- Email: [email protected]
Savings of 10 to 30% pollution reduction—air and hazardous waste
by Ron Joseph Ron Joseph
& Associates
Inc., San Jose, Calif
H
ands-on painter training has been conducted at over 23 aerospace painting facilities with the goal of lowering coating and solvent usage, VOCYHAP emissions, and hazardous waste. Direct improvements have come from improved painting techniques, which have resulted in less overspray and fewer reworks; however, many of the problems do not reside with the painters’ techniques or lack of understanding of the painting processes. Inadequate equipment, poor maintenance, and lack of understanding by supervisors and engineering managers largely contribute to unnecessary air and waste pollution from painting facilities. This article provides numerous findings and recommendations that have surfaced as a result of the training programs. INTRODUCTION
For the past 4 years painters at over 23 aerospace paint facilities have received hands-on training to improve their application techniques and become more familiar with the engineering principles of painting processes. The training programs cover a 4-day period. On the first day painters are required to apply their current coating to predefined large flat panels. Measurements of coating usage and other parameters are recorded for later calculations. The second day of the program commences with quality control measurements on the previous day’s work. Day #2 is devoted to critiquing the painters’ techniques and teaching them the finer points of painting practices with the goal of improving transfer efficiency. They are also told how spray guns work, the differences between various types of HVLP guns, and the correct steps for setting up a spray gun. A topic that few if any painters truly understand regards the pressure drop inside the compressed air hose leading from compressor to the spray gun. This subject also forms part of the training program. On the third day the painters once again apply their current coating to flat panels but this time make every effort to improve transfer efficiency. Once again their coating usage and application parameters are measured and recorded. On the fourth and last day the coated panels are evaluated for quality control, and the results are summarized so that 74
their performance on the first day and third day of training can be compared. The program is concluded with lectures on why paints and coatings cause smog, definitions of VOC and HAP, differences between solvent-based and water-based coatings, why “thinners” cannot be added to mixed paints unless so specified by the coating vendors, recordkeeping, and painters’ responsibilities for compliance. Historically, the results have shown that reductions in the use of coatings is 10 to 30% solely due to improved painting techniques. Clearly, this translates into equivalent VOCYHAP emission reductions. In almost every class the appearance of the coated panels improves significantly and in some cases improvements up to 40% have been calculated. This statistic is important because at many of the facilities at which training has been given rework on account of poor appearance has (anecdotally) been reported as ranging from 25 to 50% of the workload at the facility. If painters can achieve an acceptable finish on the first pass, rework rates drop dramatically. The supervisor at one facility reported that before his painters had taken the class the rework rate was approximately 40%, but this dropped to less than 5% after the painters had been trained. To perform rework the aircraft or components need to be scuff-sanded, the areas surrounding the damage need to be masked, the surfaces are then solvent wiped, and this is followed by the application of a primer and topcoat. Clearly, the operation is expensive and results in unnecessary environmental waste comprising VOWHAP emissions, solventladen rags, masking paper, masking tape, and mixed but unused two-component coating. While it is obvious that painters play an important role in determining the overafl efficiency of a painting operation, it has been found that supervisors and engineering managers are equally important in the equation because they control equipment purchases and maintenance scheduling, both of which are critical to the operation. In conducting these training programs many other observations have been made that contribute to poor efficiency in a painting facility. Because overall efficiency is closely related to VOCYHAP emissions and the disMetal
Finishing
posal of hazardous waste, this paper will discuss some of the most common findings and recommendations. Managers who have already taken some of the recommendations to heart have reported significant reductions in rework rates and this must, by necessity, be reflected in lower emissions and waste disposal. DESIGN
OF HVLP
SPRAY
GUNS
For the most part, painters, supervisors, and engineering managers do not understand the mechanism by which HVLP spray guns work, nor are they aware that there are three basic principles by which these guns are designed. The venturi-type HVLP guns tend to be the most popular, but not only are turbine and inductor-operated guns available, they are often capable of atomizing high solids and waterbased paints more easily and often faster than their venturi-type cousins. Unfortunately, there are times when a large-usage paint facility would be better served using one of the latter gun designs. Because the managers do not know that different designs are available, they find themselves struggling with an HVLP gun that does not have adequate capacity. The training program includes a detailed discussion of the three gun types and participants are given guidance as to when each type is appropriate. Moreover, they are encouraged to ask vendors to perform on-site demonstrations so that comparisons can be made between current and potential performance. COMPLIANCE
OF HVLP
SPRAY
GUNS
The Aerospace NESHAP as well as other state regulations require that the owner/operator of a paint facility be able to demonstrate compliance of the IWLP gun by attaching a special pressure gauge to the air cap ( see Fig. 1). According to the regulatory definition when the gun is triggered the atomizing air pressure may not exceed 10 psig. There appears to be a widespread misconception across the industrial painting industry (not limited to the aerospace industry) that IIVLP spray guns are automatically compliant. This is not so. In fact it is relatively easy to operate these guns well in excess of the lo-psig limit and out of compliance. EPA and state inspectors are becoming increasingly aware that a vast number of paint shops are currently operating out of compliance with one or more regulations and this might be due to lack of knowledge than with intent to violate. It has been observed that most paint shops, which are subject to either the Aerospace NESHAP or some other state regulation, do not own a pressure gauge/cap assembly and, therefore, cannot demonstrate compliance. Not only is this a violation of the September
1999
Figure 1. Air pressure atomizing orifice and
gauges connected to the horn of the cap, respectively.
