300MW steam power plant..Rated temperature and Pressure in inlet of Turbine is 540 deg C & 16 Mpa. Every time during warm startup (Steam temp allowable is >370 deg and pressure >5.88 Mpa, when casing temperature >150 deg), HP Diff expansion increases to more than +4.5 um where trip is +6 um. As far as i know last time we entered steam in turbine at about 400 deg.
Does this high temp steam inlet causing the diff expansion to go high?
Should the steam temp be less? or the gland steam temp causes the expansion to go high?

what may be the causes at turbine steam rolling? And after few hours when the casing temp increases to 400 deg from say 180 deg the expansion starts decreasing. plz help me

 

Differential expansion (DXD) going rotor long (+) is due to the steam being admitted is hotter than the metal temperature of the rotor and shell. The magnitude of + DXD is proportional to this mismatch. The rotor expands quicker than the shell do to the differences in mass and surface area being heated. Once the shell expands after some time, the DXD will return to normal.

Besides DXD, the rate at which a steam turbine can be rolled/loaded should be monitored by the metal temperature ramp rate. The desired amount of turbine cyclic life determines the desired ramp rate.
For a conservative cyclic life expenditure, the ramp rate is low and the + DXD magnitude during the transient will be minimum. For a severe cyclic life expenditure, the ramp rate is greater and the + DXD magnitude will approach the trip magnitude.

 

There is usually a steam temp/turbine metal temp mismatch difference that should not be exceeded.

some turbines I am familiar with say that there should not be more than a 100 degree mismatch between the steam temp and the metal temp where the steam enters the turbine. this will usually keep the expansion under control.

 

For the units I am familiar (GE LSTG), the mismatch between 1st stage shell inner surface metal and steam being admitted is desired to be <b>+100F</b> (56C) for a conservative cyclic life expenditure. The steam conditions at the 1st stage admission will depend upon the throttling losses of the controlling steam valve. For MSV FA admission,

5.88Mpa at 400C the steam at the 1st stage would be 315C
and 5.88Mpa at 370C would be 260C

So with a 1st stage metal of 180C the mismatches would be 135C and 80C or 240F and 140F. I would expect both of this to result in moderate cyclic life expenditure and +DXD till a few hours after steady state is obtained.

You should notice that for a 30C increase in main steam, the increase at the 1st stage steam would be 55C, thus a much hotter startup

 

Differential expansion Mainly increases due to Two reason one due to Higher temperature difference between Casing temp and Steam temperature Admitted to turbine and second one is the less steam flow ie.. Low Load.

It will be reduce and be on normal condition if we maintain a suitable temp difference ie.. approx max 60 deg. and increase the steam flow in turbine with low pressure Due to Less steam temperature.

Keep increasing steam Temperature, flow and Pressure Accordingly. the differential expansion will limit to 2.1 Max.

Pankaj sharma