Dogwood Energy Facility, Pleasant Hill, Mo, like many other combined cycles, opens its steam-drum doors on a routine basis to perform internal inspections. While properly aligning and closing the doors at the end of an inspection is a challenge in and of itself, the larger challenges and risks often are associated with making sure the gaskets do not get damaged during installation and that a good seal on the drum door has been established before removing the LOTO and lining up the HRSG for operation.
Depending on when a gasket leak is found, the plant would either have to drain the HRSG, if the leak was identified before starting up the gas turbine, or to bring the affected unit offline, depressurize the drum, and establish a LOTO and confined space to protect personnel performing the work.
Staff then would remove and replace the gasket, and inspect and clean gasket sealing surfaces, potentially requiring work near hot surfaces. If parts of the old gasket material were accidently dropped into the drum when making the repair, the outage might have had to be extended to locate and remove that material.
When work was complete and the LOTOs released, plant personnel would restart the unit and hope for the best. Depending on the success of the repairs, this repair evolution could have taken several hours and caused missed dispatch schedules.
The best practice described below and the plant’s method for eliminating the risk of hot torqueing steam-drum doors, described in an accompanying article, earned Dogwood Best of the Best recognition by CCJ and the 501F Users Group in 2019 Best Practices Awards program sponsored by the journal.
Dogwood is owned by Dogwood Energy, City of Independence, Missouri Joint Municipal Electric Utility Commission, Kansas City Board of Public Utilities, and the Kansas Power Pool. The facility is operated by NAES Corp; plant manager is Steve Hilger.
The best practice. After several drum-gasket leak events impacting plant dispatch had occurred at Dogwood, operations personnel put their heads together to find a way to better ensure that the drum-door gaskets would seal and hold. They found that after bottling up the HRSG, as when blanketing the unit with nitrogen, they could inject plant air via the nitrogen circuit to pressurize the drums and check for leaks.
Once the pressure reached 30 to 50 psig, staff could walk down the drum doors and listen for leaks. Dogwood has had great success in filling the HRSGs and restarting the plant when checking the doors with this process.
Caution: Before using a system for purposes outside of its original scope, engineers should evaluate the system to verify that piping design conditions are not exceeded and that there are no compatibility issues. Maintaining system integrity is an important component of safety.
Results. Dogwood has been using this method to check for drum-door gasket leaks for several years. During this time, leaks have been repaired before filling the HRSG, reducing the probability of a forced outage associated with gasket leakage. This process has produced several benefits, including these:
- Allows Dogwood to check for leaks while the plant is still in an outage.
- Checks are completed with minimal equipment in service, making it easier for operators to hear leaks.
- If a leak is identified, the gasket usually can be replaced within the scheduled outage, with the main-steam and water LOTOs in place.
- Drum shell temperatures are not an issue as all temps are at ambient conditions instead of discovering a leak during a startup, where metal temperatures may be several hundred degrees.