Mechanical hose barb connections, which consist of the hose barb, tubing and retention device, are ubiquitous in single-use systems (SuS). They look deceptively simple, but the process of bringing all three elements together to create a reliable connection that consistently meets application requirements must be controlled based on risk assessment.
The hose barb
A secure and reliable connection is determined by how well the fitting and tubing work together, so selecting the proper barbed connector is just as important as the choice of tubing and can ensure product performance, increase ease of use, and add functionality to your product designs.
Hose barbs are available in many different styles and are typically described by the nominal inner diameter (ID) of the tubing they are intended to be inserted into. This description should not be interpreted as a guarantee of performance of the connection with any specific sized tubing. There are no consensus standards for barb dimensions to fit tubing IDs. In many cases, a barb may be used with different ID tubing than described.
The choice of barb depends on a number of factors including the material, elasticity, durometer and dimensions of the tubing; the use and selection of a retention device; the ease of assembly; the intended use; and required pressure and pull-off resistance of the tube-to-barb connection. For general use in a laboratory setting, ease of assembly and convenience may influence the choice more strongly. For SuS where integrity is more imperative than the shelf life, the security and reliability of the connection may drive the decision.
The requirements for the connection should be assessed and appropriate technical due diligence used to ensure connection performance is aligned to the user’s requirements. (Refer to ASME BPE – 2019, Mandatory Appendix III, Section 7.1 for specific requirements for hose barb connections used in SuS for bioprocessing.)
Qosina offers a selection of single-use barbed tube-to-tube connectors in the styles shown below:
- Nominal size range: 1/16”–1/4”
- 50% oversized barb
- Oversized barb stem
- No parting line on barb
- Nominal size range: 1/8” – 1”
- 18% to 34% oversized barb
- Oversized straight barb stem
- No parting line on barb
- Nominal size range: 1/16” – 1”
- 12% to 57% oversized barb
- Tapered barb stem
- No parting line on barb
- Nominal size range: 1/8” – 1”
- 10% to 25% oversized barb
- Barb stem sized equal to nominal tubing ID
- Anti-rotation ribs on barb stem
- No parting line on barb
- Nominal size range: 1/16” – 1/4”
- 50% to 100% oversized barb
- Oversized barb stem
- No parting line on barb
- Nominal size range: 1/16” – 3/8”
- 40% to 60% oversized barb
- Oversized barb stem
- No parting line on barb
- Nominal size range: 1/16”–1/4”
- Straight line feature
- No parting line on barb
- Nominal size range: 1/16” – 1/4”
- 40 to 70% oversized barb
- Extended barb
- No parting line on barb
The tubing
Flexible tubing is available in a variety of materials, dimensions and mechanical properties. Typical materials include platinum-cured silicone, thermoplastic elastomer (TPE) and PVC. Tubing IDs may be available with different wall thicknesses. The elasticity and durometer of the material also influences the ease of assembly and performance of the connection.
Match materials to the application
The choice of tubing is driven by many factors including pumpability, weldability, chemical compatibility and perhaps direct experience with the tubing choice. These factors compound the importance of a diverse selection of readily available tubing choices for your single-use application.
The tubing solutions available today all exhibit different levels of flexibility, a factor that plays a large part in how well a connection performs.
In general, the softer the tubing, the more likely that a hose barb type fitting is appropriate for the application. While it is the ID of the tube that largely determines selection of the proper fitting, there are other factors to consider:
-
Pull‐off resistance: When a tube is pulled, it tends to contract and grab more tightly. A particular fitting will therefore exhibit different tensile strength characteristics for different sizes and grades of tubing. If the barb is too sharp and the tubing is very soft, enough pulling and vibration could cut the material and cause leaks and failure. On the other hand, a shallow or rounded barb mated with very stiff tubing may allow a tube to disengage with minimal pulling force.
-
Blow‐off resistance: Spikes in pneumatic or hydraulic pressure tend to make tubes expand, potentially loosening the grip of the barb. For high-pressure applications, the shape and arrangement of barbs and the relative flexibility of the tubing determines how the connection will perform under expansion of the tubing material.
Reduced blow-off and pull-off resistance is more likely if there is a mismatch between barb design and the durometer of the host or tubing. A poor match can cause other problems, too. It can make installation and maintenance much more difficult, and this can lead to leaking.
-
Ease of installation: Shape and placement of barb(s) on fittings combined with tubing flexibility determine the force required to connect the fitting to the tubing. Easing the burden of the installation technician is one issue to consider. More important, if it is too difficult to push tubing onto the fitting, the tubing may not grip properly and open the possibility for leaks and failure.
Reference: Get a Grip! An introduction to hose barbs, Colder Products Company
Hose/tubing inner diameter
Another key component of determining tubing terminations is the accuracy of the inner diameter of tubing. Given the inherent variability in tubing and hose material, it is important to carefully select a fitting or coupling that meets the inside diameter specification of the tubing being used. There are no consensus standards for barb dimensions to fit tubing IDs. In many cases, a barb may be used with different ID tubing than described.
Outside diameter of the barb
The OD of the barb(s) needs to be correlated to the ID of the tube it will be attached to. When designing a connector, the accuracy of the tube's ID, material, tube flexibility and application will be critical. For tighter seals where no clamp will be used, you'll need to increase the OD of the barb(s), but be careful as to not go overboard. If you do, installation could become your biggest bottle-neck.
Sharp vs. rounded edges: Depending on the tube/hose material used, you'll need to use a barb that is just sharp enough to grip/seal the tube, yet round enough as to not cut into the tube.
Parting line: In making a plastic part, a mold opens and closes as each piece is produced. In this process, a parting line is created: a seam that produces a slight imperfection on the plastic part where the two halves of the mold meet. On the barb of a fitting, a mold parting line invites leaks and system failure. Parting lines are especially prevalent with a poorly molded part or where the manufacturer has skimped on tooling investment or maintenance.
The retention device
Retention devices are generally placed closely behind the barb and are used to improve the connection’s pull-off strength and pressure resistance. Common retention devices are cable ties and Oetiker® clamps.
Cable ties are an inexpensive and easy solution that has been utilized for years. They’re specified by their width, length, breaking strength and color. We should note that cable ties are distributed from industrial suppliers who may not have the controls in place that are required for the bioprocessing industry. Suppliers who choose to use cable ties should consider this in their decision process.
Oetiker® clamps are specified by their width and diameter range, with a suggested compression of the tubing material under the clamp. Most Oetiker® clamp sizes are available in a degreased format, improving their suitability for assembly in a cleanroom.
Conclusion
Reliable connections for your SuS start with finding the proper combination of tubing and fittings. Matched properly with tubing or hose, the gripping and sealing power of fittings with a hose barb termination offers a connection with pull‐off and blow‐off resistance for most applications. Hose barbs are an integral part of today’s fitting and coupling solutions, providing a low cost and effective means for connecting many different types of tubes and hoses. To ensure a safe and secure system, determine how these critical elements work together to provide the best grip and seal for your application. Securing samples of each for evaluation is a great place to start.
References:
https://www.cpcworldwide.com/Portals/0/Library/Resources/Literature/WhitePapers/Documents/Hose-barb-WhitePaper.pdf https://www.cpcworldwide.com/Portals/0/Library/Resources/Literature/WhitePapers/Documents/CPC-How-to-choose-connectors.pdf
