Advances in polymer chemistry over the past two decades have resulted in improved formula- tions that have increased flexible tubing's flexibility and elasticity and have made it an ideal safe choice for potable water treatment applications.

The excellent flexibility and elasticity exhibited by modern polyethylene, poly-urethane and PVC tubing formulations provide a number of benefits for system designers and installers, including:

• Tight radius bends that can be negotiated without kinking or flow restriction,
• Simplified design layout that allows bends and turns to be made with the tubing instead of the connectors (in rigid tubing designs, additional connectors are required at each turn point which increases the complexity of the system),
• NSF Standards 61 and 51 certified flexible tubing is widely availablein sizes and configurations to meetalmost any requirement,
• A superior seal results when flexible tubing is coupled with single-barb fittings and ...
• Single-barb fittings take full advantage of flexible tubing's elastic memory properties virtually eliminating the need for clamps in many situations (see below).

A single barb fitting's engagement with an elastomer tubing comes closer to a radial O-ring seal than does a multiple barb fitting's, since a single barb allows the tub-ing to relax more snugly behind the barb.

Expanded options

With the introduction of Standard 61 certified tubing and NSF certified single-barbed connectors (see fig.1), the pos-sibilities are greatly expanded for creating compact systems that fit comfortably within confined areas. With an installation that uses rigid tubing and push-on con-nectors, a primary concern, in tight spaces, is the potential for leaks resulting from misalignment and lateral stress at the connector. This isn't a a concern with flexible tubing / single-barb connector designs. In addition there's a tendency for rigid tubing to transmit water hammer, and expansion and vibration forces to the point of connection. In a large installation, the

Figure 1. Single barb fitting

rigid tubing will absorb some of this stress. However, In an installation with shorter runs, almost all of the vibration is transferred. In a flexible tubing installation, stress and vibration are dampened and absorbed regardless of the length of tubing used. This often results in much quieter operation and increased reliability for the entire system.

Single-barb fittings

Single-barb connectors are an essential component for building systems with flexible tubing. They offer both safety and mechanical advantages when compared with multiple-barbed fittings (see fig.2). With a single-barb fitting design, it's possible to manufacture a barb without a mold parting line. This is essentially im-practical with multiple barb designs. Parting lines can be a source of leaks in a connection and provide a pathway for bacterial and fungal migration.

Figure 1. Multiple barb fitting

Multiple-barb fittings

Multiple-barb fittings manufactured with worn or misaligned mold sections are likely to have excessive mold parting lines. Make certain that the barb fittings you choose don't have a parting line on the barb surface. It requires an extra step in the molding process to produce this type of barb, but the benefits far outweigh the additional cost. One advantage of the single-barb fitting design is its ability to offer better sealing than multiple-barb fittings. A single-barb uses a straight or slightly tapered shaft behind the barb with a diameter close to the interior tubing diameter. This allows the flexible tubing to relax to its original shape behind the barb, creating a tight seal with the barb's outer radius. Some designs incorporate anti-rotation devices along the straight surface behind the barb. The devices prevent the tubing from rotating around the shaft and causing wear to the seal. In contrast, multiple barbs create a bridging effect that doesn't allow for full relaxation of the tub-ing behind the individual barbs. This bridg-ing effect can allow tubing rotation on the barb, result in less overall tube-to-barb contact, contribute to leak path formation and create dead space between the barbs where micro-biological pests can accum-ulate.

The perceptions of many people regard-ing tubing and barbed fitting combinations are based on experience with semi-rigid polyethylene tubing and multiple-barb connectors of the type commonly used in lawn irrigation systems. These compon-ents, designed for non-critical applica-tions, cut a system's cost, but are prone to leaks and pressure blowout. Because it has weak elastic memory, semi-rigid tubing exacerbates problems inherent in systems designed with multiple-barb fittings and often requires the use of extreme clamping pressures to prevent leakage and blowout. Designs utilizing flexible tubing and single-barb connectors, however, have a proven record of con-sistent reliability over many years of use in medical device, pharmaceutical, and chemical industry applications (see fig. 3).The utility of flexible tubing and single-barb fitting combinations has furthered their accepttance in a growing number of applications including reverse osmosis

Figure 3

and water conditioning industry segments, where their use has been steadily on the increase.

Conclusion

Current economic conditions have led to a renewed focus on bottom-line costs as companies are forced to operate in in-creasingly competitive markets. Employ-ing flexible tubing and single-barb fittings is an easy way to reduce system costs in many water applications. Single-barb fittings are easy to install and generally cost less than push-on or compres-sion-type fittings. Economic benefits are further enhanced because the turns and bends made with flexible tubing reduce the number of fittings required to plumb a system. Significantly, resultant savings in material and labor costs are achieved with no compromise in the quality and reliability of the system.

About the Author

• Marcia Sampson is co-founder and CEO of the Eldon James Corp., of Loveland Colorado. She has been involved in the production of plastic hose fittings for more than 15 years. Her father Kent Sampson, was a pioneer in the development of single-barb fittings for use in medical systems and invented the Kent