Efficiency of Triple Pressure Steam Turbine

Two gas turbines running at 80 percent load 300 MW each then steam turbine output is 310 MW. But at base load when both gas turbines are at 400 MW, more fuel is burning and more exhaust is coming then steam turbine is producing 380 MW while condenser vacuum is according to design value. Why steam turbine is producing 20 MW less than rated output 400MW?

Please remember our boiler feed pumps are VFD drives. Is there any issue at base load or Internal exhust is missing inside boilers? going somewhere? or issue with control valves? It’s triple pressure turbine and running at once through system. Vacuum of condenser is according to design limit.
Dear Faisal,

I was reading your last post, and I am trying to get some support on this question. I am ex Alstom power plants Overall controls concept expert engineer.

Your case need to be more detailed as what type of machine units OEMs
configuration single shaft or not .....

Efficiency of a block or plant it is very complex and need more studies by balance of power blcok by clock plus plant overall configuration should be done ( for sure you should have access on these contracts documents..).

Regarding the "loss of MW on steam turbine as you describing can be reactions of one or multiple parameters...

Is the plant connected to a industrial process?
Like smelter aluminium or other process

Steam add on?

Please try to add more details on like age of equipments, frequency of grid system, controls concept of the block and the plant, AGC if applicable..

Cause you don't tell about grid code here as it can also resulting on kind of " loss of power". For some times i have seen some sites experiencing "power losses " but with an excellent troubleshooting you can afford kind of situations!

Which controls sytems do you have in your plant?

Many kind of questions need to get response for a better understanding of the situation and an overall troublesshooting first.


The original poster REALLY needs to tell if this <b>perceived</b> problem is a recent problem and how long since it was first noticed, or if it has been ongoing since commissioning--and how long ago was the plant commissioned.

I don't recall if you asked about any extractions off the steam turbine, or if the control valves are commanded to go "wide open" at some point during loading of the gas turbines (as many Combined Cycle steam turbines do).

Most steam Combined Cycle steam turbines can only make power based on the steam coming from the HRSG (Heat Recovery Steam Generator; the "boiler"). So, if the GT(s) are only running at, say, 40% of rated output the steam turbine can't produce rated output because there's not enough steam flow coming from the HRSG(s). So, what usually happens is that at some point during loading of the plant (when the GTs are not yet at rated load) the steam turbine control valves are opened fully by the steam turbine control system and the steam flow-rate (and pressure and temperature) determines the power being produced by the steam turbine generator. And that power varies with GT load.

It's a common misconception that every Combined Cycle steam turbine will produce rated power output when the GTs are producing rated power output. But that's NOT always the case. Some GTs and their HRSGs produce more steam flow at partial GT load than they do at rated GT load. And some HRSGs have auxiliary burners to increase steam production when the GTs are at rated output.

But, the takeaway here is still the same: The original poster has not provided sufficient information for us to be of much help. Every Combined Cycle plant is not the same, and every HRSG is not the same.

I think this is more of a theory question based on assumptions. Every Combined Cycle plant should be provided with Operations Manuals to detail how the plant was designed to be operated and that should be where every question about the plant operation should start being answered.
A "Unit Rating" and the guarantee output after commissioning are going to be different. If your feed pumps are keeping your drums at operating level, you can ignore them. A few answers to some 'static' questions would help:

What's your altitude, temperature, humidity? Inlet cooling?

Do your HRSG's have exhaust by-pass dampers?

Where are your VARS between all three units?

Do you have SCR's for NOx or other?

How's your steam temps and are your de-SH's behaving?

Drain line-up correct?

What is your Main Steam pressure & temp @ 80% GT load vs 100%?

Has your steamer 'ever' put out 400?

There are a host of other 'dynamic' items for your operators to trend and study:

Steam pressure, temps, & flows. HRSG 1/2, Steamer inlet.

HP, IP, & LP Preheater by-pass controls.

HRSG's production balanced?

LP inlet pressure & spec drum flows? (carryover)

CRH flows balanced?

HRSG IP production, Flows, Temps, Balanced?

Do HRSG's have GT exhaust by-pass dampers/stack, or cascade steam by-pass to condenser?

Any way to monitor by-pass stack temps?

When last you inspected the exhaust dampers/doors? Good seal? Warped open/leaking? Melted?

Are cascade steam by-passes closed, drift open?

100% certain steamer CV's 1&2 are 100% open? CRH's? (Like Look at them from a ladder.)

Trend CT exhaust temps/flows

Steamer exhaust temps?

Condenser Circ water outlet temps the same side to side?
..and this is only for starters.
Dear Sir,

Power Plant is CCPP and two gas turbines (GE9HA c0ntro MARKVI advanced), plus Steam turbine is ALSTOM STF30 (ST control is ALSPA) Single shaft steam turbine. Overall Power Plant efficiency is 61 percent at base load heat rate combined mode 6600kj/kwh. Grid frequency is 50 hz. But boilers are not from ALSTOM, it&#8217;s from chinese manufacturer.

I need to know where issue lies. How we can calculate exhaust gases through boilers in each section LP, IP, HP?