Turbodetander (turbine) start-up issues

[Gulmi]

Hello dear fellow engineers!
My name is Gulmira.
Currently we are working on starting up the Cryogenic Nitrogen Unit.
Completed exceptional ReGex and deriming steps. Started up Turbine, but cannot reach the required RPM (42000) on the turbine. Blades are open to 50%, inlet gas Pressure 4.8 bar, RPM=32000. When turbine blades opened more, RPM increases sharply and then decreases again to lower level, also gas inlet pressure decreases to 3 bar.

because of that Liquid N2 is not collected in evaporator to start producing gaseous N2

Please help, what can be a problem?
one of the options is that gas flowrate is not enough, What else can be a problem?
will so much appreciate your help!

 

[georgeverghese]

You could try another tactic also by changing the operating conditions on the brake side of the expander to bring the expander up to speed - if you push the operating point on the brake side compressor(?) up the Q-h curve, it should increase turbine speed.