Converting a 7FA to dual-fuel

A recent project involved the installation of six Frame 7 Dry Low NOx 1.0 Dual Fuel units in Venezuela, and a decision was made to convert the fuel nozzles from a gas-only unit to a dual-fuel configuration. This decision was taken after determining that the lead time to supply all of the required sets would not meet schedule demands.

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 re-assembled 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.

Problem with positioning

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 dual-fuel 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 new passage that would 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. During operation, the thermal expansion of the mid-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.

You can read the rest of this article in the Jan/Feb issue of Turbomachinery International.