For operators of gas turbines, the introduction of more stringent emissions regulations continues to have an impact on the daily operations and maintenance strategies of the plant. In order to provide a solution, gas and steam turbine repair specialist Sulzer has developed a new solution that solves the primary fuel nozzle leak issue on two GE gas turbine models.
Richard Vogel, Fuel Nozzle & Flow Test Engineer at Sulzer, provides some background and explains how the new option has changed his approach to fuel nozzle maintenance.
The solution, which can be implemented during routine scheduled maintenance, effectively reduces previously tolerable minor leaks at the primary fuel nozzle tips to zero. This enables the turbine to run at optimum parameters for power, efficiency and component lifespan, while assisting to meet stricter emissions regulations.
The growing importance of embracing the environmental responsibilities associated with power generation has led to tighter local and national legislation being imposed on the industry. This, in turn, has led gas turbine operators to change operational parameters and make decisions to meet these tighter emissions requirements, which may conflict with what is ideal for the equipment, and in achieving targeted turbine output.
Maximizing revenue and minimizing emissions
In order to adjust emissions levels, plant operators must alter the fuel/air ratio, which then affects firing temperature and mechanical output of the turbine. In effect, there is a balancing act between the cost in terms of avoiding environmental fines for exceeding emissions targets and the lost revenue and increased maintenance costs associated with adjusting the operational characteristics of the gas turbine.
One of the main focus points when adjusting the emissions levels of the gas turbine is eliminating sources of increased emissions, such as leakage from the primary fuel nozzle.
This issue has been identified on some 6B DLN and 7EA DLN primary fuel nozzles. However, meeting more restrictive emissions limitations may require operators to adjust the fuel/air ratio, change the firing temperature or alternatively to make an attempt at reducing fuel nozzle leakage. Excessive leakage at the primary fuel nozzles comes at greater cost to the operators, which may have to implement costly emissions scrubbing at the exhaust stack.
Changes to the fuel/air ratio cause the combustion temperature to be altered and can also affect the ability of the plant to meet power demands as the mechanical output may be reduced. In addition, increased firing temperatures can have a negative effect on the longevity and the reparability of the components in the combustion system and the hot gas path of the turbine.
One practical alternative to adjusting the fuel/air mix is to address the issue of fuel nozzle seal leakage and eliminate it. Taking the GE 6B DLN and 7EA DLN gas turbines as examples, the OEM seals used on the primary fuel nozzles have been known to leak in some cases with minimal operating time.
Zero leakage, without welding
As part of a standard maintenance program, the primary fuel nozzles are typically exchanged for spare components that are kept by the plant owner; this keeps the downtime to a minimum and allows the spare set of primary fuel nozzles to be refurbished without affecting plant operations and outage duration. Ideally, operators would prefer to have fuel nozzles with zero leakage and some have opted to fully weld the fuel tip to the primary fuel nozzle body using a welded band interface to avoid seal leakage.
This solution has a short term benefit, provided the welding has been carried out correctly. However, when primary fuel nozzles using this solution need to be refurbished, the process of removing the welds means that the primary fuel nozzles may require further refurbishment than necessary to be rebuilt. Depending on the amount of base material removed, premature replacement of endcover inserts or fuel tips may be required, increasing the maintenance and repair duration.
For those operators that are looking to improve this situation, an alternative solution has been created that uses a new seal design and is now field proven to deliver zero leakage before and after installation/operation in the turbine. The collaboration between engineers at Sulzer and some of its customers has resulted in the development of new seals designed for direct replacement of the original C-seals in 6B DLN and 7EA DLN primary fuel nozzles.
Cost effective solution
The revised fuel nozzle refurbishment process follows a stringent repair methodology to ensure that the seal contact surfaces are prepared with precision to ensure the best possible fit with the improved seals. The repair technicians use custom-designed fixturing in the process of seal landing refurbishment, which ensures a high quality repair and improves the efficiency of the process.
Flow tests and dynamic leak checks are performed on a highly accurate and repeatable flow test bench. After refurbishment, final flow testing and dynamic pressure testing are performed to validate the seal integrity above normal operating pressure and confirm the zero leak status of the assemblies. Typically, the refurbishment process using the improved seal design can be completed in less than two weeks, which is both more cost effective and time efficient compared to assemblies that have employed the full tip-to-fuel nozzle body welding process.
Sulzer’s proprietary seal design can be used on both the GE 6B DLN and 7EA DLN gas turbines and initial customer feedback has shown evidence of additional benefits. Gas turbines running with the new seals achieved the required output while remaining within the prescribed emissions limits as expected. Another benefit found was improved exhaust-temperature spread, which assists in a more balanced combustion and lends itself to increased turbine reliability.
Maintaining fuel nozzle performance on gas turbines is one of the most crucial steps in actively reducing the emissions levels of the plant while achieving the required power output. Making adjustments to the fuel/air ratio to compensate for nozzle leakage is the typical solution used resulting in a reduction in output and revenue, as well as increased maintenance costs.
By reviewing the primary fuel nozzle refurbishment procedure, plant owners now have the option to achieve zero leakage. This gives operators the ability to achieve emissions limits while maintaining targeted turbine outputs, without increased expense and time involved in the alternate solution of welding. The emissions levels eliminated by using Sulzer’s improved seal design also reduce the costs associated with exhaust stack emissions scrubbing.
Sulzer is one of the largest global repair and maintenance providers for gas turbine and other rotating equipment and provides a repair and replacement service anywhere in the world.