Yes, steam turbines which exhaust to air cooled condensers ( instead of using wet cooling towers for cold circ water) must operate at both backpressure and vacuum depending on ambient conditions. The change in backpressure affects the exhaust loss and also affects the energy produsced by the last 2 rows of blades.
If the exhaust drops to too low a pressure ( hi vacuum) and the steam exhaust velocity approaches 50% of sound speed, then aerodynamic instability or "flutter" occurs and the blades vibrate and eventually can fail. Also, at too low a pressure, the last 2 rows behave as compressors instead of turbine blades and cause the steam to heat up. If the exhasut overheats, it will also cause the blades to contact the casing and damage ensues.
If we were to use a steam turbine designed for 1 barg turbine exhaust condition, in a new application where the turbine exhaust pressure is 0.14 bar (abs), would this reduced pressure cause the steam exhaust velocity to approach 50% of sound speed.
Your intended homegrown "rerate" is not a good idea without, as "hacksaw" suggested, contacting the OEM.
Many aspects of your turbine will be poorly designed for these new conditions. You will have increased the exhaust specific volume by a factor of roughly 2barA/0.14barA = 14 !! At the very least, you will have poor performance and flow limitations established by exhaust velocities (and a lot of noise too). You could experience some sort of vibratory problems as "davefitz" has suggested.
davefitz's comments about overheating applies mostly to the overheating experienced with very tall blades of turbines designed for condensing operation when they are lightly loaded, or completely UNloaded, so that is not an issue in this case.