central
air
regulation but many painters will tend to set their air pressures as high as possible to get the best looking finish. By doing so their HVLP spray guns now operate like an outlawed conventional air atomizing spray gun for which transfer efficiencies are considerably lower. During the training program painters are shown how to measure compliance and how to paint their workpiece within the lo-psig limit. ATOMIZING
VERSUS
FAN-SHAPING
AIR
Although EPA’s definition of HYLP lacks clarity a prevalent interpretation is that neither the atomizing air nor the fan shaping air may exceed the lo-psig limit. Most spray gun manufacturers who provide pressure gauge/cap assemblies measure only the atomizing air that passes through the central orifice of the air cap. Few manufacturers also make provision for measuring the air pressure at the horns of the cap where shaping of the fan takes place. Participants learn how the atomizing air pressure can change when the air leading to the horns of the cap is increased or decreased, respectively. A simple demonstration shows that if the fan adjustment knob is opened all the way and the pressure to the gun is set so that the atomizing air pressure is 10 psig, then when the fan adjustment knob is closed to provide a small fan width, the atomizing air pressure increases well beyond the regulatory limit. AIR
HOSES
AND
PRESSURE
DROP
By its very definition HVLP stands for high volume (air), low pressure (air). In other words good atomization can only take place if a high volume of air passes through the spray gun. Most of the venturi-type guns are designed so that an air pressure of approximately 50 psig on the 75
handle of the gun will be converted via the venturi to a maximum of 10 psig at the cap. (The recommended maximum inlet pressure varies with spray gun design.) Therefore, if the pressure at the inlet to the gun is less than 50 psig the atomizing air pressure will by necessity be less than 10 psig. There appears to be an almost universal lack of understanding by supervisors, and engineering managers painters, that small diameter (% in.) air hoses longer than 50 ft often cannot deliver the large volumes required by these guns. To illustrate this point consider a 50-ft length of X-in. hose for which lOO-psig air enters at the one end (usually at the regulator on the spray booth wall). For a spray gun that is designed to atomize at an air flow rate of 15 cfm the pressure drop (derived from empirical equations) is 50 psig and the pressure at the handle of the gun will, therefore, be 100 psig at compressor minus 50 psig pressure drop equals 50 psig at the gun, thus allowing the painter to get the full benefit of the spray gun; however, many HVLP guns are designed for higher air flow rates, which lead to correspondingly higher pressure drops within the hose. For instance for a flow rate of 20 cfm anyone using a 50-ft length of i/4-in. air hose will experience a pressure drop of approximately 75 psi, thus robbing the gun of atomizing air. This problem is prevalent in many paint shops where high solids coatings are used. Rework is performed because painters are not able to achieve the high-quality finishes expected by their customers. Orange peel is the most common defect that results from insufficient atomizing air, yet the problem can easily be remedied by increasing the diameter of the air hose to %6 in. or better still 3/s in. At least one shop manager all but eliminated his rework by replacing his i/4-in. air hose with one Of 3/s in. diameter. Another strategy is to purchase what the industry commonly refers to as an “air pig” (see Fig. 2). This is an inexpensive aluminum or steel manifold that is placed in a central location on the floor of the paint hangar. A large %-in. hose brings compressed air to the manifold thus preserving the pressure to the maximum extent. Several quick disconnects on the “air pig” allow for smaller diameter hoses to be attached and bring compressed air to the spray guns. Because the manifold is close to the workpiece, the pressure drop in the short, small diameter air hose is usually not significant, and the guns can be used at their full design capacity. GUICK
DISCONNECTS
While on the subject of pressure drop it has been observed that most paint shops are too liberal in their 76
Figure 2. Example of an inexpensive “air pig” that allows a %-in. or larger air hose to bring air to the manifold from the compressor, and provides for %-in. or smaller hoses to connect from the manifold to the spray guns.
