Effect of operating condition on steam turbine 1

[Jack Nicholson]

Dear all.
Hello.
I have a question.
We have a steam turbine which operated with high pressure steam (HPS). Min and Max pressure and temperature of HPS inlet is given in Vendor catalogue (min P= 39 barg, Max P=41 barg) (Min T=390 C and Max T=410 C).
Unfortunately HPS operating condition fluctuates frequently, so that T decrease to 320 C and P decreases to 18 barg (without turbine trip!). I'm surprised what is the actual effect of this variation on steam turbine?
We are assumed that everything is ok! because turbine wouldn't trip. what consequences would you have expected?
Thanks in advance.

 

[stgrme]

I am not surprised that your steam turbine operates successfully at the off-design conditions you noted. I would not expect the turbine to trip under these operating conditions.

The main consequence of operating at 18 barg and 320 °C instead of the design conditions of 40 barg and 400 °C is a reduction in the capacity (power output) of the turbine. This reduction is due to two factors: reduced steam swallowing capacity and less available energy between turbine inlet and turbine exhaust.

Swallowing capacity can be predicted by the ratio of the Square Roots of (P/v) at the off-design and design steam inlet conditions, where ‘P’ is the pressure in bara and ‘v’ is the specific volume in m3/kg. At 18 barg and 320 °C, I calculate a swallowing capacity of about 49% relative to the design conditions.

Operating at the off-design condition (lower inlet pressure and temperature) shifts the expansion line to the right on an H-S Diagram, which tends to shorten the expansion line (reduces available energy). If the exhaust pressure is held constant, I calculate an available energy of about 84% relative to the design conditions.

Assuming that the turbine exhaust pressure is held constant at the off-design inlet conditions, there is only a slight increase in exhaust moisture. If the turbine is operated at a significantly lower exhaust pressure, exhaust moisture content, and consequently, a higher erosion rate will occur in the final stages of the turbine.

Please note that if you move this thread to the Heat Transfer & Thermodynamics engineering forum you may receive additional comments.

Best of luck!