Machine Design

Weighing the advantages of tubing versus pipe

Tube bending improves fluid-flow characteristics by creating compact systems with smooth directional changes. Conversely, the 45 or 90° elbows used in piping systems frequently increase turbulence.

The demands placed on today's fluid systems are much tougher than they were a decade ago. Leaks that were once considered nuisances are now classified as fugitive emissions and hazardous spills, problems that can shut down plants. Modern processes are pushing pressure, flow, and temperature requirements to new heights every day, yet the basic concepts of connecting pipe have not changed much over the years. In fact, the integrity of most piping systems depends on sealing and fastening methods developed nearly a century ago.

Tubing provides one solution to tougher regulations and increasing stresses on fluid systems. Tubing and pipe both carry fluids in industrial applications, but tubing offers several advantages over pipe. Compared with pipe, tubing is generally easier to install and maintain, costs less, and performs better.

First of all, tube fittings are designed not to leak, even under rugged operating conditions because they have mechanical, metal-to-metal seals. Tightening fitting nuts cold-swages ferrules onto the tubing. On the other hand, threaded-pipe connections typically require heavy equipment to cut threads and pipe dope or TFE tape to minimize leakage. In addition, the greater flexibility of tubing often means systems with fewer connections. This equates to fewer potential leak points and lower costs.

Tubing is readily available in diameters up to 2 in. with a variety of wall thicknesses and materials to meet the toughest application demands. It's important to note that tubing and pipe are sized differently. A 1-in. pipe, for instance, has a 1-in. nominal bore, while a 1-in. tube has a nominal outside diameter of 1 in.

Another advantage of tubing is that it can generally be installed faster than pipe. Tubing is cut to length and deburred, and then fittings are simply tightened onto a nut. Some tube-fitting connections can be gauged to ensure correct installation before starting up the system.

Installation of a 1-in. tubing system takes about 12 min/connection. Compare that to 1-in. Schedule-40 pipe, which requires about an hour to cut, prep, align, and weld each connection. Screwed pipe fittings require an average of 48 min to connect and align. And the cost of skilled welders and pipefitters required to install pipe systems make tubing a financially wise choice.

If welding is the only option, however, tubing and an orbital welding system can still outperform pipe. Orbital welding systems are typically quicker and simpler than manual pipe welding because fittings are designed specifically for tubing.

Tube systems also reduce overall installed costs. Tubing is in nearly all cases less expensive to use than pipe. Although tube fittings carry a higher initial price than similarly configured pipe components, in the long run tube systems save money due to reduced downtime and simpler assembly and disassembly.

When adjustments are necessary, tubing is flexible and, thus, can be easily repositioned. This makes it possible to completely align systems before final hookup, which is especially beneficial when installing "field-routed" systems. Modifying an installed system only requires a hacksaw, wrenches, and a few new parts. And because tube connections have higher leaktight integrity than pipe connections, the systems can be reassembled easily and reliably.

Tube systems are also relatively easy to disassemble. Virtually every fitting provides a location for quick disassembly. Most fluid-handling systems are tweaked and modified on a regular basis, so quick disassembly is important. In tubing systems, users can replace an elbow with a tee in a matter of minutes, and most existing hardware can be reused. Disassembling threaded pipe, on the other hand, is more labor intensive because each component must be handled individually and sequentially. And welded pipe systems can only be disassembled by cutting, which is a lengthy process.

Tube systems also support high pressures and have, on average, a higher strength-to-weight ratio than pipe. Tubing systems produce better flow characteristics because the smooth internal surfaces reduce flow losses and drag. In addition, the internal diameters of the tubing and fittings are nearly identical, which reduces pressure drops and turbulence.

This information supplied by Swagelok Co., Solon, Ohio.

TAGS: Hydraulics
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