What effects on turbine performance due to high inlet steam temps

[SUTMIKE]

During startup following a major outage (turbine, boiler controls), final steam temps to turbine inlet reached 1160 degrees(Rated 1000).This peak lasted less than 20 minutes. Over the course of 4 days of startup/tuning similar spikes were encountered but max temps reached were below 1160. What effects to turbine performance, if any, could be realized due to these conditions. Since the outage we have suffered a load reduction of 20 MWs (Rated 470) that has been attributed to "loss of turbine efficiency".

 

[Feric]

Hi there:

In general, if the heat addition temperature increases, the power cycle efficiency increases.

Also, if the gas turbine inlet temperature increases, the turbine power output increases.

Assumptions are that the outlet conditions stay the same.

Here are two HTML calculators where one can see numerical output changes for given input changes.

http://members.aol.com/engware/calc3.htm

http://members.aol.com/engware/calc4.htm

Thanks,

G. Feric, PE

http://engware.i-dentity.com

 

[SUTMIKE]

I should have added that there are some who are suggesting that the elevated temperature spikes experienced during startup may have caused damage to the turbine resulting in the reduction of turbine efficiency.

 

[IRstuff]

Sure, there's some possibility that the turbine blades got warped or otherwise damaged or pitted.

What about the bearings for the blades?

TTFN

FAQ731-376

 

[SUTMIKE]

Have not had an opportunity to inpspect yet. Waiting for an outage.

 

[Feric]

Hi there:

My input is related to the ideal technical performance analysis without getting involved with what could have happened to the actual steam turbine operation and measured technical data.

Thanks,

G. Feric, PE

http://engware.i-dentity.com

 

[rmw]

Temperature limits are generally associated with pressure parts; tubing, piping, turbine shells and control valve bodies for example. Over a long period of time there would be metallurgical consequences due to operating at that much over temperature, but not for the times that you indicate.

The turbine components such as buckets, blades, diaphragm blading, etc are generally of a higher pedigree for other considerations and this little temperature for this little time probably wouldn't "warp" anything.

However, there is one possibility. The internal parts exposed to these temperatures would have expanded somewhat beyond their design points due to thermal expansion so that the first few blade rows grew larger than design and wiped out the tip seals.

Did you have any unusual rubs during start up (said LOL-how would one differentiate between usual rubs and unusual rubs?)

The same might be true of the shaft seals at the diaphragm packing but before I made that statement, I'd have to think about it some more.

That is about all I can think of that might have happened due to the temperature excursion.

rmw

 

[SUTMIKE]

Thanks for the comments. No rubs or such were observed during startup.

 

[davefitz]

I assume the original design main steam temp is 1060 F , and this is a gas fired combined cycle plant.

Exceeding design temp by 100F will:

a)temporarily improve steam turbine cycle efficiency, if increased seal leakage did not develop

b) the rate of creep life exhaustion at 1160 F will be more than 20 times faster than at design temp of 1060 F, and the rate of oxidation of ferritic superheater tubes also accelerates

c)probably violate asme B31.1 rules for not exceeding max temp for which there is publised an allowable stress for listed alloys

d)exceed the steam turbine mfr's max permitted inlet steam temp, and put their warranty in jeopardy ( by the way, they are monitoring these temps and will let you know it, someday )

e) exceed the max assumed metal temp the piping designer used when designing the pipe supports- if the pipe hit a solid bumper during its expansion, the pipeline stresses would increase dramatically, and stresses at the HP main steam valve to pipe weld would be excdessive.

f) turbine seals may have excessive wear due to expansion effects.

g) loss of effiency may be due to seal wear and also solid particle erosion SPE of the 1st stage due to exfoiliation of oxide particles formed in the HP superheater tubes due to the overheat, and worsened by throtlling of the HP throttle valve. 5% loss of HP efficiency due to SPE is not uncommmon.

h) such overheats are absolutely verboten in some parts of europe, and to avoid this during startups of gas fired combined cycle plants, some eu plants use final attemporators on the main steam and hot reheater pipes. Such overheat seems to be common practice in the US, though.

 

[davefitz]

I just reread the original post. The 470 MWE at 1000F sounds like a coal fired plant.

The 160 F overheat above original design temp is a serious problem, and should not be repeated. A 100 F overheat for 1 yr at the Mohave plant led to the failure of the Hot reheat line that killed 11 operators ( 1988).

The EPC designer that designed the transfer pipe to the steam turbine should be requested to analyze the pipe expansion at 1160 F and confirm (a) no contact with solid bumpers ) and (b) stress at the weld to the HP main steam valve.

And give a rash of sh*t to whoever let the steam temp get that high.

 

[metengr]

My expierence with very short term over temperature excursions is that minor creep damage may occur that is really not detectable on rotating blades (W) or buckets (GE) because of the lag time required for heating material beyond typical design conditions. Of course, as material heats up beyond original design conditions radial and axial clearances will be affected first, but long term creep life effects will not be noticed unless the temperature excursion have been sustained for hours on end.

Regarding inlet piping and valve bodies, this is a more serious concern and continuation of over temperature spikes should be avoided to reduce the affects of thermal stress gradients and creep deformation.

 

[tommy75]

I take it there were no indications a high vibration during the temperature excursions?

 

[abeltio]

Ask the manufacturer of the unit for recommendations regarding time and temperature excursions.

saludos.
a.