Chemical Engineering March 2023 - 33

DRUM OPERATING
PRESSURE
D.O. (mg/L) as measured
before oxygen
scavenger addition
Total iron (mg/L as Fe)
Total copper (mg/L
as Cu)
Total hardness (mg/L
as CaCO3)
pH @ 25°C
Nonvolatile TOC
(including oily matter)
(mg/L as C)
Silica (mg/L as SiO2)
Hydroxide alkalinity
(mg/L as CaCO3)
Max total alkalinity
(mg/L as CaCO3)
Max suggested specific
conductance
(µS/cm @ 25°C) to
comply with steam
purity limits
TDS (maximum, mg/L)
Sodium (estimated
from TDS, µg/L as Na)
Sodium (µg/L as Na)
<0.3
<100
SATURATED STEAM PURITY TARGET
<0.3
<0.3
<100
<100
<0.3
<100
0.1
<30
SUPERHEATED STEAM PURITY TARGET
<20 ppb (consult turbine supplier or qualified consultant for appropriate values)
Note: This is an extract from Table 1 of Ref. 1
but the compound can also concentrate
under deposits where the reaction
of the acid with iron generates
atomic hydrogen that penetrates into
the metal wall and reacts with carbon
atoms in the steel to generate
methane (CH4):
4H + Fe3C  3Fe + CH4
(7)
Gaseous methane formation
causes cracking, greatly weakening
the steel's strength. Hydrogen damage
is very troublesome because it
cannot be easily detected. The phenomenon
continues to plague many
steam generators around the globe.
While underdeposit corrosion
(UDC) can occur underneath a variety
of precipitates, porous iron oxide
deposits usually have the greatest
influence. This explains the low iron
limits shown in Table 1. Iron particulates
are generated by condensate
return and feedwater system corrosion,
sometimes by the carbonic
acid attack as outlined earlier, but
also by oxygen attack. This is a clear
example of how one form of corrosion
has potential manifestations for
other corrosion mechanisms.
The following example illustrates
the effect of organic impurities on
steam chemistry.
Author Buecker
once evaluated a fouling issue at an
organic-chemicals plant that produced
a primary product and four
derivatives thereof. Steam was generated
by several 550-psig package
boilers in which solids deposition in
the
superheaters
(and
associated
tube overheating) required tube
bundle replacements every 1.5 to
2 years. An extracted superheaterbundle
inspection revealed internal
deposits up to 1/4 in. in depth. The
inspection team also discovered
foam issuing from every boiler saturated-steam
sample line. The plant
had minimal analytical chemistry
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2023
0.1
<30
0.1
<30
0.1
<30
≤84
≤72
BOILER WATER
≤40
≤30
≤17
Book has a special note for these parameters
<210
≤1,600
<180
≤1,400
<150
≤1,150
<120
≤920
≤230
≤5.9
<10
≤1.7
<2
Book has special note
≤190
≤150
≤80
≤0.6
<2
psig
TABLE 1. INDUSTRIAL BOILER WATER CHEMISTRY GUIDELINES*
0-300
301-450 451-600 601-750 751-900
FEEDWATER
<0.007
≤0.10
≤0.05
<0.5
8.810.5
<1
<0.007
≤0.05
≤0.025
≤0.3
<0.007
≤0.03
≤0.02
≤0.2
<0.007
≤0.025
≤0.02
≤0.2
<0.007
≤0.02
≤0.015
≤0.1
8.8-10.5 8.8-10.5 8.8-10.0 8.8-10.0
<1
<0.5
<0.5
<0.5
<0.007
≤0.02
≤0.01
≤0.05
8.8-9.6
<0.2
<0.007
≤0.01
≤0.01
≤0.01
8.8-9.6
<0.2
<0.007
≤0.01
≤0.01
≤0.01
8.8-9.6
<0.2
9011000
1001-1500
1501-2000
capabilities with no
trained water chemists
on staff, but a
condensate return
data review provided
by the plant's
water treatment
vendor showed
total organic carbon
(TOC) concentrations
as high as
200 ppm. Compare
this to the recommended
upper
limit of 0.5 ppm, as
shown in Table 1.
The excessive organic
contamination
in the boilers generated
the foam that
carried over to the
superheaters.
Beyond a " water
is water " philosophy,
some plant
managers are reluctant
to install condensate
polishers
because of cost and
the extra personnel
needed to operate
the equipment.
However, such an
investment can pay
large dividends by
minimizing or preventing
boiler tube
failures and corresponding
equipment
shutdowns. Ion-exchange
polishing is a potential treatment
option for condensate that contains
inorganic ions, such as hardness,
chloride, sulfate and similar elements
and compounds. Temperature is an
ChemTreat
FIGURE 4. Shown here is carbonic acid corrosion
of a carbon steel condensate return line. Besides
direct attack, carbonic acid and other corrosion
mechanisms release iron oxide particulate matter
that can settle in boiler tubes to reduce heat
transfer and initiate underdeposit corrosion (UDC)
33
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Chemical Engineering March 2023

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