Drying a Ultra High Pressure Water Jet Re-Coat Project
June 18, 2009
Industrial blasting and coating jobs require suitable weather conditions, appropriate temperatures and dry air. Increasing the moisture load by blasting through ultra high pressure water jets makes controlling the environment even more challenging. Without proper climate control, humidity and dewpoint levels can potentially wreak havoc on plans to complete such tasks. Such was the challenge faced by Neptune Bulk Terminals Ltd. of Vancouver. In a case history first published in the first published in the 2009 Society for Protective Coatings conference proceedings, Gregg Lowes, the industrial account manager for Munters Moisture Control Services (MCS), Surrey, B.C., Canada, explains how desiccant dehumidification helped ensure a quality finish.
Neptune Bulk Terminals Ltd. was established in 1967 on the North Shore of the Port of Vancouver's inner harbor. It has an annual throughput capability in excess of 17 million metric tons. The facility includes a dedicated coal berth, two dry bulk berths, bulk liquid capability over two of three berths, storage for coal, potash, agricultural products and canola oil and site area of 71 acres plus water lots. Commodities handled include coal, potash, fertilizers, canola oil and agricultural products.
To prevent interruption of international ship loading schedules, Neptune executives sought to blast and coat two large ocean-going potash loading conveyors within a 27-day window. They selected Certified Coating Specialists (CCS) as the industrial painting contractor because CCS had successfully completed two coal ship loaders for Neptune in previous years. The Moisture Control Services (MCS) division of Munters was awarded the climate control portion of the work.
A unique component of the Neptune Bulk Terminals project was the contractor’s decision to utilize ultra high pressure water jetting (UHPWJ) for the steel surface preparation rather than the sand blast method used on the previous loader conveyors. The contractor realized there were clear advantages to using the water jet blasting vs. sand blasting. In sand blasting applications, a blast aggregate is used to blast paint off of the metal. It leaves a dry surface but one has to protect the surface from thermal rusting which is typical of surface preparation applications. In addition, sand blasting required that tens of thousands of pounds of sand blast media be shipped to site by truck, blasted and then removed from the area.
Using water jets required only bringing in the jets, managing the water stream that was leaving the enclosure, and making sure the dehumidifiers were running properly. The use of water jets required special attention to the need to rapidly dry the steel upon completion of the blast work and emphasized the importance of utilizing desiccant dehumidification equipment for drying the containment areas.
The project was bid to conform to SSPC-SP 12, “Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating” set forth by the Society for Protective Coatings. This standard describes the use of waterjetting to achieve a defined degree of cleaning of surfaces prior to the application of a protective coating or lining system. The project also specified that dehumidification be in conformance to SSPC-TR3 standard for dehumidification and temperature control during surface preparation, application, and curing for coatings/linings of steel tanks, vessels and other enclosed spaces.
Project StagesThe entire project required the management of the enclosure, blast work, drying and coating application. In the first step, CCS contracted a scaffolding contractor to build a frame around each conveyor and built a floor suspended below the conveyor for traffic. Then, the contractor shrink wrapped the area for full environmental containment of lead paint and waste water run-off control at dockside. Although they were not uniform enclosures, each containment area was nominally 175 x 40 x 30'.
Ultra high pressure water jetting began after the area was contained. Eight total guns running at 35,000 psi for 20 hours a day released an estimated 2 gal/min of water into the enclosure and blasted off hot potash buildup and paint from the steel conveyors. After the water jetting, dehumidification equipment was activated to immediately dry the bare wet steel and hold indoor air conditions for prime and finish coat painting. MCS was allotted 48 hr in the schedule to dry the assembly and ready the area for coating.
After the initial drying in the containment, the average conditions were maintained at 68 to 72°F (20 to 22°C) and ~35 percent relative humidity, with the resultant dewpoint near 40°F. This allowed the base coating - a surface tolerant high-build epoxy primer with aluminum pigment - to be applied just a few hours after drying began, and within ideal dry times for the product. The finish coat - a high build, high solids surface tolerant epoxy maintenance coating, also with an aluminum pigment - was then applied.
A unique component was how the dehumidification distribution was set up within containment. Generally in surface preparation, there is only one opening for the dehumidified air to enter, but MCS technicians for this project determined that an internal ducting distribution system would be the most effective. This system ensured that all areas of the containment received dehumidified air.
Though the decision to blast via water jet was the more efficient option, it also made the use of climate control equipment to prepare the area for coating a critical component. Water jetting leaves an extremely wet surface because it uses ultra high pressure and certainly raises the surrounding moisture and humidity levels, especially in confined spaces. It was the task of the climate control company to hold the blast and physically remove the moisture in a very short timeframe so that the bulk water from the blast did not create additional rust.
Munters used the DHI-125-ESU integrated custom air handlers (ICA), which operate on both natural gas and electric power. MCS operated two units on each conveyor for a total of between 18,000 and 20,000 cfm for each containment area. During the Neptune project, the desiccant dehumidifiers equipment ran on propane reactivation and electric generator as a cost-saving measure as using all electric power generation would have been more expensive. This decision resulted in an overall reduction in fuel costs of about $6,500 per containment compared to all electric diesel generator fuel consumption. The unit utilizes Munters dehumidifiers also incorporated an energy-recovery option that improves performance by delivering air at drier levels, while using less energy than traditional active desiccant dehumidification systems.