Recent D-11 outage experiences: The good, the bad, the ugly

Steering Committee

2017 Chair: Eddie Argo, Southern Company
2017 Vice Chair: Bert Norfleet, Dominion
Jess Bills, Salt River Project
Gary Crisp, NV Energy
Jake English, Duke Energy
Jay Hoffman, Tenaska Inc
John McQuerry, Calpine Corp
Lonny Simon, OxyChem

Perhaps the biggest benefit of user-group meetings is the access they provide to the experiences of others. You can’t come close to learning all that you probably should know by simply working at your plant—even if you’re the most diligent person on staff. It’s important to expand your knowledge horizons by gaining exposure to things you might not imagine could happen, but have. This improves your ability to make informed decisions.

The Steam Turbine Users Group (STUG), like most independent user organizations serving the electric power industry, is managed by a steering committee of veterans who relish learning, promoting safe work practices, sharing experiences, and righting wrongs (box). Be sure to attend the next meeting, August 28-31, at the Sheraton Grand at Wild Horse Pass, Chandler (Phoenix), Ariz.

What follows, based on a discussion of D-11 outages at the 2016 STUG conference, is not a typical CCJ article, but rather a few photos and explanatory captions of the many experiences shared by users. It’s a good way to learn.

Here’s how this type of session is conducted: A participant without prepared notes comes to the front of the room, flashes a photo up on the screen, and says something like, “Know what this is?” or “What’s wrong—or right—with this picture?” Impromptu discussion ensues, instant sharing of information proceeds across the meeting room. If you’re not sure what the exchange of ideas is all about, just sit there, listen, and learn. And ask questions.

The Good

Hydraulic actuators were cited as the “most overlooked” powerplant component. Actuators in Fig G1 await proper crating (Fig G2) for shipment offsite to overhaul shop. Speaker recommended a complete overhaul of actuators at every steam-turbine (ST) major; inspection and refurbishment of hydraulics (servos, fast-acting solenoids, etc) at every ST minor.

Takeaway from the user sharing his experience was that Rexroth (Bosch Rexroth Corp) makes a good actuator but it’s difficult to find an experienced and competent overhaul shop. RPM LLC, a subsidiary of Mechanical Dynamics & Analysis (MD&A), was cited as a preferred shop by one user who said the company has repair kits for all Rexroth ST valve actuators. The user went on to stress the importance of proper packaging for shipment to protect the actuator from damage.

Proper tooling is required to both protect turbine components and assure timely completion of outages. Examples: Fig G3 is a stand for properly supporting the outer shell of a D-11 HP turbine. The user sharing recommended not putting components on bare ground.

The diaphragm lifting tool in Fig G4 sits on the horizontal joint and removes all diaphragms with one crane lift, saving a great deal of time over the alternative—a series of individual lifts. Store diaphragms in a rack such as that shown in Fig G5, which can accommodate both inspection and light maintenance.

Generators got some air time in the STUG sessions. Fig G6 illustrates a user’s recommendation for assuring a complete seal around the end bell by drilling extra holes for sealant entry (arrows). Fig G7 shows a reference chamber on the generator for faster verification of hydrogen leak test—four hours versus 24 previously.

Cranes can be a significant line item on an outage budget so there’s financial upside to minimizing the number required and their time onsite. A utility engineer demonstrated a rigging technique to flip a HP casing with one crane; follow Figs G8 through G12.

The Bad

No overhead crane was provided for turbine casing and associated lifts. The outdoor steamer is on the other side of this 90-ft-tall building (Fig B1) and the only way to access it is to reach over the top of the structure with a 600-ton crane, shown at rest in Fig B2 and at work in Fig B3. A crane like this can cost half a million dollars for a major outage.

Bolts visible in balance chamber for the combined stop/control valve are prone to failure (Fig B4). Speaker recommended their replacement every outage. When the bolts break, the balance chamber drops down and rests on the control valve, impeding its operation.    

“Walk your lines,” a user told the group, using Fig B5 to show electrohydraulic control oil lines retained with U clamps (arrows). Vibration can wear lines at these points and cause leaks. On rounds, be on the lookout for obstructions and wear and insert “softeners” where appropriate.

Broken shaft (coupling) on electrohydraulic-control-oil (EHC) pump can be a real nuisance (Fig B6). To access the damaged part and replace it, you have to pull the motor located alongside the cylindrical oil conditioning unit in Fig B7. In cases where the pump and its suction strainer are installed in the oil reservoir, a forced outage may be necessary. The speaker recommended installing a longer shaft between the motor and pump to make the coupling accessible.

Failed O-ring in an EHC line to the actuator for a D-11 interceptor valve forced a 500-MW unit out of service. Control oil was found leaking from the connection. Root cause: The wrong-size O-ring (at top in Fig B8) was used.

Hose alert. No photo provided by a user who suggested that his colleagues check their blue hoses at the discharge of their D-11 EHC pumps. If Aeroquip AQP he recommended replacing them. The background: The OEM’s Technical Information Letter 1659 states, “GE does not recommend or approve the use of hoses or other flexible components in the high-pressure hydraulic tubing system.”

However, on many of the latest combined-cycle units with the EHC reservoir at the end of the lube-oil tank and with vertical pumps, GE used hoses. In some cases, these were Aeroquip AQP, which the supplier does not now recommend for use with phosphate ester fluids. Also, one station was said to have reported blistering of Aeroquip FC-636 EPDM hoses.

Jacking of the D-11 HP turbine case. A user reported flexing of the HP turbine’s outer casing when just using the OEM’s jacks at the corners. He suggested this alternative: Weld I-beams at the corners and in the center of the shell and add two jacks in the middle (on each side of the casing) to get a proper cover lift (Fig B9).

Galling of control-valve bonnet nuts can be mitigated by use of nitrided washers to take the wear (Fig B10).

Walk-around alert: Inadequate special tooling is illustrated in Fig B11. This is an unsafe practice.

Disc inspection of combined stop/control valve revealed nine linear indications (Fig B12), suggesting the casting was not properly heat-treated. Ultrasonic inspection and hardness checks of these components should be incorporated in manufacturing specs and monitored.

The Ugly

Poor supervision, or the lack thereof, is a primary reason equipment is damaged and personnel get hurt during outages. Fig U1 shows the damage to the body of a stop valve caused by arc-cutting to remove the galled pressure-seal pulldown ring in Fig U2.

Gaskets should be replaced after breaking joints, never reused. Spiral-wound Flexitallic gaskets have been an industry staple for high-pressure steam systems for decades, but close inspection before use is highly recommended; they are relatively easy to damage (Fig U3). Because they can be difficult to acquire on short notice, particularly in large sizes, the speaker recommended buying three if you need two. Also, users at other meetings have suggested special due diligence before accepting gaskets made in China, which have been said to have a high failure rate.

Welding joints (Fig U4) to prevent leakage is not an acceptable solution. However, the speaker mentioned the need to weld in a half-pipe at the horizontal joint of an operating nuclear plant’s steam turbine as an emergency fix. The half pipe was equipped with a drain.

Stellite liberation from valves continues to be an industry problem. Fig U5 shows a piece of stellite liberated from a HRSG valve that was found in the drain pot.

Parts stored outside during an outage should not be placed on bare ground and should be protected against the elements (Fig U6).

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