The editors first learned of stellite delamination in a large valve designed for high-pressure (HP)/high-temperature steam service at the 2009 7F Users Group conference. But all the attention that incident received was a brief mention late in a long user presentation profiling the major inspection of a combined-cycle plant. More than 200 owner/operators were in attendance, yet no questions were asked about the issue—at least none the editors can recall—and it was quickly dismissed. Fig A shows stellite liberated from the seat of a 20-in. hot-reheat block valve installed at that presenter’s plant and collected in the strainer for the steam turbine’s combined stop and control valve.

Another alert concerning stellite liberation came from GE Energy, which issued Technical Information Letter 1626 on Jan 30, 2009. It advised steam-turbine owners to check the condition of the stellite inlay sections used in fabricating seats for the OEM’s combined stop and control valves. Revision 1 of that TIL, dated Dec 31, 2010, recommended a “one-time seat stellite inlay UT inspection during valve installation or next planned maintenance inspection.”

Yet another alert regarding stellite cracking was sounded by Ed Sundheim, director of engineering for the North American Energy Alliance LLC, Princeton, NJ, who had intended presenting on the subject at the spring 2011 conference of the Combustion Turbine Operations Technical Forum™, but the lunch bell sounded before he got to the podium.

Instead, Sundheim provided the editors his notes to develop the CCJ article, “Don’t forget to inspect your valves,” a case history charting the inline repair of F91 parallel-slide gate valves at Newington Energy LLC. More recently, the Dogwood Energy Facility was recognized with a Best Practices Award at the spring 2013 conference of the CTOTF™ for its efforts in the identification and repair of a cracked seat on the 12-in. HP stop/check valve for one of its HRSGs. Except here, the seat material was Type-316 stainless steel; no stellite was involved.

The industry recently learned of many more incidents of stellite liberation. CFM/VR-TESCO LLC (formerly Continental Field Machining), a leading valve services company, reported earlier this year at a meeting of the Valve Manufacturers Assn (VMA) that in 2011 and 2012 it had repaired 50 valves manufactured from F91 (forged body) or C12A (cast body) and ranging in size from 12 to 24 in. More than half of these jobs involved stellite liberation.

The repair projects profiled were split roughly 50/50 between valves within the Code boundary and those that were part of the boiler external piping. Repairs on the former were performed according to guidelines presented in Section I of the ASME Boiler & Pressure Vessel Code and in the National Board Inspection Code as well as jurisdictional requirements. Valves outside the Code boundary were performed according to ASME B31.1.

Background

The editors spoke with owner/operators, a valve manufacturer, and a service firm, as well as with Kim Bezzant of Structural Integrity Associates Inc, regarding issues associated with high-temperature steam valves. It seems that many problems the industry is experiencing today can be traced to a general lack of respect for valves and inattention to detail regarding their manufacture, installation, operation, inspection, and maintenance.

Success in all aspects of equipment and system design and manufacture/construction hinges on good specifications and involvement by the owner, or its representative, in the work contracted to others. Price always is important until breakdowns force a plant out of service. Then the quality that should have been built into the equipment suddenly becomes important. As the Dutch are fond of saying, “Too soon we grow old, to late smart.”

Speaking with owner/operators you get the feeling that valves were considered pedestrian equipment. No owner the editors spoke with went to the fab shop to inspect the product during manufacture or to verify inspection results. One said valves were part of the EPC contractor’s scope of supply and only their employees were allowed in the shop, by contract. Did the EPC send someone to the shop? Certainly unlikely in one case examined where the welding was so poor even an inexperienced inspector likely would have rejected the job.

Someone else told the editors he thought his plant’s valves had forged bodies, but they actually were less-expensive castings. At least some of those castings were made in far-off places where QC didn’t exist 10 years ago, which is why the industry still hears about defects in valve bodies. You might also consider checking the bodies of your high-temperature valves when inspecting for stellite cracking and disbonding. Recall how surprised many users were when they found out that their P91 didn’t meet hardness specs, the wrong weld filler material was used, P91 was welded to P22 in error, etc.

