Chemical Engineering February 2016 - 39

topics may not be available in every boiler
supplier's company. unfortunately, many
boiler sales engineers still pull out drawings
of a boiler sold 25-40 years ago (for similar
parameters) from their archives and offer the
dated design to unwitting plant engineers,
who buy them without raising queries on
thermal design and performance aspects,
and live with superheater failures or under
performance for the rest of their lives. I have
seen this during the last twelve years of my
international consulting experience.
This article briefly outlines the features of
a good oil- and gas-fired steam generator,
mainly of the d-type design, which is very
common in the industry. The features of a
good superheater design and thermal performance
aspects that should be looked into
are also discussed. The comments hold true
for the other boiler designs, too, namely the
A and O-type boilers.
Reasons for tube failures
There are several reasons for boiler superheater
tube failures, including the following:
1.use of radiant or semi-radiant superheater
design exposed to high heat flux from a
furnace with low steam-side velocity or
low steam-side pressure drop; cross-flow
or counter-flow design; oversizing, when
steam temperature is around 400-500ºC
2.Carry-over of solids from the drum due
to poorly designed drum internals or poor
feed-water or boiler-water quality can result
in deposition of solids inside superheater
tubes and consequent overheating.
Sometimes the drum size is smaller
than it should be to prevent carryover of
moisture. load fluctuations leading to
large fluctuations in steam pressure and
drum level also cause carry-over of solids
from drum
3.Burners not tuned properly can lead to
flame impingement on the superheater. If
several burners are used, certain combinations
of burners may result in non-uniform
gas flow or temperature distribution at the
superheater inlet, leading to overheating of
some tubes
4.mechanical issues, such as thermal
stresses, creep, stress corrosion, erosion
(due to particulate matter in fluegas) and
compromised tube metallurgy
As mentioned earlier, a large percentage
of plant engineers, including the management,
take it for granted when they buy
a boiler that the boiler design itself is fine
and the superheater tube failures are only
due to operational problems. The following
sections highlight the thermal design
aspects with which plant engineers should
be familiar to avoid selecting a boiler with
dated design features or with poorly designed
superheaters.
Three decades of improvements
Plant engineers should be familiar with some
of the developments that have taken place
during the last 30 to 40 years that have
added value to steam generators in terms
of thermal performance, operating costs and
superheater life. If the boiler they are likely to
buy does not have any of the features listed
below, they can question the boiler supplier
or opt for better boiler designs available in
the market place.
Completely water-cooled furnace. The oiland
gas-fired steam generators designed 30
to 50 years ago did not have to deal with the
problem of emissions regulations, particularly
those due to oxides of nitrogen (nOx)
and carbon monoxide. The only concern
was efficiency, and oil- and gas-fired boilers
were operating at 5 to 10% excess air.
Also, not much time was spent by many
package-boiler companies in understanding
issues such as dnB (departure from
nucleate boiling), furnace effectiveness,
furnace heat flux and circulation aspects.
To protect the regions prone to high heat
flux in the furnace and possible departure
from nucleate boiling, they simply poured
refractory over the areas such as the furnace
floor, boiler front wall and in some
regions on the partition wall. Some older
designs still have a refractory-lined front
wall, which makes it difficult to ensure a
leak-proof furnace. Fluegas can leak to the
atmosphere between the membrane side
walls and refractory joints and if it contains
vapors of sulfuric acid, this can condense
on the casing to form sulfuric acid resulting
in casing corrosion.
not having a completely water-cooled furnace
also results in underutilization of the
furnace heating surface by 10 to 20% and
an increase in furnace exit-gas temperature
(FEGT). The higher the FEGT, the higher the
direct radiation to the heating surface located
at the furnace exit, namely the radiant
superheater. Pouring refractory on furnace
water-cooled surfaces is like buying a longsleeved
shirt in a department store and then
going to a tailor and paying him additional
money to make it a short-sleeve shirt. you
are not only adding to the cost of the boiler,
but also wasting a lot of labor and time on
annual maintenance of refractory. refracChemiCal
engineering www.Chemengonline.Com february 2016
39
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Chemical Engineering February 2016

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