> For non-DLN combustion when firing natural gas, is there a
> preference for steam rather than water?

No. Other than for the owner/operator who has to buy the water and then treat it before it's lost to the atmosphere with the gas turbine exhaust. It takes more steam flow to achieve the same emissions as water injection. In other words, it takes more water to achieve the steam flow required to make the same emissions reduction than just injecting water.

So, unless the site has extra steam--and the water to produce the steam--the usual "preference" is water.

Oh, and steam injection also increases dynamic pressure pulsations in combustors which increases the wear rate of combustion parts (nozzles, liners, and transition pieces).

 

hi,

please help me to know what differ installation in dry control mode & wet control mode in water injection system frame 9e, non-DLN.

 

abdi,

> please help me to know what differ installation in dry control mode & wet control
> mode in water injection system frame 9e, non-DLN.

Some turbines have different CPD-biased exhaust temperature control parameters based on whether or not water injection is enabled (wet) or not (dry). I've never personally understood why this was done on some turbines and not all turbines but I think it might be because at lower NOx guarantee levels the injection ratio was higher--but that's just my own impression.

Your question isn't clear, though, because as far as the "installation" (the physical equipment installed and required for water injection) there would be no difference. The only difference would be if the Base Load control parameters were biased based on whether water injection was enabled (flowing) or not. If you mean something other than what was described above, you'll need to let us know.

Hope this helps!

By the way, there is another occasional contributor to this forum that goes by the same name. That individual does not provide much feedback, and when he does, it usually isn't relevant or helpful to us or to others who may be reading the posts. We try to provide as much help as possible, and it's most beneficial to the responders--and to others who read these posts (and MANY people do read these posts, now and in the future)--if we get meaningful feedback on whether or not the information provided was helpful, or not, as the case may be. When questions are unclear, or when more information is asked for, the original poster must provide the necessary clarification to get a more meaningful and concise response. And, if the response is helpful it's best if the original poster provides feedback to let anyone following or reading the thread if the information was helpful. It's really what makes the community here at control.com so strong--the feedback, even if the information provided wasn't correct or useful.

So, if you're new here to control.com--Welcome! And, please, have a read of past threads (using the 'Search' feature at the top of every control.com webpage) and see how helpful the feedback is.

 

hi,

thanks for reply. please read this paragraph and please explain to me. thanks a lot.

"A hot gas path part life impact from steam or water injection is related to the way the turbine is controlled. The control system on most base load applications reduces firing temperature as water is injected. This counters the effect to the higher heat transfer on the gas side and results in no impact on bucket life. On some installations, however, the control system is designed to maintain firing temperature constant with water injection level. This results in additional unit output but it decreases parts life as previously described. Units controlled in this way are generally in peaking applications where annual operating hours are low or where operators have determined that reduced parts lives are justified by the power advantage.

An additional factor associated with water or steam injection relates to higher aerodynamic loading on the turbine components that results from the injected water increasing the cycle pressure ratio. This additional loading increases the downstream deflection rate of the second- and third-stage nozzles, which reduces the repair interval for these components. However, the introduction of GTD-222, a new high creep strength stage two and three nozzle alloy, will minimize or eliminate this factor. Maintenance factors relating to water injection for units operating on dry control, range from one, for units equipped with GTD-222 second- and third-stage nozzles, to a factor of 1.5 for units equipped with FSX 414 nozzles and injecting 5% water. For wet control curve operation, the maintenance factor is approximately two at 5% water injection."

 

abdi,

Basically the two paragraphs (which are very poorly written, and appear to be taken from some revision of GE Publication GER-3620) say that water or steam injection reduces hot gas path parts life, but that the control system can be configured to reduce or minimize one of the areas of concern. However, some owners/operators choose to take the higher output that can occur with water or steam injection along with the increased parts wear. This <b>SEEMS</b> to be the difference between wet- and dry control curves--wet control curves being those that slightly reduce firing temperature to minimize impact to hot gas path parts, and dry control curves being the control curves that do not reduce firing temperature when water or steam is being injected. <b>But</b> the passages do not clearly state which control scheme is dry- and which is wet, so the above is just my interpretation of wet- and dry control curves based on the "information" in the first paragraph. Owners/operators are expected to understand that maintenance intervals are decreased in exchange for a little more power output when water or steam is being injected when the unit is being operated with a dry control curve. The first paragraph <b>does NOT</b> describe any maintenance factors to be associated with wet- or dry control curves.

The second paragraph goes on to say that another negative impact associated with water- or steam injection can be mitigated by using second- and third-stage nozzles made with a special alloy. Finally it describes the maintenance factors to be applied to units running at Base Load based on the type of materials used in the second-stage nozzles and the amount of water injection (I believe it's expressed as a percentage of total mass flow--but, again, the passage is unclear.)

And, of course, the last two sentences of the second paragraph--which describe the maintenance factors to be applied to nozzles made of different materials--seems to contradict the first paragraph's description of wet- and dry control curves. One would think that a wet control curve (if my first presumption was correct) that reduced firing temperature when water or steam was being injected would result in a <b><i>lower</b></i> maintenance factor regardless of the material, but that's not what the last two sentences of the second paragraph seem to say.... Unless, they are ONLY referring to the effects on the second stage nozzles, in which case....

I can understand your confusion--GE documents are generally not well-written and can be contradictory and confusing. I suspect the reason there are so many revisions of GER-3620 is because of the need to clarify or correct similar contradictions or mistakes as in the two paragraphs you have copied.

