Despite the incredible range of standard quick disconnect (QD) couplings cataloged by major full-line and smaller specialty suppliers the demand for custom QD couplings for specific applications continues to grow. OEMs are always looking for ways to add value to their products, and the added functionality offered by a custom quick disconnect coupling is often a cost-effective way to do it. 

As with any design decision, a logical approach to the selection of a custom QD coupling will maximize the value it adds to your product. Here are some questions you should ask:

Do I Really Need A Special Coupling?

You should first carefully consider the use of a standard, off-the-shelf quick disconnect coupling, but this can be a daunting task. The huge number of standard choices available is the reason for a special design inquiry, because the engineer simply can’t find the right coupling among the hundreds of choices available.

Despite the number of hits you’ll get with a search for “quick disconnect couplings,” Google really can be your friend. A diligent search will very often result in finding exactly what you’re looking for.

Do I Really Need A QD Coupling?

Some typical reasons for using a quick disconnect coupling are:

1. To provide a quick and easy means of separating and reconnecting fluid lines with minimal fluid loss/spillage, minimal air inclusion, and minimal chance for dirt and debris to enter the system).
2. To simplify frequent exchanges of tools, attachments, or accessories.
3. To reduce assembly time and complexity.
4. To provide access ports for system diagnostics.
5. When time is critical, as in the case of fire and rescue operations.
6. When quick field repair or replacement of system components is critical, as in military applications.
7. To facilitate routine maintenance.

What Value Would a Custom QD Coupling Add?

Custom couplings typically cost more than standard coupling, primarily due to lower volume manufacturing and additional features. Experience has shown that in many cases, the value added by a custom coupling is not sufficient to justify the added expense.

It is ultimately a value judgment in which the benefit of the special coupling has to be weighed against the cost. Therefore, both you and your supplier will save a great deal of time, effort, and money if the real value of a special coupling is understood and agreed upon as early as possible in the selection/design process.

In some cases, such as the FD17 coupling used in a firefighter’s breathing apparatus, a quick disconnect coupling can literally save lives. In others, a coupling that can prevent an environmental spill may almost be priceless.  

FD17 Firefighter coupling.

Can I Use a “Semi-Custom” Coupling?

Engineering a custom QD coupling from scratch can be costly and involve a lengthy development process, but it isn’t always necessary. In fact, most manufacturers begin their custom design process with a review of both existing standard designs and all of the approaches considered for previous special projects.

Chances are that your initial search found some standard couplings that were close to meeting your needs. In almost every case, it is quicker and less expensive to modify or add features to an existing standard than it is to produce a true one-of-a-kind special.

Creating a “semi-custom” coupling may involve something as simple as changing seal materials or applying a special protective coating, or adding a safety feature. For example, “crashworthy” aerospace valves that automatically seal when damaged have been adapted for new applications as diverse as fuel system connections in Indy-type racing cars and fuel tank lines on over-the-road semi-trucks.

What Features Do I Need?

Start by defining the connection method. The most common ones are push-pull ball latch, thread-together, cam locks, and twist locks (bayonet).  Your choice may be influenced by limited accessibility requiring one-hand operation, or the need to connect under high pressure.

Next, consider the need for valves. Most couplings incorporate valves in at least one coupling half to seal the lines upon disconnection. The type of valve design is often a key consideration, because it can affect flow rate, spillage, dirt entrapment, and the ability to connect under pressure. 

One of the earliest and most common valve designs is the opposed poppet style. Although this is a simple cost effective solution, these typically spill a small amount of fluid when the coupling is disconnected.

If the fluid is benign and the system is not affected by a slight loss of fluid, then this usually presents no problem. However, even if the fluid is air or water, a release under pressure can present a hazard to the user, the equipment, or the environment.

Recently, the flush face valve design has become very popular due to nearly zero spillage. These couplings also offer resistance to dirt entrapment that can lead to contamination and damage of system components.

Another key consideration is the need to connect/disconnect the coupling under system pressure or trapped residual pressure. With most standard couplings, the valves must be forced open against the fluid pressure as the coupling is connected.

If the pressures involved are high, it is often impossible to connect a push–pull coupling by hand. For these applications, the solution may be to select a thread-together coupling. Some custom couplings, such as the one in the example above, include balanced pressure valve designs that make it possible to connect by hand up to 4500 psi.

The Achilles’ heel of most valve couplings is that they restrict fluid flow and increase pressure drop. However, it is possible to design a custom coupling that offers both maximum flow and an unobstructed flow path.

One such coupling, the FD83 shown here, is used to supply liquid coolant to the operating components of supercomputers. This application does require a custom coupling because maximum flow is required and absolutely no leakage can be tolerated.  

FD83

The coupling includes ball style valves and a bayonet-style connection with safety interlocks. First the halves are connected with a ¼ turn motion to engage them and make the seal. Then each of the ball valves is individually opened to complete the fluid path. The result is a no-spill coupling with excellent, straight-through flow characteristics when the valves are open.

The design is also “piggable” which means a cleaning pig can be moved through the coupling for line maintenance. While this is not a requirement in a supercomputer cooling application, the same principle can be used in couplings for pipelines, refineries, and processing plants where the need for cleaning does exist.

What Are The Operating Parameters?

Of course there are many other factors to consider in choosing a coupling that will perform satisfactorily in your particular application. While some of these may not add to the value of a coupling, any of them can totally destroy the value of your system if not selected properly.

Start with the basics by quantifying pressure, temperature range, flow rates, and fluid compatibility before contacting the coupling manufacturer. Then detail any special conditions in the operating environment. These operating parameters will impact the choice of base materials and sealing materials used in the coupling and/or the use of secondary processes to add corrosion protection.

Typical choices for base materials include stainless steel for offshore, chemical processing, and computer applications, titanium or aluminum for aerospace couplings, and various grades of steel for other industrial applications. Special protective coatings, like Eaton’s Eatonite laser clad material and NiFlor Nickel/Teflon compounds can also be applied to couplings used in harsh environments.

What Should I Look For In Choosing A Supplier?

If your research determines that you really need a custom solution, then bringing the coupling manufacturer’s custom product development team onboard as early in the process as possible is the best way to ensure a satisfactory outcome. And, if you really need a custom solution, chances are you also need a really experienced supplier.

The broader the experience base your coupling supplier has, the more likely it is that they have produced something similar to what you need. That improves your chances of finding a cost-effective semi-custom solution.

You should also look for a supplier with both engineering and manufacturing capabilities earmarked for custom projects. Without dedicated support, your custom products may not get the priority attention required to meet your schedules. Here again, a larger supplier is more likely to have resources dedicated to custom projects than a small supplier.

But no matter who you choose to work with, you will achieve the best result by taking the time to answer the basic questions outlined here first. Knowing in advance what you want, why you want it, and how much value it adds to your product is the key to making smart, cost-effective decisions about both the coupling you need, and the supplier you need to work with.