Hoffman Engineering Saves $32,000 Annually by Reducing Water Use
Process Background
Hoffman Engineering manufactures metal and composite enclosures that house electrical and electronic controls, instruments and components. They employ 1,700 people.
Hoffman Engineering worked with a MnTAP intern to reduce fresh water use by at least 3.5 million gallons a year, saving $32,000 annually.
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% to 7%.) Overall water use has increased. But, without the changes from the intern project, Hoffman believes that their current water use would be far higher.