Wet steam/saturated steam Turbines 5

[JohanOlsson]

I'm looking for steam-turbines that operates with saturated steam, which means that the turbine in question must be able to handle a lower steam quality than an ordinary steam turbine. I know that there is applications in large scale power plants, like nuclear power plants, but I'm looking for smaller turbines.
The actual case is about a plant of 50 MW working with saturated steam at about 20 bar.
Does anybody know about any company that makes turbines for these conditions?

Johan

 

[hacksaw]

are you referring to the lack of available superheat?

 

[JohanOlsson]

hacksaw:

Exactly! The steam cannot be superheated, so we have to use the saturated steam (~20bar).

Johan

 

[Montemayor]

Johan:

Steam turbines don't have to be fed superheated steam to function well. I've used both saturated and superheated and I, quite frankly, prefer the saturated variety. I used them for years with economic and mechanical success. But I always kept an expert and experienced millwright/mechanic on the payroll. The turbines didn't give problems once they were setup and maintained expertly. They just need, like any high-speed machine, someone who knows how to set them up and maintain them.

Be aware that we are talking about pure, saturated steam - not wet steam! There is a big difference between both feeds to the turbine. Any steam turbine is going to suffer and be subject to damage when subjected to entrained water particles or droplets - let alone slugs of water. You must maintain your steam saturated and devoid of entrained water. Make liberal use of steam traps all the way up to the entry to turbine - and with no low, un-drained spots in your saturated steam piping.

I used and liked Terry Turbines as well as Elliot. There are still many steam turbine companies out in the market place - in Europe and USA with good operational reputations. I realize you are a mechanical engineer; however I would emphasize that the steam quality of the saturated steam is not "low" - unless you allow it to be contaminated with condensates. It is merely saturated, with no superheat. If you trap and drain well you can ensure that your turbines will receive good quality, saturated steam. Discuss this well with your turbine manufacturer. I'm sure he'll tell you the same thing. Lots of luck. I hope this experience helps out.

Art Montemayor
Spring, TX

 

[poetix99]

I agree, for the most part, with the discussion so far. There is no fundamental difference in what a steam turbine with (dry) saturated inlet steam would look like compared to one designed for superheated steam. Some details might be different, particularly so as to direct some of the accumulating moisture away from the blades, and out of the steam path. It will be better for you that the manufacturer would have some experience with these "wet" applications.

Part of the risk is that "dry, saturated steam" at the inlet actually means a few percent wet at the inlet. Putting "slug" of water through a steam turbine can cause a lot of damage. Trapping the water out of the inlet piping is as "Montemayor" stated, very important.

Moisture in the steampath increases blade erosion, and diminishes stage efficiency because the droplets increase the effective surface roughness of the blading. Exhaust moisture content could be as high as 18 -22% for saturated inlet (as compared to 10 - 15% for 75 - 150 C° of superheat).

50MW at 20 barG corresponds to an inlet volume flow that could be too big for some of the small manufacturers and (maybe) too small for some of the big guys to be interested. Possible manufacturers include: Toshiba, Elliott (maybe), Siemens, GE (through Thermodyn or Pignone), etc.

I know that Toshiba has some experience in this size with geothermal heat recovery applications. Small combined cycle plants with low amounts of superheat from the HRSG are possibly a comparable application.

 

[rmw]

It is a good discussion. Unless you are careful, D&S steam is an oxymoron. But it can be had. The quality issue is not necessarily limited to the issue of drainable (via traps, etc) water, but the moisture in steam due to low quality. You can have moisture in the steam, and be seeing little flow from the traps, because it is entrained in the vapor. This entrained moisture will cut turbine parts to ribbons.

In addition to the proper trapping recommended above, a good moisture separator is mandatory if the steam is suspected to be wet.

Let me pause and say that I have clients in an industry that uses lots of turbine drivers, and in one plant, the steam will be superheated, and in one down the road doing exactly the same job in the same way, there won't be a superheater in the plant. Who is right and who is wrong? Neither.

The key, to my way of thinking is good piping design. When the steam leaves the boiler, assuming good moisture separation in the steam drum, it is D&S. As it travels along the piping, it loses heat to the surroundings, and cools off, and the quality suffers. However, at the same time, the steam loses pressure, which tends to superheat the steam, offsetting the moisture formation due to the temperature loss.

Therefore, a well designed piping system can minimize quality problems, assuming that we are talking about a fairly constant steam flow for 50 MW.

I am not totally knowledgable about Terry, Elliot, or even Dresser Rand's capability to build machines in this power range, but I think that you are in the realm of GE, Siemens, Mitsubishi, Hitachi, et al.

A machine to produce that much power with that low of a inlet pressure might be a bridge too far. I could not get the conditions to run at

http://www.dresser-rand.com/steam/calc/main.asp

and that may be the reason why. I did not play with it long enough to see how low the power input would have to be to get it to run. Using Katmar's steam flow estimation tool at

http://www.katmarsoftware.com

and assuming 50% efficiency, and 3"/hg back pressure, shows a steam flow of over 1 million lb/hr.

