The number of shafts generally relates to the number of power-producing shafts, not counting those in the transmission to the load.

The LP and HP shafts are coaxial and are within the enginer casing. On some designs the power turbine drving the load is on a third shaft independent of the LP and HP shafts, on others such as the LM6000 the load is coupled to the LP shaft with the HP shaft acting as a gas generator. The gas flow passes from LP intake to LP compressor discharge to the HP compressor intake, then from the HP turbine exhaust to the LP turbine intake, then from LP exhaust to the power turbine intake if applicable.

Have a look for an image of an LM6000 or LM2500 for examples.

single shaft : in general used for power gen apps where operation is at fixed speed (example 3600 rpm - 2 poles 60 Hz generator)
two shaft : typically used for mechanical drive apps where speed variation is applied to cover a more or less wide operating envelop of the driven equipment.

This is valid for aero derivatives or heavy duty. Some single shaft are used for mechanical drive as well (big size, LNG market is a good example)

On a two shaft, typically you have the gas generator part (compressor, combustor, turbine) on one shaft and a power turbine on second shaft.
If you take the example of the PGT 25 which is a GE aero derivative gas turbine for mechanical drive, the gas generator is an LM2500 'same' as used in the aircraft industry and the addition of a power turbine makes it a PGT25. That is to say the gas power at the outlet of the gas generator after the LP/HP turbines is expanded into a dedicated power turbine to generate shaft power.

The power turbine is coupled aerodynamically via a governor (adjustable nozzle) which adjust the path of the flow to the power turbine thereby varying the gas turbine output speed/load.

I don't recall model numbers or manufacturers, but several of the early aero (jet) gas turbines were dual-shaft: Inside shaft linked the forward set of compressor blades and their large diameter bypass air blades and the last set of turbine blades. A separate (faster but shorter) outside shaft was concentric of course to the inner shaft, but linked the compressor blades with the nbr one set of turbine blades.

Intent as I understand it was to get the bypass air around the engine to the largest diameter but the slowest speed possible, while the compressor blades inside the casing driven by the outside shaft were at maximum speed and optimum diameter for the fuel flow to the burners inside.

Steam turbines have all sorts of multi-shaft, multi-turbine, multi-geared parallel and combined arrangements.

scotty, rotary, racook hello!

I got three answers all very good ones

the way they have been worded, in a way confirms my obsevation that there is not a clear-cut definition of a single shaft-turbine versus two shaft

one answer migt say "if this is like ehis, and that is like that", it is this kind, but remember there can be a combination of this and that also ---

another "85% of power turbines are aero-derivative, single shaft, except when this happens here, and that happens there, in whih case ---

Plese do not misunderstand it is just that the solution is not a clear black and white

please see if the following statement is reasonably accurate, without going into too may details

in a single-shaft turbine, the compressor shaft and turbine shaft are concentric. Any change in concentricity by a gear-box can be ignored
in a two-shaft turbine, the power turbine shaft and the compressor turbine shaft are not concentric


Thank you
Aamir

The nomenclature applies to industrial type gas turbines and not to aeroderivatives

Single shaft gas turbine: compressor rotor is bolted to power turbine, used mostly for power generation. NOTE: the gearbox is a reductor from 5105 rpm (typical speed for a GE frame 5 unit) to 3000 rpm (50 Hz) or 3600 rpm (60 Hz)

Two shaft gas turbine: the compressor rotor is bolted to the gas generator and the power turbine is freewheeling. Between the gas generator and the power turbine a stage of variable nozzles help control the power output.

Two shaft gas turbines are mostly used in the Oil & Gas industry as prime movers for load compressors.

See the GE website for the definitions

saludos.
a.

QUOTE
The LM2500 is derived from the CF6 family of aircraft engines used on a variety of commercial aircraft and is a hot-end drive, two-shaft gas generator with a free power turbine.
UNQUOTE

quote from a GE website.

This refers to an aero-derivative turbine with two shaft ; nomenclature applies here too.
And yes it is true this nomenclature is more used/common for heavy duties but I think you made an over-statement.

@rotaryw,

No, don't think it is an over-statement... the original post is asking for the difference between single and two shaft gas turbines.

The quote from GE is perfectly clear:

QUOTE
The LM2500 is derived from the CF6 family of aircraft engines used on a variety of commercial aircraft and is a hot-end drive, two-shaft gas generator with a free power turbine.
UNQUOTE

It says: two-shaft gas generator, which is the [LP Fan-LP gas generator(turbine section)] + [HP compressor-HP gas generator(turbine section)] the two shafts are concentric (LP goes inside HP), WITH a free (turning) power turbine which is a THIRD shaft that takes the combustion gases to generate power.

In the LM (aeroderivative) world we never talk about "two shaft gas turbine" because it is a known fact that to use the unit for power generation a free turning power turbine must be present. This is not true with the industrial type units, which are ALMOST ALWAYS single shaft for power generation, with the two shafts reserved mostly for use as mechanical drives.

saludos.
a.

To my knowledge a pgt25 has a gas generator (purely an LM2500) and a high speed power turbine.
The gas generator has an axial compressor coupled to a HP turbine and it is rotating at constant speed (3600 rpm). So this is two shaft.

On an LM6000 there yes, exists LP/HP turbine and compressors systems. But you see already there are differences in the "LM world" you mention.

And still a "three shaft" terminology is for pure aeronautical turbines, as there you have a fan (1 fan stage which is coupled to the LP turbine system).