I feel confident in saying that if you contacted GE--which you probably should to get the clarification that is necessary--that they will have a very sound reason for why they described this the way they did. One thing to always remember about GE documentation is that it's the LAST thing that's done in the product development life-cycle, and it's almost never reviewed by peers or product designers for accuracy. And, when it is reviewed by product designers, or when product designers have input on the documentation, the result is usually very cryptic and contradictory. And on that note, I rest my case as the passage cited is both cryptic and contradictory.

 

good answer of you, but I have a question. why the water inject is involved only to the liquid fuel?

 

Most gas turbines these days use DLN (Dry Low NOx) combustion systems for gas fuel. Before DLN was developed, water injection was used for NOx control on gas fuel.

> good answer of you, but I have a question. why the water inject is
> involved only to the liquid fuel?

 

Most GE-design F-class heavy duty gas turbines have combustion systems that reduce NOx (Nitric Oxode) emissions without using water as a diluent to reduce emissions. If the unit also burns liquid fuel, then usually water (or steam) is necessary to reduce NOx emissions when burning liquid fuel (distillate).

Older combustion systems used water (or steam) injection for NOx emissions reduction when burning either natural gas or liquid fuel.

Even older combustion systems didn't use any diluent for emissions reduction (emissions reduction wasn't considered necessary).

Hope this helps!!!

 

>I'm not sure about the statement regarding lowering
>temperature to increase efficiency....

Water or steam injection for power augmentation, creates more mass flow. Also water injected upstream of the compressor will be compressed to excess of 700*F before entering combustion wrapper, water vapor.

 

GT fuel flow is controlled to maintain a constant firing temperature (Tf). It's not practical to measure the temperature in the combustors so exhaust gas temp (EGT) is measured and Tf is calculated from that value. EGT varies as mass flow varies because the temperature delta (dT) across the expansion turbine is different for different pressure ratios.

The turbine is exhausting to atmospheric pressure so the exhaust pressure is always about the same. When compressor discharge pressure (PCD) is high, due to increased mass flow as would occur on a day when the ambient temperature is low, the expansion ratio is greater. When the pressure ratio across the expansion turbine is greater, due to higher PCD, more expansion cooling occurs, so EGT naturally reduces. Conversely, when PCD is low, there is less expansion cooling, so EGT goes up.

Therefore, using EGT to maintain a constant Tf means the measured EGT temp must be biased based on PCD. When PCD is high, fuel flow is reduced to target a lower EGT because that ensures a constant firing temperature. If this correction to targeted EGT were not made, Tf would go up when PCD goes up because a higher PCD means a larger pressure ratio, which means more expansion cooling, which means a lower EGT. If the EGT were not biased the fuel flow would be increased to get to the targeted EGT, which would cause a higher firing temperature.

Water can be added to the GT working fluid mass flow by evaporative cooling (fogging or media type), by wet compression (spraying water droplets into the compressor), or by water or steam injection into the combustion chamber (for NOx control and/or power augmentation). When water is added, and if no correction is made to the GT controls, Tf will naturally decrease.

This reduction in Tf with increased water content is due to the fact that water vapor has a higher specific heat than the air/fuel mixture, which means there is less expansion cooling, so EGT increases. If the control curve (ETG vs. PCD) is not changed, the controls think the higher EGT is due to increased fuel flow, so fuel flow will be reduced to get back to the targeted EGT.

A "wet curve" can be employed to correct for the reduction in Tf that is caused by increased water vapor. This will increase turbine output because the Tf will be increased back to where it was with dry operation. However, changing to a wet control curve does not come without cost. The higher specific heat of water vapor means there is more heat transfer to hot-section components when Tf is the same. Using the dry curve during wet operation compensates for this additional heat flow so there is no increased maintenance interval with wet operation. However, there is less gain in output than there would be if the wet curve were used.

Evaporative cooling increases output because it makes the air denser, so mass flow is increased, and because it takes less work to compress cooler air. Wet compression increases output by reducing the power consumed by the compressor, in the same way an intercooler reduces compressor work, and because there is a small increase in mass flow due to the injected water droplets (usually about 2% of the air mass flow). Water or stream injection increase output because mass flow is increased.

Any water added to the working fluid will reduce NOx production. NOx is mostly caused by oxidization of atmospheric nitrogen and production increases exponentially with an increase in combustion temperature. Therefore, most NOx is produced in hot-spots in the combustors. Water vapor quench's hot spots due to its high specific heat. Adding a given mass of water by inlet fogging or wet compression will reduce NOx by about half as much as adding the same amount of water by water injection into the combustors. But adding water at the compressor inlet gives a much larger power boost. Water injected for NOx control can be decreased when water is also introduced into the compressor.

 

Dear All,
would You Please why not to inject water injection at turbine load above 85% of the base load? could the unit be affected when its at T7 limit and then you start the injection?

 

Hi

You can have a read on this good thread..
https://control.com/forums/threads/water-injection-in-gas-turbine-effect-on-efficiency.26674/

 

Why ? What is the meaning of this OM reply ?

"It is not recommended to start the water injection at full load since this could lead to instabilities in the water injection.

Our recommendation is that the water injection is not started above 85% load."

 

Hi

You can have a read on this good thread..
https://control.com/forums/threads/water-injection-in-gas-turbine-effect-on-efficiency.26674/

Thank You Very Much - But i Still Have the concern-----Why We need to lower the load under the base before stating the injection