effects of running steam turbine in lower capacity than it's rated

Moisture content with respect to the Wilson line can lead to stress corrosion cracking in the rotor which is a very serious issue.
Operating in partial arc admission at 50% of the nominal design may excite certain harmonics of vibration and induce high cycle fatigue.
There are actually a lot of potential impacts, but these are two off the top of my head without any knowledge of the specific unit.

Thank you csk62, but what I am tying to understand is how lower steam flow rate going into the nozzle and casing can causing more moisture if that is the case. is it because of steam flow profile change due to low volumetric steam flow entering with lower velocity?
by the way, inlet pressure is 450 psi and exhaust is 110 psi.

I presume we are discussing a non-condensing turbine, which operates with constant pressure (450 psig), saturated steam at the inlet and exhausts at a constant pressure of 110 psig. The general effects of operating at reduced load can be seen by using a Mollier diagram. If you plot the expansion line for the rated load case, it will have a relatively steep slope. My guess is that the isentropic efficiency will be about 70% at rated load. Based on an assumed efficiency of 70%, the quality of the exhaust steam will be about 93.2% (6.8% moisture).

If the same conditions are maintained at the inlet and exhaust of the turbine, and if the load is reduced to a value less than rated, the expansion line will have a shallower (flatter) slope because the turbine is less efficient at loads lower than rated. With lower efficiency, the quality (and enthalpy) of the exhaust steam will increase (moisture will be lower). Therefore, I would expect a lower erosion rate at reduced load.

I have attached a Mollier diagram (form) for the 1997 steam tables if you would like to plot the expansion lines.

I do not foresee any major problems operating at half capacity. In general, excitation forces are reduced at lower loads so I would not anticipate vibration of components leading to high cycle fatigue if problems have not been encountered operating at rated load.

Please note that the same analysis might not apply to a condensing steam turbine.

Best of luck!

In answer to OP: it depends...

To add to the exellent responses stgrme and csk62 have already posted, the design of the turbine and the exact loading will have a noticeable effect on thermal efficiency...

The immediate thought that springs to mind is that if all steam valves are raised by a common lifting bar or equivalent and therefore all open and close together, at any load but full there will be a throttling loss through the steam admisssion valves, which translates to a pressure reduction at all nozzle inlets.

If on the other hand the machine uses partial arc admission and sequential valve opening, it may be the case that, depending on loading, some of the steam admission valves will be wide open whilst the others are fully closed, in which instance there will not be any throttling loss.

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

Crshears and Stgrme , thank you for your input.

Contrary to intuition, there will actually be less moisture in the stages than at full load.

A less efficient turbine will exhaust higher energy though the exhaust than a more efficient turbine.
With fixed inlet pressure and fixed exhaust pressure, the reduction in the work done by the turbine must go somewhere, and it goes to the exhaust straight through.

operating at part load will absolutely be less efficient even if the first few stages are partial arc (unless the whole turbine is partial arc).

At part load, the turbine is extracting less useful work, thus the energy content as compared to full load will be higher in the later stages. So the moisture content will be less (higher energy) and can potentially push the wilson line further downstream.

Throttling decreases pressure, but temperature is relatively constant unless work is extracted. An ideal throttle is a constant enthalpy process.

long story short, less moisture (good), worst efficiency (bad), more energy in the exhaust steam (can be good or bad depending what the process it feeds)