The basic parameters for tool selection in pipe cleaning applications are pipe diameter, whether the pipe is straight or has an elbow, and the length of the run. The location and number of orifices in the cleaning head are dependent on whether the pipe is blocked (requiring forward jets) or has scale on the walls (requiring outward or radial jets) and how much pulling force is needed (requiring rear thrusting jets). Centralizers and extension arms may be used to reduce the distance a jet must travel through the air from the exit of an orifice to the surface being cleaned (standoff distance) in larger pipes (see Figure 1).
Tube cleaning may be done using rigid or flex lances, with the tube size determining the maximum size of the lance and nozzle. In general, the nozzle diameter should be no larger than 2/3-3/4 of the tube diameter and greater than any couplings or hose ends to prevent material from catching. Tube nozzles are typically nonrotating tips with as many as 20 orifices, self-rotary nozzles with 2-7 orifices installed on the end of a nonrotating lance, or tips with 2-7 orifices on a rotating lance.
The two basic types of tube cleaning jobs are polishing (removing scale from the walls of open tubes) and clearing completely plugged tubes. Flex-lance nozzles are usually jetted to produce several pounds of pulling force (thrust) accomplished by the use of rear-facing jets. In unplugging patterns, about 60 percent of the water is directed to the rear jets, which are too poor in quality for effective material removal. When a rigid lance is used on a securely supported lancing machine, there is no need for thrusting jets and more power can be directed forward to clear the blockage.
Large-diameter tanks, vessels and stacks are challenging due to their sizes, access limitations and internal geometries. Open vessels can most effectively be cleaned by slowly raising or lowering a rotating 2-D tool along the axis of the vessel. Complete coverage is achieved with the jets directly aimed at the wall at a close, constant standoff distance and controlled speed of rotation. Unfortunately, this is often not possible due to central obstructions such as agitators inside the vessel. In these cases, 3-D tools can be deployed. A 3-D tool is most effective when left in place to operate through a cleaning cycle. Positioners for 3-D tools allow placement of the tool closer to the surface to be cleaned and out of the way of obstructions.
Self-rotary tools have the advantage of using fewer and thus larger orifices than nonrotary heads, while also achieving complete coverage of the walls of the pipe being cleaned. The rotation speed of the head is determined by the size of the pipe and the nature of the deposit to be removed. Maximum rotation speed is dependent on the diameter of the pipe and the standoff distance. Rotating slower than necessary will increase the time it takes to achieve complete coverage. However, in the case of very thick deposits, slower rotation may be most effective. As a pipe gets larger, the jets move faster across the surface at the same rotation speed. This becomes an important consideration when very large pipes, tanks or stacks need cleaning. In a 10-foot-diameter vessel, the rotation speed should be no more than 80 revolutions per minute (rpm) to maintain a surface speed of 40 feet per second (ft/s) or less. The surface speed in a round pipe or vessel is calculated as: Surface speed (ft/s) = rpm x diameter (ft) / 19.1.
Feed rate can be estimated using the rotation speed and the number and size of jets. Typically, a jet spreads and has an effective impact path greater than the orifice diameter; this factor may be included as a multiple in this equation: Feed rate (in/min) = rpm x number of jets x orifice diameter (in) x jet spread.
When selecting tooling for your next waterblast system, you should adopt a methodical approach to parameters such as pipe or vessel size, standoff distance, jetting and rotation to help you achieve successful results.
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