I have a steam turbine that suddenly undergoes high radial vibration at the front bearing, this then subsides after about 10 minutes and returns to normal running conditions. There is not much vibration on the bearing casing... only the shaft. Any ideas what is causing it?

When does this occur? during start-up? out of the blue?  does it reoccurr? other?  Does the front bearing have a governor and oil pump stub-shaft?  Where is the shaft vibration monitored on this bearing?

where are bearings #2, #3, and #4 arranged on the steam turbine?  Are these vibrations monitored?  any blips on these during the same time?

did you have any other issues going on at the same time or slightly before?

I think there a number of possibilities but some additional info would be required to narrow those down...

This is not a lot of information.  

Try to extend the period of time that the rotor is on turning gear (or rotating at low speed if you don't have a turning gear).  If the rotor is relatively heavy and "bowed" while shutdown, running on turning gear will help to minimize ill effects of this.  (Running on turning gear immediately after shutdown is also important to minimize rotor distortion in the first place.)

Try to extend the warm-up period before going to normal load, normal speed.  Beware, however, of the danger of overheating the last stage of condensing turbines when operating for extended periods of time at no/low loads.  (Most OEMs make safeguards against this with cooling steam to LP stages, temperature alarm in exh. casing, and/or hood sprays.-Bellows Seal Valves)

Many times it is simply not possible to attain anything close to normal operating temperatures within the unloaded (or lightly loaded) warm-up sequence, and you perhaps are already doing the best you can.

Is is also possible that as the difference between cold and hot alignment is causing a problem during start-up.  If the turbine is set too low with respect to the inlet bearing case, you might see this transient vibration until the turbine grows into hot alignment.  Check your cold and hot alignment readings to verify that they're within OEM specs.

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