POWER July 2017 - 40

ENVIRONMENTAL ISSUES
Table 2. Phosphorus-free test results. The water treatment program at a Midwest
refining plant was changed to a phosphorus-free chemistry with the results shown here.
Source: U.S. Water
Parameter
pH
Conductivity
" M " Alkalinity
Chloride
Calcium
Magnesium
Silica
Sulfate
Units
N/A
µmhos/cm
ppm as CaCO3
ppm
ppm as CaCO3
ppm as CaCO3
ppm as SiO2
ppm as SO4
to degradation by oxidizing biocides.
Hydroxyphosphonic acid has shown susceptibility
to chlorine, even at low levels.
This presents a problem because oxidizing
biocides are a necessary part of any successful
water treatment program for control
of potentially harmful bacteria, such as
Legionella. Organic inhibitors based on the
phosphorus molecule do not meet the discharge
requirement for P due to the reversion
of some of the organic phosphonate to
orthophosphate (O-PO4
) and eventually to
phosphorus. Unfortunately many alternative
programs have failed to properly control the
corrosion rates in systems without the use of
P-bearing inhibitors.
Table 1 illustrates a " green chemistry " alternative
evaluation conducted by U.S. Water
Services Inc. (U.S. Water), a company based
in St. Michael, Minn. The table contains information
related to the various alternatives
where the values are the subjective ratings determined
by the authors of " The Pursuit of a
Green Carbon Steel Corrosion Inhibitor: Part
1 of 4 " published by Modern Pumping Today
in January 2015 (4 is the best score and 1 is the
worst). Based on this information, U.S. Water
developed a unique cooling water treatment
technology, called PhosZero, a zero-P cooling
water chemistry.
Finding the Best Option
A Midwest refining plant reached out to U.S.
Water for help achieving its goal of reducing
P concentrations in its discharge. The plant
had two different cooling towers due to different
process conditions and both utilized
induced-draft counter-flow cooling towers.
Traditional water treatment programs were
being utilized in the treatment of the cooling
towers, but due to discharge regulations, the
client needed to find a viable chemical treatment
alternative. The new P-free program
40
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Tower 1
8.59
2,117
422
123
774
665
37.3
732
was implemented at the plant.
In order to ensure proper system protection,
the corrosion rates were carefully
monitored using corrosion coupons as well
as an online corrator. The client wanted to
maintain the same water efficiency, that
is, not reduce the cycles of concentration,
which yielded a Langelier Saturation Index
(LSI) value of approximately 2.25.
This LSI represents a relatively high scaling
potential and the heat transfer surfaces
were monitored closely as well. During
the course of the entire trial, there was no
observed accumulation of scale that would
impede the heat transfer efficiencies. Table
2 lists the pertinent cooling water values
during the trial.
Tower 2
8.58
2,299
446
315
570
500
36.8
678
After the zero-P program was implemented,
the corrosion rates improved drastically
within the first five hours of application.
The online corrator corrosion rates for the
mild steel (Figure 1) showed the significant
improvement during the
implementation
phase. The corrosion rate in mils per year
(mpy) was reduced from greater than 2.5
mpy to less than 0.5 mpy.
In this study, corrosion coupons were employed
in addition to the corrator to monitor
the cumulative corrosion rates during a 45to
60-day exposure period. The results are
shown in Figure 2.
The results were better than anticipated
and the primary objective of eliminating P
from the cooling water chemical treatment
program was attained. In addition, the zeroP
treatment program provided a significant
reduction in the corrosion rates. The corrosion
rates dropped from an average of about
4 mpy to an average of around 1.3 mpy.
Another Effective PhosZero
Implementation
In a second study, Minnesota Power's Arrowhead
High-Voltage Direct Current Terminal
was experiencing scale and white rust
buildup inside its cooling tower condenser
tubes, creating obstacles cooling the inverter.
If left untreated, the buildup could have
resulted in a costly asset replacement and
unscheduled downtime.
At the same time, the Minnesota Pollution
Control Agency (MPCA) was instituting
new phosphate discharge limits and
encouraging plant management to meet the
1. Reduced corrosion rates. The water treatment program at a Midwest refining plant
was changed to a phosphorus-free chemistry with an almost immediate improvement in the
mild steel corrosion rate. Source: U.S. Water
Time (hours)
POWER | July 2017
Mild steel corrosion rate (mils per year)
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POWER July 2017

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