Stage 1 Nozzle Refurbishment

On many combustion turbines, certainly on the GE 7FA, the Stage 1 Turbine Nozzle is probably the most critical component for turbine performance and emissions margin. The throat area plays a big factor in the amount of hot gas flowing through the turbine, and it also plays into the compressor pressure ratio (CPR) which affects firing temperature. This component also has the potential of using too much of the precious compressor discharge air whether by excessive cooling air consumption, or bypassing this air around the chordal hinge seal.

who cares if it is round? I do.

A perfectly round nozzle ring is not necessary, but if it is out of round / elliptical by more than 1/8 inch, not only will it make transition piece installation more difficult, but it is a sign that something is wrong. It could be that the support ring itself is warped, but that is what usually gets all the attention and is not typically the problem. The horizontal joint seals may not be fully engaged, but that can be remedied. The main problem is if the duplex airfoil segments get warped (too much heat during weld repairs) and subsequently jammed into the support ring. This will warp the overall ring and could cause gaps in the chordal hinge which is supposed to seal axially against the stage 1 shroud blocks. A good after-the-fact check for this is to look at the downstream side of the used nozzle ring during an outage after a run and look for rusty-colored streaks. Those are tell-tales of a leaky chordal hinge.

throat area

Through a long series of precise measurements, the “throat” between the forty-eight stage 1 nozzle airfoils is calculated. The turbine is designed for a specific throat area. If it is too large, it will cause one type of problem, and if too small, it will cause another type of problem, both of which will affect overall turbine efficiency and MW output. There is an optimal range for S1N throat area, and your refurbishment vendor should know this target and report how close they got. If this is not a part of your repair spec, we can help.

all those little cooling holes

The stage 1 nozzle airfoil segments are cooled by compressor discharge air through ports in the outer support ring. The air passes through perforated inserts and out through small cooling holes strategically located in the airfoils and platforms. There are two circuits, one to the leading edge and one to the trailing edge. GE issued a Technical Information Letter (TIL) which describes performance issues (high NOx / low output) after installing refurbished stage 1 nozzles, and they point to excessive cooling air flow. Some vendors take this very seriously while others prefer to ignore it because it is complicated to measure and expensive to remedy. If your repair vendor does not look at this, you need to find someone who does.