One owner’s experience

Discussions with a boiler engineer for an owner of multiple combined-cycle plants equipped with HRSGs and valves from several different suppliers revealed how pervasive the stellite cracking and disbonding issue is. His company implemented a fleet-wide survey of its large steam valves and now tracks on an ongoing basis the inspections, detailed findings, and repairs for each critical valve.

This owner’s best practice today regarding inspection is to open and inspect valves on a two-year cycle, and have a capital spare. All components are checked—including springs, stems, discs, and seat rings. Straight-beam ultrasonic examination is used to locate any disbonding between the hardfacing material and the valve body. This is important: Where there is disbonding, cracking usually follows.

Don’t chintz on inspections, the boiler engineer advised. A proper job requires top talent. The company you select should be experienced in this work, have the proper coupons for calibrating its instrumentation, and provide a savvy technician to conduct the inspection.

Corrective action by this owner, when necessary, is guided by inspection findings and may differ from plant to plant depending on the damage encountered, because there are no industry standards for hardfacing and some other required repairs. The utility expects the EPRI task force discussed in the main text to provide a much-needed guideline for applying hardfacing alloys in a manner that minimizes, and possibly eliminates, the damage being experienced today. Its expectation of a viable EPRI solution was echoed by virtually all industry participants interviewed—including other owners, operators, manufacturers, and repair firms.

At the first of this owner’s F-class combined cycles to inspect its HP steam valves, engineers found cracks in the integral seat for a stop/check valve that propagated through the stellite and into the valve body. A field repair crew machined off the hardfacing, chased the cracks to the bottom and repaired the body with Grade 91, and then refaced the seat with stellite.

Operability issues experienced with a parallel-slide gate in HP steam service at another plant in the fleet raised a red flag and engineers inspected all four HP valves on its two HRSGs during an outage planned for 10 days. That outage was extended by about a month and half because of the valve work. Cracking of seats and discs was found on both parallel-slide gate valves (Fig B) and both stop/checks (Fig C). Some of the liberated stellite still has not been found. All four valves were removed and send to a qualified shop for repairs.

The parallel-slide gates had serious issues. The retainer plates in both either broke or bent and contributed to body damage; pieces of stellite liberated from the seat rings on both gates. In addition, the disc came off the stem on one of the valves and resultant chattering caused still more damage. These valves were completely overhauled and repaired; Grade 91 material was built up where necessary and Stellite 21 hardfacing was applied in wear areas. In effect, the valves were restored to their original condition.

For the stop/checks, hardfacing was machined off and the Grade 91 material dressed; stellite was not reapplied. Given the company’s experience and the state-of-the-art in hardfacing, Grade 91 seating surfaces were viewed as the least-risk alternative.

Another F-class 2 x 1 combined cycle in the fleet recently inspected its HP valves. Seats for the gate valves are integral with the bodies and dye penetrant identified cracks in both, but there was no evidence of disbonding. Cracking of the stop/check seats also was in evidence, with no disbonding. However, the guide ribs were galled. Stellite was removed and the guides are now Grade 91 with no hardfacing.

Lessons learned. Before the owner inspects for the first time valves on another of its combined cycles approaching 10 years of service it will take delivery on two new HP gates and stop/checks. Experience indicates that the chances of finding valves as-new is unlikely and alternative with least schedule impact is to cut out the existing valves and drop in new ones.

In sum, the owner’s near-term strategy has been to minimize operational risk by verifying the integrity of the fleet’s HP valves first. These valves, it believes, have the most severe service conditions. Long-term thinking is that EPRI will have established best practices for steam valves before inspection of hot-reheat and other valves is necessary.

The manufacturer’s view

Dr Nabil T. Tarfa, who recently joined Velan as VP materials and process technologies, shared his thoughts on cracking and disbonding of hardfacing in combined-cycle service. Montreal-based Velan is one of the electric-power industry’s leading suppliers of valves for high-pressure/high-temperature steam service. Most of company’s valves have forged bodies, Tarfa said, because forgings have a more uniform structure than castings and generally are of a higher quality. 
Asked why he believes steam valves for F-class combined-cycles are experiencing so many problems of late, Tarfa summed up his thoughts succinctly: There’s a gap between the service conditions specified by the buyer and how the equipment is being operated. He said Velan is investing considerable time and effort to determine both the root causes of the problems experienced and how to address them.

