TECH CIRCULAR NO X-27. DATED: 27-5-89

 

DETERMINATION OF PERMISSIBLE DIFFERENTIAL EXPANSION LIMITS OF HP, IP AND LP ROTORS AT SITE: 210 / 500 MW STEAM TURBINES (KWU DESIGN).

1.0 INTRODUCTION 

The differential expansion limits are set at site considering the actual axial flow path clearances. The guidelines for the same are given below:       

1.1 ASSEMBLY ASPECTS

                                  The HP and IP turbines are assembled in the plant. The axial and radial clearances between moving  / stationary blades and at shaft seals are checked and recorded. The rotor position with respect to external surfaces of the casing is also checked and recorded. After the assembly of HP and IP turbines, axial movement of the rotor in positive and negative direction is also checked and recorded. All the above shop records are sent to site. 

                                   During erection at site, the reference dimension for axial position of the rotor with reference dimension for axial positions of the rotor with reference to casing is again checked and maintained as per shop records. This ensures the desired internal axial clearances as maintained in the shop . Axial movement of the rotor in positive and negative direction is also checked and compared with the shop readings. The rotor position is considered “O” when rotor is placed with equal axial clearance on both sides of the thrust collar in the thrust bearing.               

1.2 DESIGN ASPECTS 

                               

  The differential expansions, at various stages of HP and IP rotors, are proportional to their distances from the thrust bearing. Therefore the differential expansion pickups are installed at the farthest end of each rotor.                                

                       Due to large distance between LP rotor and thrust bearing, high valve of LP differential expansion has been foreseen .The LP shaft seals are of  SEE THROUGH’ type i.e. without castallations, to unable unrestricted axial expansion at the shaft seals. The vertical central line of the LP rotor is kept displaced towards the thrust bearing with respect to vertical central line of the LP inner casing by 10 mm in case of 210 MW and 13 mm in case of 500 MW Steam turbines so as to ensure that the vertical central line of LP rotor approximately coincides with the vertical central line of the LP inner casing during normal operation of the set with normal value of the differential expansion of LP rotor. The drawing no. 0-10744-01000 for 210 MW and 0-10744-02000 for 500 MW steam turbines may be referred in this regard.

 

2.0     EVALUATION OF PERMISSIBLE DIFFERENTIAL   EXPANSION LIMITS

2.1    DIFFERENTIAL EXPANSION LIMIT FOR HAND TRIP  :  

                    Permissible differential expansion values (DEp) are decided based upon the actual axial flow path clearances is kept to prevent chances of any rubbing in axial direction. The turning is to be tripped if the differential expansion has reached to the value DEp.The actual axial flow path clearances are plotted in the log sheet 016.40 enclosed herewith in annexure-I . Now a straight line originating from thrust bearing is plotted in such a way that it has safety margin of not less than 20% of the corresponding axial clearance at the location of the pick –up gives the value of the permissible differential expansion  (DEp)

                2.2      ALARM SETTINGS:

               2.2.1    HP AND IP TURBINES:

                                      The alarm setting is kept at 1.0 mm less than the permissible differential expansion values i.e. DE  (al ) =DEp –1 mm

               2.2.2        LP TURBINE: The alarm setting for LP turbine is kept at approximately 80% of the permissible differential expansion values. i.e. DE  (al) =DEp   -0.2  DEp 

3.0      TENTATIVE SETTINGS FOR DIFFERENTIAL EXPANSION: With axial clearances of HPT, IPT  & LPT within design tolerance the setting for differential expansions can be kept as follows. 

 

 

 

          HPT

           IPT

           LPT

 

 

  210 MW

 

     

T

A

T

A

T

A

     POSITIVE

    + 6

    +5

   +6.5

    + 5.5

+30

  +25

      NEGATIVE

    - 4

   -3

   - 3

     - 2

   - 7

 - 5

   

 500 MW

      POSITIVE

   + 6

 + 5

 + 7.5

  + 6.5

+35

+ 30

     NEGATIVE

  - 3.5

 - 2.5

  - 3

   - 2

 - 6

 - 5

                                                 In case of deviation flow path clearances the actual values of the clearances are to be plotted in the log sheet 016.040 and the permissible values and the alarm settings are determined at site.


 

 

 

Back to Technical Services