Steam turbine start-up degree of superheat

[Justin Powell]

Trying to find more information on steam quality requirements on a start up of a 70's vintage GE turbine ( impulse type 2 casings compound double flows reheat condensing turbine). I've read in detail manufacturers recommendations regarding rate of metal temperature change in the 1st stage and RH bowl temperature change limits. I cannot find any particular recommendations of the required degree of superheated steam to introduce to the turbine on the initial roll off. I believe there is a required amount of SH (above the saturation temperature for the associated pressure) that would allow for steam to still be above saturation when it completes through the HP section and returns to the boiler for reheating. I think this will vary depending on the turbine temperature conditions as well. Can anyone provide some information or opinion.

 

[anorlunda]

I believe there is a required amount of SH (above the saturation temperature for the associated pressure) that would allow for steam to still be above saturation when it completes through the HP section and returns to the boiler for reheating. I think this will vary depending on the turbine temperature conditions as well.

Yes that will vary greatly as the turbine warm-up. Earliest stages warm the fastest. I think it would be very difficult to keep the steam dry at every stage during warm up, so I'm skeptical that it is a hard requirement.

The consequence is that some steam will condense and spin off the bucket tips. However, the amount of steam flow during roll off is very small, so the moisture mass flow will also be very small. If you have no explicit moisture removal paths, then I expect that the water will run out via the steam extraction points located at the low points in the casing.

You also have sealing steam prior to roll-off. Some of that condenses also, so there must be a path for the water to get out.

I'm most familiar with nuclear turbines, where the steam is saturated always. They have moisture removal grooves in the ends of some of the buckets.

The hazard would be if big globs of water get into the RH section of the furnace. They would cause a thermal shock. To avoid that, does not mean eliminate all condensation, but rather to make sure the moisture is removed by some path other than the RH piping.

Remember that column in Power Magazine? Was is "Ask Gus?" @jim hardy would remember the name. That would have been a great question for him.

Don't you have written startup procedures for your plant? That would be the place to look, not the manufacturer's literature.

 

[jim hardy]

Things are changing in the industry as plants age and get cycled more frequently.
this article suggests the answer to you question is about 25°F
and it describes a novel way to roll a turbine - valves wide open and let it follow the boiler up to temperature..
https://www.power-eng.com/articles/...edure-speeds-cold-start-protects-turbine.html

Traditionally we rolled the turbine by cracking open the throttle valves,
letting the steam expand and cool by throttling of course

i honestly don't know at what steam condition we rolled our fossil turbines off turning gear (they ran at 1000F 2400 psi)
control of superheat temperature would be tricky at such low flow...

on the nuke units we rolled the turbine with 1005 psig saturated at the throttles
which is very near the peak of the saturation curve

Those nuke turbines were designed for saturated steam .
Isentropic expansion through them (straight down on Molier chart) takes you wetter, and extraction points for feedwater heating remove the moisture pushing you back up the chart along a constant pressure line toward saturation to a lower % moisture..

old jim

 

[Justin Powell]

Things are changing in the industry as plants age and get cycled more frequently.
this article suggests the answer to you question is about 25°F
and it describes a novel way to roll a turbine - valves wide open and let it follow the boiler up to temperature..
https://www.power-eng.com/articles/...edure-speeds-cold-start-protects-turbine.html

Traditionally we rolled the turbine by cracking open the throttle valves,
letting the steam expand and cool by throttling of course

i honestly don't know at what steam condition we rolled our fossil turbines off turning gear (they ran at 1000F 2400 psi)
control of superheat temperature would be tricky at such low flow...

on the nuke units we rolled the turbine with 1005 psig saturated at the throttles
which is very near the peak of the saturation curve

View attachment 238855

Those nuke turbines were designed for saturated steam .
Isentropic expansion through them (straight down on Molier chart) takes you wetter, and extraction points for feedwater heating remove the moisture pushing you back up the chart along a constant pressure line toward saturation to a lower % moisture..

old jim

Jim,

Thank you for your response. This Unit is a GE turbine with the parameters you spoke of above (2400 PSI 1000F). We have read the starting and loading information and have not seen any definitive number in terms of degree of superheat in the GE starting recommendations. I have read multiple things that suggest 100 degrees to roll the turbine. I guess our thinking is to ensure that we have steam above the saturation point in the later stages on the HP. In this day and age of coal scrutiny and older plants being shut down, we are trying to make sure that we don't self inflict any damage that could speed up the shut down of these units particularly because of a lack of knowledge.

Justin

 

[anorlunda]

If the rotor is cold, then there is no way to avoid some condensation. At least for a brief period of time.If the GE starting recommendations don't mention it, I would not worry about it. These recommendations are pretty thorough.

 

[jim hardy]

there's so little pressure drop across the turbine at that low flow rate
that first stage pressure is going to be probably only around one atmosphere

so imagine yourself inside the turbine

moisture will get hurled to the outside by centrifugal force
and run down to the bottom of the turbine casing
where it'll find a drain line with its valve open,
that's why we open the drains at startup.

You will need measurement of first stage pressure and temperature to know steam condition at the blades
but you can estimate them from the Molier chart and throttle conditions

i put a green line starting at what, maybe 475F saturated?
isenthalpic expansion(throttling) horizontal to the 1 atm pressure line, looks like maybe 325F
which looks like about 113F of superheat
from there isentropic expansion(doing work on blades) vertical down to saturation maybe 190F at 5 or 6 psi??
i don't know if that will get you all the way through the HP turbine but i expect it would.

I will see if any of my old power plant friends are familiar with fossil turbine roll procedures.

I tend to agree with @anorlunda
open the drain valves and let-er-rip
we always had a hold at low rpm anyway to check for vibration and noise
and if it'd been off turning gear to let the shaft straighten back out
and you have that meter or recorder for differential expansion to alert you of uneven heating

with awareness of throttle conditions and the Molier chart you can write procedures that'll keep you out of trouble.

@dlgoff has considerable fossil experience
and i'll try to interest an old friend, a procedure writer, in this thread.

old jim

 

[jim hardy]

these guys suggest 100C superheat

https://www.imp.gda.pl/files/transactions/126/126_12_.pdf

my old friends recollect
full operating pressure
steam lines fully warmed
all drains aligned to condenser

nobody yet quite remembers temperature
so it was probably same as operating, 1000F

will update if i get an answer that differs.

old jim