The company is a participant in the EPRI research program on cracking and disbonding of hardfacing alloys and also is sponsoring a research project with a Canadian university to identify factors contributing to the failures and how to address them. He believes that an effective solution hinges upon an open exchange with users, to learn more about how plants are operated in the competitive generation industry.

Tarfa said there are several important factors to consider in analyzing the cracking/disbonding problem, including these:

• The metallurgical nature of the bond between the hardfacing material and base metal.
• Temperatures involved, and the times at those temperatures. Plus, numbers of cycles, ramp characteristics, thermal shock caused by quenching of hot metal by condensate, etc. 

Pentair, which provides a wide range of valves to power producers under several brands—including Crosby, Sempell, Dewarance, and Clarkson—offered a brief overview of hardfacing issues at recent meeting. The speaker acknowledged that “stellite failures are an industry issue,” and stated the following:

• Each OEM must have a written procedure, which must consider (1) the shape, dimension, and accessibility of parts to be welded, (2) range of dilution to achieve a specified hardness, (3) where the job will be performed—in the field or in the shop, (4) welding position, and (5) level of automation.
• The application is a consideration that must be reviewed.
• The application of weld overlay is critical.
• The right material for the right application. 

Finally, “For determination of hardfacing procedures and application of weld material, shape of components, used materials, and operating conditions must be considered. A general rule to have a unique standard for deposition of of hardfacings is not feasible. Each manufacturer has to select and qualify individual processes for hardfacing.”

Field repairs

CFM/VR-TESCO LLC (it stands for Continental Field Machining/Valve Repair-Technical Service Co LLC) does the lion’s share of its business in powerplants—about 90%, in fact. Several people the editors spoke with believe it to be the “go to” firm for in-situ valve repairs. An overview of what CFM presented before the VMA meeting mentioned earlier offers combined-cycle owner/operators valuable perspective on the extent of the industry’s valve challenges.

Most of the ASME Section I repairs the company has completed can be segregated into these three groups:

• Valves requiring seat replacement.
• Removal and rewelding of integral seats.
• Repair of cracks in the valve body, guide ribs, and/or disc guides.

Half of CFM’s “R” stamp repairs were on valves with SA217-C12A bodies that required replacement of seat rings. The partial-penetration welds holding the seat rings to their respective valve bodies were cracked—the majority all the way around, allowing the ring to fall out of the seat pocket. Portions of the hardfacing was missing from some of the seats. All of the valves with Type-316 stainless steel seats were cracked 360 deg and were replaced with seats of A182-F91or A387 Grade 91 Class II materials.

The remainder of the repairs were on integral valve seats. Most (85%) of those valve bodies also were made from SA217-C12A material. All valves had cracks in the seat-area hardfacing going back into the base material. The majority also had cracking in the guide ribs. In some cases, the stellite had become disbonded from the base material and had moved downstream. This required removal of the remaining stellite and undercutting of the base material. After the base metal was built up, hardfacing was applied.

The majority of the valves repaired under B31.1 (those outside the ASME Code boundary) were parallel-slide gate valves with bodies of A217-C12A. Seat rings were of F91 and P22 materials. Defects were the same as those found on the Section I valves: (1) Seats cracking in the heat-affected zone on the body side of the body-to-seat-ring weld. (2) Stellite breaking off the seat-ring face and entering the steam system.

Welding procedures. Users might consider investing time to learn about stellite and welding procedures for hardfacing. If you have responsibilities on the steam side of the plant, you’re likely to be involved in valve repairs at some point. The Deloro Stellite website offers a good backgrounder on hardfacing materials and such pertinent welding processes as manual metal arc, tungsten inert gas, metal inert gas/metal inactive gas, and plasma transferred arc.