How to choose the right connector for low-pressure fluid handling

by Grant Wilhelm, Senior Design Engineer, Colder Products Co.

Selecting the ideal tubing connector can ensure product performance, increase ease of use and add functionality to your product designs. Here’s a three-step guide.

A range of products function by transferring liquids or gasses through small-diameter tubing—from patient beds and in vitro diagnostics in the medical market, ink management systems in printers, and liquid cooling systems in server racks to condiment delivery in food applications and fuel lines in marine and engine systems. The primary need in each application is to move fluid from point A to point B. While this sounds simple, the fluid-handling process can be complicated by media characteristics, flow rate or pressure requirements, physical space limitations, air inclusion and spillage concerns, user or environmental safety issues, and much more.

The tubing connector is a critical component in solving these fluid-handling challenges. Often bumped to a last-minute design decision, the connector you choose can make a big difference in product reliability, performance and user acceptance. There are thousands of connector options available, and choosing among the differing capabilities, materials and types can be an overwhelming task. To simplify this process, connector selection can be broken down into three steps. Following each step outlined below will help ensure successful connector choices for any product design requiring fluid handling with tubing.

Step 1: Define the functional needs of the application
The operative demands of your application determine the parameters for tubing and connectors. Fittings, luers and quick disconnect couplings are the most common tubing connectors used in low-pressure environments. While each is appropriate for certain applications, they are not necessarily interchangeable. For example, in applications where spill prevention is desired, a valved connector is a better choice than a luer (see figures 1 and 2). If the application involves frequent connecting and disconnecting, a quick disconnect designed for long-term reliability is a good choice.

Overall, consider the following design options when analyzing your application:

• Tubing—What size tubing, both inner and outer diameter, are you using? The inner diameter (ID) size of the tubing is generally the first consideration for the size of the connector.
• Termination type—The most common termination styles are hose barb, compression fittings and push-to-connect (see figure 3).

• Hose barb terminations fit inside tubing and provide a secure connection over a range of tubing styles and materials.
• Single-hose barbs work well with softer tubing, such as silicone rubber. Connectors with multiple well-made barbs (featuring sharp edges and free of parting lines) provide a secure connection that is not easily disconnected over tubing styles and materials.
• Compression fittings fit over the outside of tubing and use a nut and ferrule to make the connection.
• Push-to-connect terminations allow tubing to be pushed into the fitting.
• Flow requirements—What is the required flow rate and flow pressure? How much pressure drop is allowable? Pressure drops across connectors and valves vary greatly by manufacturer, and some designs exhibit less turbulence and resistance to flow than others. Be sure to allow for the effect of shutoff valves and tubing connections in your calculations.
• Size—You will need to know ID for hose barbs, OD for push-in-fittings and tube ID and OD for compression.
• Valve options—There are many styles of valves, and their flow rates and pressure drops vary. Connectors with integral valves prevent spills upon disconnection and prevent air from entering the system during connection. Connectors with precision flush-face valves, compared to a poppet-style valve, are considered true “dry-break” or “non-spill” connectors and should be selected when there is a critical need to avoid any spills, contamination or the introduction of air into the system upon connection (see figure 4).

• Mounting options—How is the connection going to be configured into your application? Common mounting options include panel mount, or direct mounting into NPT thread or ports.
• Connector quality—The level of manufacturing quality of a connector dramatically affects its performance and can also play a role in aesthetics. On molded plastic couplings, parting lines and mold defects (where two halves of the mold come together) should be minimal on the connector body and absent from the first hose barb. Imperfections on barbs or threads can lead to leaks. The molded connector’s gate (the point where the plastic was injected into the mold) should be free of jagged edges that can catch or tear gloves or tubing.

Step 2: Match materials to the application
The type of media influences the selection of a connector. The connector and tubing materials selected need to be compatible with the fluid being used. Fortunately, connectors are available in a ranges of both plastics and metals (see figure 5). In addition to considering media compatibility, the viscosity and corrosiveness of the fluid going through the connection should also be factored in to any materials decision.

Couplings may also include components such as O-rings and panel nuts, as well as internal components such as springs. The chemical compatibility of these parts with your media should also be considered. For example, applications involving bleach solutions will require a chemically resistant material in the flow path.

