POWER March 2015 - 43

COMBINED CYCLE GAS TURBINES
Table 1. HRSG design tube temperature comparison. Source: Tetra Engineering
Tube location Design tube temperature (F)
HPSH4
RH31
RH32
HPSH3
RH2
HPSH21
HPSH22
RH1
HPSH1
1,125
1,125
1,125
1,065
1,061
1,125
1,125
1,100
935
and 1,210F-1,215F.
Recently, a large GTCC plant in the U.S.
implemented an extended turndown with
a GT performance upgrade. The increased
turbine exhaust temperature was around
1,208F-1,215F at about a 50% output level.
Problems were observed with the existing
desuperheater spray valves, which prevented
raising spray flows, so steam temperatures
rose from 1,048F to 1,058F-1,060F. This
raised owner concerns about exceeding design
tube metal temperatures in the superheaters
and reheaters.
The maximum tube temperatures set for
ASME Boiler and Pressure Vessel Code
calculation is the design midwall (average)
tube temperature allowed. The design allowance
for spread in tubes temperatures is
typically around 25F. Thus, the average tube
temperatures should be 25F below the design
temperature. The tube temperatures at the actual
operating conditions were checked at the
higher steam temperature for acceptability, as
shown in Table 1. The values were acceptable
but close to limits. Operation was not feasible
until the desuperheater spray valves were
modified to allow greater spray amounts.
Keeping Steam Cool Enough for Making
Power. With lower steam flows and
higher GT exhaust temperatures, the final
steam temperatures from main steam and
hot reheat can be more difficult to keep at required
values. Often at low loads, the steam
turbine will also have reductions in allowed
steam inlet temperatures.
Almost all modern drum type HRSGs regulate
final steam temperatures (main steam
and hot reheat steam) with interstage spray
attemperators. These are typically located
between the primary and secondary stages of
superheaters (SH) and reheaters (RH). This
arrangement avoids risks of water intrusion
to the steam turbine and allows some control
of tube metal temperatures in the final stages
of superheat and reheat.
Desuperheaters usually have a minimum
March 2015 | POWER
Max. operating tube
temperature (F)
1,100
1,100
1,100
1,040
1,036
1,100
1,100
1,075
910
Notes: HPSH = high-pressure superheater, RH = reheater, TEG = turbine exhaust gas.
steam velocity and upstream enthalpy requirements
set by the OEM to ensure good
droplet evaporation. The area of the SH/RH
surface is fixed, and at low flows the effectiveness
() of the surface is much higher than
at higher flows. Effectiveness is defined as:
 =
Ch(th,in - th,out)
Cmin(th,out - tc,in)
The values Ch
and Cmin are the heat capacity
rates of the hot fluid (gas) and the minimum
(steam) rate as mass flow x heat capacity.
As flow is reduced (steam side), Cmin
is reduced,
increasing effectiveness because the
outlet steam is more easily heated to the gas
temperature range.
The derivative of Tc,out
to Tc, in is simply
(1- ).
At low flows (<50%) the change in outlet
temperature for a given inlet temperature
change is only 40% or so of its value at full
flow. Large changes in inlet temperature after
desupereaters are required for even small
reductions in final steam temperature. This
high spray water to steam flow ratio can lead
to incomplete evaporation and liquid water
accumulation on pipe and header walls.
Improved sprays and spray controls can
allow additional spray capacity without violating
limits on approach to saturation temperature,
but they still cannot fully compensate
for reduced steam flow in some units. The addition
of terminal attemperation sprays in the
outlet steam lines is possible, but the installation
should be in compliance with ASME
TDP-1 (Prevention of Water Damage to Steam
Turbines Used for Electric Power Generation:
Fossil-Fuel Plants).
Adding Steam Attemperation. Some
newer HRSGs have steam attemperation to
help control final steam temperature. Typically,
some amount of colder steam is taken
from the saturated steam outlet of the steam
drum (for main steam) or the cold reheat piping
(for hot reheat steam). This colder steam is
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Tube temp. at 50%
load; TEG 1,214F
1,095
1,098
1,090
951
965
All well below design
All well below design
All well below design
All well below design
then piped to the steam outlet to cool the steam
flow to the turbine. With no liquid water, the
risk of thermal shock damage to the piping or
steam turbine is eliminated. However, using
this bypass steam reduces the steam flow to
the superheater and reheater sections in the
HRSG. This can result in higher tube metal
temperatures due to inadequate cooling.
A newer HRSG has been equipped with
steam attemperation instead of interstage
desuperheaters in the reheat steam. At extended
turndown, the steam attemperation
was successful in maintaining final RH temperatures,
but because the system reduced
steam flow in the RH tube panels, local steam
temperature limits were exceeded. These
were set to prevent overheating of the tubes
and headers in the RH system.
Managing Inlet Exhaust Gas Attemperation.
Cooling the inlet exhaust gas to
lower temperatures is another method of
controlling metal temperatures in the HRSG
at low loads. This cooling can be done by
water spray or ambient air fed into the hot
exhaust gas. In both cases, the actual process
of mixing with the highly turbulent, swirling
exhaust gas must be carefully designed
to achieve a uniform cooling and avoid damage
to the HRSG inlet duct or pressure parts.
Failure of air attemperation components can
result in consequential damage to pressure
parts-typically, the finish high-pressure
(HP) superheater or reheater tube panel-
immediately downstream.
Figure 1 shows a system where water is
sprayed into the inlet exhaust gas. It worked
well, but overspray can damage the liner
plates, as seen in Figure 2. At other plants
with water sprayed into the duct, repair of
spray nozzles has become a regular maintenance
issue.
Colder ambient air can be used to reduce
exhaust gas temperature. Figure 3 shows a
system to blow cold ambient air into the inlet
exhaust gas at a CC unit with a GE Frame
7FA gas turbine. The system works, but the
highly turbulent inlet duct flow can lead to
damage in the air inlets and consequential
damage to HRSG heat transfer surfaces, as
seen in Figure 4.
Keeping Gas Hot Enough. At the inlet
to the HRSG, the problem is exhaust gas that
is too hot, but as the exhaust travels through
the HRSG, it can be cooled to an excessively
low temperature. In many cases, additional
operational constraints are required.
For example, plants with NOx
control by selective
catalytic reduction systems (SCRs) will
have a specific temperature range for operation.
SCRs are usually located just after the HP evaporator
sections for this purpose. At low loads
in sliding pressure operation, the HP evaporator
pressures can be low enough that the low satu43
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POWER March 2015

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