Metal Painting and Coating Operations

Table of Contents  Background  Regulatory Overview  Planning P2 Programs  Overview of P2  Surface Preparation
Alternatives to Solvent-Borne Coatings  Application Techniques  Curing Methods  Equipment Cleaning

Equipment Cleaning

When a painting process is completed, a color change is needed, or maintenance is required, the metal coater must clean the equipment. There are numerous P2 opportunities for reducing waste and air emissions in equipment cleaning operations.

General Description

All coating practices require some type of equipment cleaning. For spray painting, the most common coating operation, spray guns and accessories must be cleaned between color changes, when orifices clog and often at shift changes (IWRC, p. 15).

Pollution Problem

External equipment surfaces generally are cleaned by soaking, wiping or flushing with solvent. If equipment cleaning is done in an open container, a significant quantity of solvent is lost to evaporation. Internal parts and passageways as well as paint guns are commonly cleaned by flushing solvent through the gun and orifice. This practice also results in significant evaporation and loss of usable product (IWRC, p. 15).

P2 Options

A cost-effective method for reducing wastes is to eliminate unnecessary cleaning. For equipment that requires cleaning, making improvements in operating practices that minimize solvent use and reduce evaporation should be implemented wherever practical. Using a gun washer to clean spray guns is one example. Various solvent recovery and reuse technologies are also available. In addition, alternative cleaning solutions can be used. Each of these options is discussed below.

P2 Tips for Equipment Cleaning

  • Eliminate unnecessary cleaning
  • Improve current operating practices
  • Use a gun washer
  • Recover and reuse spent solvents
  • Use alternative nontoxic cleaning solutions

Scheduling Improvements

Implementing better operating practices and scheduling can significantly reduce waste generated from cleaning operations. The amount of waste generated is directly related to the number of times paint color or paint types are made. For this reason, scheduling improvements have perhaps the largest effect on the volume of waste produced from cleaning equipment. Making large batches of similarly produced items instead of small batches of custom items, increases the time between cleaning. Additionally, scheduling paint jobs so that they move from the lightest color to the darkest can also reduce the need to clean.

Eliminate Unnecessary Cleaning

When assessing the cleaning process, all the typical cleaning tasks should be reviewed to learn whether cleaning is necessary. While most coaters assume that spray guns, tips and lines must be cleaned for reuse, cleaning some low-cost items might not be advisable. Costs from cleaning solvent purchases, solvent waste disposal and solvent emissions could be higher than simply replacing the item being cleaned. However, the costs of proper disposal must be factored into any decision (MnTAP, p. 5).

Improve Current Operating Practices

A technical assistance provider should also help a client company review the ways in which cleaning solvents are handled. All solvents should be stored in covered containers when not in use. Leaving solvents in the open air creates unnecessary solvent waste and VOC emissions. In addition, the company should set a standard for the minimum strength necessary for cleaning in order to ensure that used solvent is disposed of or recycled only when it loses its cleaning effectiveness, not just because it looks dirty (MnTAP, p. 5).

CASE STUDY:

Crenlo, Inc. - Spray Nozzle Selection Reduces Solvent Waste Volume

Crenlo, Inc. manufactures products from steel and aluminum. Finished products are coated with baked enamel paint. Most colors are prepared onsite. Paint from any batch may be stored for future reuse. To ensure proper quality, the paint is remixed and strained to remove solids larger than the 90 mesh screen size before delivery to spray booths.

In the past, the straining equipment was cleaned using fresh solvent sprayed from a hose fitted with a nozzle spraying a flat fan of solvent from a 0.172 inch diameter orifice. This nozzle is rated for 4.3 gallons per minute (gpm) flow with a 30 pounds per square inch (psi) supply pressure. Annual cleaning of the straining equipment produced about 14,000 gallons of waste costing at least $16,000 per year. The cleanup solvent is a recycled blend that is distilled offsite and returned to Crenlo. A single charge covers both purchase and processing costs. The 4.3 gpm nozzle was originally selected because this size nozzle was already in use on an aqueous spray wash line at the plant, so a supply was available onsite. A technical assistance assessment identified that nozzle size was the key factor affecting the volume of solvent used.

