Approach to feedwater heater corrective maintenance - Energy-Tech Magazine: Heat Exchangers

Approach to feedwater heater corrective maintenance

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Posted: Friday, February 1, 2013 12:00 am

In the April 2012 issue of Energy-Tech magazine, we wrote about the importance of developing a long-term life-cycle management program for your Feedwater Heaters (FWH). That article stressed that the utility must be committed to periodic condition assessment and trending over time. We identified a number of complimentary inspection and testing techniques to accomplish that objective. As heaters degrade and eventually fail, the understanding of why damage is occurring is just as important as the short term resolution of arresting the current leakage event.  It is for these reasons that failure cause analysis (FCA) is the primary objective in FWH life-cycle management and, as such, must remain an integral consideration throughout the corrective maintenance process.

This article will discuss details in maintenance repair procedures from two distinctive approaches once failures are experienced. One perspective is based on forced outage situations with very limited time available. Procedural guidelines are established about the approach of satisfying immediate resolution of leaks under these circumstances. The other perspective is a planned maintenance shut-down with a longer time frame and better working conditions. In either case, the station must have a comprehensive repair approach established for its FWHs, and not just a reactive “fix it and forget it” mentality.  

Forced outage approach

Based on an indication of leakage during operation, the heater should be isolated and removed from service as soon as practical to prevent collateral damage from leak impingement. The decision to authorize, or when to actually initiate a forced outage to address a leaking FWH, is strictly a Plant Management judgment. Certain FWHs pose different financial impacts on unit operating loss, depending on the respective locations within the system. Operations might be able to quantify the severity of the failure based on a tube-side pressure test. Maintenance should pre-stage tooling and equipment to address anticipated activities. Some of these activities include channel access, lift/rigging device operation, replacement gaskets, etc. Managers should review past job files (if applicable) regarding the specific heater to be addressed to look for trends.  Once access is gained, and the partition plate covers removed, station mechanics should conduct channel side inspection, keeping in mind that working conditions might be significantly hampered by the constraints of the situation. The following are the recommended subsequent steps based on a forced maintenance outage.

  1. Shell Side Test: With the heater isolated, conduct a shell side air test (approximately 30-40 psig), to locate leaks in the tube field. Leaks must be differentiated as either tube failures vs. tube weld joint or roll joint failures, where leakage is apparent through the ligament surrounding the tube. Repair procedures are predicated on this determination.
  2. Limited Individual Tube Hydrotesting (ITHT): Forced outages often preclude the necessary time or required conditions for NDE/ECT activities. The specific situation might limit maintenance personnel to do nothing more than shell side testing to locate leaks and move directly to plugging the failures (temporarily) and plan to re-address them at the next scheduled opportunity. The risk in doing so must be understood and accepted by management, since leak impingement may have caused collateral damage to adjacent tubes. Also, failures may be catastrophic and/or tubes may be totally severed. Without the time available to employ the more fragile instruments like NDE probes or fiberoptic videoprobes, maintenance should – at a minimum – attempt to perform ITHT of 2-3 rings of the failure’s adjacent tubes to identify any weakened tubes and minimize the possibility that they will not subsequently fail upon return to service. This approach is preferred to “insurance” plugging of surrounding tubes. ITHT procedures also were previously discussed in our April 2011 Energy-Tech article.
  3. Tube Leak Location Determination (TLLD): If remaining forced outage time frame and conditions allow, maintenance should try to ascertain the location (in the tube span) for all failures identified by shell side test and ITHT procedures. If this cannot be done at this time, it should be addressed during the next planned outage.
  4. Videoprobe Inspection: Under the restrictions of a forced outage situation, videoprobe inspection of the failed area(s) might not be possible. Typically, videoprobe inspections are tabled for planned outage situations, but if the quantity of failures and the possibility of catastrophic damage is a concern, then further inspection utilizing fiberoptics should be considered. Of major concern is when the severity of failures indicates a tube might be severed and damaged so extensively that return to operation without addressing some sort of stabilization of loose and broken sections could cause additional collateral damage, even though tube ends might be plugged.
  5. Tube Plugging Under Forced Outage Conditions: Failed tube plugging under forced outage conditions must consider the primary objective for FWH maintenance – Failure Cause Analysis. Assuming that the limited outage time frame and the adverse conditions within the heater have prevented maintenance personnel from making all the necessary tests and inspections to permit analysis and condition assessment, it is recommended that plugging be categorized as temporary under these circumstances. Management must therefore be committed to revisiting the specific heater in question at the next available planned unit outage. To this end, the selection of a type of plugging device that allows reliable quick installation under the potential of adverse conditions, while offering the ease in removability to readily allow future inspections/tests, is recommended for forced outage situations.

