Written by Brad Buecker | Power Plants
Over the last two decades, many coal-fired power plants have been retired for both economic and environmental reasons. Wind and solar have replaced a significant portion of this generation, but with renewable energy production subject to the influences of nature, rapid dispatchable sources are critical to maintaining grid reliability. This “bridge” capacity has largely been filled by simple and combined cycle power generation, whose plants are typically fueled by natural gas. The high temperatures produced in combustion turbines and the typical on-and-off cycling of these units induce high stress in system components, including downstream ducts and internal components such as noise silencers.
What Maintenance Issues Were Identified?
The combustion products that power gas turbines can reach 2,700 degrees F. Even though much of this heat is converted to mechanical energy in the combustion turbine, the exhaust gas temperature may exceed 1,100 degrees F. In a combined cycle unit, most of this heat is consumed in the heat recovery steam generator(s) to produce additional power in one or more steam turbines. This arrangement significantly increases the overall efficiency of the plant. More important than efficiency in simple cycle plants is to quickly provide power during times of rapid load increases. Most units can reach full load within ten minutes, but the turbines may be online for only a few hours before cycling off.
Simple cycle turbine exhaust is not utilized for additional power production, and the flue gas is discharged into the atmosphere. Therefore, the outlet duct and stack are subject to hot, rapidly flowing gas and temperatures that fluctuate over a very large range. Thermal stresses and mechanical degradation can be substantial. Periodic equipment repair and replacement is critical to maintain reliable and safe operation. The following list highlights important equipment maintenance issues that had arisen at a simple cycle power plant in Idaho.
- Degradation of original silencer baffles and support brackets in the outlet duct
- Replacement of the internal duct liner in some spots
- Need for new test ports on the outlet duct
- Cracks in duct and stack welds
Noise regulations are common for simple and combined cycle plants, as many facilities are located within or near residential areas. Duct silencers are necessary to keep noise levels below local guidelines. Over time, silencers suffer wear from the high-velocity flue gas, and bracket supports may crack and break loose from the duct.
Internal duct liners are also critical. The insulation holds heat within the duct to reduce thermal stress and minimize the formation of hot spots on duct walls. The liner and supporting mounts suffer wear and tear from the hot, rapidly flowing flue gas.
Test ports are important for the evaluation of flue gas flow dynamics and conditions. The original test port design may not provide the most accurate information. A unit down for maintenance is an ideal time to install additional test ports in strategic locations. This task was a key part of this project.
In many high-energy applications, cyclic stress placed on welds will cause cracking and through-wall penetrations. These penetrations can allow the release of process fluids or exhaust gases into the environment. Also, once cracking begins, it continues to perpetuate until structural failure may occur.
What Solutions Did SVI BREMCO Recommend?
Following a complete system survey, the SVI BREMCO staff addressed each of these issues and others during a several-week maintenance outage. A primary task was, of course, silencer baffle and support bracket replacement.
This work required coordination of multiple personnel, including equipment specialists, welders, crane operators, and safety personnel. These specialists also installed new test ports and made duct liner repairs during the maintenance window.
The SVI BREMCO website includes additional posts that provide information about more extensive liner replacements.
SVI BREMCO personnel repaired duct/stack metal cracks.
They also discovered and repaired a crack in a generator lubricating oil line, which obviously is not part of the duct and stack. This is a profound example of the importance of regular and thorough inspections of all plant equipment during outages and, where conditions permit, during normal operation.
Safety – Of Paramount Importance
As is evident from the photos and discussion above, much of the work took place at relatively high elevations and in outdoor conditions that required moving, lifting, and anchoring heavy loads. Safety is always of primary importance at SVI BREMCO, and a safety meeting is held at the beginning of every morning and evening shift of a project. Each meeting for this project included a discussion of a particular topic to remind staff of critical safety issues. Topics included:
- Fall protection
- Electrical safety
- Hot work issues
- Lock out/tag out (LOTO) procedures
- Scaffolding and ladder safety
SVI BREMCO has a highly experienced staff to evaluate and offer solutions for your power industry maintenance needs. Please reach out to us at:
P.O. BOX 1491 SHIPPING ADDRESS:
CLAREMONT, NH 03743 680 JOHN STARK HIGHWAY
TEL: 603/863-9071 FAX: 603/863-9014 NEWPORT, NH 03773
www.bremco.com sales@bremco.com