use of quick disconnects within the air hose. These small devices are both a blessing and a curse. On the positive side they allow for two or more lengths of air hose to be attached to each other quickly and easily, and they can conveniently be attached to regulators, pressure pots, etc. The hidden curse is that they contribute significantly to high pressure drops, often reducing air flows to such an extent that IIVLP guns cannot properly atomize high solids coatings. Paint facilities pay dearly for the use of these inexpensive fittings. Larger compressors are needed to overcome the pressure drops caused by these small villains and it would be interesting to monitor the number of unnecessary paint reworks that can be traced to them. In selecting quick disconnects paint shop managers are encouraged to shop around for those that have large internal diameters and minimize restrictions in the hose. Better still there are many instances in which hoses can be attached to regulators or linked to each other by means of a large-diameter connector or ball valve. MANIFOLD
AIR
HOSE
MANAGEMENT
In large aircraft paint hangars where two-component polyurethanes are applied three hoses are often used; one to carry the atomizing air, the second for the coating, and the last for the breathing air used by the painter. Some painter hangars are so large that hoses in excess of 100 ft are required to stretch from the wall of the spraybooth where compressed air is accessed, to the extremities of the aircraft where painting will take place. At the beginning of a shift painters often spend a fair amount of time straightening out their hoses to insure that the painting process will continue without interruption; however, when large aircraft are Metal
Finishing
Figure 3. Typical hose reel, which locations throughout the spraybooth.
can
be placed
at various
coated several teams of painters commence their work at the tail, wings, and nose, respectively. As the coating process continues the painters move closer to each other and the hoses often become so entangled that helpers are required to organize the mess. More importantly, the group of three hoses is often pulled over ladders and work stands, picking up dry overspray and other contaminants. Worse, the hoses are often dragged over surfaces, such as the wings or fuselage, where an underlying primer coat might have been applied only a few hours earlier. The damage that occurs can easily account for subsequent rework. Large painting facilities should consider investing in hose reels located at strategic points within the paint hangar (see Fig. 3). Engineers are advised to evaluate the feasibility of suspending hose reels from the ceiling so that the hoses can be pulled down to the painter without the need to drag them across recently painted surfaces. PRESSURE
POTS
When only small quantities of paint are to be applied HVLP guns are usually fitted with a one-quart cup that is attached directly to the gun. In most facilities, however, much larger quantities are required and 5-, lo-, and X-gal pressure pots are commonly used to feed coating to the spray gun. Some paint facilities equip their pressure pots with a single air regulator, which serves to regulate the compressed air inside the pot where the coating will be forced through a fluid hose to the spray gun. A second stream of compressed air bypasses the regulator and leads directly through the air hose to the spray gun. In the absence of a dedicated regulator on the pressure pot for the atomizing air the painter should have a second regulator attached to the handle of his/her HVLP spray gun. Now he/she can insure that the maximum inlet pressure to the September
1999
gun does not exceed the manufacturer’s recommended limit and he/she can also confirm compliance with EPA regulations. Unfortunately, regulators and their associated fittings are relatively heavy, especially when held for hours on end by men and women alike, Many of them complain of unnecessary fatigue, shoulder, wrist, elbow, and finger problems. An alternative to having the regulator on the gun is to attach a second regulator to the pressure pot where the air pressure can be adjusted so that compliance with the lo-psig limit can still be maintained. By investing in a second regulator it is not necessary for the painter to have a third regulator on the handle of the spray gun and his/her load is often lightened by up to 1 lb. It has frequently been observed during training programs that transfer efficiency decreases rapidly as the painter becomes fatigued; hence, by making an inexpensive investment, usually less than $40, the health and welfare of the painter is enhanced, while the economics of the painting operation are improved. There is a definite environmental benefit and it will quickly lead to a cost payback. COMPRESSOR
No matter how talented a painter is, paint defects commonly occur when the atomizing air pressure changes unexpectedly during the operation. The most frequent reason is that the paint facility does not own a compressor dedicated to the painting process. In a typical scenario a painter sets up his/her spray gun by adjusting the air and fluid pressures to achieve the desirable finish. Soon after starting to paint, someone, often in a different building, draws compressed air to operate some pneumatic device such as a sander. Immediately the compressed air to the spray gun drops and the coating can no longer be properly atomized. This is reflected in the paint finish, which now has one or more defects such as orange peel or worse-runs and sags. The painter must now take steps to limit the defects. An experienced painter will notice that the atomization has changed and he/she will immediately increase the atomizing pressure, provided that additional air is available. On the assumption that appropriate adjustments can be made the painter continues to produce a good-looking finish until, without warning, the person operating the pneumatic device no longer needs compressed air. Now there is a surge of compressed air to the spray gun, the coating is over atomized and dry paint dust settles on freshly painted surfaces. Another defect has occurred. More rework results, leading to increased emissions, disposal of hazardous waste, and costs. 77