The following lists generally available connector and O-ring materials, plus guidelines for using these materials.

Thermoplastics
• ABS—Economical medical-grade thermoplastic that withstands gamma and E-beam sterilization
• Acetal—Strong, lightweight and economical material that has good rigidity over a broad temperature range, with toughness and durability
• Polyamide (nylon)—Resists wear and abrasion, with good mechanical properties at elevated temperatures
• PEEK (polyetheretherketone)—An engineered thermoplastic with high-temperature, chemical and fatigue resistance
• Polycarbonate—Resistant to some chemicals, transparent and withstands sterilization for medical applications.
• Polyethylene—Low-cost, chemically resistant, opaque thermoplastic
• Polypropylene—Excellent general-purpose resin that is highly resistant to attack from solvents and other chemicals
• Polysulfone—Rigid, strong and chemically resistant, it withstands repeated sterilization and higher temperatures better than most thermoplastics

Fluoropolymers
• PVDF (polyvinylidene difluoride)—A tough engineered thermoplastic with a balance of physical and chemical properties that makes it suitable for high-performance applications

Alloys
• Aluminum—Lightweight metal with a high strength-to-weight ratio that is available with a durable anodized finish
• Chrome-plated brass—Rugged and attractive, this metal is excellent for high- pressure and high-temperature applications
• Die-cast zinc—Weighing about 20% less than brass, this durable and lightweight material withstands high pressure and high temperature
• Stainless steel—Offers excellent rust resistance and is often used for connector components such as valve springs

O-Ring selection
• Buna-N—This is the most common O-ring material due to its solvent, oil and water resistance
• EPDM (ethylene-propylene-diene- monomer rubber)—Also known as EPR, this material offers excellent chemical resistance
• FKM (fluorocarbon)—Known for its outstanding resistance to heat, oxidation, weathering and ozone
• Silicone—Has as good or better temperature resistance as FKM; medical- grade silicones also meet FDA Class VI requirements for biocompatibility in life science applications.

Other media considerations include the range of temperatures (-40 to 93º C) and pressures the connectors might be subjected to in the application, in storage and during shipping, as well as cleaning solutions that may be used or other environmental exposure. Testing potential product materials is always a good idea in order to evaluate material performance in the actual application.

Step 3: Consider enhanced connector function
In addition to connecting tubing to facilitate and control fluid flow, today’s connector technologies can bring increased functionality and performance attributes to product designs. For example, intelligent connectors equipped with radio frequency identification (RFID) technology (see figure 6) allow data exchange at the point of connection. Communicating couplers transmit information that can protect equipment, improve processes and even save lives.

These intelligent couplings communicate by sending RFID signals between the two separated coupling halves. Data is stored on an RFID tag embedded in the passive half of the coupling or insert. Looking for the tag is an RFID reader housed in the active half of the coupling or body. When the two coupling halves are brought within a few centimeters of each other, the reader detects the tag, reads it, and sends the tag data to the control unit running the system. The control unit can also tell the reader to write new information to the tag.

One application of RFID-enabled couplers is for ink management in commercial printers by confirming that the correct ink color is used.

Intelligent couplings can also be used to ensure a product is retired at the end of its useful life, to prevent inferior media from damaging equipment, to enforce single- or limited-use consumable devices, and to ensure correct connections in multiport environments.

Another connector approach that adds value to products is the multifunction quick disconnect (see figure 7). These connectors handle the transfer of power, signal and fluids (liquids and air) through a single device.

Multifunction connectors eliminate the need for multiple connections and help prevent misconnection. They also simplify the user interface between remote tools and a device. In medical applications, these connectors enable technicians to quickly and safely change or replace modular tools, umbilicals or hand pieces.

Connectors with enhanced functions can provide competitive advantages for your product in the marketplace. Available through select manufacturers, custom connectors with advanced capabilities can make your product easier to manufacture and easier to use.

Although connectors seem like a minor component in most products, they deserve more than an afterthought in your design process. Connectors are often a user’s primary interface with your product, and they can they play a key role in meeting product performance and ease of use goals. By following the preceding steps in your connector selection, you will not only successfully move fluids from point A to point B—but you could also move your product to a more intuitive, compelling and functional design.

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