Where unacceptable damage has been found (as determined by the structural evaluations of Subsection 5.7.1), it is necessary to repair or replace the affected components. One mitigating approach that is sometimes used is to replace only those fittings that have experienced significant wear. This approach is satisfactory if the wear is very localized. This is the case in which the wear is concentrated downstream of a flow control valve or an orifice. In most cases, though, the wear is widespread throughout a susceptible line or system. Unless changes to cycle chemistry can be implemented to control FAC damage, it is only a matter of time until upstream or
downstream fittings will also need to be replaced. This fitting-by-fitting replacement approach is less expensive in the short term, but is generally not cost effective over the long term. Plants using this selected replacement technique have also experienced unexpected failures in components scheduled for future replacement. It is recommended that when making repairs,
(Cr content > 1.25%). Again, unless changes to cycle chemistry can be implemented to control FAC damage, replacement/repair with carbon steel will result in repeat FAC damage.
5.9.1 Repairing and Replacing Components
The following items should be considered in making replacement decisions: • The cost and availability of replacement fittings.
• The need for skills and procedures to weld alloy steels and clad material to carbon steel, or apply weld overlays or use the temper bead technique.
• The pre-and post-weld heat treatments required for welding chrome-molybdenum fittings. This heat treatment may affect the outage schedule.
• The piping stress analysis required if a large portion of a carbon steel line is replaced with stainless steel.
• The feasibility of replacing the entire system with a more wear-resistant material.
If repair is decided upon, the weld buildup technique is commonly used for the temporary repair of piping. Interior weld buildup is generally preferred to exterior buildup for the following reasons:
• Interior weld repair results in a smoother internal surface.
• By using interior weld repair, the resulting, smoother internal surface reduces the difficulty of making future UT inspections.
• An exterior weld buildup tends to result in a more complex state of stress.
Temporary clamping devices or furmanite boxes are often used to make temporary repairs to low pressure piping. However, permanent repairs should be made at the first opportunity in the event that the damage is growing and may cause the component to lose structural integrity (i. e., completely rupture).
If repair or replacement of a component is necessary, it is recommended that the plant owner develop a strategy so that the wear process does not continue. This essentially means not
repairing/replacing with carbon steel material. However, there are cases in which use of like-for- like (i.e., non-FAC resistant) material is appropriate. These cases include:
• The plant has optimized the feedwater chemistry (Sections 2 and 6) or the line will experience less damaging operating conditions (e.g., a higher steam quality) such that the replacement is projected to last the remaining life of the plant.
• Procurement of a resistant material would delay plant restart. In this case, consideration should be given to upgrading the replacement with a resistant material at the next outage.
• The remaining life of the plant is such that a like-for-like replacement will perform satisfactorily.
• Life cycle costs and risk considerations associated with like-for-like replacement, including associated inspection costs, do not support change to FAC resistant material.
5.9.2 Use of FAC Resistant Materials
It has been widely demonstrated that materials containing chromium are resistant to FAC damage. Lesser improvements come from molybdenum and copper. Replacing carbon steel piping with chrome-molybdenum alloy (SA335, Grade P11 or P22) (1.25 or 2.25% Cr alloys) or stainless steel (normally a 304 alloy) should alleviate FAC damage for the life of the plant. The benefit can also be achieved by coating the piping surface with a high-alloy layer (flame
spraying or weld overlay) or using a clad pipe with a high-chromium or stainless steel inner layer surrounded by a carbon steel outer layer. In all cases replacement should be with a minimum of a 1.25% Cr alloy. Recent EPRI work has reviewed the use of weld overlay for repair of FAC
damage to deaerators(11). This review notes the need for further weld procedure development to
allow use of chromium levels greater than 1.25% without performing post weld heat treatment. EPRI work is ongoing.
In the specific case of two-phase FAC damage to both conventional fossil plant components and to HRSG components, use of FAC resistant materials will likely provide the most cost-effective, longterm control of FAC damage since cycle chemistry control options are limited as discussed in Sections 2 and 6.
Table 5-2 presents the degree of improvement associated with common piping alloys, as
predicted by CHECUP™, which is based on the data of Ducreux(12)
. This data is generally
considered the definitive work in the area of the influence of material composition on FAC wear rate. It is clear from the values shown in the table that FAC can be effectively eliminated through material improvement.
Table 5-2
Performance of Common FAC-Resistant Alloys
Material
Nominal Composition (Chrome & Moly only)
Ratecarbon/Ratealloy
_________________
P11 1.25% Cr, 0.50% Mo 34
P22 2.25% Cr, 1.00% Mo 65
304 18% Cr >250
Material changes can be used to replace an entire system or to repair an especially troublesome area. However, material replacement may not reduce the wear rate if the damage is caused by a mechanism other than FAC. This is the case, for instance, if the damage is caused by cavitation or liquid impingement.
5.9.3 System Design Changes
When repairing a section or line of piping damaged by FAC, some consideration can also be given to design changes, particularly when a like-for-like replacement is made. However, design changes generally result in only small reductions to the rate of FAC damage. For example, reducing the flow velocity by changing the diameter of a piping system from 12 to 14 inches (30- 35 cm) will only reduce the FAC rate by about 20%. One instance, however, where design change can be effective occurs in increasing the pipe diameter to reduce the velocity in control valve stations. Valve stations are typically designed to accommodate the flow capacity of the control valves. This typically results in a reduced diameter of about 60% of the line size and a consequent increase in the fluid velocity. This locally increased velocity has often caused damage downstream of the valve. Redesigning the valve station to reduce the local velocity and turbulence can greatly reduce the rate of FAC damage.
5.10 References
1. CHECWORKS™ Computer Program Users Guide, TR-103496, EPRI, Palo Alto, CA:
August 1994.
2. CHECWORKS™ Fossil Plant Application - CHECUP™ Code, Version 1.0 User Guide,
EPRI, Palo Alto, CA: 1998. TR-103198-P5.
3. CHECUPweb Version 1.0 a web application on the EPRI Solutions production server, EPRI, Palo Alto, CA: 2004. 1008127
4. NDE of Ferritic Piping for Erosion/Corrosion, NP-5410, Electric Power Research Institute, September 1987.
5. FAC Wear Rate Assessment Through Insulation, EPRI, EPRI NDE Center, Charlotte, NC:
2000. 1000114.
6. On-Line Flow-Accelerated Corrosion Assessment of Large Diameter Piping Through Insulation with Radiographic Techniques, EPRI, Palo Alto, CA and Florida Power & Light,
Juno Beach, FL: 2004. 1009594.
7. Assessment of the Pulsed Eddy Current Technique: Detecting Flow-Accelerated Corrosion in Feedwater Piping, EPRI, Palo Alto, CA: 1997. TR-109146.
8. Interim Guidelines for the Nondestructive Examination of Heat Recovery Steam Generators,
EPRI, Palo Alto, CA: 2004. 1004506.
9. In-service Feedwater Heater Condition Assessment Using the Pulsed Eddy Current NDE Technology, EPRI, Palo Alto, CA: 2001. 1006372.
10. American National Standard Code for Pressure Piping, “Power Piping”, ANSI B31.1. 11. Repair of Deaerators, EPRI, Palo Alto, CA: 2004, 1008069.
12. J. Ducreux, “Theoretical and Experimental Investigation of the Effect of Chemical
Composition of Steels on Their Erosion-Corrosion Resistance”, presented at the Specialists Meeting on the Corrosion-Erosion of Steels in High-Temperature Water and Wet Steam, Les Renardieres, France, May 1982.