Facility managers are often reluctant to invest in compressors dedicated to the paint facility but they should realize that this investment can often be quickly cost justified. LIGHTING
New, modern spraybooths do not lack overhead and side lighting and many spraybooths are adequately bright; however, with few exceptions the spraybooths in which training programs have been given have lacked adequate lighting from the floor up or at such angles that painters could see the “wet edge” of their spray fan when coating vertical surfaces. In aircraft paint hangars one of the most prevalent problems is inadequate lighting under the belly, fuselage, and wings of an aircraft. This problem is not confined to aircraft hangars but is also experienced in component and truck spraybooths. Despite an abundance of overhead lights painters cannot see their spray patterns when inadequate light prevents reflection from surfaces that are in the shadows of the overhead permanent lights. Observations taken during training programs have shown that under these circumstances painters often leave voids in the paint finish such that the metal might shine through the primer or the primer might show through the topcoat. Alternatively, painters who cannot see what they are doing tend to be conservative and apply twice as much paint as is required. From an equipment perspective additional lighting does pose a problem. Portable fluorescent lights must be explosion proof and have sufficient luminosity to be useful. They are expensive, often costing several thousand dollars for a single fixture and facility managers find it difficult to calculate the payback. The fact is that in many old and new paint booths portable lights are desperately needed and the lack thereof simply contributes to more rework and coating films that are heavier than they need to be. MAINTENANCE
Painters can be taught how to use paint conservatively, increase transfer efficiency, prevent defects due to improved techniques, and more; however, no matter how well a painter has been trained, defects will continue if the painting equipment is poorly maintained. Common maintenance problems are frequently observed during training programs and they include pressure gauges that do not function, gauges that are so covered with overspray that one cannot read the needle, old and cracked air and fluid hoses, pressure pots for which the seals are so worn that air pressure within the pot cannot be maintained, spraybooth lamps so covered with overspray 78
that spraybooths are dark and reflections cannot be seen. Fortunately, some EPA and state regulations require that spraybooth filters be changed when the pressure drop across the filters exceeds the manurecommended limits. Unfortunately, facturer’s many spraybooths do not fall under such regulations and overspray is allowed to build up to levels at which air flow through the booth falls even below OSHA levels of 100 ft/min. In such booths overspray cannot be efficiently removed from the workpiece and defects, such as dry spray, abound. Unless a proper maintenance schedule is implemented and those who perform the maintenance do so diligently and with knowledge of the equipment, problems within the paint shop will continue and nothing much will be achieved by simply purchasing new equipment. CONCLUSIONS
Hands-on training programs have proven valuable at many levels. Painters have consistently been found to improve their techniques and have shown that they can dramatically reduce VOWHAP emissions and hazardous waste, sometimes by 10 to 30%. Many of them have learned for the first time how their equipment is intended to work and what issues they need to be aware of. At a higher level the programs have been invaluable in identifying equipment and maintenance problems that persist in host spraybooths; namely those in which the training programs have been conducted. Supervisors and their managers have often taken recommendations to heart and have made appropriate changes to equipment and maintenance procedures. In those cases where painters’ supervisors and managers have participated, verbal reports have indicated that emissions and hazardous waste disposal have been reduced primarily due to a decrease in rework. Limited success has been achieved during those training programs in which only painters were present, while supervisors and engineering managers have shown little or no interest in attending the program. Anecdotal feedback from environmental managers has indicated that painters and their supervisors have become considerably more aware of and sensitive to environmental regulations and compliance issues. Even during those programs where supervisors were not present it has been reported that painters have advised their seniors of their responsibilities toward compliance. For this reason alone many environmental managers have found justification for funding such training programs. NW Metal
Finishing
& Associates
Inc., San Jose, Calif
H
ands-on painter training has been conducted at over 23 aerospace painting facilities with the goal of lowering coating and solvent usage, VOCYHAP emissions, and hazardous waste. Direct improvements have come from improved painting techniques, which have resulted in less overspray and fewer reworks; however, many of the problems do not reside with the painters’ techniques or lack of understanding of the painting processes. Inadequate equipment, poor maintenance, and lack of understanding by supervisors and engineering managers largely contribute to unnecessary air and waste pollution from painting facilities. This article provides numerous findings and recommendations that have surfaced as a result of the training programs. INTRODUCTION