FWH maintenance action plan for next scheduled outage

As mentioned, the constraints of the forced outage might not offer more than the quick opportunity to locate leaks and perform straightforward, standard plugging of the failures. It is recommended that maintenance personnel try to gain as much data and additional information as possible. Forced outages very seldom allow complete condition assessment and other pertinent information necessary to optimize the repair. Plant engineering and maintenance management should generate a corrective action plan directly following a forced outage situation where all documented data, results of tests, layout plots, inspection results or the lack thereof, dictate the follow-up course of action to be planned for during the next planned unit outage. The contingency plan should include but not be limited to:  

  • Specific NDE/ECT surveys
  • Selective ITHT
  • Videoprobe inspection
  • Failed tube sampling
  • Laboratory metallurgical analysis
  • Expert consulting support
  • Use of FWH service specialists
  • Special tools/equipment
  • Stabilizing hardware
  • Special studies to be conducted if required

Planned outage FWH maintenance

Planned outage FWH maintenance is typically scheduled in direct response to either a follow-up action plan based on a recent forced outage incident, or based on normal scheduled proactive life-cycle inspections and tests. Temporary plugs/repairs installed during the forced outage event are then usually removed, allowing the necessary testing/inspections to gather data for complete condition assessment of the failures, including – as applicable – all incurred collateral damage. It is important that corrective repairs be based on the details of failures and the results of the condition assessment of damage. This includes the selection of the proper plugging device for the application.

It is beyond the scope of this article to discuss the detail of all considerations in selecting the best plug for a particular set of circumstances. There are many plugging options available in today’s marketplace, each with their own respective limitations. All plugging methods have pro and con issues related to their use, and all have failed in service for one reason or another. For the purpose of this article we will simply say that selection of the plug device should always be based on the specific FWH application and the results of the condition assessment of the damage incurred.

Basics for FWH corrective repairs

In general, the most obvious form of corrective maintenance with regard to FWHs is simply tube plugging. However, there can be number of problems within the heater that can make this trivial task not so simple. The tube might be severed and might be required to be staked and restrained in its ability to vibrate and cause additional damage. In some cases, there might be a tube-to-tubesheet joint leak that requires removal of a tube section. Additionally, previous repair failures can lead to additional damage and might require restoration of the tubesheet itself.

Questions related to plugging repair

Whenever a leak is identified within a FWH, the responsible engineer should have a lot of questions. Typical items to think about include:

  • Is this a new leak or a failure of a previous repair?
  • If a new leak, is the failure in a new area of the heater or near an area of previous failures?
  • Is it a tube leak or a joint leak?
  • If it is a joint leak, will a tube section need to be removed?
  • If it is a tube leak, is the tube severed?
  • Will the tube or the tubesheet hole be plugged?
  • Are there any problems with working clearances that would prevent normal plugging procedures?
  • What type of plug will be used?
  • Finally, why did the failure occur and what can be done to prevent future failures?

Most utilities run into problems with their maintenance programs when they do not take the time to properly plan and think about these items. Often, the short term expediency of trying to get the heater opened, tested and plugged, trumps the steps necessary to conduct a proper condition assessment and optimal repair. FWHs with tight working clearances can hamper repair efforts when the failures are located in the inner row or peripheral tubes. Special tooling/equipment might be required. Improper preparations can lead to inadequate repairs that can fail again in the future and cause additional damage.  

It also is important to know the materials of construction of the heater when plugging. These details will have an impact on potential damaging mechanisms to anticipate, as well as the material requirements of the plugs to be used.

Tube-to-tubesheet joint leaks

If the leak is due to a tube-to-tubesheet joint failure, the repair becomes much more complicated than simply plugging the tube, especially in high pressure FWHs. In these cases, care must be taken to remove the weld joint and not disturb the surrounding joints or tubesheet ligaments.  Then, the tube must either be drilled out or internally cut and hydraulically extracted from the tubesheet. Once the tube is removed, the tubesheet hole should be inspected to ensure that no damage has occurred to the ligaments and potentially affect the surrounding tubes, or the ability of the plug to be installed and form a tight seal.

If tube to tubesheet joint leakage is allowed to exist, then typically catastrophic tube damage can occur just behind the tubesheet where the high pressure feedwater rapidly expands and blasts the tubes. This is especially prevalent on the outlet pass where the fluid could flash into vapor or become entrained with the incoming high velocity steam in the Desuperheat zone. Typically, a tube joint failure will result in “wormholing” down the length of the tube as the high pressure feedwater works its way through the tubesheet. Ideally, the tube will take the majority of the damage. If the wormholing occurs into the tube sheet ligaments and impairs surrounding integrity, then additional tubes might be required to be plugged or removed as well. In the worst instances, field machining might be required to return the tube holes to concentricity or more uniform IDs in preparation for plugging.