Three nozzles were purchased and tested in the cleaning system. Flow rates for these nozzles ranged from one-fourth to one-fiftieth of the original flow rate. The smallest of these nozzles' orifices (0.026 inches) cleaned the equipment at an acceptable level in 60 to 90 seconds at 30 psi, and used 80% less solvent than the original nozzle. Waste accumulation from this source was monitored over the next two months and confirmed the improved efficiency using the new nozzle.

Foreign particles (such as rust) in the solvent feed line plugged the nozzle orifice frequently over the first 2 weeks of operation. Plugging was eliminated by installing a small in-line basket filter to remove solids before they reached the nozzle. Cleaning time with the low-flow nozzle was doubled or tripled compared with the original nozzle. The 60 to 90 second cleaning time was judged acceptable, although operators were not pleased with this change. Cleaning time was reduced by 30 seconds by instituting a presoak step. The presoak used a dirty solvent bath to remove or loosen most of the paint. The equipment was then sprayed with fresh solvent for a final rinse. The presoak resulted in additional waste reduction.

Savings

There was no capital investment for this project. Supplies included the purchase of three nozzles for testing ($70) and a small, in-line basket filter ($50). Six hours of labor were needed to test the nozzles, and approximately 4 hours were spent unclogging the nozzle orifice for the first 2 weeks of operation. Total implementation costs were approximately $270. Reduction in waste resulting from the new nozzles came to about 11,000 gallons less of spent solvent waste generated per year with savings of approximately $13,500 per year.

(MnTap 6/91-83)

Use a Gun Washer

The use of a gun washer can also help to reduce wastes generated during equipment cleaning. An automatic gun washer operates like a dishwasher.

The paint gun is partially disassembled and placed in the unit. Cleaning is accomplished by recirculating solvent sprays. These units reportedly reduce solvent waste by 50 to 75%. VOC emissions can be reduced by up to 20%, and a 60% labor time savings can be achieved (IWRC, p. 15).

Units range in cost from $600 for small units to approximately $1,500 for industrial type units (i.e., gun and paint hose wash). Similar units may also be leased through various chemical suppliers and waste management companies at a cost of $165 to $195 per 5 gallon waste solvent change out interval (IWRC, p. 15-17).

Pressure Pot Liners

For maintenance of pressure pots, many companies use a polyethylene inner liner with the pressure pot. The main advantage of this practice is that only a small amount of paint comes into contact with the steel or stainless steel body, and cleaning the liner requires only a small amount of solvent. After pouring solvent into the liner, the operator should swirl it around for a few seconds. The operator can then discard the spent solvent into a hazardous waste drum and the liner is ready to be reused.

Some operators choose to allow the paint that sticks to the side of the liner to dry out, which causes it to flake off with ease. If the solid paint is shown to be hazardous per RCRA guidelines the facility must manage it as a hazardous waste. If it is not hazardous, it can be discarded with the rest of the solid waste. The liner should then be reused (EPAq, p. 137).

Use Alternative Cleaning Solutions

Because of the increased need to reduce VOC emissions, alternative cleaning solutions are available. They include dibasic esters (DBE), N-methyl-2-pyrolidone (NMP), and a variety of other alkaline-, citric-, and water-based solvents such as d-liminone, naptha, and terpenes. These chemicals have reduced VOC emissions due to their lower evaporation rate. Although toxicology information specific to these chemicals is relatively limited at this time, many researchers believe that the relative safety of similar chemicals indicate that they are a feasible alternative to organic solvents in certain applications (MnTAP, p. 5-6).1

1 For more information on these alternative solvents, see Project Summary: SAGE 2.1, Solvent Alternatives Guide: User's Guide. Research Triangle Park, NC: Air and Energy Engineering Research Laboratory. EPA/600/SR-95/049.