For the past 4 years painters at over 23 aerospace paint facilities have received hands-on training to improve their application techniques and become more familiar with the engineering principles of painting processes. The training programs cover a 4-day period. On the first day painters are required to apply their current coating to predefined large flat panels. Measurements of coating usage and other parameters are recorded for later calculations. The second day of the program commences with quality control measurements on the previous day’s work. Day #2 is devoted to critiquing the painters’ techniques and teaching them the finer points of painting practices with the goal of improving transfer efficiency. They are also told how spray guns work, the differences between various types of HVLP guns, and the correct steps for setting up a spray gun. A topic that few if any painters truly understand regards the pressure drop inside the compressed air hose leading from compressor to the spray gun. This subject also forms part of the training program. On the third day the painters once again apply their current coating to flat panels but this time make every effort to improve transfer efficiency. Once again their coating usage and application parameters are measured and recorded. On the fourth and last day the coated panels are evaluated for quality control, and the results are summarized so that 74
their performance on the first day and third day of training can be compared. The program is concluded with lectures on why paints and coatings cause smog, definitions of VOC and HAP, differences between solvent-based and water-based coatings, why “thinners” cannot be added to mixed paints unless so specified by the coating vendors, recordkeeping, and painters’ responsibilities for compliance. Historically, the results have shown that reductions in the use of coatings is 10 to 30% solely due to improved painting techniques. Clearly, this translates into equivalent VOCYHAP emission reductions. In almost every class the appearance of the coated panels improves significantly and in some cases improvements up to 40% have been calculated. This statistic is important because at many of the facilities at which training has been given rework on account of poor appearance has (anecdotally) been reported as ranging from 25 to 50% of the workload at the facility. If painters can achieve an acceptable finish on the first pass, rework rates drop dramatically. The supervisor at one facility reported that before his painters had taken the class the rework rate was approximately 40%, but this dropped to less than 5% after the painters had been trained. To perform rework the aircraft or components need to be scuff-sanded, the areas surrounding the damage need to be masked, the surfaces are then solvent wiped, and this is followed by the application of a primer and topcoat. Clearly, the operation is expensive and results in unnecessary environmental waste comprising VOWHAP emissions, solventladen rags, masking paper, masking tape, and mixed but unused two-component coating. While it is obvious that painters play an important role in determining the overafl efficiency of a painting operation, it has been found that supervisors and engineering managers are equally important in the equation because they control equipment purchases and maintenance scheduling, both of which are critical to the operation. In conducting these training programs many other observations have been made that contribute to poor efficiency in a painting facility. Because overall efficiency is closely related to VOCYHAP emissions and the disMetal
Finishing
posal of hazardous waste, this paper will discuss some of the most common findings and recommendations. Managers who have already taken some of the recommendations to heart have reported significant reductions in rework rates and this must, by necessity, be reflected in lower emissions and waste disposal. DESIGN
OF HVLP
SPRAY
GUNS
For the most part, painters, supervisors, and engineering managers do not understand the mechanism by which HVLP spray guns work, nor are they aware that there are three basic principles by which these guns are designed. The venturi-type HVLP guns tend to be the most popular, but not only are turbine and inductor-operated guns available, they are often capable of atomizing high solids and waterbased paints more easily and often faster than their venturi-type cousins. Unfortunately, there are times when a large-usage paint facility would be better served using one of the latter gun designs. Because the managers do not know that different designs are available, they find themselves struggling with an HVLP gun that does not have adequate capacity. The training program includes a detailed discussion of the three gun types and participants are given guidance as to when each type is appropriate. Moreover, they are encouraged to ask vendors to perform on-site demonstrations so that comparisons can be made between current and potential performance. COMPLIANCE
OF HVLP
SPRAY
GUNS
The Aerospace NESHAP as well as other state regulations require that the owner/operator of a paint facility be able to demonstrate compliance of the IWLP gun by attaching a special pressure gauge to the air cap ( see Fig. 1). According to the regulatory definition when the gun is triggered the atomizing air pressure may not exceed 10 psig. There appears to be a widespread misconception across the industrial painting industry (not limited to the aerospace industry) that IIVLP spray guns are automatically compliant. This is not so. In fact it is relatively easy to operate these guns well in excess of the lo-psig limit and out of compliance. EPA and state inspectors are becoming increasingly aware that a vast number of paint shops are currently operating out of compliance with one or more regulations and this might be due to lack of knowledge than with intent to violate. It has been observed that most paint shops, which are subject to either the Aerospace NESHAP or some other state regulation, do not own a pressure gauge/cap assembly and, therefore, cannot demonstrate compliance. Not only is this a violation of the September
1999
Figure 1. Air pressure atomizing orifice and
gauges connected to the horn of the cap, respectively.
