With the location of the repair being so
close to the 6th disk, some additional
repairs would be needed to this disk, which
would extend the overall time to complete
the project. in order to save time, Sulzer
proposed both stage 6 disks would be
removed and possibly reinstated at a later
date. This was then agreed by customer.
REPAIR EXPERTISE
Joining two sections of a turbine rotor
requires considerable expertise, including
computer modelling and finite element
analysis (FEA) to ensure that the proposed
design would withstand the stresses of
normal operation. The FEA was also carried
out at an overspeed of 3’600 rpm to ensure
that the centrifugal loading on the disks
would not cause any damage to the rotor
shaft after the repair. With all the necessary
analyses completed, the machine shop
started to prepare the two rotor shaft
sections for the addition of the stub shaft.
In-house precision machining enabled
the stub shaft to be shrink-fitted into the
prepared connections before the whole
joint area was preheated prior to the
welding process. Using precision-controlled,
submerged arc welding equipment, the
stub shaft was built up to a level that would
allow it to be machined back to the required
dimensions. Once the original dimensions
had been achieved, a series of non-
destructive tests (NDT) was carried out to
ensure there were no flaws in the completed
rotor assembly.
These processes were repeated to remove
the cracks in the thrust end of the rotor
as well, bringing the completed assembly
back to finished dimensions. Once all the
machining was complete, the rotor was
dynamically balanced before being shipped
back to the customer.
IMPROVING PERFORMANCE
While the repairs were being completed
on the rotor, the field service team was
working at the customer’s site to repair
the diaphragm and improve the sealing of
the casing. This work would be influential
in improving the efficiency of the steam
turbine.
Prior to the project being started by Sulzer,
the turbine required 393 tonnes of steam
per hour to produce the 53.4 MW of energy.
Despite one set of disks being removed, the
repair to the static and rotor components of
the turbine delivered by Sulzer, enabled it
to maintain an output of 55.1 MW but using
only 374 tonnes per hour of steam, which is
an 8% improvement in efficiency.
When the repaired turbine rotor arrived
back on site, the field service team carried
out the installation and commissioning,
which included vibration testing at full
load. All the results were well within the
original specifications and the generator has
remained at full capacity ever since.
Andrianto concludes: “The customer was
very impressed with the results of this
project. In total, the whole repair took only
16 weeks, which is considerably less than
the estimated lead time for a new rotor
from the OEM, which is closer to 52 weeks.”
www.sulzer.com
Issue 37 PECM
111