Dear Sir,
We are operating GE frame 7FA DLN 2.6 Gas turbines & HRSG (without diverter damper).

As per GT stop sequence, M/C will coast down gradually from base load (as per ramp down setpoint), breaker open, flame off & turning gear. It will take 24 hours to stop the "cooldown off" sequence.

To save the outage down time (of course money!!!!!!!) we used to do six hours of natural cooling just after M/C reach to turning gear & after we used to start the M/c on crank mode until wheel space temperature drops down to <65 degc.

During force cool, we also monitor the drum metal temperature difference (steam side drum top metal temperature- water side drum bottom metal temperature) if the drum metal temp difference increased to >40 degc, we used to stop the cranking & wait until drum metal temp difference drops to acceptable value.

I wonder we should not do any force cooling in gas turbine, force cool will affect the metal stress, damage the hardware at longer run.

As we don't have the diverter damper at GT exhaust, during force cooling we are giving stress to GT combustion hardware as well as WHRSG metal hardware also.

Is it advisable to force cool the Gas turbine & HRSG. Shall we have any impact at longer run? if we continue force cool the machine at this way?

Please advise.

Regards
[email protected]

 

Forced cooling isn't recommended for use on every shutdown; it's intended to be used on an "emergency" basis. But, reducing outage down time and saving money is an emergency for most plant managers and -owners, so it's become a relative term. And it will be used as normal shutdown method at many sites probably until such time as it's proved to be harmful to the machine.

Personally, I haven't seen or heard of any ill effects on gas turbine components. And, based on the scenario you describe for your site, the units are probably fine.

Your constraint seems to be the HRSG components, and the stresses induced on them. As you have noted, without a bypass or diverter damper you have to pay close attentions to HRSG temperature differentials as recommended by the HRSG manufacturer.

Again, based on the scenario you describe you're probably fine. And I wouldn't really refer to it as forced cooling because you're going through a normal cooldown cycle of six hours before you start trying to cool the entire unit (gas turbine and HRSG) faster by cranking the unit.

I would remind you that the cranking motor runs at approximately 150% of rated current during each purge (highospeed) portion of a CRANK sequence, and starting that motor also has an effect on the motor longevity.

There's more to be considered than just the gas turbine hot gas path parts.

 

Nowadays I think forced cooling has become a normal practice for all utilities as it saves a lot of outage time. Forced cooling is also practiced in my plant (mine is also a combined cycle without bypass damper). Here usually we crank the GT for about 1 hour after 2hours from shutdown. Then the cranking is repeat after about 3 hours from the 1st crank. This has been in practice since the unit commissioning on 1993 and until now its not an issue at all.
So dont worry.. As long as the cranking motor is healthy, you are fine

 

Additional to CSA's comments, I would like add the following. Beside the thermal shock to the HGP parts and downstream equipment such as the HRSG, the following parts might be affected also.
1) The starting motor
1.1) Overheating of the E-motor, which might affect the electrical protection.
2) Clutch
2.1) SSS clutch failure due to extended torque.
2.2) Jaw Clutch failure, likelihood to fail is more than the SSS clutch. In particular if the actuator cylinders are leaking.
3) Residual unbalance due to rotor "bowing". This problem will reflect in the bearings, usually the radial vibration will increase. You will see immediately increase of seismic vibration (0.3-0.8mm/s)during transient conditions.
4) Last but not least the torque converter.
4.1) Usually this equipment will be overheated which will lead to premature failure. I have seen this happening on a FR5001!
Again, there is not any evidence of the forced cooling negative influence on the HGP parts (no assessment done by the OEM and its neglicable), especially if you practice this once to twice a year (during off line WW). However for the HRSG, there might be some affects such as distortion of the inner wall plates due to thermal shock. To monitor this, you should inspect periodically the HRSG's internal parts.
Note: During cranking, there is a possibility to have minor leakage through the SRV and GCV. Remember that these valves are control valves and they always leak a bit. If your FG system is equipped with a FG block valve or so called stop valve, ensure that this valve is tight shut off. If the valves will pass FG, the HRSG as well as in the downstream exhaust system gas will accumulate and might lead to explosion during re-start of the GT. There are some cases recorded in the Caribbean (probably the Bacardi and the ladies were much more attractive then the GT....)

Good Luck...TEMPUS FUGIT, save money (6hrs cool down) risk reputation; jeopardize HSE and risk availability and integrity of the GT Unit.

