Clashing, the term used to describe the contact between rotating blades and stationary vanes that has occurred in some compressors for GE frame engines, generally does not stop after the blade profile has been changed by impact wear. Recent inspections conducted by Advanced Turbine Support LLC confirm the following:
Clashing damage in terms of severity and the number and location of affected airfoils is increasing in at least some compressors.
Damage from clashing, which had been characterized solely by wear and tear on the trailing edges of rotor blades near their platforms and on the leading edges of adjacent stator vanes at their tips, now also may include cracks in vanes a few inches from the bottom of the airfoil in the middle of the convex side (Fig 1). This location is identified as an area of concern in Technical Information Letter (TIL) 1884, “7EA R1/R2 Inspection Recommendations,” issued Apr 26, 2013.
An inspection team from Advanced Turbine Support found cracks in 7EA S1 vanes in the area of concern for the first time the week before the 7F Users Group’s 2014 annual meeting in Phoenix, May 19-23. Recall from previous coverage that in 7EAs, clashing almost always is found in Stage 1;in 7FA compressors, it usually occurs in the second and/or third stages.
When clashing was first reported by an owner/operator at a 7EA User Group meeting several years ago, the damage was found in vanes located at or near bottom dead center (6 o’clock position) of the first-stage ring segment. Since then damage has been identified in vanes at many other locations, including ones in the top half of the casing. In the unit where inspectors found the cracks shown in Fig 1, 20 of the 53 stator vanes exhibited clashing damage.
One user told the editors the OEM’s engineers attribute clashing to increased tip deflection caused by a rotating-stall-driven S1 vane frequency response during startups and shutdowns. The frequency response, together with a loss of damping attributed to ring-segment “lock-up” caused by corrosion, produces higher-than-normal operating stresses. But not everyone is convinced this is the only cause.
Vane cracks. Advanced Turbine Support’s service manager, Mike Hoogsteden, told the editors that the cracks in Fig 1 were found during the dye penetrant inspection recommended in TIL 1884 for all airfoils damaged by clashing. Fig 2 shows clashing damage to the tip of one of the cracked vanes shown in Fig 1. The service manager said many customers have eliminated the dye-penetrant inspection portion of the TIL 1884 recommendation from their inspection specifications because, until three weeks ago, no cracks had been reported publicly in the area of concern. The decision to not perform this inspection obviously should be re-evaluated.
Hoogsteden added that the cracks were validated when Advanced Turbine Support performed a follow-up in-situ eddy-current inspection on all of the stator vanes in this stage. During that inspection, a third vane was found to be cracked. This crack was less than 30 mils deep and did not yield a dye-pen indication. Engineers recommended the engine be removed from service at once and the vane row replaced.
Proof that the severity of clashing damage is increasing over time in at least some units is offered in Figs 3-7. The first photo in the series shows the condition of a 7EA R1 vane in 2013 after 2276 hours of service and 424 total starts. Fig 4 is the same vane a year later after only 89 additional hours of service and 20 more starts. The vane profile following an in-situ blend by technicians from Advanced Turbine Support is shown in Fig 5.
Figs 6 and 7 characterize the damage to the trailing edges of R1 rotor blades, which ranges from rub marks in the former to torn and displaced metal in the latter.
Clashing also was mentioned in an OEM presentation at the CTOTF™ 39th Annual Spring Conference in Palm Beach Gardens, Fla, April 6-11, 2014. A couple of users who attended the session said they couldn’t recall many questions from the other owner/operators present. One attendee told the editors he has been looking for signs of clashing on Frame 7s in the Northeast for years, but has found none.
To the contrary, he believes he has found more loose vanes than tight. He wasn’t happy about this, but in retrospect, loose vanes may be less of a concern than locked-in ones. The engineer added that replacement Type 431 stainless steel blade rings may be the way to go to prevent lock-up, as well as to address pitting and erosion issues identified with some old units.
Another user told the editors he knows R1/S1 clashing has occurred at a few of his company’s plants equipped with 7EAs, but the damage to date is nothing like the ripped metal shown in a few of the pictures taken by the Advanced Turbine Support inspectors. However, damage is being tracked fleet-wide and if it gets worse, replacing vane rings with the stainless steel offering could be incorporated into a future outage.