Steam Turbine performance with variation in inlet steam flow rate

[feghbalib]

Dear All,

I am estimating a future problem while still in erection phase for 1350KW steam turbine. I need your help to estimate a solution.

Due to some reason, the flow rate of high pressure steam goes to the turbine is decreased at the desired pressure (other parameters are remain same). Therefore I want to know the effect of reduce inlet flow rate in the HP Steam turbine output. What’s the effect of reducing power output on RPM? Can we control this effect by changing output pressure? Is it possible to reduce drop power output by changing in RPM or output pressure of turbine?

The steam parameters are mention below:

1- Current condition

Manufacturer : SHINKO
HP Steam inlet pressure : 30 bar
HP Steam inlet temperature : 236 Degree C
Steam Consumption : 30 (ton/hr)
HP Steam Turbine outlet pressure 3.5 bar
HP Steam Turbine outlet Temperature 148 Degree C

2- New Condition
Steam Consumption : 20 (ton/hr)

There is 1 extraction of LP steam from the turbine to Back Pressure Receiver

Can somebody suggest the role of flow rate in steam turbine. Will steam turbine able to work properly even at low flow rate inlet?
Thanks,

 

[racookpe1978]

It will "work" at the reduced flow of the steam.

It will NOT make your original 1350 KWatt. (Depending on internal flow degradation, and on the heat extraction percent changes) it will probably be close to 20/30 x 1350 KWatt It will also be less efficient that if it were running at design pressure, temperature and steam flow.

Why do you think reducing rpm will somehow increase or negate the effect of cutting off 1/3 of your steam flow?

What effect will reducing RPM have on your load? If a pump or similar centrifugal (fan) load, then the lower rpm means significantly lower pump output. (You're much lower on the pump curve.)

 

[feghbalib]

Dear racookpe1978;
thanks for your comment.
I just want to know the relationship between power and RPM.

 

[georgeverghese]

Reduced flow ( away from BEP ) usually results in reduced efficiency.

Directionally, decreasing the backpressure on turbine exit should give you more power, but the increased velocity on the last few stages of the turbine may result in sonic flow / blade vibration and fatigue. So dropping the exit pressure down may not be possible. To confirm how far you can decrease exit pressure to attempt to get back to 1350kW at 20t/hr at constant rpm ( ie. constant torque) , the steam turbine vendor should be able to help.

 

[davefitz]

The "fan laws" indicate that the flow will vary linearly with rpm, the pressure across the turbine vary by the 2nd power of the rpm, and the power produced vary by the 3rd power of the rpm.

"Nobody expects the Spanish Inquisition!"

 

[racookpe1978]

Back to my original question: What are you driving with the steam turbine?

 

[gerhardl]

Let me try with the simple view:

Energy (steam) input in the turbine is diverted in three: 1350kw, loss and recovered/diverted energy.

Less energy in = less energy out, equal in percentage or probably a bit larger in percentage of loss as well as same for recovered energy.

RPMs can probably be regulated and will be as is if load (KW) are reduced.

Relationships :see davewitz (?) Linear with rough calculation?

Advice: study the manual for operational and regulation trimming and adjustments.

If the turbine has a regulated bypass and a large amount of steam is diverted at normal operation less input from boiler may not be critical, as more could be directed to turbine.