 

Thanks for your valuable inputs, I did not mentioned starting method of gas turbine, sorry for that!!!!, we have Load Commuated Inverter(LCI) we are not starting Gas turbine through carnking motor

Thanks

 

My bad for not remembering that you had told us that the unit was an F-class machine, which usually has "static starters".

 

As other friends said now a days many are adopting forced cooling to save time as time is money. No doubt forced cooling immediately after shutdown impose thermal stress on the combustion and turbine hardware. But after 2 hours natural cooling with all compartment doors closed forced cooling can be done.

In absence of divertor damper HRSG cooling curve is also to be considered. Cranking can be done at slightly less speed than the rated so that we can lower the motor current to normal.Torque convertor angle can be reduced to lower the cranking speed. in your case you need not bather cranking motor loading.

 

Forced cooling can be done after min. 2 hours of natural cooling only to reduce stresses in GT. In some machines provision provided for fast cooling by L4CD i.e, fast cooldown in perticular F-class machines where LCI for starting means. In this machine will roll with 4% speed in MS6101FA and with 9% speed in MS9351FA. Rolling with lower speeds will not be much problem than in craking where rpm will be 23.3% in 9FA and 30% rpm in 6FA machine.

regards
venu

 

At my plant, the overriding factor following any turbine trip is availability. That's because availibility is a big part of the bonus equation for management. Doing what's right to protect the machine from damage is of little or no concern to those who gain more by cooling the machine off too rapidly than they do by being prudent.

Consequently, I have seen some of our 7FA's crank cooled immediately following a high load trip, with doors open and the compartment vent fans going. This definitely cools off the machine more quickly, but it also "liberates" compressor blade tips by shrinking the compressor casing faster than the rotor.

You can live with a fair amount of tip liberation, but these blade tips damage the downstream blades and turbine buckets. If a blade root is damaged even slightly near the root, the compressor is at high risk of catastrophic damage, and that row of blades must be replaced.

So far we've been lucky in that the blade damage downstream of the "liberated" blades has been toward the outside. But really, folks. Is a few hours of availability really worth the risk of all the downtime and expense? I think not. Of course being in maintenance, I do make more $$ when we have to tear a machine for an emergency repair....so, me worry? Nah....

 

Hi I do not see why we have to go for force cooling always. We all know the stress developed failures as some friend says nothing will happen which may not be the case with every one. Every machine will not be so LUCKY to with stand the internelly developing failure. The outage time may be short but then the life of the machine may be also short. Until otherwise its an emergency force cooling should be avoided.

 

Some managers have figured out that it's more economical to have short outages and get back on line sooner (to make more money) than to pay for the possible stress-related damages caused by forced cooling. Especially if they get bonuses for increased production.

And especially if they think they will be moving to another plant before the increased maintenance costs will have to be paid for while they're still around.

Force cooling is really meant to be an option for emergencies; it's just that the definition of emergency varies from manager to manager.

Also, forced cooling of the turbine (without throwing the compartment doors open and turning on all the compartment vent fans!) doesn't really hurt the turbine, but it can be injurious to the HRSG or exhaust components.

Everything has to be considered. And that includes trading maintenance costs for increased production. At certain times of the year, it's probably worthwhile (though that bit about opening the compartment doors and running vent fans is really pushing the limit). Someday, the compressor bucket isn't going to just get rubbed down, it's going to break off at the root, and when that happens, well, the new manager will have to deal with the consequences of what the previous manager established as a standard practice!

Accountability? How do we quantify that?

 

we recently do GT 9FA force cooling - 8 hrs after natural cooling.

our machine is a single shaft machine - coupled to GE D10 ST.

we did force cooling by cranking to purging speed w/o opening the enclosure door open.

will it hurt the compressor vanes?

isn't that will cause uneven cooling ie. GT turbine gets cooled faster than the casing?

 

After almost 1 day natural cooling for 9FA, we are doing 4 ~ 5 hrs GT force cooling till the max wheelspace dropped to 65 oC. My question is, it is advisable to crank the unit using LCI for 4 ~ 5 hours? Is there any disadvantage in the Generator which act as a Motor for that long cranking? Or is better the do 1 hr forced cooling after 2 hrs from shutdown and repeat after 3 hrs thereafter just like "sd" practice forced cooling?

Does anybody know about GE recommended forced cooling practice for 9FA?

Ankarao,
Can you tell us the HRSG cooling curve? Thanks.

 

Im very sure that GE never recommend any kind of forced cooling for their machine. As for the off cooldown mode change over, the setpoint for my machines is set at 121 degC. I was told that the max temp value was changed long long time ago before my time to expedite gt force cooling. Any one have any experience on this matter?