Quick disconnect couplings offer a practical, timesaving, solution for hoses that are frequently engaged and disengaged. Unlike regular threaded fittings, they require no tools, and some can even be disconnected single-handed. This convenience, however, comes with a price: more risk during the very operations the couplings were designed to facilitate. The following will explore what quick disconnect couplings are, how they work, and what safety precautions they require.
By Davi Correia, Senior Mechanical Engineer
Hydraulic and pneumatic systems are used in a variety of industries for many purposes, such as powering tools and moving loads. They both rely on fluids to function, but with a very important difference: pneumatic systems use a compressible fluid (air, in most cases) and hydraulic systems use an incompressible fluid (which are oil or water-based).
Compressible fluids can, as the name suggests, be compressed. This means they can act as a spring in storing energy. If you put your finger over the tip of an empty syringe, you can push the plunger inward some distance; when you release it, the energy stored in the compressed volume makes the plunger move back. If there is water (an incompressible fluid) in the syringe, the plunger will not be able to move it at all. Actually, this is not quite right: water, and some other fluids deemed incompressible, can be compressed to some degree; it just takes enormous amounts of pressure to do it, and lots of pressure means lots of stored energy.
Some of the applications that use hydraulic and pneumatic systems require lines that, once connected, remain connected for an indefinite amount of time. Examples of this situation are a hydraulic press or a pneumatic actuated valve. There are other applications, however, that require repeated connections and disconnections, sometimes several per hour, as in the case of pneumatic power tools in a shop.
When dealing with agricultural implements, a farmer may need to connect different hydraulic equipment in a tractor during the day. If the connections are secured with regular threaded connections, as in Figure 1a, the farmer will require two wrenches and ample time to hook up the new equipment. However, if using quick couplers, connecting and disconnecting is almost instantaneous (see Figure 1b). It is worth mentioning that quick couplers are known by a variety of names: quick disconnect fittings (QDs), quick connect, quick coupling, and quick release couplings.
How they work
An ideal QD fitting must be capable of achieving at least seven goals at the same time:
- Deliver a leak-proof seal;
- Prevent loss of fluid (this is critical when dealing with hydraulic fluids for environmental and economic reasons);
- Prevent air entering the system (for hydraulic applications);
- Prevent dirt from entering the system;
- Sustain repeated cycles of disconnection, connection, and pressurization;
- Have a low pressure drop;
- Have a low cost.
Although one can strive to have all seven of these goals met, compromises must be made for each application. A brief example of these compromises can be seen when comparing two of the most common hydraulic QD fittings: poppet and flat-face style (Figure 2). They both consists of two halves, a male half and a female half, and rely on a ball-lock mechanism to secure the two halves.
Poppet versus Flat-Face
“When connected properly, coupling parts seal and lock joints effectively by containing internal pressures and resisting any tensile forces that tend to pull the joint apart. The parts are easily disconnected without tools by disengaging a locking mechanism and separating the parts. Each half contains a valve, which is held open when the coupling is connected. This action allows fluid to flow in either direction through the coupling. When the coupling is disconnected, a spring in each half closes the valve, preventing the loss of fluid and entrance of air. The union nut has a quick-lead thread that permits connecting or disconnecting the coupling by turning the nut.”
Poppet style fittings offer a more compact and less expensive coupling than the flat-face option (see Figure 3). On the other hand, poppet style will lose roughly 15ml of hydraulic fluid with every disconnection, see Figure 4. Poppet style also has a bigger cavity that is open to the atmosphere, which can collect dirt. In the worst-case scenario any remaining dirt upon connection could travel to the pump and cylinders; in the most benign scenario it could help clog the filter. Flat-face style coupling are bulkier, but lose almost no fluid when disconnecting. They are easier to keep clean, offer a lower pressure drop, and provide little space where dirt can accumulate.
Selecting the right coupling is similar to selecting hoses, but with some extra steps. Before getting into typical considerations such as pressure, temperature, and fluid compatibility, QD selection requires some questions to be answered: 
- Does the fitting need to operate with the use of only one hand?
- Do you need a shut-off mechanism in either or both ends of the connector? Or is no shut off mechanism(s) preferred to maximize flow capacity when connected?
- Is ‘no-spill’ required? That is little to no leakage during disconnection? No-spill characteristics may come at the expense of flow performance.
- Are there multiple fluid lines that require simultaneous connection/disconnection?
- What type of tubing or hose will be connected to the quick disconnect?
- Is the application subject to vibration or side load? These are two conditions that may prevent the use of QD fittings.
- Is contamination an issue? Low dead volume fittings, the ones with little to no cavities to entrap dirt, are preferable in this case.
Having answered these questions, the selection process moves to filling out a datasheet. Now the operator responsible for the application will specify traditional variables such as:
- Pressure – Not only normal operation pressure, but also take into account process upsets that may cause pressure spikes or vacuum.
- Temperature – Couplings must meet any temperature requirements, fluid and ambient.
- Materials – Metallic and non-metallic materials must be compatible with the fluid being carried.
- Flow – What is the required flow through the coupling?
As mentioned previously, pneumatic and hydraulic system contain stored energy. The sudden release has been the cause of many accidents; hose whip, in pneumatic applications and injection injury, in hydraulic ones. Both can lead to life-threatening injuries, especially the latter.
In order to avoid accidents and near-misses with these systems, some general rules should always be observed. The first and most obvious one is to make sure the fittings have the right specification for the task at hand. As discussed in ‘selection’, there are some conditions that can be too challenging for QD fittings, such as vibration.
The second rule concerns mixing and matching of different manufacturers. Although fittings may follow the same standards, this is no guarantee that parts are interchangeable across different manufacturers. Tolerances and surface finish may cause the crucial male and female coupling to not engage correctly or fail under otherwise normal conditions.
Finally, the most important guidance is to understand and follow operating instructions and legislation regarding the safety of a specific installation. For example, using tools to force the disconnection of QD fittings or disconnecting them with the line still pressurized. In hydraulic applications, this type of error may cause a jet of fluid to penetrate the skin and provoke severe tissue damage. If you ever feel the need of skipping lunch, just search for images related to ‘hydraulic injection injury’. Reference is a good start to learn more about safety while working with hydraulic systems.
- R. G. Sebastian & S. Kumar S., DESIGN OF QUICK CONNECT-DISCONNECT HYDRAULIC COUPLING, International Journal of Recent advances in Mechanical Engineering (IJMECH) Vol.3, No.3, August 2014, DOI: 10.14810/ijmech.2014.3304
ABOUT THE AUTHOR
Davi Correia is a Senior Mechanical Engineer who has worked at a major Brazil-based oil company for the last 15 years. Correia is part of multidisciplinary team that provides technical support for topside piping and equipment of production platforms. During this period, he began to work with materials and corrosion, and later moved to piping and accessories technology, where he has become one of the lead technical advisors on valve issues. Correia was part of the task force that revised the IOGP S-562 standard, and wrote the S-611 standard. Correia has a master’s and a doctor’s degree in welding by the Universidade Federal de Uberlandia.