Dear All;

My greetings to all. Please, I want anyone to explain the swirl effect in the gas turbine GE frame 9FA. What does it mean and how does it work, and how do I identify the cans that give a spread or cold spot in exhaust, and from the cold spot in exhaust, how to identify the cans that cause this cold area.

Thanks

 

Most heavy duty gas turbines use thermocouples in the exhaust of the machine to measure the exhaust temperature--which along with other operating parameters--most importantly axial compressor discharge pressure) can be used to control and protect the turbine section of the machine. And, most manufacturers use a number of thermocouples arranged symmetrically around the periphery of the gas turbine exhaust to monitor the exhaust temperature.

Some gas turbine manufacturers, such as GE, use what is referred to as a can-annular combustion system--which means there are multiple combustors ("cans") annularly arranged around the machine, each one having its own fuel nozzle(s) injecting fuel into the combustor to produce hot gases which are then directed into the first stage turbine nozzle sections to flow through the turbine and into the exhaust. The hot gases from the combustion zone(s) of each combustor are directed into the first stage turbine nozzles through what are usually called transition pieces--which take hot gases in a "round" flow and squeeze them down into an arc of a circle whey they flow into the first stage turbine nozzle.

The ideal combustion situation--and the best combustion situation--for the machine is when precisely the same amount of fuel flows into each of the Individual combustors and produces the exact same combustion gas temperature. Another aspect of ideal gas turbine operation is that the same amounts of combustion air and cooling (dilution) air also flow into each one of the individual combustors. In this way each section of the first stage turbine nozzles receives hot combustion gases that are VERY similar in temperature to all the other sections of the first stage turbine nozzles. And, as these hot combustion gases flow through the turbine section and into the exhaust the temperature of the gases as they are expanded (and cool) are very similar, also, which results in uniform exhaust thermocouple readings around the gas turbine exhaust area. This is the intent of the system--to produce nearly identical hot combustion gas temperatures in each individual combustor that will be directed into the turbine section. This is the best for the components of the turbine section (nozzles, buckets (blades) and exhaust section) and results in optimal parts life (meaning longer periods between shutdowns for maintenance inspections and parts replacement).

A problem arises when one (or more) of the combustors gets more--or less--fuel, or more or less combustion air, or more or less cooling air. This causes the temperature of the combustion gases in those combustors with uneven fuel and/or air flows to be higher or lower than other combustors. And, this is NOT good for the turbine components (combustion liners; transition pieces; nozzles; buckets; exhaust).

BUT, there's no way to know precisely which combustors have more or less fuel flowing, or more or less combustion air, or more or less cooling air. For machines running on gas fuel, the individual combustors are fed from a common manifold for each set of fuel nozzles, so if the problem is uneven fuel flows to individual combustors it's very difficult to know which combustor has the problem.

Combustion liners (where the fuel is injected from the nozzles and combusted) can develop cracks and enlarge cooling holes. The seal (called a "hula skirt" seal) between the open end of the combustion liner can also develop crack and holes. The seal strips where the transition pieces fit into the first stable turbine nozzle segments can degrade or not have been installed properly.

But, fear not! The surprising thing is that the combustion gases as they flow through the turbine DO NOT MIX! So that means that if Combustor #4 has a fuel problem (say some of the fuel nozzle orifices are plugged with debris) the combustion gases from Combustor #4 will be lower than the other combustors and that low temperature will be sensed by one or more of the thermocouples in the gas turbine exhaust. This causes an exhaust temperature spread (differential) between the temperaltures being detected in the gas turbine exhaust--the higher the exhaust temperature spread (differential) between adjacent exhaust thermocouples the worse the problem is in a particular combustor.

While the combustion gases do not mix with the combustion gases of other combustors as they flow through the turbine and into the exhaust, they do not travel through the turbine in a straight line. So the low combustion gas temperature from Combustor #4 will not necessarily be sensed by the exhaust temperature thermocouples directly downstream (axially) from Combustor #4. And, in fact, the "cold spot" (because in our example there is less fuel burning in Combustor #4 because one or more orifices of the fuel nozzle(s) in Combustor #4 are plugged) will shift with the load and as the axial combustor IGVs (Inlet Guide Vanes) open and close. So, as the machine is started, synchronized and loaded the cold spot caused by less fuel flowing into Combustor #4 may be measured by exhaust thermocouples #12 and -13. And as the machine is loaded (and the IGVs open) the cold spot may shift to exhaust thermocouples #11 and -12, and as the machine is loaded further the cold spot will move closer and closer to the exhaust thermocouples almost directly downstream of Combustor #4. As the machine is unloaded the cold spot will move in the opposite direction. This phenomenon of a "moving" hot- or cold spot is called "swirl."

The point is while the colder combustion gas temperatures from Combustor #4 do not mix with or get warmed up from the combustion gases from the other combustors they do not flow straight through the turbine and into the exhaust. And, the fact that the cold spot caused by Combustor #4 will "move" in the gas turbine exhaust as sensed by the exhaust thermocouples, so it IS possible to determine which combustor is experiencing a problem (might be plugged fuel nozzle orifice(s), or a crack or hole in the combustion liner or transition piece) by knowing how much the combustion gases from a particular combustor experiencing combustion trouble will shift in the gas turbine exhaust as a function of load and machine design (and IGV angle). And, gas turbine-specific swirl charts, or swirl angle charts, have been developed and can be very useful when trying to understand which combustor is experiencing a problem when an exhaust temperature spread is increasing to pinpoint which combustor is having the problem.

So, swirl is the action of a cold spot or hot spot "moving" around the gas turbine exhaust as the machine is loaded and unloaded. The amount of "movement" (swirl) in the exhaust from a combustor in the exhaust is a function of load (and IGV angle) and machine design.

It's very important to note that swirl angle charts can be very specific to some machines. Some PC applications which can be used for various families of gas turbines have been developed and are in limited circulation--but these are not always very precise for every machine in a particular family of gas turbines.

 

Thanks Friend for your good explanation