POWER June 2013 - 32

HEAT RECOVERY STEAM GENERATORS
tubes, into the riser header, through the risers,
and into the steam drum and separators. Each
individual riser and feeder is modeled.
The Chisholm correlation is used to determine
water/steam slip. The modified Martinelli-Nelson
correlation is used to determine
two-phase pressure drop. The thermal inertia
of the tubes and fins is simulated by specifying
the actual tube inside and outside radii
and multiplying the metal density by the ratio
of the finned tube weight (tube + fins) to the
bare tube weight.
Heat transfer coefficients in the riser are
calculated as the greater of the natural convection
and forced convection (liquid) coefficients.
The forced convection heat transfer
coefficients are calculated from the DittusBoelter
correlation for fully developed turbulent
pipe flow. The natural convection heat
transfer correlation is calculated from the turbulent
vertical wall correlation.
Drum and Downcomer Analysis. A
one-dimensional model was constructed of
the drum using the in-house Foster Wheeler
computer code CONDRAD. Heat transfer coefficients
in the lower drum (below the water
level) are calculated from the turbulent horizontal
wall correlation (cold plate facing up).
The heat transfer coefficient in the upper drum
(above the water level) is calculated from the
laminar film condensation correlation.
Boundary Conditions. The evaporator
flue gas inlet temperature and flow rate and
drum pressure are obtained as functions of
time from the HRSG system model. Feedwater
flow rate is added to maintain the drum
water level near the normal water level by the
model controller.
A cold startup with the 300F/hr drum water
temperature rise was one of the scenarios
analyzed. Figure 4 presents the riser outlet
quality versus time for some of the eight ris4.
Riser outlet quality. The steam quality in different riser outlets is illustrated during a
cold plant startup using a 300F/hr drum water temperature rise. Saturated steam has a quality
of 1.0. Source: Foster Wheeler North America Corp.
Cold
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
2,000
4,000
6,000
8,000
Time (sec)
5. Efficient heat transfer. The internal heat transfer coefficients for the risers and the
drum during the same startup scenario described in Figure 4 are shown. Source: Foster Wheeler
North America Corp.
Riser 1 (cold)
4,500
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
32
2,000
4,000
6,000
8,000
Time (sec)
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10,000
12,000
14,000
Riser 3
Riser 5
Riser 7
Upper drum
Lower drum
10,000
12,000
14,000
4th
6th
Hot
ers for this scenario. The first riser is on the
cold end, while the eighth one is on the hot
end. Figure 5 presents internal heat transfer
coefficients for the risers and the drum.
Cycle Life Calculations. The HRSG is
subjected to cyclic load changes, especially
during accelerated startups. The temperature
gradient that exists between the inside diameter
intersections of the drum riser and the
drum outside diameter causes thermal stresses
in the component and combines with pressure
stresses (the steam drum is at operating
pressure) to cause fatigue damage. The maximum
Von Mises stresses of the combined
(temperature and pressure) load case were
used to determine the fatigue life for each
case, as well as the fatigue damage per cycle.
Total stresses, using the combined thermal
and pressure loads, were used in the fatigue
calculations. The fatigue life evaluation is
based on the analysis methodology described
in ASME Boiler and Pressure Vessel Code,
Section VIII, Division 2, Appendix 5.
A preliminary screening analysis of the
headers and drums was made to determine
the most critical components of the HRSG
from the consideration of thermal cycle fatigue.
The components consisted of HP
headers and drums and their connection to
thin-walled tubing and piping. The riser connections
to the steam drums were found to be
the most highly stressed areas during a cold
startup transient.
Based on the results of the preliminary
screening analysis, the connection between
the 5-inch riser and the drum was identified as
the component and location most susceptible
to thermal cycle fatigue. The thermal transient
models were run to determine the temperature
distribution in the drum-riser connection, up
to the end of the main startup ramp. The thermal
fatigue analysis that follows is focused on
this particular drum-riser connection.
Four startup transient scenarios were selected
for analysis of the drum-riser connection
using a general purpose finite element
program:
■ Cold startup at 200F/hr
■ Cold startup at 300F/hr
■ Warm startup at 200F/hr
■ Warm startup at 300F/hr
Each startup condition was simulated by
a transient thermal analysis, which was followed
by a stress analysis at the drum-riser
connection at the specific time during startup
when the maximum thermal stresses occurred.
The stress analysis included the combined effects
of thermal and pressure loadings.
The temperature and stress results obtained
by the finite element analysis were
then used to determine the fatigue life of the
POWER | June 2013
Heat transfer coefficient (Btu/hr ft2 F)
Steam quality

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POWER June 2013

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