Engine vibration continues as a top concern of 501D5-D5A Users

D5 D5AThe 501D5-D5A Users can be characterized as an intimate group, historically attracting about two-score attendees to its annual conferences and vendor fairs, fewer at the “mid-year” meetings in January. Most attendees have been repeat participants, several from far-away places like Korea and Indonesia, with the typical user knowing perhaps half of the owner/operators in the room.

The chairman, Gabe Fleck, recently promoted to manager of gas plant operations at Associated Electric Cooperative Inc, has been at the front of the room for a decade, supported by Vice Chair Barry Mayhew, maintenance manager at Cardinal Power, and Director Lonnie Grote, compliance coordinator for NAES Corp.

Fleck’s low-key demeanor and unflinching dedication to the group is critical to the organization’s success. He works diligently at technical program development, and with sponsors to provide users new experiences in terms of meeting locations and social events. This June’s conference was in Louisville, affording visits to Churchill Downs, the Louisville Slugger Museum, and a local brewing company—plus bourbon tasting on a private yacht. The Seelbach Hilton Hotel was an apropos conference location for a mature group, dating back to 1905 and listed in the National Register of Historic Places.

That time always seemed to stand still at D5-D5A meetings was not surprising because of the continuity of attendees and discussion threads. Technical topics are linked from meeting to meeting by interactive messaging via the group’s robust online forum. Over time, conclusions are drawn and solutions developed—with help from the OEM, Siemens Energy Inc, and participating third-party equipment and services providers, of course.

Fleet characteristics contributing to the group’s stability include the following:

• Both engines served are no longer in production (although there are three new D5As in storage), the two fleets holding steady with a total of 150 units worldwide.

• The service factors (operating hours divided by period hours) for engines not in cogeneration service are relatively low, based on information shared by asset owners. One reason for this undoubtedly is the increased use of more-efficient F-class engines in peaking service.

Given the foregoing perspective, why these “surprises” at the 2013 annual meeting:

• Record attendance, breaking the 50 mark for the first time in the 17-year history of the group.

• Most first-timers ever, 22; plus a dozen who had participated in only one other meeting previously.

The only logical answer may be that the retirements of many senior plant professionals, predicted for several years, is now occurring and that their replacements have little or no experience in operating and maintaining D5s or D5As. Participating in this and other user groups is the only practical way new O&M personnel can come up to speed on what they have to know, and whom they should know to get the help needed going forward.

The technical program began after breakfast on Tuesday, June 4, with presentations by Mitsubishi Power Systems Americas Inc and a few suppliers participating in the vendor fair later that afternoon. Four hours of closed user discussion rounded out the day. The second and third days of the meeting were divided between Siemens technical presentations and closed user discussions.

Engine vibration occupied a considerable amount of discussion time. Identifying and correcting sources of vibration has been a major initiative between the users group and the OEM for the last two years. The collaborative Vibration Improvement Program is guided by an executive committee consisting of user representatives and Siemens management. Work packages were developed and teams formed to perform root-cause investigations, testing, rotor-dynamics modeling, and data analysis, and to make operational recommendations. Creative Power Solutions, an independent turbomachinery consultant based in the Phoenix area, was retained by Siemens to assist in the effort.

A user started the discussion by saying his D5A trips on high vibration about once every 10 starts, with the trip setting at 10 mils. It takes two hours for the vibration to build to that point. He thought the turbine blades might be locking-up on startup, so root springs were installed in the gaps between the R4 blade roots and the discs to make rotor vibration levels more repeatable and to facilitate balancing.

The low-cost consumable part was developed to minimize disc wear by preventing blade rock during turning-gear operation. However, springs were not the answer to this user’s issue. Where springs do provide benefit, a recommendation was made to replace them at each hot-gas-path (HGP) inspection. More information on blade-root springs is available in the Mitsubishi presentation, which is posted on the D5-D5A website in an area accessible only by registered users. The steering committee urges qualified owner/operators to register today.

Another attendee said vibration on his machine was traced to coke buildup on the bearing exhaust line; when coke broke off, the balance would shift. He learned that running unbalanced—example: x=2, y=5—contributed to more stable operation. Each time technicians tried to do a proper balance, vibration increased and the unit tripped. Someone suggested that a successful vibration analysis demands that you know what you’re looking for.

Yet another user got into the passionate exchange, suggesting that overtightening of bolts on the marriage coupling to adjust run-out can cause vibration when the bolts relax. He said this happened on his unit.

Someone else asked if everyone with a vibration issue is sure their bearing probes are properly located. Here’s why: Installing exhaust-bearing vibration pickups on the bearing housing rather than outboard can reduce readings by up to one-half during startup, full speed/no load, and at base load. This location, which is consistent with ISO standards, is said to minimize the contribution of bearing-housing motion to the vibration signal. Redundant probes were suggested when making this modification, to improve serviceability.

The discussion continued with an attendee who was puzzled as to why vibration on his machine was at 4 to 5 mils—until plant personnel really dug into the matter. Turns out the stack was pushing down on the end of the machine and causing the unbalance. The solution was to lift the stack by about an inch.

The air separator also was mentioned as a possible source of vibration. One of the Mitsubishi speakers had addressed that at the beginning of the meeting. He said the original air separator uses spring force to maintain contact with the R1 disc and that relative movement at the R1 disc face can cause fretting. Retrofit with a bolted air separator was said to reduce R1 disc fretting potential and increase the through-bolt compressive load on turbine discs—effectively decoupling the air separator. This mod, developed for the M501F gas turbine, is applicable for the D5 and D5A turbines as well. A drawing is provided in the Mitsubishi presentation cited earlier.

You could sense the frustration of a user who told that group that the vibration gremlin has resided in his D5A since 2001, tripping the unit every dozen starts or so. It seems that rust/dirt accumulated and then released, upsetting the engine’s balance. His experience suggested that balance weights aren’t necessarily the solution. The unit had so many weights it was ridiculous, he said. Every service technician believed he had the solution, the user continued, and added weights. All were wrong.

Posted in D5-D5A Users |

Comments are closed.