Turbine - Drop in Back Pressure and Increase in Nozzle Box Pressure

Have you checked your nozzle isn't blocked?

Maybe the LP steam pressure is maintained, but how do you know the flow is maintained? The decreased back pressure suggests otherwise, leading to 34point5's question.

Not much info, but another possibility, if I'm reading your description right, is bearing problems on turbine causing undo friction.

If turbine is binding up, with steam flow the same, this could also create back pressure drop and pressure buildup in nozzle box.

I guess I interpreted the question a little differently. To me "back pressure" is the equivalent of the condenser pressure / turbine outlet; And "nozzle box pressure" would be the supply pressure. Is this strictly an LP turbine or is there an HP also? (I'm trying to understand if the LP steam flow you are referring to is the main supply, admission, extraction, etc)

A lower back pressure (assuming it is the turbine outlet) and an increase in supply pressure is what normally happens when you shut the throttle to a steam turbine. Assuming that this is only an LP turbine (no HP flow), then I would also say that the steam flow is not the same.

You have to stop the turbine and clean your turbine blades. There will be heavy deposit on the turbine baldes which had caused drop in back pressure and increased in nozzle pressure. This problem is due to heavy Silica/Phosphate contents in your steam.

I hope this will solve your problem.