Effortless assembly

Sept. 18, 2003
New coupling method eliminates the juggling act commonly associated with pneumatic-tube installations.

Leland Dick
H-P Products Inc.
Louisville, Ohio

Over the past several decades, pneumatic conveying of bulk powder and solids has become widely used as a method of material handling within processing plants. Pneumatic conveying pipelines traditionally use compression couplings to join tubing, bends, and fittings. Since their introduction 50 years ago, however, the basic design of compression couplings has changed very little. But a new coupling device promises easier assembly and a strong sure grip. Results are that pneumatic conveying pipes move less in high-vibration environments and users save on installation expenses.

Traditional compression couplings use galvanized steel for the outer shell and a slotted, inner sleeve as well as a slotted rubber gasket. To boost stiffness, two metal bars run parallel to each other at the top of the outer shell. The bars also provide an anchor for the adjusting bolts to pass through. The coupling slips around two adjoining tube ends and is compressed by tightening anywhere from two to five bolts depending on coupling size.

The bolts are tightened in sequence, drawing the outer shell down around the tubes and squeezing the slotted inner sleeve and gasket together for a smooth, uniform fit. A static strip placed between the gasket and the tube eliminates static electricity build-up. In vacuum applications, a thin metal gasket protector between the rubber gasket and the tube OD eliminates the chance that gasket material will be sucked into the conveying line.

In devising the new coupling, input came from sources that included integrators, installation contractors, and plant engineers. They helped identify key design issues not only with the compression coupling but also with other available methods of tube joining. These discussions revealed six design criteria: Ease of installation, minimal leakage, grip strength, tube alignment, repair/maintenance & reuse, and total cost.

The resulting Instalok coupling provides solutions to all six issues by reducing the number of assembly components via use of metal-on-metal connections and O-ring seals. Here's the breakdown on how the new design came about:

Ease of installation: This was the number one issue identified. Compression couplings require a synchronized effort of joining tube ends and sequentially tightening bolts. At times this forces the installer to perform a midair juggling act and attach the coupling while holding all the interior components in place. The new coupling eliminates most interior components and incorporates independent clamps that only compress the ends of the coupling. As a result, the Instalok is lightweight, easier to handle, and can install on one end of a component before final pipeline assembly. Additionally, because the clamps are independent of each other, they can be moved anywhere around the coupling to avoid obstacles and tight places.

Leakage: Air leaks degrade how pneumatic-conveying systems perform. In large systems with numerous tube joints, multiple small leaks add up to poor performance. With the Instalok design, embedded O-ring seals pinch against the tube OD when the coupling clamp is tightened. This is an improvement over the compression coupling which has a gasket that tends to "bunch" at pinch points and around the static strap creating small air leaks. Independent tests under vacuum have shown the new coupling is leak-free at up to 29 HG. It also withstands 100 psi under positive pressure.

Grip strength: This is critical in high stress or high-vibration areas. Compression couplings provide a comparatively low coefficient of friction between the rubber gasket and the metal-tube surface. In some cases, there is enough of an initial jolt when a system activates to move the tube end within the coupling. This can lead to tube ends separating, becoming misaligned, or possibly even pulling out of the coupling altogether resulting in loss of product and system downtime. In contrast, Instalok uses a metal-to-metal grip that bites into the tube surface and delivers approximately three times the grip strength of traditional compression couplings. Independent pull test results show the 3.5-in. Instalok begins to lose its grip at 1,800 ft-lb of separation force. For comparison, the same size compression coupling begins to separate at 600 ft-lb of force.

An additional benefit of the metal-to-metal grip is conductivity. Static dissipation is a critical aspect in pneumatic conveying systems. Friction between conveyed material and the interior wall of the conveying tube produces a static charge. If left unchecked, it may build up enough to shock employees or in some cases spark a dust explosion. Traditional compression couplings use static strips to dissipate charge. The strips represent one more complicating factor during installation and are a potential air leak source. They can also break or eventually wear out.

Tube alignment: Proper alignment of tube ends is key to good system performance. Misalignment can block the line. In plastics processing, misaligned pipes may cause pellet shear resulting in the development of "angel hairs," fines, or streamers that may potentially harm downstream processes. The design of the Instalok coupling provides a "self-centering" feature. Its middle section, located between the O-ring seats, is sized (reduced) to more closely match the OD of the conveying tube. As a result, the tube end centers in place when passing over the O-ring ensuring alignment.

Repair and maintenance: A major concern for installers and maintenance engineers is the time needed to remove compression coupling gaskets that have bonded over time with the conveying tube. Friction can make operating temperatures of conveying lines frequently exceed 200°F. As a result, an installer doing maintenance or relocating a line normally must remove the compression coupling, scrape the old gasket away, then replace it with a new one. There's a cost for the new gasket and considerable time invested in scraping, prepping, and installation. But with O-rings, installers just loosen the bolts, slide the coupling back, and separate the tubes. If an O-ring needs replacement, the cost is a fraction of that for a compression coupling gasket and the installation process is easy.

Total cost: Instalok couplings install anywhere from 35 to 50% faster than compression couplings. Their relatively simple design and use of few components also drops costs about 20% compared to compression couplings. Ease of disassembly and reuse potential offers lower maintenance costs as well.

Instalok couplings are made of galvanized steel for superior corrosion resistance. Coupling OD ranges from 1.5 to 4 in. with a standard O-ring made of black nitrile. Grade five stress relieved bolts help ensure a trouble-free installation.

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