Chemical Engineering February 2016 - 46

p
p
Total loss
Total loss
h
p
Mass loss
Upward flow
Figure 8. Flow versus pressure
drop in upward- and
downward-flowing tubes
Gravity loss
Friction loss
Gravity loss
m1
Ambiguous region
-p
Downward flow
is located at the turning section. Hence,
prediction of its performance and tube wall
temperature is less accurate compared to
that of the convective design. when buying
new boilers, plant engineers should
give preference to designs with convective
superheaters and be aware of the likely
problems with a radiant design. At partial
loads, the steam flow inside the tubes
also will have a non-uniformity due to low
steam pressure drop and this can also add
to the increase in tube wall temperature in
certain tubes.
4.Effect of adding FGr: As far as the steam
temperature is concerned, the variation
in steam temperature is much larger between
100 and 50% loads with a radiant
superheater compared to the convective
design, where the screen section dampens
the large variations in fluegas flow
and temperature.
Figure 9. The problem of tube
failures discussed in the text
was solved with baffles and a
parallel flow design
+
Failure analysis
The author was asked to investigate frequent
tube-failure problems in an invertedloop
superheater with cross-flow design as
shown in Figure 6A. This boiler could neither
be operated near 90-100% load nor
at lower than 40% load, as tube failures
would occur. This is a poor superheater
design to start with and the author would
have advised the plant against purchasing
this boiler and to go for a design with a convective
superheater. There were a few major
Superheated steam out
p
m1
m2
Mass loss
Friction loss
concerns with this design:
* Superheater location is at the turn around
section at the furnace exit with uncertain
gas flow pattern and non-uniformity in fluegas
flow and gas temperatures
* It is a single-pass cross-flow design with
low steam-mass velocity inside the tubes
and hence low steam-side pressure drop
* The superheater is facing the furnace and
hence has direct radiation from the furnace
in addition to convective and non-luminous
heat fluxes
* low steam-side pressure drop impacts
the downward flow of steam in long tubes,
as explained below
One of the problems with tall invertedloop
superheaters is the effect of steamside
pressure drop at low loads as shown
in Figure 8 when steam flow is in the vertical
downward direction.
when steam flows upward, both the
gravity loss and friction loss are additive
and hence their sum is always positive and
the total pressure loss versus flow curve is
monotonic [2].
In the downward flow section, at low
Gas flow
+
+
+
Mud drum
46
Saturated steam in
present design
Saturated steam in Saturated steam out
Suggested modification
Upper
header
Lower
header
Gas flow
loads, the enthalpy pick up will be higher
in a radiant superheater compared to a
convective one (due to the contribution of
furnace radiation) and the density of steam
decreases and hence lower gravity loss.
As steam flow increases, the specific volume
reaches a steady value and the steam
density also will be higher while friction loss
increases as the square of flow. The total
loss curve will have two operating points as
shown at low loads while at higher loads
the total loss curve is monotonic and hence
stagnation is not a concern. This trend is
exhibited with a convective superheater
also, but due to lower gas temperatures
at lower loads, the tubes are much safer.
Hence superheaters of inverted-loop design
should be designed with a reasonably
high steam-side pressure drop, which ensures
gravity loss is not low enough at low
loads to cause flow stagnation or ambiguity
in flow. A " safe " pressure drop is arrived
at in the design stage at 100% load based
on steam pressure, height of superheater
tubes and steam pressure. One has to also
perform partial-load calculations and ensure
flow stagnation will not be an issue. unfortunately,
many boiler designers do not understand
this, and design superheaters with
a low steam-side pressure drop resulting in
tube failures at low loads.
Solution. nothing much could be done
about the location of the superheater. It was
determined through calculations that the
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