POWER March 2021 - 32

GAS TURBINE
3. A typical piping scheme for a 2-2-1 combined
cycle gas turbine configuration with
HRSGs; high-, intermediate-, and low-pressure
(HP, IP, and LP) turbines; hot reheat (HRH); and
cold reheat (CRH) lines shown. Courtesy: Tata
Consulting Engineers Ltd.
cleaning will be accomplished by generating
steam velocity in the pipeline that
is greater than what typically occurs in
each steam line during normal operation.
In a 2-2-1 configuration, the typical
piping arrangement is such that individual
lead lines (HP, HRH, CRH and LP) from
one HRSG are tied to respective lead
lines from the other HRSG at a certain
distance to form a header line. The header
line is routed down to the steam turbine
inlet. The outlet piping from the HP
steam turbine is a header that is again
branched out to individual lead lines returning
to individual HRSGs. The header
lines are sized bigger when compared to
individual lead lines. Figure 3 indicates a
typical scheme of piping connection between
HRSG and ST.
The Challenge
In this project, the material selected for
the HP and HRH individual lines and
header, as well as for the steam bypass
lines to the condenser, was alloy steel.
During the design stage, the outlet steam
parameters for different operating conditions
at the HP steam turbine exhaust
were evaluated and carbon steel piping
was selected for the CRH line (header
and individual lines to each HRSG). The
decision to select carbon steel material
for the CRH lines led to challenges during
the SBO.
While blowing the headers, both the
gas turbines and their respective HRSGs
were operated to achieve the desired
CFR. However, the CFR in the individual
CRH and HRH lead lines could not be
achieved in this mode for two reasons.
First, the normal operating pressure in
the lead lines is lower during 1-1-1 operation,
which results in a higher specific
volume of steam, and subsequently,
maximizes momentum conditions (mo
× vo
2
). Second, operating both GTs at the
same time during steam blowing results
32
in higher steam flow through the piping.
This results in increased pressure drop
across the piping. As the pressure drop
is higher in the steam headers and temporary
piping, the steam blowing pressure
at the individual lead lines is also
expected to go up. This results in lower
specific volume of steam, and in turn,
leads to lower CFR values.
Therefore, to achieve the required
CFR in the individual CRH and HRH lead
lines, only one train was operated (one
GT and one HRSG). While increasing the
GT load to meet the CFR requirement, it
was observed that the main steam temperature
was rising close to 500C, which
is not acceptable for carbon steel piping
(CRH lines). The acceptable temperature
limit for carbon steel piping is 427C.
One option to bring the temperature
of the steam down to an acceptable level
was to add an external desuperheater,
upstream of the CRH line. This would
spray water in the steam prior to admitting
it into the CRH line to bring down the
temperature to the desired level. However,
a desuperheater was not anticipated
during the design stage. Considering
that the project was in the commissioning
phase, procurement of a desuperheater
would have delayed the process,
as this item was not readily available in
the local market and had to be procured
and shipped from a neighboring country.
Furthermore, piping erection was also
completed at site and adding a desuperheater
at that point would have been
complicated and resulted in a schedule
delay. Hence, this option was ruled out.
During the design stage, an intermittent
type of blowing was proposed to the STG
manufacturer. However, the STG manufacturer
later insisted on following the continuous
blowing methodology to meet its
requirements. Considering the shortage
of time available and to avoid any schedule
delay, the only option was to successfully
execute the continuous steam blowing operation
with the existing design.
The Solution
The root cause of the problem was hightemperature
GT exhaust gases that in
turn produced high-temperature steam
while trying to achieve the required CFR
values. This had to be brought to a permissible
level for the carbon steel CRH
piping. All the other critical lines identified
for steam blowing were alloy steel
piping, and hence, the higher temperature
was acceptable.
The GT is the main driver that increases
the temperature of the steam
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while increasing the load. The key requirement
to meet targeted CFR values
was increased mass flow of steam at
desired lower temperatures acceptable
to carbon steel piping. To increase the
mass flowrate of steam, the GT exhaust
gas mass flowrate must also be
increased, which is possible only when
the GT load is increased. Subsequently,
the GT exhaust gas temperature also
rises to undesirable levels.
If the exhaust gas temperature is controlled
from the GT side, then subsequently,
the temperature of the steam is
also brought down to acceptable levels.
Based on discussions with the GT manufacturer,
it was understood that the turbine
exhaust temperature (TET) could be
brought down by pre-setting the value in
the GT control system to the desired temperature
at rated GT exhaust flow condition.
This could temporarily be modified to
meet the steam blowing requirements,
and then be reset to the normal operating
conditions at a later stage. Achieving the
desired lower temperature without software
modification was not possible.
GT in Normal Operation Mode. The
gas turbine is operated in such a way that
the temperature remains constant up to
full-speed no-load (FSNL) conditions. After
synchronization and as GT load gradually
increases, the exhaust temperature
also rises and reaches rated temperature
(550C) between 50% and 60% of the GT
load. The inlet guide vanes (IGVs) are in
tracking mode and follow GT loading beyond
this point. The IGVs start to open
up gradually, thus increasing the mass
flowrate to rated conditions. The fuel
supply is also increased in parallel, such
that the exhaust temperature remains at
rated temperature until the gas turbine
reaches full load.
GT in Steam Blowing Mode. In this
mode, the TET is pre-set to the required
temperature by modifying the GT control
software. The gas turbine operation is initiated.
There is no change in the operation
until the GT reaches FSNL. After synchronization,
the exhaust temperature is controlled
to the pre-set temperature as the
GT continues to load further. The exhaust
mass flowrate continues to increase, while
the modified TET continues to remain constant
until the rated load is achieved.
Figure 4 illustrates the normal and
modified GT exhaust gas temperature
profile for both of the above described
modes. The variation of GT exhaust gas
temperature and mass flowrate for varying
loads during Steam Blowing Mode is
indicated in Table 1.
POWER | March 2021
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POWER March 2021

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