| intern summary |
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Hoffman Engineering Saves $32,000 Annually
by Reducing Water Use
Reducing Water Use in Painting Pretreatment
| Company |
Hoffman Engineering
Company Anoka, Minnesota |
| Results |
Reduced fresh water
use by at least 3.5 million gallons a year, saving
$32,000 annually. |
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Process Background
Hoffman Engineering manufactures metal and composite enclosures
that house electrical and electronic controls, instruments
and components. They employ 1,700 people.
The intern project focused on the painting
pretreatment process for one of Hoffman's production lines.
The pretreatment process removes surface contaminants, then
applies a phosphate coating to metal surfaces which improves
powder paint adhesion and increases corrosion resistance.
Fabricated metal enclosures are conveyed through the washer’s
four spray stages: 1) chemical cleaning, 2) rinsing, 3) phosphate
conversion coating, and 4) final seal rinsing.
Incentives for Change
Although Hoffman has reduced its overall water use by 11 million
gallons since 1992, its water treatment costs have increased
30 percent.
At the start of the intern project, the
pretreatment system used 201,000 gallons of water per week,
more than any other process in the plant. Annually, the system
costs over $90,000 for water and $200,000 for chemicals. Water
use was erratic, varying weekly by over 100,000 gallons.
Hoffman set a goal to reduce water use by
50 percent. The intern looked at all of the spray stages except
stage 3 which used the least water and was the most critical
process step.
Options Implemented
Stage 1: Chemical Cleaning. The first stage of the
pretreatment process is alkaline cleaning which removes most
soils and oils. This stage used approximately 15 percent of
the system's fresh water.
The tank was continuously overflowed with
both water and chemicals. A mist spray rinse between stages
1 and 2 drained back to stage 1. It provided more water than
was lost to dragout and evaporation.
A shut-off valve was added to the
tank overflow pipe. During periodic wash line interruptions,
this valve prevents the tank's contents from overflowing when
the washer's riser pipes drain back into the tank. A pressure
gauge and control valve were installed on the freshwater
mist spray rinse after stage 1 to let operators see the flow
rate.
Stage 1 savings: Approximately $9,000
for almost a million gallons of water per year. Its share
of the system's water declined from 15 percent to less than
one percent.
Stage 2: Rinsing. The rinse stage
washes off residual soil and alkaline solution. This stage
uses approximately half of the fresh water entering the system.
It overflows continually with fresh water to keep the total
dissolved solids (TDS) below 1,000 parts per million.
Because the electrodes on the TDS probe
were in the water, they developed a film which caused inaccurate,
elevated readings. Until the electrodes were cleaned, extra
water would flow into the tanks in response to the elevated
readings. These probes also needed to be replaced every six
months.
An inductive conductivity meter ($2,100)
replaced an automated conductivity meter to control total
dissolved solids (TDS) within a narrower and higher concentration
range, which reduces overflow of usable water. Its probe is
not in contact with the water. This prevents false elevated
TDS readings and decreases cleaning maintenance.
Stage 2 savings: Will reduce water by
almost a million gallons per year, saving about $9,000 annually.
Requires less scheduled maintenance.
Stage 4: Final Seal-Rinse. The final
seal-rinse stops the phosphatizing reactions and flushes unwanted
residue from surfaces. The pH of part surfaces is adjusted
to improve corrosion resistance. This stage uses approximately
30 percent of the fresh water entering the system, through
diluting the tank to meet the TDS level.
The tank was often over diluted, using excess
water and chemical. Because TDS was manually measured and
controlled, the freshwater feed was frequently left open to
ensure low TDS. It was also occasionally left open after the
line was shutdown for the day.
The automated conductivity meter
from stage 2 and a control valve were installed to adjust
the TDS level. A new solenoid prevents the unintended open
valve.
Stage 4 potential savings: $10,000 for 1.1 million gallons of water a year.
The intern also adjusted a valve
to minimize overflow from stage 4 when diluting the tank with
fresh water. The overflow was directed to the stage 2 mist
arc for reuse. This water has a low pH and helps neutralize
the higher pH rinse water that is carried over from stage
1.
Savings: Included in stage 2 savings.
System Improvements.
Six-foot drain zones between the
stages reduced carryover and overspray but still inadequately
isolated the stages. Drain zones were modified to return more
solution to each respective preceding stage.
Savings: 400,000 gallons of water per
year, saving $3,800 annually.
Flow meters were installed on all
of the washer's stages. These meters accurately show the flow
patterns within the system, making it easier to better control
the system and address problems.
Chemical use is expected to decrease
as overflow from the stages is controlled. Less chemicals
will be needed to maintain each stage's chemistry.
Additional Options
Tank Cleaning. Set procedures for contract personnel
who descale the pretreatment system weekly. State that they
should shut off water and chemical feeds when emptying tanks.
Prerinse Riser. Install a prerinse
step before stage 1 to reuse overflow from stage 2. A prerinse
would remove the heaviest soils, reducing the load on the
rest of the system.
TDS Set Points. Determine if TDS
settings are lower than needed for adhering paint adequately.
If so, raise set points. If not, improve incoming water quality.
Results
The overall effect of the intern project is difficult to quantify
because of changes to the operation. But, individual changes
to better control water use in the paint pretreatment process
appear successful. For brief periods in fall 1996 and winter
1997 when operations were most similar to those during the
project, water use declined 21 percent. This translates into
a 3.5 million gallon per year reduction, saving Hoffman $32,000
annually. Additional savings from decreased chemical and water
treatment costs are expected but were not quantified.
A number of factors make before-and-after
comparisons difficult. Production volume and part sizes have
increased significantly. Also, the city changed wells and
incoming water now has higher TDS. (The differential between
incoming TDS and the control set point decreased from 50 percent
to 7 percent.) Overall water use has increased. But, without
the changes from the intern project, Hoffman believes that
their current water use would be far higher.
More Information
MnTAP has a variety of technical assistance services available
to help Minnesota companies manage and reduce their industrial
waste. If you would like assistance or more information about
MnTAP's Intern Program, call 612.624.1300 or 800.247.0015
from greater Minnesota.
This project was conducted in 1996 by MnTAP
intern Mike Hoit, a physics senior at the University of St.
Thomas.
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