central
air
regulation but many painters will tend to set their air pressures as high as possible to get the best looking finish. By doing so their HVLP spray guns now operate like an outlawed conventional air atomizing spray gun for which transfer efficiencies are considerably lower. During the training program painters are shown how to measure compliance and how to paint their workpiece within the lo-psig limit. ATOMIZING
VERSUS
FAN-SHAPING
AIR
Although EPA’s definition of HYLP lacks clarity a prevalent interpretation is that neither the atomizing air nor the fan shaping air may exceed the lo-psig limit. Most spray gun manufacturers who provide pressure gauge/cap assemblies measure only the atomizing air that passes through the central orifice of the air cap. Few manufacturers also make provision for measuring the air pressure at the horns of the cap where shaping of the fan takes place. Participants learn how the atomizing air pressure can change when the air leading to the horns of the cap is increased or decreased, respectively. A simple demonstration shows that if the fan adjustment knob is opened all the way and the pressure to the gun is set so that the atomizing air pressure is 10 psig, then when the fan adjustment knob is closed to provide a small fan width, the atomizing air pressure increases well beyond the regulatory limit. AIR
HOSES
AND
PRESSURE
DROP
By its very definition HVLP stands for high volume (air), low pressure (air). In other words good atomization can only take place if a high volume of air passes through the spray gun. Most of the venturi-type guns are designed so that an air pressure of approximately 50 psig on the 75
handle of the gun will be converted via the venturi to a maximum of 10 psig at the cap. (The recommended maximum inlet pressure varies with spray gun design.) Therefore, if the pressure at the inlet to the gun is less than 50 psig the atomizing air pressure will by necessity be less than 10 psig. There appears to be an almost universal lack of understanding by supervisors, and engineering managers painters, that small diameter (% in.) air hoses longer than 50 ft often cannot deliver the large volumes required by these guns. To illustrate this point consider a 50-ft length of X-in. hose for which lOO-psig air enters at the one end (usually at the regulator on the spray booth wall). For a spray gun that is designed to atomize at an air flow rate of 15 cfm the pressure drop (derived from empirical equations) is 50 psig and the pressure at the handle of the gun will, therefore, be 100 psig at compressor minus 50 psig pressure drop equals 50 psig at the gun, thus allowing the painter to get the full benefit of the spray gun; however, many HVLP guns are designed for higher air flow rates, which lead to correspondingly higher pressure drops within the hose. For instance for a flow rate of 20 cfm anyone using a 50-ft length of i/4-in. air hose will experience a pressure drop of approximately 75 psi, thus robbing the gun of atomizing air. This problem is prevalent in many paint shops where high solids coatings are used. Rework is performed because painters are not able to achieve the high-quality finishes expected by their customers. Orange peel is the most common defect that results from insufficient atomizing air, yet the problem can easily be remedied by increasing the diameter of the air hose to %6 in. or better still 3/s in. At least one shop manager all but eliminated his rework by replacing his i/4-in. air hose with one Of 3/s in. diameter. Another strategy is to purchase what the industry commonly refers to as an “air pig” (see Fig. 2). This is an inexpensive aluminum or steel manifold that is placed in a central location on the floor of the paint hangar. A large %-in. hose brings compressed air to the manifold thus preserving the pressure to the maximum extent. Several quick disconnects on the “air pig” allow for smaller diameter hoses to be attached and bring compressed air to the spray guns. Because the manifold is close to the workpiece, the pressure drop in the short, small diameter air hose is usually not significant, and the guns can be used at their full design capacity. GUICK
DISCONNECTS
While on the subject of pressure drop it has been observed that most paint shops are too liberal in their 76
Figure 2. Example of an inexpensive “air pig” that allows a %-in. or larger air hose to bring air to the manifold from the compressor, and provides for %-in. or smaller hoses to connect from the manifold to the spray guns.
