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The 4 Keys to Choosing the Correct Crawler for Your Needs

Mon, 10/01/2007 - 12:02pm

By Richard Lindner and Mark Burcham


As with any maturing technology, the inspection crawler market has evolved from a handful of basic models to a vast array of configurations, each uniquely adapted for nearly any chemical pipeline inspection application. For buyers, this can be a double-edged sword since finding the right crawler among the myriad of choices can require research, forethought, and inevitably some compromise. So, here are some important issues to consider when determining the ideal crawler size for your application.
1. Basic Compatibility


Crawler size can determine how you maneuver to overcome common pipeline obstacles.
A large mainline crawler won't fit into a 4-inch line, nor could a 4-inch-capable crawler comfortably navigate a 60-inch trunk line with high flow. Special applications require special crawlers. Fortunately, most inspection work falls closer to the center of the bell curve — 8- to 24-inch lines — and most crawlers sold today target these applications. Your job is to decide what size crawler best handles this range of pipe sizes.

Historically, larger crawlers have normally been the first selection. In the beginning, camera technologies were bulky so crawlers had to be big — no other option existed. In addition, some crawlers must also be large enough to carry heavy auxiliary equipment such as cutters and lateral launch cameras.

However, the latest logic says to pick the largest crawler that performs well in the smallest line. In other words, it's easier to scale up than scale down. Many smaller crawlers are fully modular, which means they reconfigure to various line sizes with interchangeable wheels, lamps, cameras, and camera elevators.

2. Traction & Range

Weight improves traction but can get you stuck in sediment. Lightweight cable and high-grip wheels compensate for reduced crawler weight.
Larger crawlers are heavier, and weight can improve traction, offering greater travel range. Smaller crawlers compensate by using narrower-gauge conductors (motors in lighter crawlers draw less current), and some also incorporate networked control architecture, which reduces the number of conductors a crawler cable must carry. As a result, these cables weigh up to four times less than traditional types, and yet they maintain exceptional pull strength with tough Kevlar jacketing.

To further offset the traction advantage of weight, many small crawlers offer a modular design that lets you swap out wheel sets for maximum grip: spikes for PVC, abrasive for grease, and so on. Since traction is determined by the static friction between two surfaces (the wheel and the pipe wall) and the force pushing them together (crawler weight), improving friction with purpose-built wheels compensates for lower weight. Nor is weight always an advantage. Consider thick sediment, where a large four-wheeled crawler may bog down, but on top of which a smaller crawler with six wide wheels can continue traveling.

Ultimately, range means nothing if you can't get past the first obstacle. Users who regularly encounter obstacles mustn't ignore issues of maneuverability and steerability. More than anything, it determines the distance you can expect to complete, and the frequency with which you must back-crawl from an adjacent point of entry to finish a line.

3. Maneuverability Around Obstacles


If you have the short wheelbase and steerability to pass through a bend, you can boost productivity by skipping access points.
Generally speaking, the larger the crawler and the greater its overhead clearance, the bigger the obstacle it can roll over. However, this conventional wisdom is subject to many exceptions since the best strategy — and sometimes the only viable one — is not to roll over the obstacle in the first place.

Protruding pipe flanges, for instance, cannot be rolled over. The only alternative is to steer around them or abort the inspection entirely. Effective steering typically requires a short wheelbase because longer wheelbases on larger crawlers demand a turning radius unavailable in many pipelines.

A shorter wheelbase also gets a crawler past other obstacles. With offsets, for example, the shorter your wheelbase in relation to wheel diameter, the steeper the offset you can climb. (Better still is your crawler having six-wheel drive.) And only a short, steerable wheelbase will get through tight bends without flipping or wedging.

Where garden-variety debris and sediment are concerned, the capabilities of crawlers also 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. Even better, when you combine steerability with a short wheelbase, you can always take the path of least resistance around debris.

4. Beyond Pipe

Crawlers with interchangeable parts adapt to a broad range of pipe sizes, materials, and conditions.
If you're like most operators, a lot of time is spent on travel, setup, and maintenance. Crawler size impacts these areas, too, and while in-pipe performance is paramount, it's worth looking beyond, especially if budget and productivity are important.

Sometimes getting to the jobsite is half the battle. No doubt, some larger lines may require the full armada of support gear. More common work in smaller lines, however, doesn't have to be so unwieldy. Crawler size has a cascading effect on support gear. For example, the narrow cable of a compact crawler spools up on a much smaller reel. Smaller crawlers and reels demand less electrical power, so your generator can be smaller, too. Factor out other items you won't need — crane, winch, space for large equipment trunks — and pretty soon you're looking at a compact, maneuverable "sprinter" chassis rather than a high cube van, or perhaps even a trailer or pickup bed console.

In addition, smaller crawlers pull with less force on their lightweight cables, and steerable crawlers require no periodic yank into alignment. Reducing these sources of cable strain reduces the frequency of cable re-terminations, the most common service procedure. Moreover, when service needs do arise, smaller crawler systems are generally easier to maintain and cheaper to ship.

Concluding Advice
Finally, request a field pipeline inspection demo for each crawler under consideration. Then, throw in all the tests — partial collapses, protruding flanges, bends, offsets, debris, grease, flow — and let the competition begin. In the end, your company will be the true winner.

Richard Lindner, president of Envirosight LLC, has 15 years of experience in the video inspection industry. Envirosight manufactures video pipeline inspection equipment for the maintenance and rehabilitation of underground infrastructure. The company's Rovver crawler, SuperVision crawler, and patented QuickView zoom inspection camera serve environmental and industrial applications around the world. Mark Burcham is the trenchless technologies manager for Lyttle Utilities, a contractor specializing in trenchless repair, manhole rehabilitation, lateral lining, point repair, new construction, inspection, and cleaning. More information on the technologies mentioned in this article can be found at www.envirosight.com.

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