwater heater tubing metallurgy. More critically for HRSGs, the FAC prioritization should include a review of low pressure evaporator tube failures and the chemistry treatment used for the feedwater as discussed in Section 2. Units currently operating, or previously operated, on all-volatile treatment (AVT) with addition of a reducing agents such as hydrazine should be given priority for FAC assessment.