Hydrocarbon Processing - November 2021 - 71
Water Management
K. KRAETSCH, ChemTreat, Inc., Chesterfield,
Virginia; and B. BUECKER, ChemTreat, Inc.,
Lawrence, Kansas
Advanced methods for controlling boiler tube
corrosion and fouling-Part 2
In Part 1 of this article, the authors covered techniques for
minimizing corrosion and contaminant ingress from condensate
return and feedwater to industrial steam generators. While some
corrosion mechanisms can cause severe damage to condensate
and feedwater piping, heat exchangers and other equipment, the
release of corrosion products or the transport of other impurities
to the boilers can also cause major problems. This article examines
internal boiler treatment programs, which include several
critical functions, including:
* pH control within a mildly alkaline range
to minimize general corrosion
* Reaction with impurities to keep contaminants in
suspension and minimize deposition on boiler internals.
As suggested in Part 1, even well-treated boilers may have
chemistry underneath existing deposits that varies greatly from
the bulk boiler water, and severe corrosion under these deposits
may lead to tube failures and unit outages. This article includes a
direct example of this scenario.
Internal boiler water treatment. As power generation units
increased in number and size in the 1930s, tri-sodium phosphate
(Na3
PO4, or TSP) became a popular water conditioning chemical
for drum boilers. At that time, phosphate treatment served
two primary functions. The first was to establish moderately alkaline
conditions in the boiler to minimize general corrosion of
carbon steel boiler tubes, drums and headers (Eq. 1):
Na3PO4
+ H2O } Na2
HPO4
+ NaOH
(1)
This function is still critical today.
The second function of phosphate was-and in some cases,
still is, for industrial boilers-control of hardness ingress.
Phosphate will react with hardness to form soft sludges that
may be blown down, as opposed to the hard scale shown in
FIG. 11 of Part 1. However, as high-pressure units evolved in
the last century, some boilers were plagued by under-deposit
caustic corrosion generated by the rather high concentrations
of TSP needed for scale control. This led to the development
of coordinated and congruent phosphate treatment programs
that were often a blend of tri- and di-sodium phosphate (DSP),
but sometimes included a small amount of monosodium phosphate
(MSP). DSP and MSP will shift Eq. 1 to the left and reduce
the NaOH concentration.
The development of congruent treatment was also influenced
by the discovery that sodium phosphates become reversely
soluble at temperatures above approximately 250°F (FIG. 13).
In high-pressure boilers, most of the phosphate added for
pH and, if necessary, hardness control may precipitate on boiler
internals. This precipitation, known as " hideout, " is accentuated
by boiler tube deposits. Congruent treatment was developed
to maintain similar sodium-to-phosphate ratios between
the chemical remaining in solution with that in precipitate, although
this often does not occur.
Even so, phosphate treatment remains a strong choice for industrial
boilers, particularly because the potential for hardness
ingress to many industrial units is much greater than for utility
units. TSP is now the only phosphate species recommended for
power boilers and is applied at low concentrations to minimize
hideout. Industrial boiler treatment should be evaluated on a
case-by-case basis. If heavy deposits are present, then chemistry
control may " run on the razor's edge, " so to speak. If only TSP is
used, then excess hydroxide could potentially concentrate under
the deposits and generate caustic attack. Conversely, under
a congruent program, acid phosphate compounds might form
and attack the tube metal.
As the following example illustrates, significant improve60
70
80
90
100
50
40
10
20
30
100
200
300
Temperature,
°F
FIG. 13. Solubility of tri-sodium phosphate as a function of temperature.
Hydrocarbon Processing | NOVEMBER 2021 71
400
500
600
Sodium phosphate, g Na3PO4
in 100 g of water
Hydrocarbon Processing - November 2021
Table of Contents for the Digital Edition of Hydrocarbon Processing - November 2021
Contents
Hydrocarbon Processing - November 2021 - Intro
Hydrocarbon Processing - November 2021 - Cover1
Hydrocarbon Processing - November 2021 - Cover2
Hydrocarbon Processing - November 2021 - Contents
Hydrocarbon Processing - November 2021 - 4
Hydrocarbon Processing - November 2021 - 5
Hydrocarbon Processing - November 2021 - 6
Hydrocarbon Processing - November 2021 - 7
Hydrocarbon Processing - November 2021 - 8
Hydrocarbon Processing - November 2021 - 9
Hydrocarbon Processing - November 2021 - 10
Hydrocarbon Processing - November 2021 - 11
Hydrocarbon Processing - November 2021 - 12
Hydrocarbon Processing - November 2021 - 13
Hydrocarbon Processing - November 2021 - 14
Hydrocarbon Processing - November 2021 - 15
Hydrocarbon Processing - November 2021 - 16
Hydrocarbon Processing - November 2021 - 17
Hydrocarbon Processing - November 2021 - 18
Hydrocarbon Processing - November 2021 - 19
Hydrocarbon Processing - November 2021 - 20
Hydrocarbon Processing - November 2021 - 21
Hydrocarbon Processing - November 2021 - 22
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Hydrocarbon Processing - November 2021 - 24
Hydrocarbon Processing - November 2021 - 25
Hydrocarbon Processing - November 2021 - 26
Hydrocarbon Processing - November 2021 - 27
Hydrocarbon Processing - November 2021 - 28
Hydrocarbon Processing - November 2021 - 29
Hydrocarbon Processing - November 2021 - 30
Hydrocarbon Processing - November 2021 - 31
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Hydrocarbon Processing - November 2021 - 42
Hydrocarbon Processing - November 2021 - 43
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Hydrocarbon Processing - November 2021 - 90
Hydrocarbon Processing - November 2021 - Cover3
Hydrocarbon Processing - November 2021 - Cover4
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