Do you really have that much steam available at that low pressure?

rmw

 

[Montemayor]

rmw:

Great discussion on your part; good empirical facts and information as well. I agree totally with your observations and experience. What I originally envisioned was not one solitary steam turbine. Although I am an avid advocate of economy of scale, I would never put all my eggs in the one basket. I'm used to hedging my bets with multiple units for flexiblility and controlled turn down.

It should be interesting to hear from Johan and what his detailed plans are.

Art Montemayor
Spring, TX

 

[JohanOlsson]

Montemayor, rmw & poetix99: Thanks alot for the help!!!

The plant is located in southern Sweden. Today it only produces heat for the local district heating system. The boiler has a heat output of ~40 MW, but there is also a flue gas condensing part which contributes with about 10~12 MW. Just next to the plant, two geothermic holes has been drilled. Unfortunately these won't provide the amount of heat that was calculated before the drilling. Therefore a heat pump will be used to extract the geothermal heat. This heat is to be used in the plant. The installation of the turbine will make the plant able to provide the compressor of the heat-pump with electricity. But the turbine should also be large enough to produce "extra" electricity that can be sold on the market during the seasons when the need for heat is low. This means the turbine will probably be in the powerrange of ~5-25 MW. I'm sorry if my lack of information in the first post caused any misunderstandings about the size of the turbine!!!
Since the plant is fired with biomass fuel, electricity sold on the market will provide the owner with "green certificates". This is the main reason for the conversion of the plant as well as the heat pump used for the geothermal energy.

Johan

 

[EdStainless]

I know that turning all of the shaft work into electricity makes the contols easy and offers the potential for selling power, but it is very inefficent.
Using multiple radial expansion turbines with some of them direct driving the heatpump compressors would be more effiecent.
One source is

http://www.mafi-trench.com/index.html

You could also have some power generation in the mix. The balance might be tricky.


= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[rmw]

Johan,

At what pressure(s?) do you send the steam out to the district?? The answer to this question has a lot to do with the quality of the steam at the turbine exhaust, whether it is wet or not. It is something that has to be dealt with by a turbine manufacturer, and your piping designer. I just need it for my thinking about this project.

Do you plan to condense steam used in generation of electricity in times of low district heating steam demand??

EdStainless's suggestion to do the heat pump mechanical work with mechanical drive steam turbines is excellent. I am going to give Ed a well deserved star for that one.

That would seem to me to be good constant load, year in, year out for these mechanical drives, and a good place for you to let down (drop pressure on) a good portion of your 20 bar saturated steam.

Then, with what is left, the Electricity generation turbine size is starting to get manageable for selling electricity by letting down the remainder of the steam throught the generator turbine, whether extracting, or condensing.

Depending upon your answer(s), I still have some thoughts, but don't want to throw them out until I refine the project parameters a little more.

And by the way, I agree with Art Montemayor on the viability of using multiple turbines for redundancy sake, but I also recognize that capitol costs sometimes govern those types of things. That is why my mind went to a single unit first on. Since your district steam load demand would probably be variable over a years span, multiple units might have more merit on that basis.

rmw

PS, Thanks for the compliment, Art. It is most welcome at this time.

 

[poetix99]

This thread is becoming old, and, in fact, JohanOlsson will figure certain things out for himself. Nonetheless, I'd like to say that although redundancy might be a nice thing, this is all about making money - whether we might like to admit that or not.

From that perspective it is virtually impossible that, for a total plant output of 5 - 25 MW, multiple units will be justified on the basis of any rational life cycle analysis; this is too small an output. Even the original mistaken 50MW is too small to (easily) justify multiple units; the capital costs are far too high to justify the redundancy, and it is not as if one is placing all the eggs in a basket of unproven technology.

 

[athomas236]

About 10 years ago I was involved in a project that involved installing 2x30MWe back pressure steam turbines into an existing desalination plant.

The plant had GT's/HRSG's that produced saturated steam at 20bar. This pressure had been selected by the consultant because it was necessary to drive the ejectors on the desalination plant. The steam for the brine heaters was reduced to about 2bar through pressure reducing valves.

Then along came another consultant who proposed that back preesure steam turbines were installed in parallel with the PRV's. The result was an extra 60MWe of power generating capacity and a happy client.

Regards,

athomas236

 

[stgerd]

Johan did not told if it's an backpressure or condensing
ST. Condensing ST would be difficult because of last stage erosion. With a backpressure ST this would not be a serious problem. In any case at this steam conditions
the ST works in an area with a very large rate of erosionkorrosion, this means that material will be wash away by the steam but this you can manage with the selection of materials ( 3 % to 13% cr steel ).