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THE ISSUE
Currently approved corrosion control coatings removal processes are expensive, time consuming and generate significant volumes of hazardous waste. Removal of many coatings, such as non-skid and deck coatings, is particularly time consuming and expensive. Processes currently in use by the Navy generate large volumes of airborne dust and debris that can contaminate other corrosion control processes and maintenance activities. They are among the most hazardous industrial occupations, and are increasingly regulated both from a worker safety as well as an environmental standpoint, which adds to the time involved as well as expense. The noise and other hazards associated with conventional methods can preclude the scheduling of other maintenance activities that might otherwise occur at the same time.
Conservatively the U.S. Navy performs corrosion control on more the 750,000 ft2 (69,700 m2)of steel annually for its ships, tanks and voids , generating more than 9 million pounds of mixed abrasive and paint waste. This waste must be disposed at a cost of nearly $2.7 million annually. These numbers however do not include the additional corrosion control and maintenance painting performed at the facilities level which could be estimated to be at least equal to that for ship maintenance.
The adoption by the Navy of suitable cost effective coatings removal processes that have higher production rates, produce less hazardous waste and allow concurrent scheduling of other tasks in the same area will result in significant reductions in downtime and the overall cost of maintaining DoD assets.
PROPOSED SOLUTION
Utilization of a new magnetic induction based technology as an alternative cost-effective technique for coatings removal could significantly reduce costs. This technology has a number of benefits including very high removal rates (even for tough coatings such as non-skid and deck coatings), reduction of hazardous waste, airborne pollutants, and noise, as well as significant cost savings. The system was developed and is currently manufactured by the Norwegian company, RPR Technologies (see Attachment A). Officials at the Naval Research Laboratory in Washington, DC are familiar with the technology.
Operationally:
a) The process increases productivity, resulting in significant reduction in surface preparation time, total downtime, and associated costs.
b) The process significantly reduces the risk to workers who are both directly in indirectly involved in corrosion control and coatings maintenance.
c) In addition to scheduled corrosion maintenance in port or dry dock the system could be used on ships at sea for efficient and effective emergency repairs and for onboard maintenance of tanks and other structures.
d) Because induction heating is noiseless and does not produce dust and other airborne hazards, other tasks may be carried out in nearby areas allowing for greater flexibility in scheduling.
COATINGS REMOVAL USING INDUCTION HEATING
The development by RPR Technologies of a practical induction heating process for coating removal is relatively recent event, utilizing known & proven technology (induction). At the present time the machines are used primarily in Europe for the purpose of industrial and ship coating removal.
The operation is performed by passing a high frequency magnetic coil over the surface where the coating is to be removed. The coil is mounted within a water-cooled device which is similar in size and configuration to a vacuum cleaner head. The magnetic field produced in the substrate creates a localized current, and resistance causes the surface of the substrate to heat rapidly. Only the first few tenths of a millimeter of the substrate are heated, typically to a nominal 250°F to 400°F (120-200°C), for a very brief period. This heating instantly disbonds the coating at the substrate, whereby the coating can be effectively removed, either by hand or with certain coatings on vertical surfaces by allowing it to fall off under its own weight. Although the surface of the substrate is heated to a high temperature, because the depth of heating is quite shallow the heat dissipates quickly. The coating itself is not heated directly, and the process does not damage coatings on the reverse side of the substrate. Chart A shows the results from a Navy test performed March 20, 2007 at Earl Industries Ship Yard. An RPR sensor was passed over an area of a steel plate multiple times in order to determine the effect of heat. The results show a maximum temperature of 380°F (193°C). According to the official procedure description issued by the USNavy, the maximum allowable temperature for operation has been set to 400°F (200°C) . This temperature is sufficient to cause disbanding and removal of most common types of paint and coatings used in industrial applications. Use of the RPR machine’s “automatic mode” is required, hence reducing the likelihood of achieving temperatures above 400°F. The minimum steel thickness has been set to 3/16 inch (4.7mm).
Chart A: Thermal Exposure with Multiple Pass
The process is rapid and depending on the design of the substrate structure, it removes 90 - 100% of the old coating, leaving only a minimal amount of additional surface preparation to be performed where required. Remaining cleanup and profile establishment, if needed, can be obtained via abrasive blasting, but at significantly faster rates, under more controlled conditions, and with substantially less grit consumption (i.e., waste generation). The use of grit is minimized because the bulk of material has been removed and HAZMAT (hazardous materials) costs are significantly reduced. Where the old coating has protected the substrate from corrosion, the original profile would be preserved without damage, and if appropriate to the new coating to be applied, would require no abrasive blasting, but only final rinsing with fresh water or the use of other methods to eliminate dust or other contamination. Coating removal rates using induction heating can theoretically be as high as 1000 ft²/hour (90 m2) on flat surfaces. In practical terms this would depend largely on the type of structure from which the paint is to be removed. Open decks and walls will approach the 1000 ft²/hour removal rate. Under almost any circumstances, the induction unit's rate would be a significant improvement over abrasive blasting or ultra high pressure water jetting alone and it can be performed concurrently with other operations.
Table 1 shows a comparison of conventional abrasive blasting rates verses comparable induction heating scenarios.
Table 1 Comparative Rates for Abrasive blasting and Induction Heating Processes forCoatings Removal
Abrasive Blasting Induction Heating w/Sweep Blasting Removal Rate (ft2/hr) Grit Consumption (lbs/ft2) Removal Rate (ft2/hr) Grit Consumption (lbs/ft2) 50-110 8-12 500-1000 1-2
The induction heating unit can be operated at the standard shipyard power of 440V 3ph and consumes 40kW of electricity. Current electrical costs are $0.15/kW/hr. Thus, it is estimated that the operating cost is $6/hr or roughly $50 per day. Because the unit contains few moving parts, there are no consumable components which require frequent replacement.
