7FA DUAL-FUEL NOZZLE CONVERSION

Arecent project involved the installation of six Frame 7 Dry Low NOx (DLN) 1.0 Dual Fuel units in Venezuela. After determining that the lead time to supply all of the required sets would not meet schedule demands, a decision was made to convert the fuel nozzles from a gas-only unit to a dualfuel configuration.

The resulting re-engineering project utilized new sets of dual-fuel end covers and secondary nozzles. The fuel nozzles were disassembled down to their smallest component. The details of the parts were laser scanned and dimensionally inspected to create 3D models which were converted into SolidWorks 3D modeling software to allow for the virtual model to be manipulated. This re-engineering phase took three weeks to complete.

Utilizing 3D models, the parts were reassembled in the computer to ensure proper fit up before manufacture. Part drawings were created to facilitate the procurement of everything required for manufacture, inspection and delivery within six weeks of project start date.

Primary end covers An early challenge was how to introduce an atomizing air cavity into the gas-only end cover. The initial consensus was that this would easily be achieved as the dualfuel nozzles would already have this cavity installed. However, closer review of the covers revealed that the gas passage on the gas-only end covers was installed on the cold side of the end cover whereas this passage is located on the hot side of the dual fuel end covers. The position of the cavity overlapped the location of the atomizing air passage of the dual fuel covers, leaving it impossible to install a passage similar to the dual fuels.

To address this problem, designers used 3D modeling to identify the best location to install the newpassage thatwould not affect the minimal wall thicknesses of the internal passages nor would it reduce the cavity cross sectional area.

Unfortunately, the chosen location of the gas passage on the cold side also posed an issue for mounting the fuel blocks (dog bones). The fuel blocks’ mounting bolts would now pass directly into the gas passage. This was overcome by deepening the passage and installing isolation blocks.

As part of the modification/conversion, designers looked to minimize nozzle boss braze cracks. These cracks are caused by thermal stresses induced by the captive mid-section of the boss. This boss has lower and upper braze joints (Figure 1). During operation, the thermal expansion of themid-section causes the upper braze joint to crack. The condition was eliminated by constructing the insert out of two pieces. Each of the two parts has only one point of retention, thereby reducing the stresses typically experienced.

Gas-only end cover and secondary nozzles

During the processing of the end cover, it became necessary to remove a small amount of material across the entire surface to ensure the new boss’ were perpendicular to the end cover mounting face. Removal of this material, however, meant that the liquid core would now insert too far into the atomizing air cone and impinge on the water injection holes. This condition was corrected by installing welded shims under the fuel blocks allowing for interchangeability of the modified end covers hardware and all other DLN1.0 dual fuel end covers. (Figure 2)

Issues arose too with regards to the conversion of the secondary nozzles. Initial design changes included the installation of the liquid and water inlets, and replacement of the blank liquid cartridge with a live one.

After further review of the main body, it was discovered that in order to prevent trapped air in the liquid passage, the housing has a cross channel passage installed from the liquid passage into the flame scanner port. To prevent liquid fuel from entering this passage, designers engineered a way to separate these passages by inserting and brazing a sleeve between the ports.

This required the removal of the flame scanner inlet pipe to gain access to the area identified as the best location for this sleeve. Since there would be no access to the sleeve after assembly, the properties of Gold Nickel Palladiumbrazematerial were harnessed so the part sleeve joint would outlast the useful life of the original design. After this modification was completed, the secondary conversion was as straightforward as initially anticipated.

According to the customer, this conversion project provided 25% savings over aftermarket parts while utilizing the excess inventory of gas only end covers and secondary fuel nozzles. Similar conversion projects for the GE Frame 7FA(+e) and S/W501F units have been embarked upon.

All six 7FA Gas Turbines at the site are now capable of operating on blended fuel. To date, these units have accumulated more than 5,100 fired hours on blended fuel.

Authors

Chris Garrett is Fuel Systems Manager at ProEnergy Services, an integrated service provider to the energy sector. Tel 660.829.5100 or visit www.proenergyservices.com