Respected Sir;

Good day. I am working on a gas turbine two shaft Centaur 4502. We measure the temperature at inlet of third stage nozzle with 6 thermocouple identify by T5. The readings of thermocouples are different and from these readings the delta and spread are calculate.

The some turbines measure the EGT and also calculate the temperature delta and spread.

I want to ask what is the difference between delta and spread and what effect of these readings on gas turbine health?

Please help me to clear this point.

Thanking you i will remain
James

 

James,

I'm not familiar with Centaur turbines, but I am familiar with GE-design heavy duty gas turbines which monitor exhaust temperature spreads to protect the turbine against combustion problems which can cause catastrophic failures of turbines.

There are almost always multiple fuel nozzles for any type of gas turbine, and while some gas turbines have an annular combustor (one that is continuous around the outside of the turbine) some have multiple can-type combustors (called can-annular combustors). The idea is that when there are multiple fuel nozzles and/or combustors that the amount of fuel flowing through every combustor and fuel nozzle should be as nearly identical as possible. This ensures that the profile of the hot gases entering the turbine nozzles are as uniform around the gas turbine as possible.

Let's consider a gas turbine that rotates at 5100 RPM, or 85 RPS (Revolutions Per Second). This means that every turbine bucket passes a fuel nozzle/combustor location 85 times per second. If the temperature of the gases entering the turbine section from the combustion section are uniform around the turbine then the turbine nozzles and buckets will have a very long life.

If, however, let's say one of the fuel nozzles is badly plugged and so very little, if any, fuel is flowing out of it. The temperatures of the hot combustion gases downstream of that fuel nozzle (or combustor) will be much lower than those on either side of it. This means that every time every bucket rotates past this fuel nozzle/combustor it is cooled and then heated again as it rotates further around the unit. In our example, evey turbine bucket will be subjected to repeated cooling and heating, cooling and heating--at the rate of 85 times per second. This represents a thermal stress on every turbine bucket, this repeated cooling and heating.

If the temperature differential is very high between adjacent fuel nozzles/combustors then the thermal stresses are very high--and the resultant damage can quickly cause failure of the turbine buckets, which when liberated can cause quite an expensive repair problem.

Even if the temperature differential (sometimes referred to as a "spread") isn't very high if allowed to persist for a long time it can still cause premature weakening and eventual premature failure of turbine buckets.

So, spreads are usually the temperature differentials between high and low exhaust values, since many turbines do not actuall measure the temperatures of the hot gases entering the turbine nozzles from the combustor(s). And, I would imagine "delta" is similarly used to indicate the magnitude of the temperature differential(s).

This is about as much as I can offer you. Measuring spreads and deltas is a way of determining the severity of a combustion problem. Or, let's say there's a serious crack or leak of cooler axial compressor discharge air into the combustor, that, too, could cause a "cold spot" which would have the same damaging effects as low fuel flow-rate to one or more fuel nozzles/combustor(s). Many times, turbines are either shut down or tripped on high spreads to protect against catastrophic failure. The turbine manufacturer or packager decides what is an "excessive" spread or delta and what action the control system should take in the event the operators fail to recognize a problem or take corrective action in order to protect the turbine.

Most sites always insist that spreads (deltas) are "caused" by problems with the gas turbine control system--but that's almost never the case when the control systems are properly maintained. Spreads are the result of mechanical problems (presuming the control system is properly maintained) or plugged fuel nozzle(s) (also a "mechanical" problem in my estimation).

Hope this helps! There is a lot of information here about GE-design heavy duty gas turbines and control systems, but not very much about Centaur units (which I believe are manufactured by Solar).