From a materials consumption and waste disposal standpoint, the current costs for abrasive blasting are approximately $3.80 to $4.00 per square foot. Including labor which is generally taken at 0.75/ft2, a conservative cost for abrasive blasting based on grit purchase, hazardous waste disposal and operator labor is $4.65/ft2. Equipment depreciation costs rated at 10% and overhead (5%) are not included as they would also apply to the proposed solution. Using induction heating on a typical job, the removal costs would be labor at $0.75/ ft2, power requirements of $0.075kWhr/ft2 and waste disposal costs $0.50/ft2. Thus the surface preparation costs using the proposed induction heating process would be$1.32/ft2or a savings of $3.33/ ft2.
For calculation purposes, the following provides a hypothetical comparison against a recent conventional job on USS WASP (LHD-1). Employing the traditional methods using only abrasive blasting, the waste generated from preparation of 19495 ft2 on the USS WASP was 233,940 lbs. If we use the previous example pertaining to the USS WASP (LHD-1) but instead employ induction heating, the waste generation using induction heating would be 2,924 lbs of coating and 29,242 lbs of abrasive grit for a total of 31,166 lbs of waste. Using a combination of induction heating for coatings removal and sweep blasting to re-establish the surface profile, the amount of coating waste for removal of a 20 mil thick coating would be 0.15lbs of coating per ft2. Abrasive grit consumption to achieve Sa 2.1/2 (NACE/SSPC SP-10) would be 1.5 lbs per ft2 or a total of less than 2lbs/ ft2 of waste generated for prepared surface. The combined use of induction heating and sweep blasting would reduce the waste generation by 202,774 lbs or a waste reduction of nearly 80% over the traditional method.
Table 2 shows costs for an abrasive blasting job verses a comparable induction heating process requirement.
Table 2
Comparative Operating Costs for Abrasive Blasting and Induction Heating Processes
PRODUCTION AND LABOR REQUIREMENTS
In Table 3, below, the removal rate has been listed and some clarification is needed to accurately express the costs, for comparative purposes. Abrasive blasting typically consists of one blaster and one safety man. Typically only two men are in a tank for safety reasons. For the 19,495 ft2 tank on USS WASP the surface preparation took one week (7 days) with two, two-man teams working 10-hour shifts (280 hours). At $60/hr per man the total labor cost was $16,800. For this job the effective removal rate was 70ft2/hr. For coatings removal using the proposed induction heating method the process would require two men (one safety and one operator) working for 19 hours (or 38 man hours). The total labor cost would be reduced to $2,248 or a savings of $14,520. From a labor standpoint the cost savings can be clearly seen with the use of the proposed process.
Table 3
Comparative Time and Waste Disposal Savings using Induction Heating for Coatings Removal (sweep blasting included for nominal re-establishment of surface profile)
Abrasive Blasting Induction Heating Comparative Savings Surface Area (ft^2) 19495 19495 - Grit Consumption (lbs) 233,940 29,242 204,698 Removal Rate (ft^/hr) 80 1,000 >90% Waste Disposal Cost ($) 70,000 9,349 60,651 Painting Cost ($) 48,900 48,900 -
OTHER METHODS
There are other existing alternative methods to abrasive blasting which are certainly well-known to the U.S. Navy. Hydroblasting, which is the second-most employed surface preparation process typically requires high startup and maintenance costs. Hydroblasting presents significant problems with waste water containment and treatment as well as flash rusting of prepared surfaces which leads to premature coatings failure unless subsequently removed using abrasive blasting. Production rates for hydroblasting are usually higher than those for abrasive blasting or mechanical methods, but are significantly lower than those possible with induction heating. Dry ice blasting systems requires highly specialized equipment and operator training, and still require removal and blasting to obtain original profiles. Chemical methods are sometimes used for coating removal, but with large-scale projects they pose challenges for waste disposal as well as concerns for worker health and safety.
SAFETY CONSIDERATIONS
Coating removal using induction equipment is inherently safer than methods currently used by the Navy. By contrast, from a safety standpoint, abrasive blasting is extremely hazardous. Typical operating pressures are 150 to 160 psi and injury is commonplace. At these operating pressures, abrasive embedment into skin and severe limb (hands and foot) injury are very common. Typically ventilation equipment must be shut down for cleaning and maintenance, thus blasters must stop working while environmental control equipment is repaired or serviced. Finally, noise levels within and around abrasive blasting operations are very high, and hearing loss to unprotected workers in the vicinity of abrasive blasting is common. Alternatives such as hyrdoblasting present similar safety problems.
CONCLUSION
In summary, the cost of using induction heating to remove coatings for corrosion control and maintenance painting is also very low when compared to other marketed processes. Induction units have very high production rates on even the most difficult coatings, such as non-skid and deck coatings. Induction machines have few moving parts, produce almost no noise, and produce no hazardous waste (other than the disbonded coating), generate no air pollution, and can be used safely outdoors, even in inclement weather.
The use of induction heating for the removal of maintenance coatings is a cost-saving alternative to conventional grit blasting and alternative techniques. Use of the system will cut downtime significantly and would allow greater flexibility in the scheduling of other maintenance activities. The technology can be demonstrated to the Navy and other joint-services at any time.