POWER March 2021 - 33

GAS TURBINE
The key was to maintain the GT exhaust
temperature at a pre-set desired
level and continue increasing the mass
flowrate so that the mass flowrate of
steam generated will also increase. This
will ensure that the desired steam flowrate
at the lower temperature is generated,
and hence, CFR requirements for
the HRSG lead lines are met.
Any risk on operating the gas turbine
under Steam Blowing Mode was also
evaluated. Considering that it is a heavyter
completing the operation.
The procedure adopted in this project is
applicable for the continuous steam blowing
methodology. The decision to select
the method of steam blowing depends on
the ST manufacturer's recommendation.
The above technique of executing the
steam blowing operation is not the conventional
method practiced in the power
industry. This method is the result of
addressing a unique challenge encountered
in this project.
While the situation encountered in this
project was challenging, the methodology
adopted to resolve it has opened the door
for cost avoidance in future projects.
duty gas turbine, it was understood from
the OEM that it could be operated in this
mode temporarily for a limited period.
Prior to implementing the above modifications
in the GT control systems, the
proposed revisions in the software underwent
various tests (using the simulator)
to understand any other issues that
may come up while operating the GT
under this mode. It was observed that
there were no errors or alarms generated
during the simulations. The next step
was to implement this modification in
the GT control systems.
The Result
After modifications, two options were
available in the GT control system when
the GT reaches FSNL conditions. They
are: GT in Normal Operation Mode and
GT operation in Steam Blowing Mode.
Steam blowing was successfully completed
using the modified approach, with
the GT operating under Steam Blowing
Mode. The steam blowing mode in the
GT control system was made inactive afWhile
the situation encountered in this
project was challenging, the methodology
adopted to resolve it has opened
the door for cost avoidance in future
projects. If the above methodology is
planned and executed from the project
initiation stage, there is a good chance
for the EPC contractors to save money
on the piping material for the CRH line
and other temporary piping. It is understood
that this methodology of steam
blowing is more appropriate in combined
cycle projects where E-class and advanced
E-class GTs are selected for the
topping cycle.
However, engineers must carefully
operating
review different
conditions
of the steam turbine and analyze the
steam temperatures at the turbine exhaust
before deciding to implement this
methodology. If the steam temperature
at the HP turbine exhaust for different
operating conditions is ≤400C, then it is
possible to select carbon steel piping for
the CRH line and other temporary piping.
Further, the requirements of operating
GT load
(%)
100
50
40
30
20
10
Fullspeed
no-load
343
1,235
278
Table 1. Variation of GT exhaust gas temperature
and mass flowrate in Steam Blowing
Mode. Source: Tata Consulting Engineers Ltd.
the GT in Steam Blowing Mode need to
be clearly stated to the GT manufacturer
in the initial stage of the project and the
feasibility of the same needs to be evaluated
jointly. In a scenario where the proposed
modification is not agreeable to
the GT manufacturer, the following two
methods can be explored:
■ Intermittent type of blowing (in consultation
with the STG manufacturer).
■ Adding an external desuperheater in
the temporary piping.
Intermittent
blowing
increases
the
commissioning duration by several
weeks. In case of selecting an external
desuperheater, the main challenge would
be to address the limited space below the
STG deck for installation of temporary piping
along with the desuperheater. Also,
this option would increase the commissioning
cost due to the addition of the desuperheater,
control valves, instruments,
cabling, spray water piping, and alloy steel
piping upstream of the desuperheater.
To understand the scope of savings,
a quick calculation was made to compare
different materials for piping. In
this project, it was observed that if alloy
steel was selected instead of carbon
steel for the CRH piping (excluding other
temporary piping used for SBO), the additional
cost to be incurred would have
been about $250,000. Based on the experience
gained in this project, the new
option can be explored in future projects
for implementation, and thereby, potential
savings may be realized and/or longer
4. This graph shows the gas turbine (GT) exhaust gas temperature profile. Load, turbine exhaust
temperature (TET), modified TET, and exhaust mass flowrate (Mexh) are indicated in percent
on the y axis. Time after synchronization is shown on the x axis in seconds. Courtesy: Tata
Consulting Engineers Ltd.
March 2021 | POWER
www.powermag.com
commissioning periods avoided. ■
-Rajarajan Rathina is deputy general
manager and Senthil Rajendran is senior
manager for Tata Consulting Engineers
Ltd.'s Power Business Unit.
33
Mexh
(kilograms/
sec)
Mexh
(tons/hr)
491 1,768
369 1,328
344 1,238
344 1,238
343 1,235
343 1,235
Texh
(C)
412
412
409
377
344
311

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POWER March 2021

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