Steam turbine minimum load

[rotw]

Hi All,

I am looking after the minimum load acceptable (turndown) for a steam turbine. I guess this is something to obtain for the OEM, but I am just looking for a typical order of magnitude. The steam turbine is rated around 15-20 MW and is for power gen application.
So if we assume the steam inlet conditions and the speed unchanged, how far can we reduce the load (or the steam flow) and still stay within machine operating limits ? Just a rule of thumb.

Does it make a big difference to have a back pressure or condensing machine and to have an extraction or not ?
what is the most important criteria to look for turndown limit?

any input would be greatly appreciated.

Regards

 

[byrdj]

I'll first clarify my background is in maintencace/operation of steam turbines, not design.
most utility size condensing units do not like to go below 30/40% for nightly load reduction. the problem being condesation occuring before steam exits the last stages. Normally the initial steam conditions are also reduced

 

[ccfowler]

rotary world,

It would be most helpful if you could provide more details regarding your posting. Obviously, steam turbines can tolerate the necessary brief start-up and shut-down sequences of changing speeds at zero load and brief periods at near zero load. The interest in presumably extended periods of very low load is much less obvious.

Extended operation at very low loads inherently implies substantially lower operating efficiency. As byrdj stated, altered initial steam conditions can mitigate some of the likely damage risks. If your concern is regarding a very infrequent need to operate at very low loads to deal with potential emergency conditions, incurring some modest turbine damage may be a trivial issue when compared with the nature and risks of the contemplated emergency situations. If extended operation is required for some sort of process need, then your study will need to include much more than just the potential turbine damage. Presuming that the stean is from sort of fired steam generator, flame stability and emissions problems are likely to become more controlling issues. Additionally, boiler feed pumps do not take kindly to very low flow rates, and serious consideration will need to be given to sufficiently large by-pass flows to avoid rapid damage to the boiler feed pump(s).

The development of problems with other items should be considered, too. Likely problems include feedwater heaters and turbine extraction flows, and potentially very troublesome is the matter of deaerating heater performance and operating problems.

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.

 

[rotw]

Byrdj and ccfowler,

Thank you for your valuable info. I will try to give more details hereby.
The issue is related to process needs and the check is intended for an operation at part load continuously.

With regard to the auxiliary items and the steam generation system in general, the concern is noted and surely has to be investigated as I want to consider the full picture. I want a rule of thumb but at any price (like sales people do sometimes) such as give incorrect input from oversimplification.

Lets assume there is a steam balance in the plant and that hypothetically it is possible to feed continuously the steam turbine with very low amount of steam flow, compared to rated capacity, at the same level of pressure and temperature at the steam turbine inlet. In such a way, I would investigate the matter focusing on steam turbine only. From here, do I have to go for a detailed study because things change dramatically versus manufacturer, technology, quality of steam, etc. OR is there an order of magnitude for load reduction (turndown) that we can consider as a reasonable shot to begin with ? This will be an input for a preliminary design, so please help me to be the safe side.

Thanks if you can share your insights both from design and field experience point of view for both condensing and back pressure type steam turbines and also the implication of having an extraction. It will be of great help.

Thanks

 

[rotw]

I meant "I want a rule of thumb but NOT at any price (like sales people do sometimes) such as give incorrect input from oversimplification"

 

[ccfowler]

It is a rare situation where there are truly hard limits on anything. One is always dealing with a balance of trade-offs some economic, some physical, and some a mix of both. You indicated that this steam turbine is for power generation, so it seems likely that there may be some balance of concerns, interests, and enemies where the turbine may reasonably continue to be operated at proportionately greater loading despite reduced process needs.

About a century ago, a 15 to 20 MW turbine would have been considered truly enormous, but now it is a relatively small unit. Utility size turbines in the 1000 MW range and larger are normal, and units ten times the size of yours have been common for decades. You have not indicated the age of your turbine or it's general configuration. For pure power production, multiple extraction flows are very important for cycle efficiency, but if your turbine is tied more intimately with process needs, then the extraction flows may well be chosen more for process purposes than for steam cycle efficiency. If so, operation at reduced loadings will probably result in the extraction flows not serving the process needs adequately. It may make more sense to operate the steam turbine cycle at higher loads just to better serve process needs. Similarly, while process steam may be reduced from the extraction points, more optimal use of extraction flows for feedwater heating may improve cycle efficiency enough to mitigate some of the burdens of reduced process needs. I suspect that your optimal solutions may be much more elegant and complex than you are anticipating.

