Flow vs. impact. If you need to increase impact, increase flow, not pressure. Go to a larger-flow unit and keep the pressure down.
Nozzle size vs. quantity. Watch the nozzle count. Fewer nozzles keep the individual orifices large, and large nozzles generate large droplets with higher impact. Although fewer nozzles usually are better, there is a trade-off. When only a few nozzles are present, they must be very wide angle, and wide-angle flat fans tend to have lower impact. The optimum nozzle count is four, with three coming in second. Five or more nozzles necessitate smaller orifices.
Distance vs. impact. Distance does not help. The farther the tank wall is from the nozzle, the less the impact. Keep the nozzle sized appropriately for the tank. See Fig. 2.
Figure 2. Tank-Washing Nozzle Selection
This chart can assist in the selection of a tank-washing nozzle by calculating the flow rate required based on interior surface area.
What is the operational flow of the unit, and how is it distributed? "How much liquid does it take to wash my tank?" is a common process-specific question, but guidelines do exist.
One of the most common rules of thumb is 2.5 gpm to 4.0 gpm per foot of linear circumference for vertical cylindrical tanks with a height of less than four times the diameter. Another is 0.25 gpm to 0.40 gpm per square foot of interior surface for square tanks or those with more complex shapes. Sometimes the values go even higher.
These guidelines originated around the process of tank washing with stationary spray balls and assume all surfaces are being sprayed at the same time. The testing suggested this process is not efficient.
Instead, the tests indicated that the sweeping action of a rotating nozzle increases the cleaning efficiency substantially. For an appropriately sized unit in which the soil is not highly aggressive, it would be better to use a guideline closer to 1.0 gpm per foot of circumference. For other tank configurations, a guide of 0.1 gpm per square foot of interior also will work.
These numbers might be too low if the cleaning action is particularly difficult. The ultimate selection is something you will have to base on testing and experience.
The liquid's placement in the tank can be just as critical. Is the spray hitting where the soil is? Tank washers offering 360-degree coverage ," a common design ," cover the entire interior, including the top and bottom. Some shadow exists directly overhead, but a good design minimizes this. In a cylindrical tank of roughly equal height and diameter, this means less than half the spray hits the vertical walls. Most solution washes the top and bottom. However, units are available that omit straight-up or straight-down spraying, concentrating more of their efforts on the walls.
Tank-washing machines that rotate on two axes spend most of their effort washing the areas immediately above and below the unit and have the least power directly opposite themselves horizontally. Although this scenario is unfortunate, a better practical approach to washing large tanks is currently not available.
What is the distance from the target surface to the tank washer? Distance is the enemy of effective cleaning. Get as close as you can to the walls. To accomplish this, you might have to wash with more than one unit or from more than one insertion point.
Keep in mind that vertical distance is just as important as horizontal distance. If your tank is only 4 feet (ft.) in diameter but 10 ft. tall, do not fool yourself into thinking the wall is only 2 ft. away from the tank washer. As mentioned earlier, no specific tank washer design jumped out of the selection as a clear front runner. No guarantees are possible without a direct analysis of a specific cleaning process.
Available in an array of shapes, sizes and configurations, tank-washing nozzles and machines are available from more than a dozen manufacturers.
Suggested cleaning guidelines
Easy cleaning processes. A good example of an easy cleaning process is when paint is rinsed out of a tank using a process solvent. The solvent dilutes and rinses away the residue, assuming the residue has not dried.
If the cleaning task is not complex and the tank is not very large, just about any tank washer will work. If the tank is less than 8 ft. to 10 ft. in diameter, a small, inexpensive free-spinning unit in stainless steel or plastic should fit the bill. The orifices can be flat fans, slits or solid streams.
For simple rinsing, remember to:
Make sure the nozzle has sufficient spray coverage to hit all areas needing rinsing.
Verify that the spray can reach far enough at the operating pressure to hit the most distant points.
Keep the pressure appropriate for the design. More is not always better.
Make sure the flow rate is at least 0.1 gpm per square foot of interior surface. A flow less than that might not provide sufficient wash down the walls to cover areas where the nozzle might be weak.
Place the nozzle in the center of the tank about one-third of the way down from the top.
Consider the need for more than one nozzle to cover everything if internal obstructions exist.
Moderately difficult applications. As the cleaning task becomes more difficult with a more aggressive soil, the washer choice becomes more critical. An example of this type of application involves removing a fat- or grease-based product residue using a hot solution of sodium hydroxide (0.5 percent to 2 percent typically). Free-spinning units still might work well, especially those with higher flow rates.
Designs with friction bearings (no rolling elements) tend to turn more slowly naturally, helping to maintain spray effectivness. Controlled rotation designs also can improve the impact. Tank-washing machines probably are not needed at this level, however.