The 501G Users Group only serves owner/operators of Siemens/Westinghouse engines. Users supporting G machines made by Mitsubishi Power are not typically members. This differs from the 501F Users Group, where owner/operators of all 501F engines share experiences.
The 501G is a small organization of well-connected engineers and technicians who have “grown up” together—so to speak—and generally quite familiar with each other’s plants and equipment.
501G Steering Committee
Chairman
- Jody Lumpkin, plant manager, Hillabee Generating Station, Constellation Energy Corp
Members
- Scott Wiley, outage manager, Vistra Corp
- John Wolff, technical support/compliance manager, Ironwood, EthosEnergy Group
- Kevin Robinson, operations manager, Lakeland Electric
Background: The first Siemens/Westinghouse 501G, installed by Lakeland Electric, began commissioning operations in April 1999, but COD wasn’t until March 2001—only a few days after the second machine began commercial operation at Millennium.
Fleet size is small by industry standards—24 engines at 13 sites in the US and one in Mexico. Four plants are equipped with one engine each; seven have two gas turbines; two are equipped with three machines each, arranged in 1 × 1 combined cycles.
User meetings typically host several first-timers, so many discussions are similar from year to year because newcomers have to be brought up to speed. But there’s not much turnover in the top positions at G facilities which means each meeting pretty much picks up where the last one left off. This contributes to presentation efficiency, given the minimum amount of repetition.
G users came together with their 501F colleagues for Sunday social events (Feb 4, 2024) and the Monday technical program described in the 501F report referenced earlier—including the annual safety roundtable, vendor presentations (a/k/a Vendorama), reception, and vendor fair.
User presentations. A few of the user presentations made during the 501G portion of the conference are summarized below with the objective of providing enough information for you to decide if the content of a given slide deck should be reviewed in greater depth.
If that’s the case, access the PowerPoints of interest at forum.501Gusers.org. But be aware that you must be a registered user to gain access. If not, you can begin the registration process with a simple mouse click. CCJ readers who already are registered will find the presentations in the folder “2024 Conference Materials.” (Click on the magnifying-glass symbol at the top right-hand side of the page.)
Seal-oil modification
In spring 2022, Unit 3 at this plant, equipped with three 1 × 1 combined cycles powered by 501G gas turbines, tripped offline because of high relative vibration at the turbine end of its gas-turbine/generator. The OEM’s diagnostic center noted fluctuations in the pressure and temperature of the air-side seal oil. Its suggestion was to restart with intensive monitoring of the vibrations and seal-oil parameters.
During this activity, a technician noted that during startup the local analog gage, located downstream of the thermocouple, was reading almost 10-deg-F lower than the t/c providing data to the DCS.
Further investigation into historical data, and trending when the generator had excessive vibrations, revealed that typically there was a large variation in the air-side seal-oil temperature at the turbine and collector ends of the machine.
The plant team developed a plan to remove the local analog gage and install a t/c in its place. There were open terminals in a nearby I/O cabinet, and the well where the t/c was installed, was a dry well, making the swap quick and easy. The new t/c senses oil temperature about 14 ft further downstream from where the existing t/c was installed.
The plant and OEM agreed with the install and logic changes to have the cooler bypass valve controlled by the new t/c. Finally, the plant techs decided to use the old t/c in the new logic as a fail-safe in case the new t/c faulted.
Good news: The seal-oil mod eliminated the vibration and there have been no issues associated with seal-oil temperature regulation since the project was completed. To dig into the details, scan the nearby QR code with your smartphone or tablet.
Condenser event: January 2022
This case study of a condenser event may well be one of the most valuable safety briefs you’ve ever received if your combined cycle is equipped with a fuel gas heater (FGH). An abridged account of the incident follows. It was discussed at the previous 501G meeting, but the safety-conscious steering committee wanted to be sure personnel fleetwide are aware of the incident.
As a result of the failure of a main lead in the steam turbine/generator, the facility was in a steam-turbine outage in January 2022 during which the GTs remained “available” to the grid (with fuel at pressure up to the FGH stop valves). With the plant experiencing numerous issues with the FGH (for example, leaking gas isolation valves) and its leak-detection system (prone to false alarms), and other factors, natural gas found its way to the condenser via the IP water-side supply line (the source of heat for the FGH).
The plant was down for 10 days, but with cold weather in the forecast, the gas turbines were started to build up pressure to about 50 psig in the HRSG to prevent freezing. The steam pressure in the HRSG provided the motive force to move the gas that had been accumulating in the IP drum to flow down the steam piping to the condenser.
When a welder arrived to repair a 1-in. pipe connection on the outside of the condenser, the arc ignited the gas and the subsequent explosion blew out six rupture discs on top of the unit, caused bolting threads on the LP steam-turbine cover to fail, and damaged internal structural supports. Rupture-disc parts were found all over the plant site. The explosion was heard and felt throughout the plant.
No one was injured (not even the welder), no condenser tube leaks occurred, and the LP section of the turbine suffered no internal damage.
Recommendations to avoid a similar incident elsewhere, included the following:
- Eddy current test FGH tube bundles and perform leak testing at least every six years.
- Use more robust (expandable) inserts when plugging tubes.
- Ease plant staff access to vent valving and instrumentation.
- Develop preventive-maintenance guidelines for FGH systems.
- Perform comprehensive FGH system design review.
- Install permanent redundant methane detectors on air ejectors with feedback to control system.
Reheat-header replacement
Presentation covers ever so briefly the replacement of lower reheat headers on the plant’s two heat-recovery steam generators. Project was completed successfully in eight days according to the personnel involved; they credit Viking Vessel Services’ Tuff Turbe Transition (TTT), a part qualified to Section I of the ASME Boiler and Pressure Vessel Code, for the success achieved.
According to Viking, TTT reduces thermal expansion for a stronger, more secure tube-to-tube and tube-to-header connection than a conventional open-root butt weld, as well as the purge requirements associated with it. The connection reportedly can be made in a fraction of the time required for a conventional weld.
Row-4 blade: Lessons learned
A quick look through these slides might encourage you to attend the next meeting to see how the R4 blade saga discussed concluded—if it indeed is over. The case history begins with the OEM capturing six images of R4 blades in spring 2023, reporting no issues. Shortly thereafter new borescope requirements were announced by Siemens and R4 was reinspected at the OEM’s request that fall.
Photographs distinguished between normal wear and unanticipated wear into the base metal. Based on this “evidence” the OEM recommended immediate replacement of the damaged R4 blades. But Siemens said it wouldn’t have these parts until December and cleared the gas turbines for another 2000 hours (EBH) of operation. Note that both engines were scheduled for HGP inspections in fall 2024.
The plan was to replace R4 blades in Unit 2 in spring 2024 with the expectation of a 13-day turnaround. R4 in the sister Unit 1 was to be reinspected in spring 2024 with the intention of replacing damaged components that fall. However, the OEM then advised it did not have the labor to support the replacement of Unit 1’s R4 blades if they had to be replaced.
