Normally in gas turbines (GE machines) alarms/trips are based on seismic vibration probes, but in steam turbines mostly it is based on proximity probes. What is the reason?

 

"In the beginning..." there was only seismic vibration sensors. And, also, in the beginning GE gas turbines and GE steam turbines were produced by two different divisions of GE, though the engineering and philosophies of the gas turbine division were originally derived from those of the steam turbine division.

But, over the years, steam turbine changed their philosophy and control schemes to start using proximity probes for vibration sensing (and shell expansion, differential expansion, etc.) and the gas turbine division tended to stay with seismic sensors which were deemed adequate protection for gas turbines (and which are still adequate for protection, though they don't provide much in the way of vibration analysis information which can be very useful at times).

With the development of the newer gas turbines, and with the requirements of many owners and purchasers of new gas turbines, the gas turbine engineering division has begun using proximity probes for both sensing and alarm/trip.

Of course, it also doesn't hurt that GE has assimilated the Bently-Nevada company (the poster child for proximity vibration monitoring).