Chemical Engineering June 2018 - 68

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HRSG performance
calculations
Often, heat-recovery steam generators
(HRSGs) are designed for a
particular set of gas inlet conditions.
However, once installed, due to various
plant limitations, such as gas
turbine load, steam or feed water
parameters, the HRSG may not operate
at the designed gas-inlet conditions.
Then how does one know
that the HRSG will perform as guaranteed
once the appropriate gas
inlet parameters are provided? This
example shows what can be done.
HRSG performance calculation
Example 1: An unfired HRSG is designed
with the parameters shown in
Tables 4 and 5, and the supplier has
guaranteed performance as shown
at 500,000 lb/h exhaust gas flow
and at 1,100°F inlet gas temperature
with a steam flow of 81,000 lb/h of
600 psig steam at 892°F. In operation
however, the HRSG generates
only 55,600 lb/h steam at 500 psig
at 863°F. Inlet and exit gas temperatures
are 1,000°F and 350°F, as
shown in Table 6. Plant engineers
want to know if this performance is
acceptable. The boiler supplier says
that, due to inadequate exhaust gas
flow and temperature, the HRSG is
making less steam than guaranteed,
and that the vendor's design is acceptable.
Is the vendor correct?
Solution: Tables 4, 5 and 6 show
the information given by the HRSG
supplier and the field data. This is a
problem commonly faced by plant
engineers. The newly purchased
HRSG operates at some gas/steam
parameters that are not close to the
design parameters. How can plant
engineers determine if the HRSG
was sized properly and if it will meet
guaranteed performance after a few
years, when gas inlet conditions for
the guarantee case are provided?
Suppliers' warranties typically last
for one year and any performance
problems must be corrected by the
vendor within that period. However,
now since the plant is unable to provide
the design gas inlet conditions
within a year, the plant engineers are
in a difficult position - that is, they
are not sure if the HRSG has sufficient
surface or not. However, using
the simplified approach, the plant
engineers can get a good estimate
of how the HRSG will perform in the
guarantee case or whether it will
meet or not meet the guaranteed
steam production.
Let us first establish the complete
field data. It will help if we have accurately
measured data, such as
the water temperature entering and
leaving the economizer, the drum
temperature, the gas temperature at
inlet and exit of the HRSG, the water
and steam flows and steam temperatures.
Using these data, it is possible
for a process engineer to get
a good idea of what the HRSG will
do when the exhaust gas inlet is at
" design " (guarantee case) conditions
(namely 1,100°F and flow 500,000
lb/h). Presently, the gas flow is much
lower. Exhaust gas-flow measurement
is generally not accurate and
hence it is obtained by energy balance
based on energy absorbed by
steam.
The data shown in Table 6 from
the field should be used to generate
the complete HRSG gas/steam
temperature profiles. Since exhaust
gas, steam and water temperatures
and water flow are accurately meaFig
6: Expected performance based on simulat
Fig 6: Expected performance based on simulation Fig 7: Arrangement of the fluid heater
FIGURE 7. This diagram shows the expected boiler performance, based on the simulation
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ENGINEERING WWW.CHEMENGONLINE.COM
JUNE 2018
FIGURE 8. This diagram shows the arrangement of the fluid heater in the steam
system
67
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Chemical Engineering June 2018

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