use of quick disconnects within the air hose. These small devices are both a blessing and a curse. On the positive side they allow for two or more lengths of air hose to be attached to each other quickly and easily, and they can conveniently be attached to regulators, pressure pots, etc. The hidden curse is that they contribute significantly to high pressure drops, often reducing air flows to such an extent that IIVLP guns cannot properly atomize high solids coatings. Paint facilities pay dearly for the use of these inexpensive fittings. Larger compressors are needed to overcome the pressure drops caused by these small villains and it would be interesting to monitor the number of unnecessary paint reworks that can be traced to them. In selecting quick disconnects paint shop managers are encouraged to shop around for those that have large internal diameters and minimize restrictions in the hose. Better still there are many instances in which hoses can be attached to regulators or linked to each other by means of a large-diameter connector or ball valve. MANIFOLD
AIR
HOSE
MANAGEMENT
In large aircraft paint hangars where two-component polyurethanes are applied three hoses are often used; one to carry the atomizing air, the second for the coating, and the last for the breathing air used by the painter. Some painter hangars are so large that hoses in excess of 100 ft are required to stretch from the wall of the spraybooth where compressed air is accessed, to the extremities of the aircraft where painting will take place. At the beginning of a shift painters often spend a fair amount of time straightening out their hoses to insure that the painting process will continue without interruption; however, when large aircraft are Metal
Finishing
Figure 3. Typical hose reel, which locations throughout the spraybooth.
can
be placed
at various
coated several teams of painters commence their work at the tail, wings, and nose, respectively. As the coating process continues the painters move closer to each other and the hoses often become so entangled that helpers are required to organize the mess. More importantly, the group of three hoses is often pulled over ladders and work stands, picking up dry overspray and other contaminants. Worse, the hoses are often dragged over surfaces, such as the wings or fuselage, where an underlying primer coat might have been applied only a few hours earlier. The damage that occurs can easily account for subsequent rework. Large painting facilities should consider investing in hose reels located at strategic points within the paint hangar (see Fig. 3). Engineers are advised to evaluate the feasibility of suspending hose reels from the ceiling so that the hoses can be pulled down to the painter without the need to drag them across recently painted surfaces. PRESSURE
POTS
When only small quantities of paint are to be applied HVLP guns are usually fitted with a one-quart cup that is attached directly to the gun. In most facilities, however, much larger quantities are required and 5-, lo-, and X-gal pressure pots are commonly used to feed coating to the spray gun. Some paint facilities equip their pressure pots with a single air regulator, which serves to regulate the compressed air inside the pot where the coating will be forced through a fluid hose to the spray gun. A second stream of compressed air bypasses the regulator and leads directly through the air hose to the spray gun. In the absence of a dedicated regulator on the pressure pot for the atomizing air the painter should have a second regulator attached to the handle of his/her HVLP spray gun. Now he/she can insure that the maximum inlet pressure to the September
1999
gun does not exceed the manufacturer’s recommended limit and he/she can also confirm compliance with EPA regulations. Unfortunately, regulators and their associated fittings are relatively heavy, especially when held for hours on end by men and women alike, Many of them complain of unnecessary fatigue, shoulder, wrist, elbow, and finger problems. An alternative to having the regulator on the gun is to attach a second regulator to the pressure pot where the air pressure can be adjusted so that compliance with the lo-psig limit can still be maintained. By investing in a second regulator it is not necessary for the painter to have a third regulator on the handle of the spray gun and his/her load is often lightened by up to 1 lb. It has frequently been observed during training programs that transfer efficiency decreases rapidly as the painter becomes fatigued; hence, by making an inexpensive investment, usually less than $40, the health and welfare of the painter is enhanced, while the economics of the painting operation are improved. There is a definite environmental benefit and it will quickly lead to a cost payback. COMPRESSOR
No matter how talented a painter is, paint defects commonly occur when the atomizing air pressure changes unexpectedly during the operation. The most frequent reason is that the paint facility does not own a compressor dedicated to the painting process. In a typical scenario a painter sets up his/her spray gun by adjusting the air and fluid pressures to achieve the desirable finish. Soon after starting to paint, someone, often in a different building, draws compressed air to operate some pneumatic device such as a sander. Immediately the compressed air to the spray gun drops and the coating can no longer be properly atomized. This is reflected in the paint finish, which now has one or more defects such as orange peel or worse-runs and sags. The painter must now take steps to limit the defects. An experienced painter will notice that the atomization has changed and he/she will immediately increase the atomizing pressure, provided that additional air is available. On the assumption that appropriate adjustments can be made the painter continues to produce a good-looking finish until, without warning, the person operating the pneumatic device no longer needs compressed air. Now there is a surge of compressed air to the spray gun, the coating is over atomized and dry paint dust settles on freshly painted surfaces. Another defect has occurred. More rework results, leading to increased emissions, disposal of hazardous waste, and costs. 77