Severed tubes

If investigation of the tube failure reveals that the tube has been severed, then the tube should be stabilized in order to prevent the unsupported failed ends of the tube from colliding with other tubes and causing additional damage in operation. In some cases, particularly in the nuclear industry, wire rope has been used down the length of the tube, however, in our opinion, this does not provide the strength and rigidity required to properly stabilize the tube. Solid bars or “stake rods” should be installed down the length of the tube to span across the break and should extend at least to the depth of the next support plate in order to provide the proper restraint. In some cases however, it might not be possible to span across the severed portion of the tube, as the severed ends might have become misaligned. In these cases, the failed tube should be “imprisoned” by inserting the stake rods in the surrounding tubes. This way, if the severed tube collides with one of the adjacent tubes, this will not result in the spreading of the damaged area.  If severed tubes are a common failure mechanism for the station’s feedwater heaters, then it is advisable to have a ready supply of stake rods in stock to minimize delays, especially in a forced outage situation. Typically the rods must be custom made and cut to manageable lengths based on the working clearances within the heater. Additionally, proper provisions for securely anchoring the rod in place also must be made. Severed tube ends that are not properly stabilized frequently lead to secondary failures when the heater is placed back in operation.

Tubesheet repairs

In attempting to repair failures, some utilities have historically compounded the problem by pad welding areas of adjacent tubes, some of which are non-failed and plugged for insurance. Due to the difficulty in preheating the tubesheet and heat treating these welded “clusters,” they become surrounded by a large heat affected zone. Repeated failures in these areas as a result of thermal stresses can often lead to tubesheet damage. When tubesheet damage occurs, it can often be very costly and time consuming to repair. Tubesheet repairs are always custom repairs, the scope of which is determined by the extent of damage. Therefore, prior to conducting a tubesheet repair, it is imperative to remove all previous repairs and get back down to the base material so a full condition assessment can be performed. It must be determined how deep the damage extends into the tubesheet before the ligaments return to the nominal thickness. Care must be taken when removing the prior repairs in order to prevent causing additional damage. Additionally, the boundary of the area of concern needs to be clearly determined. Successful repairs have been conducted on feedwater heaters either by welding a “flower plate” within the excavated cavity, or through tube sheet restoration via weld filler metal deposit. The latter method is typically only effective when the damage to the tubesheet is not very deep. Even under the best circumstances it is difficult to predict how long this type of repair will last. Since these types of custom repairs can be very costly, and their longevity is uncertain, decisions to undertake them are justified on a case by case basis.

Conclusion

The best chance for optimizing FWH corrective repairs includes the following essential functions:

  • Record and analyze data
  • Evaluate conditions and establish the repair/plugging procedures
  • Make the proper preparations for repair
  • Carry-out established plugging details and conduct post repair leak test
  • Analyze detailed results to determine failure cause
  • Identify and carry out any possible remedial actions

Understanding that outage opportunities to address heater failures are driven by other economic considerations, utilities must have organized approaches established for both forced outage and planned outage situations. Corrective repairs typically only satisfy the immediate failure situation; the best programs go further to identify the root cause. Satisfying this primary objective offers the sole possibility to preclude similar future failures.


Michael C. Catapano has more than 35 years of experience in the operation, design, procurement and maintenance of feedwater heaters, condensers and other shell and tube heat exchangers, including 7 years with PSE&G and 28 years as president of Powerfect Inc. His current work at Powerfect is primarily devoted to consulting, troubleshooting problems and assisting utilities with feedwater heater replacement and operating and maintenance activities. Catapano is an ASME fellow and has assisted ASME and EPRI in numerous feedwater heater projects, seminars and publications. He also holds three patents pertaining to feedwater heater testing and repair. Catapano has a bachelor’s degree in Mechanical Engineering from Newark College of Engineering. You may contact him by e-mailing editorial@woodwardbizmedia.com.

Eric Svensson graduated from the Georgia Institute of Technology in 1993 with a bachelor’s degree in Chemical Engineering. He joined the Naval Nuclear Propulsion program shortly after graduation, where he received training in Nuclear Power Theory and Operations. In 2000, he received a master’s degree in Operations Management from University of Arkansas. His current role as vice president of Engineering at Powerfect is devoted to consulting, troubleshooting problems, as well as operations and maintenance activities. Since joining Powerfect, he has been involved in writing the specification and conducting quality control checks for more than 20 replacement feedwater heaters. He also is a member of the ASME Heat Exchanger Committee and has co-authored several technical papers. You may contact him by e-mailing editorial@woodwardbizmedia.com.

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