Loss in Steam Turbine Output

[joeh2o]

2 on 1 combined cycle plant. 2 7FA with ATS HRSG to an ABB reheat steam turbine. 4 years old.

Perfromed hot gas path inspection on both GT in October 2002. Noticed a drop in STG output. This was expected due to the lower TEG temperature after the inspection. In February of 2003 we had a limited STG overhaul involving reduction gear re-alignment, bearing inpection, gland steam modification, borescope and relocation of the excitation control module. We also installed new duct burners on both HRSGs. After we started up after this outage we noticed another step change in the STG output. Right now we are experiencing a loss of about 3 to 4 MW. The GTs are determined to be performing up to standards. A HRSG perfromance audit was performed by a company out of minnesota and found that the HRSGs were performing as they should. However trending shows that steam production in the HP and IP sections of the HRSG have decreased and the LP section steam production has increased. HP turbine exhaust pressure and temperatures are lower when compared to similar plant conditions. Also HRH steam pressure has decreased. While we are troubleshooting all we can think of is there any more input that some one could give me on what else to look at in order to track down this problem?

 

[pablo02]

How is your bypass system set up?

 

[pablo02]

Also, what fuel are you using, gas or oil?

 

[joeh2o]

Bypass system is HP bypass to CRH, then back to the HRSG to HRH and then bypassed to the condenser. And the LP steam dumps to condenser separately. Have shut all three bypass valves for 10 minutes and saw no increase in STG output. We use natural gas as fule here.

Also we see our % of CRH return at 90%. Industry norm is 98 to 99%. We recalibrated our steam flow instruments but will do so again to verify that the number is correct. Also another thing I will try next is isolating the HRH dump to the condenser for a longer period of time and then check the CRH return % as well as STG output.

 

[hacksaw]

curious about your statement about the loss of HP steam production and the claim that the SG are performing to spec.

how is the HRSG performance being taken? Thermal profile only?

You may also need to check the exciter controls as the control the power factor and therefore the real power delivered.

 

[joeh2o]

HRSG performance was performed with thermal profile only. Is there other more effective ways to perform a profile?

The I&E technicians have checked the exciter controls.

 

[hacksaw]

Thermal is the usual method for defining efficiencies, but the steam flows determine the HP(KW).

We experienced a loss of STG output shortly after an overhaul, shutdown was taken several months later only after vibtration(thrust) shot up. A steel coupon(1"x1&quot had worked it's way into turbine and wiped all of the blades (HP/IP/LP sections)as it worked its way through.

I'd focus on what is driving the LP sections to start picking up the load. It coupple be inlet temp to the HP section or loss of pressure.

 

[joeh2o]

Other than the HP and IP sections of the HRSG under performing and driving the LP section to work harder. Is there anything in the STG that would cause the HP and HRH to steam conditions to decrease while allowing the LP section to increase? Our STG is HPT section (10 stgs) with inlet conditions of 1485 psig/1000 degree exhausting to the CRH back to the HRSG. The HPT operates at 5530 rpm coupled to the generator through a reduction gear. On the other side of the generator is the MPT/LPT (14 stgs) the LP section of this turbine is after the 9th stage. The inlet conditions of the MPT is 320 psig/1000 degrees. The inlet conditions of the LP section is 65 psig/450 degrees. The MPT exhausts to the condenser axially at around 1.65 psia.

 

[hacksaw]

Other than blade damage or solids buildup (depends on you water chemistry: salt, silica, etc), you could be having problems in your superheater section. The later would show up as a reduced super heat (and or pressure) at rated HP flow.

If the loss of output was showing up before the inspection (and assuming that you got a chance to inspect the turbine sections) then you need to examine the loss of supeheater performance.

Get a hold of a calibrated temperature element and check your superheat. Loss of high temp. accurracy is always a concern.

 

[joeh2o]

What would one expect to see if HP desuperheater control valves were leaking by. Lower superheater outlet temperatures are certain to be one. But would HP steam flow decrease as well. Would HP steam pressure also decrease.

 

[hacksaw]

SH bypass valves in heat recovery do not, generally, reduce steam flow. Yes leakage or loss of valve calibration would cause a reduction in steam outlet temperature. At a minimum you will have a single steam single bypass valve. Some systems also have provision for hotgas bypass. There are other valve setups but these are the minimum.

Some systems also use steam blow offs that relieve excess steam to the lower pressure headers (operate on loss of TG demand). Generally, such blow off valves are located downstream of the SH to protect it from overheating; mis-operation will cause loss of HP steam but with out loss of SH at the turbine inlet, etc.,etc.

If you are producing the steam (check your BFW flow) then it has to go somewhere. You should be able to account for all steam use in each flow path.



Fouled SH (or tube leaks) equipment would show up as added pressure loss and reduced SH, at the turbine inlet for the rated flow.

The catch is that your controls probably look at SH temperature and will confuse the issue by shifting load to the IP/LP sections on reduced superheat.

 

[Adam01]

You seem to have sufficient answers to you questions so I will not add to them. I'm interested in the statement you made regarding realignment of your load gearbox. We are in a similar situation I was interested to know if you found a cause for the gearbox movenment or if the movement was not unexpected. Will start a new thread if this is going to go anywhere.

Adam01