Single Shaft vs 2 Shaft Design turbine for use in power generation

[RobsVette]

Hey guys,

I am going to be looking at a project next week that involves the use of a 2 shaft turbine produced by Solar. I believe this is the Mars 100 Unit. In the past we have had great experience with Solar, but none of the units have been 2 shaft designs.

I am personally familiar with using a single shaft design in power generation applications because the speed control for the turbine is much easier, since the shaft speed is directly proportional to the position of the input valve (then load is directly proportional to the input valve) when operated with a utility electric grid.

I have seen many large gas compessor turbine drives operated as dual shaft designs as well. I always thought the reason for this was to allow for some flexibility between the compressor load and the turbine fuel valve.

Can you guys please explain why a single shaft turbine or a dual shaft turbine would be used for gas compressrion / oil pumpign vs power generation and what the advantages and disadvantages of each are? Also, anyone that has any experience with the Solar Mars 100 unit could chime in on their personal experiences, it would be greatly appreciated.

thanks,
Rob

 

[rmw]

Two shaft machines are run in compression/pumping applications because output shaft speed is not necessarily fixed as it is in power generation where it is fixed to the frequency of the grid.

That said, do some Googling around on aeroderivative turbines used for power generation and you will find that they are 2 shaft and 3 shaft machines as opposed to their single shaft frame cousins which are of course frequency fixed speed.

The 2 or 3 shaft designs allows portions of the turbine to spool up to speeds where turbines are very efficient resulting in higher overall efficiency or better heat rates for the engine.

rmw

 

[ccfowler]

For power generation, the multi-shaft aeroderivative turbines have advantages of lesser starting power requirements for "black plant" starts, and they can usually be brought to full load quicker than single-shaft machines. For power generation, the multi-shaft machines tend to have better part-load efficiency than "otherwise comparable" single-shaft machines. Since all gas turbines tend to be more than a bit fuel hungry at less than full load conditions, it may be better to think of the multi-shaft machines as having less-poor part-load efficiency than single-shaft machines. Large single-shaft power generation gas turbines are usually optimized for maximum efficiency at their constant operating speed and maximum rated continuous loading condition, so they commonly achieve higher maximum efficiencies than multi-shaft machines. As long as the single shaft machines can be operated at or very near their maximum efficiency loading, the relative fuel savings will justify their selection over multi-shaft machines. When part-load operation can be expected for a non-trivial portion of the machine's service life, then multi-shaft machines may present a more attractive choice.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.