Steam Turbine Bleed Pressure/Flow

[marvalar]

Greetings!

I have a question regarding steam turbine bleed pressure.

I'm studing a condensation turbine with 2 bleeds. The Bleed 1 feeds the deaerator, while bleed 2 feeds the LP Preheater.

On the turbine vendor balance sheet I have for the bleed 2, let's say: 1.0 bara and 2000 kg/h.

Now. I know that my LP preheater at 1.0 bara will condense more steam than 2000 kg/h, it has enough area for it.

How will the system react. What will be the equilibrium point.

I would say that at the equilibrium, the pressure will be lower than 1 bara and the flow will be greater than 2000 kg/h. The problem is that I can't understand the impact on the turbine pressure profile.

Please, can somebody advice?

 

[rmw]

Your flow from your bleed point (if not controlled by valves) will be determined by the pressure differential between the bleed point and the condensing pressure of the preheater. The flow rate will be determined by how much the preheater condenses. All steam condensed by the preheater-which helps the thermodynamics of the boiler side of the cycle-is steam that doesn't finish the trip through the turbine and make real power for the generator.

When the preheater has absorbed all the heat its surface is capable of absorbing no more flow will be drawn from the bleed point (unless valve controlled.)

The pressure inside the turbine at the bleed point will also vary as a function of generator load. Your design pressure at the bleed point is different at full load and at partial loads. That too will affect how much the preheater draws and what temperature water it produces.

Your specification from the vendor is merely a snapshot of one operating condition, normally full load or even possibly 5% overload.

rmw

 

[marvalar]

You got it absolutely right. Thanks a lot.

I've got vendor balances for 100%, 75%, 50% and 25% load. And yes, the bleed pressure decreases when the load decreases.

Pre-heater sizing is on my side and I'm sizing it for the 100% load case. I feel that I must give some feedback to the vendor regarding the expected performance of the pre-heater at the other scenarios.

My approach is going to be. Check with the estimated bleed pressure for the partial load cases, how much steam the pre-heater will condense, and then give that figure back to the vendor.

Do you agree with the approach?

 

[rmw]

Yes I agree. If you system is pretty straight forward (meaning that all the condensed turbine exhaust passes through the preheater and nothing else) and your turbine vendor knows that, they will know the approach temperature to the heater from the condenser and will know how much extraction steam will be drawn from the turbine.

You are correct to size your heater for the 100% condition (unless your turbine has a overload condition.)

rmw

 

[stgrme]

Since you have heat balances at 100%, 75%, 50% and 25% load, you can plot the pressure in the extraction zone to the preheater versus flow to the downstream blade group. Typically, this is a straight line. Then, you can estimate the pressure in the extraction zone of the turbine, based on the actual extraction flow versus the extraction flow on the vendor heat balance.

If you are also responsible for the deaerator, you need to consider operation with the preheater out of service if that is a possibility. I would also suggest informing the turbine vendor, if operation with the preheater out of service is possible.

Best of luck!