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.

 

[rotw]

Thanks ccfowler,

Quoted
I suspect that your optimal solutions may be much more elegant and complex than you are anticipating.
Unquoted


Do you think that looking for a typical turndown figure, is an oversimplification of how steam turbine works which will lead, to a certain extent, to something dangerous ?

I really want to be on safe side. My knowledge in steam turbine is poor and I am afraid to give any "rely upon" input that will turnout bogus later, even if it is in the context of a pre screening phase and the design will be adjusted/tuned (so any guess is ok as long as the magnitude of deviations can be manageable).

Thanks in advance.

 

[ccfowler]

Very commonly for pure power generation purposes, a turndown ratio of 3:1 would be very great with flame stability and emissions being the most likely controlling factors (with cycle efficiency and fuel economy already being significantly sacrificed). The details of specific steam cycles for power generation are always controlled by the expected duty (base load, mid-range, or peaking duty) with cycle efficiency, fuel choice (or mix), emissions, all being important factors. Since your steam cycle seems to be tied to some process need, those needs almost certainly controlled the design of the specific steam cycle details. Yours would almost certainly be considered to be a cogeneration design, and typically very important net energy savings result from cogeneration cycles when compared to power generation only steam cycles. Even where your process needs may be substantially reduced, relatively substantial net energy savings may still be realized.

The simple answer to your question is that looking only at a turndown ratio for analyzing a steam cycle is almost certain to be dreadfully misleading. You would serve your needs much better by looking very thoroughly at the entire process and looking for alternative means to achieve the intended goals. You may be pleasantly surprised by options that you may find by more thorough and detailed evaluation of the entire interconnected process. A steam turbine in the size range you indicate is still large enough for multiple extraction points to be able to figure significantly and economically in the performance of the steam cycle. Altering the initial steam conditions for the turbine may be an item for consideration, but it is something that may involve very complex issues for your steam generator.

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.

 

[snoopnoon]

the critical factor in what we call "light load" operation is the amount of steam flow required to keep the low pressure end of the turbine cool. this is called cooling steam. The effect of light load operation has a significant impact to the amount of energy each stage of the turbine is "using". as the load demand decreases each stage uses less and less of the available energy of the steam and the temperature going into the last stage increases. For turbines that have the bearing bracket supported within the LP casing, this can case the LP end to expand and cause misalignment and other thermal expansions that the turbine was not designed for.

 

[snoopnoon]

Also of note a couple things. under severe low flow conditions, the LP stages will subtract net power due to windage and freewheeling. this causes a significant temperature rise of the materials of the rotating and stationary components.

If this is a sound OEM turbine, the first inlet control valve or the control system is typically set up so that it provides enough cooling steam such that you avoid this conditions. If this is an extraction machine, only control system will be able to accomplish this because all of the inlet flow can escape out the extraction and leave the LP end to heat up under minimal flow condition.

Tyipcall all of the stage component materials are checked and design against a known light load operation point such that the minimal flow does not damage any components. But only the manufacturer will know this information but often times is provided with your instruction book or operating parameters one way or another.

Erosion is typically not a factor when the inlet conditions are maintained and is often just the oposite.

 

[rotw]

Thank you all for your inputs

 

[davefitz]

The low load limit on most steam turbines is defined by the aerodynamic instability ( flutter) of the L-0 blades.At loads below 10% MCR mass flwo the last 2 rows actually operate as a compressor, absorbing energy from the shaft. For LP blades that are tied together this minimum permitted load may be 10% MCR mass flow thru the L-0 blades ( net load about 5% MCR), but a high backpressure will worsen this issue. Free-standiing blades without ties may need a higher mass-flow- the OEM will need to advise.

"Whom the gods would destroy, they first make mad "