The need for waterjetting standards and guidance in meeting them has become more urgent as ultra-high-pressure (UHP) waterjetting has become more feasible for large-scale surface preparation projects previously restricted to dry abrasive blasting. Production rates for UHP technology have increased dramatically as the equipment has evolved from handheld tools, to semi-automated tools, to fully-automated machines that provide 100 percent containment of spent water and paint debris. This article reviews the evolution of UHP equipment. Other factors relating to the viability of UHP are detailed elsewhere.
Handheld Tools
One of the largest elements limiting the widespread use of UHP waterjetting has been productivity. Until recently, the most common type of UHP tools were handheld tools. And only three years ago, productivity levels of handheld UHP lances were one-quarter to one-third that of handheld abrasive nozzles, which limited commercial viability. UHP waterjet coating removal rates generally fell int the 20-30 sq ft/hr (2-3 sq m/hr) per handheld UHP tool, compared to productivity ranges of 90-120 sq ft/hr (8-11 sq m/hr) per nozzle for abrasive blasting.
Reasons for the low productivity included operating pressures that were in the 30,000-36,000 psi (4,400-5,200 Mpa) range as well as slow nozzle rotation speeds. But recent advances have increased the productivity level of these tools dramatically. For example, pressure ranges have increased to 40,000 psi (5,800 Mpa); higher nozzle rotation speeds in the 3,000-3,500 rpm range augmented advances in operating pressures; and advances in nozzle technology have improved productivity. These higher pressures and nozzle advancements have pushed productivity to rates comparable to handheld abrasive blasting. Typical removal rates for UHP are now 80-100 sq ft/hr (7-9 sq m/hr), making UHP comparable to dry abrasive blasting of similar coatings from similar substrates.
handheld tools are versatile and will always be required for detail work, smaller projects, and areas that large machines cannot reach. However, despite advances in their technology, they are limited in productivity because of operator fatigue and the amount of thrust that an operator can handle. Theoretically, productivity could be increased by adding more volume (power), but human operators can safely handle only a limited volume, about 3 gal./min (11 L/min) at 40,000 psi (5,800 Mpa). This volume equates to about 30 lbs (14kg) of thrust, which limits productivity.
One other limitation of handheld tools is that they may leave surfaces wet long enough for light flash rusting to form. Of course, most manufacturers now have coatings available that can be applied directly over light surface rust with no additional surface preparation steps. Occasionally, however, such as in conditions of high humidity or rain, an additional surface preparation step after handheld waterjetting is needed before coating application.
Semi-Automated Systems
Modifications of handheld tools led to semi-automated wheeled systems that operate much like a lawnmower. This development helped eliminate operator fatigue and thrust limitation, thus increasing overall productivity. Semi-automated systems enable one operator to put the full horsepower of one pump on the work surface. Semi-automated tools can operate with as much as 6.5 gal./min (25 L/min) at 40,000 psi (5,800 Mpa).
Typical production rates for a single operator using these tools range from 300 400 sq ft/hr (27 to 36 sq m/hr).
However, semi-automated tools still have limitations. An operator must be in proximity to the waterjetting; some form of external containment is required; the tools are limited to horizontal surfaces; and the cleaned surface is still susceptible to flash rusting.
Fully Automated Systems
To overcome the limitations of other types of waterjetting tools, fully automated robotic systems have been developed. One recent example is a vacuum-attached free-crawling vehicle. The system features four main components: a 40,000 psi (5,800 Mpa) waterjet pump; a vacuum-attached free-crawling robot; a vacuum pump module that both adheres the robot to the work surface and transports the water and paint chips; and a final filtration system where the paint chips are filtered our into a large disposable fabric filter bag. For lead-based or other hazardous paint removal, a high efficiency particulate air (HEPA) filter can be added.
The productivity of this system is not limited by human factors such as thrust and fatigue. Productivity is also enhanced through robotic control of the waterjet nozzle, which provides the waterjet a very controlled motion to fully optimize the cleaning pattern.
Production rates up to 1,000 sq ft/hr (90 sq m/hr) can be achieved with this type of system. Only a single operator is required to operate the system. At least one model is easily operated by a joystick-type controller that can be as far as 250 ft (76 m) from the system. The high productivity of this tool has led to the use of UHP waterjetting on very large area removal projects that were not feasible with handheld tools.
All water and paint particles are immediately picked up by the robot and the surface is heated by the waterjets, resulting in a clean and dry surface that is suitable for immediate repainting. A robotically cleaned surface will remain free of flash rust unless rain or very high humidity contacts the surface.
Early robotic systems could operate only on flat horizontal or vertical surfaces. Models recently developed incorporate a vacuum-attached free-crawling vehicle that can clean horizontal, vertical, overhead, or curved surfaces.
Conclusion
Technological developments in waterjetting equipment have allowed the process to become feasible for large scale projects where productivity demands are high. As the technology continues to be refined and industry acceptance continues to grow, the need for consensus guidance for industry practice will continue to increase, also.
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