Facility managers are often reluctant to invest in compressors dedicated to the paint facility but they should realize that this investment can often be quickly cost justified. LIGHTING
New, modern spraybooths do not lack overhead and side lighting and many spraybooths are adequately bright; however, with few exceptions the spraybooths in which training programs have been given have lacked adequate lighting from the floor up or at such angles that painters could see the “wet edge” of their spray fan when coating vertical surfaces. In aircraft paint hangars one of the most prevalent problems is inadequate lighting under the belly, fuselage, and wings of an aircraft. This problem is not confined to aircraft hangars but is also experienced in component and truck spraybooths. Despite an abundance of overhead lights painters cannot see their spray patterns when inadequate light prevents reflection from surfaces that are in the shadows of the overhead permanent lights. Observations taken during training programs have shown that under these circumstances painters often leave voids in the paint finish such that the metal might shine through the primer or the primer might show through the topcoat. Alternatively, painters who cannot see what they are doing tend to be conservative and apply twice as much paint as is required. From an equipment perspective additional lighting does pose a problem. Portable fluorescent lights must be explosion proof and have sufficient luminosity to be useful. They are expensive, often costing several thousand dollars for a single fixture and facility managers find it difficult to calculate the payback. The fact is that in many old and new paint booths portable lights are desperately needed and the lack thereof simply contributes to more rework and coating films that are heavier than they need to be. MAINTENANCE
Painters can be taught how to use paint conservatively, increase transfer efficiency, prevent defects due to improved techniques, and more; however, no matter how well a painter has been trained, defects will continue if the painting equipment is poorly maintained. Common maintenance problems are frequently observed during training programs and they include pressure gauges that do not function, gauges that are so covered with overspray that one cannot read the needle, old and cracked air and fluid hoses, pressure pots for which the seals are so worn that air pressure within the pot cannot be maintained, spraybooth lamps so covered with overspray 78
that spraybooths are dark and reflections cannot be seen. Fortunately, some EPA and state regulations require that spraybooth filters be changed when the pressure drop across the filters exceeds the manurecommended limits. Unfortunately, facturer’s many spraybooths do not fall under such regulations and overspray is allowed to build up to levels at which air flow through the booth falls even below OSHA levels of 100 ft/min. In such booths overspray cannot be efficiently removed from the workpiece and defects, such as dry spray, abound. Unless a proper maintenance schedule is implemented and those who perform the maintenance do so diligently and with knowledge of the equipment, problems within the paint shop will continue and nothing much will be achieved by simply purchasing new equipment. CONCLUSIONS
Hands-on training programs have proven valuable at many levels. Painters have consistently been found to improve their techniques and have shown that they can dramatically reduce VOWHAP emissions and hazardous waste, sometimes by 10 to 30%. Many of them have learned for the first time how their equipment is intended to work and what issues they need to be aware of. At a higher level the programs have been invaluable in identifying equipment and maintenance problems that persist in host spraybooths; namely those in which the training programs have been conducted. Supervisors and their managers have often taken recommendations to heart and have made appropriate changes to equipment and maintenance procedures. In those cases where painters’ supervisors and managers have participated, verbal reports have indicated that emissions and hazardous waste disposal have been reduced primarily due to a decrease in rework. Limited success has been achieved during those training programs in which only painters were present, while supervisors and engineering managers have shown little or no interest in attending the program. Anecdotal feedback from environmental managers has indicated that painters and their supervisors have become considerably more aware of and sensitive to environmental regulations and compliance issues. Even during those programs where supervisors were not present it has been reported that painters have advised their seniors of their responsibilities toward compliance. For this reason alone many environmental managers have found justification for funding such training programs. NW Metal
Finishing