Back Pressure Steam Turbine Modification 1

[hd167]

We have a SIEMENS back pressure steam turbine with the following design data:
- Input steam pressure: 38 bar (abs)
- Input steam temp.: 450 °C
- Exhaust (back pressure): 4.5 bar (abs)
- Output: max. 7080 kW
- Steam flow (at 7080 kW): 63 t/h
- Min. steam flow: 18 t/h

During the last years the process steam demand has changed, so that we can operate the turbine only with 18-30 t/h steam (mostly 20 t/h). This means that the daily efficiency of the turbine is now much lower than the former efficiency (full load efficiency).

Questions:
1. Is it possible to modify and adapt the turbine to the new load conditions, so that a better efficieny is obtained?
2. What modifications are reasonble to be made (to reduce the no. of stages?)?
3. Is it possible to calculate the efficiency of this turbine at part load (20 t/h), without knowing the details of the blades? My calculation of the efficiency at full load gives me a value of 74.5%.
4. Is it possible to reduce the min. flow of 18 t/h (at present) to 12 t/h?

I would appreciate your help.

 

[rmw]

Check with the turbine OEM. They will have to advise.

What you are seeking is major surgery, and will probably require a new nozzle block, or nozzle ring, as starters. Much of what a turbine does downstream depends on what goes on in the first stage.

Stages might have to be removed, or rebucketed (rebladed).

Your overall calculation of efficiency is a product of the summation of the stage efficiencies of all the stages of the machine.

The efficiencies of a stage is a function, among a lot of other things, of the volumetric flow, which determines the volumetric efficiency of the machine. In other words, when the steam path is full of steam, the machine is most efficient.

These are general sayings. The specific questions you are asking can only be answered by Seimens.

It should be possible to do what you want to do. However, the question becomes does the cost justify itself?

rmw

 

[byrdj]

Besides asking Seimens, there are a lot of consultant companies that may help. Along with the engineering, most can provide the parts and the installation. In Post 605-47965 there is referance to a company that provides training courses in turbine efficency. That, or similar companies my be more responsive.

 

[poetix99]

This is a major turndown of the turbine; 31.5% of the original rating. As "rmw" pointed out, a turbine is a volume flow device, and you have reduced the design volume flow by a factor of 3.

As you have been advised already, you should consult with Siemens. They should be willing to quote a couple of rebuild options, with different levels of performance.

You should realize that you will not obtain, via any economically feasible rebuild of this existing machine, the same performance that you could get from a machine that was actually designed and newly constructed for the 12 - 30 t/h in the first place.

I think that a reaction design (such as what I assume the Siemens design to be) handles turndown a little bit better than impulse designs, but it a lot to ask nonetheless.

(The Siemens turbine is probably between 8 and 10 stages for your application conditions.)

Here are some general points concerning any steam turbine:

The optimal number of stages is determined by steam conditions, turbine speed and stage diameters; NOT by the amount of steam flow.

The amount of flow determines the size of the stages (and therefore might determine the combination of stage count & speed)

When the flow is decreased from the design flow, the first and last stages are moved further from their design points than the other stages. The energy on the first stage increases dramatically, and the energy on the last (few) stage(s) decreases.

I am not at all sure what the "minimum flow" corresponds to. A non-condensing turbine does not require very much of a so-called "cooling steam" flow.

Good luck.