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Since inspection crawlers were introduced, they have evolved from a handful of basic models to a vast array of configurations addressing lines as small as 4 in. and up to and beyond 60 in.
Unfortunately, this wide selection has also been joined by an uncertainty about crawler size among many municipal, operations and maintenance professionals. What size will deliver the most productivity and offer the best results in relation to specific pipeline characteristics? How is the need for traction balanced against the requirements for overcoming obstacles or steep inclines?
Like most carefully planned decisions, the answers include properly understanding the specific size attributes of each crawler and then making an informed investment based on a thorough understanding of the applications. For instance, some decisions are made easily by realizing that a large mainline crawler will not fit into a 4-in. pipeline or a 4-in. capable crawler lacks the ability to navigate a 60-in. trunk line with high flow.
Many applications, however, are not as extreme. Most inspections fall closer to the center of the bell curve and are performed on pipelines within the 8- to 24-in. diameter range. As a result, in recent years many municipal and maintenance personnel have chosen more compact designs that offer the flexibility to tackle larger applications.
This is because many compact crawlers are now fully modular and can be easily reconfigured to various line sizes through the use of interchangeable wheels, lamps, cameras and camera elevators. Another benefit provided by compact crawlers—especially those with a short wheelbase and six-wheel drive—is their ability to maneuver around protruding pipe taps, over thick patches of sediment and up offsets.
Furthermore, while bulk is often associated with sturdiness, size and weight can also work against a crawler in certain applications. In contrast to their larger counterparts, compact crawlers tend to pull with less force on their lightweight cables, which in turn reduces cable strain and related service issues. Moreover, when service needs do arise, compact crawler systems are cheaper to ship, maintain and repair.
While larger crawlers have traditionally been used for applications requiring increased traction due to their added weight, smaller crawlers have recently been designed for similar applications by allowing users to swap out wheel sets for maximum grip on a variety of pipe materials. Common wheel options include spikes for PVC, abrasives for grease, etc.
In addition, weight is not always an advantage in some applications. For instance, a large four-wheeled crawler may bog down in thick sediment, whereas a more compact crawler with six wide wheels is more likely to traverse such areas.
Range & Maneuverability
Range is ultimately meaningless if the crawler cannot maneuver over and past obstacles. As a result, users who regularly encounter debris, offsets, protruding taps and curved inverts must not ignore issues of maneuverability and steering ability. More than anything, it determines the percentage of segments the crawler can be expected to complete and the frequency with which it must backcrawl from an adjacent manhole to complete a segment.
In addition, while larger crawlers can be expected to roll over larger obstacles, a better plan of action may include not rolling over obstacles in the first place. Protruding pipe taps, for instance, cannot be rolled over. The only alternative to aborting an inspection is to steer around them.
Furthermore, effective steering typically requires a short wheelbase, because the longer wheelbase of a larger crawler demands a turning radius unavailable in a pipeline. In comparison, a shorter wheelbase can also help a crawler past other obstacles such as offsets. For example, the shorter the wheelbase in relation to wheel diameter, the steeper the offset a crawler can usually climb. (Better still if your crawler is six-wheel drive). And only a short, steerable wheelbase will provide maneuverability through the tight bend of a curved invert without flipping or wedging.
Where garden-variety debris and sediment are concerned, the capabilities of a crawler depend largely on ground clearance and the ratio of wheel diameter to axle distance. In other words, holding ground clearance and wheel diameter constant, debris-prone environments favor shorter wheelbases.
Sometimes getting to the jobsite is half the battle. Access points for municipal lines often lie on narrow streets, busy thoroughfares or remote easements requiring off-road capability. No doubt, some mainline inspections may require the full armada of support gear. More common work in lines 24 in. and smaller, however, does not have to be so unwieldy.
Fortunately, crawler size can have a cascading effect on support gear. For example, the narrow cable of a compact crawler spools up on a much smaller reel. Compact crawlers and reels demand less electrical power, so generators can be smaller, too. When other items are factored out—crane, winch, space for large equipment trunks—you are soon looking at the use of a compact, maneuverable Sprinter chassis rather than a high cube van, or perhaps even a trailer or pickup bed console.
While there is much to consider when purchasing the best crawler, the decision does not have to be purely academic. Municipalities and contractors should request a field demo of every crawler under consideration. This demonstration should also be performed in arduous situations that include partial collapses, protruding taps, curved inverts, offsets, debris, roots and grease. Only through exhaustive field trials will the best crawler for your specific needs be revealed.