Loss of Cooling Water to Steam Turbine Condenser

S

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sriram sandep

our system configuration is such that we need to run 2 main CW pumps all the time when steam turbine is in operation with two HRSG's. when steam turbine is at 200 mw load suppose if both main CW pumps tripped due to some or some other reason and there is no cooling water supply to the condenser, i know that condenser protection will be acted and steam turbine trips.

my question is when steam turbine is in operation. if there is no cooling water supply to condenser, then vacuum drops very fast and possibility of condenser tube damage. as the steam turbine got tripped on condenser protection but again all the bypass is dumped to condenser how it can take this load?

does steam turbine trip on condenser protection activates the diverter damper to trip or not?

if steam turbine not trip acted what is the operator action required?

GT and ST are GE make GT 9fa and ST D11
 
You will have to look at the protection logic for the diverter damper, but I believe tripping the steam turbine should trip the damper to bypass mode. Also, loss of vacuum should trip the steam turbine bypass valves closed, but, again, you should verify that the logic does this.

Obviously, with both the steam turbine tripped and the steam bypass valves tripped, it is likely that the steam relief valves will open to limit pressure.
 
sriram sandep,

>does steam turbine trip on condenser
>protection activates the diverter damper
>to trip or not?

The answer to this question lies in the plant design and operations manuals and documents. Some diverter dampers cannot change position when the gas turbine is running--there's just too much force on the damper from the exhaust gases flowing out of the gas turbine. You will need to look at the manuals provided with the plant during the turnover after commissioning, or examine the programming of the turbine and DCS control systems.

>if steam turbine not trip acted what is
>the operator action required?

In my experience, the turbine will trip on decreasing vacuum, and that will, in turn, set in motion another set of operational considerations/problems for the operators. I don't believe it would be a good idea to dump steam to the condenser if the condenser vacuum was insufficient, so safeties are probably going to open and vent a lot of steam (noisily) to atmosphere.

Without cooling water flow through the condenser, the vacuum is going to decrease relatively quickly. The water in the condenser tubes is also going to be heated by the steam that's not condensing. There is usually a rupture disk on the condenser that will "operate" if the condenser pressure gets too high, and that's also going to create another issue (noise and steam release) for the operators. The tubes can sag and even rupture if the heat is too great.

So, these are some of the reasons for tripping the steam turbine--to prevent continued flow of steam into a condenser that isn't condensing. The last stages of the steam turbine are also going to quickly heat up, and that's not good for the steam turbine, either.

But, the specific answers to your questions should be contained in the manuals provided with the plant. They define the design criteria (including contingencies--if any--for problems such as you are describing) and intended operating procedures for the plants.

And, the turbine control systems contain the programming and configuration to implement the design and operation guidelines defined in the plant design and operations manuals.
 
In addition to this reply, there is chance of breaking of condenser rupture disc if both CW pumps tripped and ST not not tripped. However ST must trip on condenser vacuum protection, there must also be HRSG protection which acted on condenser pressure protection as Turbine steam bypass must have vacuum protection.

In my opinion, if ST not tripped, operator has to trip ST/Bypass immediately on loss of total CW.
 
M
What I understood from the conversation that when the machine trips on cooling water not available to the condenser, how can it take the heat load of bypass steam dumped in the condenser?

Now when the machine trips, it not trips on loss of cooling water. It actually trips on "Condenser Cell Pressure High" that is usually 0.3 KSC (absolute) for all the machines.

After that the bypass steam is dumped into the condenser which rises the condenser cell pressure further. At 0.4 KSC (absolute), bypass valve trips and you cannot open the bypass valve now.

After that the further pressure in the boiler is released through safety valves.
 
On both CW pumps tripping, first ensure turbine to be tripped on low vacuum (0.3 kg/sq.cm absolute). secondly open vacuum breaker, then to prevent any steam entry make PRDS pressure to 0 kg/sq.cm by closing control valves either MOV.

This will prevent turbine & condenser from any loss...
 
sriram sandep,

in our combined cycle configuration, a bypass trip is initiated on condenser high pressure, condenser high level, high steam temperature in the bypass or high exhaust temperature in the steam turbine. A bypass trip closes the diverter damper. If it doesn't close, a gas turbine trip is initiated.
 
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