Recover and Reuse Spent Solvents

Onsite recycling of used solvent is another way to reduce waste and save money. Savings come from reducing the amount of solvent purchased and the volume of spent solvent that must be sent offsite for costly disposal. Two common methods of solvent recycling are settling and distilling (MnTAP, p. 5-6).

Settling involves putting used solvent in a container and letting the particulate matter settle out. The container should be designed to allow for removal of solvent without shaking up the sludge that has settled out (MnTAP, p. 5-6). Solvents can be used for gun cleaning and then can be placed back into the storage container for subsequent settling and reuse. Eventually, sludge will make up the majority of the container and offsite hazardous waste disposal will be necessary. At this point, the processes can be repeated using a different container. Solvent waste reduction of up to 33% can be accomplished with this simple method (IWRC, p. 15-17). Filtering equipment, which removes the particulate matter from solvents, also is available (MnTAP, p. 5-6).

Waste solvent also can be collected and processed through distillation equipment. Approximately 80% of the used solvent is recovered with basically the same cleaning properties as a new product. The remaining 20% sludge (still bottoms) must be collected for offsite hazardous waste disposal. To help maintain the cleaning properties of the recycled thinner, certain paint and solvent wastes should be segregated. Waste gun wash solvent and any waste lacquer paint and thinner mixtures can be included for recycling. All waste urethanes, enamels and enamel reducers should be placed in a separate container; enamel and urethane products will not clean as well as pure lacquer thinner. By segregating the two, the reclaimed solvent will possess cleaning properties like a virgin thinner. This waste management technique has the advantage of reducing the volume of virgin thinner purchased as well as the amount of waste thinner generated (IWRC, p. 15-17).

Onsite distillation equipment comes in a wide range of capacities, from 5 gallons per 8 hour shift batch operations to more than 100 gallons per hour flow-through units. Costs for 5 gallon batch units start at approximately $1,500 with an average cost of $3,000 (IWRC, p. 15-17).

CASE STUDY :

Solvent Reclaimer

The Marmon Motor Company of Garland, Texas, spent $10,000 to install a solvent still that reclaims thinners from paint-related wastes. By installing this standard technology, Marmon reduced its disposal of thinners from 34 drums to 3 drums and cut procurement of new thinner from 4,000 gallons to 2,000 gallons per year.

Savings

As a result, waste disposal costs were reduced from $6,200 to $1,400 per year. Purchasing costs for new thinner decreased from $9,500 to $4,750 per year. Even with additional labor costs at about $5,000 a year, the annual savings were approximately $4,500 with a 2.2 year payback period (PPIFTI 1994).

 

CASE STUDY:

Paint Gun Cleaning

Thermon is a electronics manufacturer in San Marcos, Texas, with 300 employees. To clean paint guns, the company traditionally soaked the guns in solvent. This method generated significant amounts of solvent waste and even clogged the guns when inexperienced personnel performed the cleaning.

To remedy this situation, Thermon purchased a spray gun cleaning system that circulates solvent through the gun. The cleaning solvent is filtered to remove particles that clog the paint gun, enabling the solvent to be reused several times before disposal. The system decreases solvent purchases and effectively cleans the paint spray gun even when inexperienced personnel perform the cleaning.

To prepare the paint guns for the cleaning system, the remaining paint is emptied from the canister and the canister is washed with a small amount of solvent ( pint). The remaining solvent is poured out before the gun is loaded in the cleaning system. This extra rinsing step significantly reduces solvent use.

Savings

With the new cleaning system and methods, Thermon reduced solvent use by 60%.

The cost of the spray gun system was $700. The system was found to be very economical due to reduced solvent costs and, more importantly, improved spray gun performance due to increased cleaning effectiveness. Despite the fact that Thermon does a relatively small amount of painting, the cost of the unit was recovered in 6 to 8 months (PPIFTI 1994).