Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) Steam Turbine

Dear All,

We are operating a large Ammonia Urea Complex in Pakistan. Following are details of Steam Turbine driving Boiler Feed Water pump at our expansion unit.

Type - Single stage impulse with one nozzle ring, one ring of reversing blades and two rows of moving blades

 Following are permissible steam inlet and exhaust operating parameters;

 

Minimum

Normal

Maximum

Inlet Pressure     (bar gauge)

36.3

37.3

 

Inlet Temperature (0C)

375

380

 

Exhaust Pressure (bar gauge)

 

3.43

3.92

 As per OEM

 At normal conditions with 37.3 bar gauge inlet pressure, 380 0C inlet temperature and 3.43 bar gauge exhaust pressure turbine will deliver 1742 KW at 23040 Kg/Hr steam consumption with 215 0C exhaust temperature. Refer to attached Annex-I for steam flow diagram at normal conditions.

At most unfavorable conditions with 36.3 bar gauge inlet pressure, 3750C inlet temperature and 3.92 bar gauge exhaust pressure the turbine will deliver 1935 KW at 26510 Kg/hr steam consumption with 220 0C exhaust temperature. Refer to attached Annex-II for steam flow diagram at most unfavorable conditions.

 Power requirement of boiler feed water pump to produce desired flow is 1742KW @2985RPM; which is same as nominal power of the turbine; hence turbine is undersized for the application. Refer to attached Annex-III for specification sheet of the turbine.

Turbine is able to meet power requirement of the pump after initial startup following a cooling cycle, however its performance deteriorates with the passage of time. Following is the comparison of operating parameters of the turbine;

 

Date

Steam inlet Pressure

Kg/cm2

Steam Inlet Temperature

0C

Exhaust Steam Pressure

Kg/cm2

Exhaust Steam Temperature 0C

Turbine Steam Consumption

Ton/Hour

Condition

11-10-16

38.4

386

4.45

244

26.4

Parameters of Start-up after a cooling cycle

13-12-16

38.6

389

4.4

256

27.3

Parameters after 02 months of operation

 

As per my understanding, increase in exhaust temperature of this type of turbine is a clear indicative of its performance degradation.  Interestingly, performance of the turbine is restored after a cooling cycle of 10~12 hrs without doing any maintenance work. This abnormal behavior of the turbine is exhibited every time after a cooling cycle. The turbine has been overhauled several times but no abnormality is ever found in its internals.

 It would be a great favor if anyone advise us on following points;

  • What can be the possible reason for this gradual increase in exhaust temperature of the turbine? Why the exhaust temperature and steam consumption restored after a cooling cycle?
  • Is this performance degradation with time related to overloading of the turbine?

 

Thank you in advance.

 

Attachments

Original Post

Sounds like the steam is slipping by the turbine blades and going directly to exhaust. As the turbine heats up, internal clearances will expand, I would focus my attention on how the steam can short circuit from the nozzle to the exhaust. If this were happening I would expect a loss of HP/KW, loss of speed, increased exhaust temp.

If the turbine is not meeting performance expectations, then there is something wrong. The distance from nozzle to the turbine blades, the condition of the nozzle/s, steam supply temp can also affect the performance of the turbine.

Have  you contacted the OEM for assistance?

Dave

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