Why Are Impulse Stages Used Before Reaction Stages in Steam Turbines?

[engnr_arsalan]

I have seen modern steam turbines with velocity compounding, they start with impulse stage then reaction stages,can anyone tell me, why There is a impulse stage first,why not a reaction stage first?

 

[SteamKing]

When steam is first admitted into the turbine casing, it is traveling at relatively low velocity. The impulse stages are placed first on the turbine rotor so that the steam expanding thru them will increase in velocity, after which reaction stages can be used to take advantage of the now faster flowing steam.

 

[engnr_arsalan]

When steam is first admitted into the turbine casing, it is traveling at relatively low velocity. The impulse stages are placed first on the turbine rotor so that the steam expanding thru them will increase in velocity, after which reaction stages can be used to take advantage of the now faster flowing steam.

Doesn't nozzle drop steam pressure and increase its velocity upto super sonic level?