Gland Seal Thermal Stresses

[et1ying]

Hello everyone,

We have 2 GE combustions turbines supplying a 300MW Toshiba steam turbine (3 pressures, HP:1200-2400, Reheat: 185-380, LP: 40-60)through 2 HRSG's. During normal operations, our gland sealing steam comes from our Cold Reheat maintaining 4 psig at about 700 degrees Farenheit. When we perform overnight shutdowns, some of our operators run our auxillary boiler to supply gland sealing steam, instead of using the residual heat of the HRSG's to supply the steam (the aux boiler does not swing gland seal pressure during the shutdown period and the HRSG purge period). When our aux boiler supplies steam to the seals, the best steam temperature we get (using an electric superheater) is roughly 250 to 300 degress Farenheit. There is much debate between a few individuals, over the possible damage being done to these seals when cooled from roughly 700 down to an initial 250 degrees and then allowed to build to 300 (due to lag in the superheater response and the heating of the piping between the aux boiler and the gland seal system). Could I get some of your insights and guidance?

Thanks you and have a good day,

Dan

 

[balu1]

Unfortunately this is not my field so I can't answer your question. Not even make an engineering gues since I don't even know what a "gland seal" is. Not to mention HRSGs.

But you should also try Engine & turbine engineering forum since there may be some more gurus not visiting this site.

Good luck,

Balu

 

[rmw]

Do you have any feel for how fast the temperature changes?

Since you have to cool down the supply piping to the seals as well as the seals themselves, it would appear that this would be a gradual temperature gradient, not a sudden temperature change. If it is real gradual, and not a shock situation, I don't think I would be worried about it.

However, is it is a rapid cooldown, then I am with the other side.

How is that for taking both sides of the issue?

rmw

 

[abeltio]

What i would do is to get the OEM to approve on the procedure...
they are the best suited to evaluate the differential expansions that take place during the transient conditions.

in any case the seals affected most by the change are the ones where the casing is hotter than the steam (HP and IP)...

with the unit shutdown seals are used to keep the condenser vacuum, perhaps the question you should ask yourself is: do we really need the condenser vacuum?
quite a few plants break condenser vacuum as soon as the unit is at turning gear speed to avoid the common effect of the unit over-running and disengaging the turning gear because of the vacuum.

aux sealing steam is only used during a cold start.

HTH


saludos.
a.

 

[poetix99]

I have the same question as "abeltio": Why run the steam seal supply at all when the turbine is shutdown? (I assume that "overnight shutdown" means the turbine is shutdown.)

Perhaps the time required to restore normal vacuum is too long for a relatively short (i.e. overnight) shutdown...

In any case, I also agree with "abeltio" that this is something to review with OEM, and that it is PROBABLY not a big deal for the (outer) steam seals.

 

[cub3bead]

The main reason to maintain sealing steam is to hold vacuum which will speed up the next mornings restart.

As a water witch, if I can keep that nasty O2 out of the system during shutdowns I am a lot happier and the water side and steam side equipment will operate better over the life of the unit.