steam turbines operating at different conditions 2

[revata]

we are hoping to purchase a backpressure type steam turbine with the following design conditions.
1. inlet steam pressure 30 bar (abs)
2. inlet steam temperature 370 deg. C
3. outlet steam pressure 1 bar (abs)
4. steam flow rate 24.75 ton/hr
5. power generation 3.0 MWe

the steam conditions that are available at our process plant are as follows;
1. steam pressure 30 bar (abs)
2. steam temperature 370 deg. C
3. steam flow rate 15 ton/hr (60% of design capacity)

what will be the outlet steam pressure from the steam turbine.

 

[TD2K]

The outlet pressure is set by your system, it's not a property of the turbine. If you put it in a steam system running at 5 bar, that will be the exhaust pressure. If you exhaust it to atmosphere, the outlet pressure will be close to the original 1 bar(s) and if you put it on a surface condenser, the outlet pressure will be quite possibly subatmospheric depending on the cooling medium you use.

 

[davefitz]

TD2K is right.
I assume that if you want a backpressure turbine, you are using the exhaust heat for process purposes. It would then exhayust to a heat exchanger that operates with an inlet steam temp of 101 or greater. Operation slightly above 1 bar abs will ensure no air inleakage contamination of condensate.

The turbine vendor will design the blades to provide max efficiency while using your specified inlet and exhasut conditions.

 

[poetix99]

By way of answering a question that you haven't asked, but perhaps meant to (or should have):

When operating at 60% flow (or any part-load point), a greater pressure drop will occur on the inlet control valves (sometimes called governor valves, or governing valves). This is how, although, as TD2K said, the exhaust pressure is not by changing the turbine flow, the turbine flow can change with the overall pressure drop appearing to remain the same.

As recently discussed in another thread, my statement is not true of all turbine designs, but is a good general explanation.

Also, you should assume that the turbine efficiency will be less at the part-load point, and that you will therefore obtain less than 1.8MWe (60% of 3MWe) because of the poorer efficiency.

 

[revata]

poetix99, thanks for your comment. As per your suggestion wouldn't the following be the operating conditions for the steam turbine if the governing valves were operating to control the flow to 60% of the rated flow?
1. inlet steam pressure to the governing valve - 30bar (abs)
2. inlet temperature - 370 deg. C
3. inlet steam pressure to the turbine (after the governing valve) - 18 bar (abs)(60% of rated inlet pressure)
4. steam flow rate 15 ton/hr (60% of rated flow)

that is the net pressure difference across the steam turbine is still 30 bar (abs).

 

[poetix99]

Yes, that is more or less correct.