Safe couplings for fluid systems

Quick-disconnect couplings can significantly improve the serviceability of fluid systems by saving time, reducing spills and increasing safety for owners, service technicians and manufacturers. Quick-disconnect couplings incorporating valves can automatically shut off fluid flow and maintain pressure and vacuum on both sides of the fluid line. Quick-disconnect couplings with valves can also be used when hazardous or costly media, if spilled, poses a potential regulatory concern.

The importance of selecting the right component cannot be overstated. Each element comprising the fluid system contributes to the performance of the overall product and can affect its durability, serviceability and reliability. Despite their compact size, quick-disconnect couplings can make a difference in the safety and servicing of fluid systems.

Factors to consider when selecting a fluid line coupling

Materials

Manufacturers have used a variety of different materials for their fluid system couplings. It is important to consider the chemical compatibility of the media as well as the strength and functional requirements of the application when determining the material to be used.

One of the first decisions is the material selection or preference. Plastic couplings are often a cost-effective option, appealing to manufacturers keeping their eye on the bottom line.

Plastic couplings used in fluid system applications have included acetal and nylon, which are cost-effective and compatible with a wide range of fluids, such as fuels, coolants, window-washer fluid and engine lubricants. Plastic can also be moulded in a variety of colours to distinguish between different fluid lines. It is important to specify a product and material that satisfies the application requirements.

Metal couplings are typically chosen when design, environmental conditions and user requirements call for greater durability and strength and higher pressure, vibration or temperature requirements.

OEMs should consider a metal component when the application is in a demanding environment or when the user may potentially over-stress the coupling. Manufacturers can utilise a metal male half with a plastic female half to strike a balance between performance requirements and cost and weight concerns.

Brass and zinc are common metals used in components for fluid systems. Many engineers are accustomed to working with brass components and often specify them in their systems. Diecast zinc enables manufacturers to reduce the weight of the metal couplings and enjoy the benefit of melding in terms of a more streamlined design. Diecast zinc combines the strength of metal with the manufacturability of plastic. Nickel-plating further protects the material and adds to its aesthetic appeal. Other diecast materials are available but are more expensive.

Shut-off valves

For many manufacturers, the incorporation of automatic shut-off valves in couplings is a primary driver in the decision to specify a coupling, over simple fittings, in fluid systems. Shut-off valves in couplings promote safety by their ability to prevent spills from occurring. With shut-off valves in place, fluid lines using valved couplings minimise trapped air in the system.

Couplings are available in a choice of single or double shut-off valves. Single shut-off couplings are typically placed in a line to preserve the fluid on only one side of the coupling when the liquid on the non-valved side does not have to be contained. Double shut-off couplings, on the other hand, maintain pressure and retain fluid on both sides of the connection.

O-ring seals

O-rings perform a critical function in the fluid connection by preventing fluid leakage in both the connected and disconnected positions. O-rings are used on the internal valves to shut off flow when the couplings are disconnected. Another O-ring is used externally on the male half of the coupling to seal the fluid media while in the coupled position.

It is important for the O-ring to be both durable and chemically resistant to the media. If an O-ring is not chemically resistant to the fluid, the O-ring can break down and become hard or swollen, which prevents the coupling halves from connecting or disconnecting properly and could cause fluid leakage.

For fuel applications, it is important to note that a material can be compatible with gasoline but react adversely to the additives in the fuel. While there are many different choices in O-ring compounds, many sources recommend buna-N and fluorocarbon (FKM) compounds for fuel applications. FKM materials have been shown to be durable and exhibit good chemical resistance to a wide variety of fuels. However, according to some materials research and field experience, the FKM F-Type O-ring, which has a high fluorine content, provides even better compatibility with fuel and common additives.

Mounting and tubing connections

Since tubing and couplings are often the last elements to be specified, it is important to work with suppliers who have the broadest array of mounting and hose termination options. Coupling suppliers work closely with manufacturers to determine whether inline, rigid mount, panel mount or elbow is the best solution to fit the design, space constraints and performance needs of the fluid system. Hose barbs can come in single, double and triple barb options as well as a variety of barb profiles. Coupling providers can also provide both fractional and metric sizes. Collaborating closely with your coupling provider can help you select options that will suit your cost and design needs.

Conclusion

A multitude of options exist for couplings in fluid systems. Some coupling manufactures have the capability to produce state-of-the-art coupling designs that are build-to-order to meet your requirement or specification. While the available choices may be numerous, the considerations outlined in this article are the first steps towards selecting a coupling based on a balanced consideration of the manufacturer's design, quality, service and cost requirements.