Quick Disconnect Fittings for Liquid Cooled PCs

What is a Quick Disconnect Fitting?

A quick disconnect fitting (also called a “quick disconnect coupling”, "QDC", "quick disconnect", or “QD”) in PC liquid cooling is a specialized fitting type which facilitates quick disconnection of tubing from components while maintaining a seal so coolant does not leak. This in turn allows users to remove components without having to drain the liquid cooling system.

How does a Quick Disconnect Fitting work?

QDC fittings consist of two halves with one being female and the other being male. Internal shut-off valves automatically open when the halves are connected, and close when disconnected. Internal o-rings then ensure the connection is watertight and prevents leaks when the QDC is disconnected. Male and female halves are usually sold as a set but can sometimes be purchased as individually.

(Female QDC halves - left, Male QDC halves - right)

(Shut-off valve being manually pressed on male QDC half for display purposes - It is not advise to do this as it could ruin the valve)

There are many quick disconnect styles to choose from depending on budget and application requirements. Here are the various attributes to consider when selecting QDC fittings for your liquid cooled server, HPC workstation, or desktop PC.

Disconnect Types:

QDC fittings come in various disconnect types, each with unique advantages and drawbacks.

Push-in – The push-in QDC type is generally the most convenient since it typically requires the least amount of action to connect and disconnect. It features either a slide or hinge latching mechanism to join male and female QDC halves together. While the push-in type usually the most expensive option, they often offer the best performance. There are however some lower cost options that require a tool to disconnect them.

(Push-in type quick disconnect fittings connected. Nylon - top, Brass - bottom)

(Push-in type quick disconnect fittings disconnected)

Twist – The twist QDC type offers convenience similar to push in QDCs but instead require a 45–90-degree rotation to connect and disconnect. However, this design can be challenging to use in tight spaces where the ability to grasp and rotate is limited.

(Twist type quick disconnect fittings connected)

(Twist type quick disconnect fittings connected)

Screw – The screw QDC type is the least convenient to use but has the advantage of being the most secure since it does not rely on a mechanical latch. Instead, the male and female halves are screwed together forming the most secure seal. Another advantage is the ease of manufacturing which reduces cost.

(Screw type quick disconnect fittings connected. Nylon - top, Brass - bottom)

(Screw type quick disconnect fittings disconnected)

Thread Genders & Sizes:

While QDCs have female and male halves, the threads on these halves can be either male or female so they can be attached to components or other fittings in your liquid cooling loop.

(female threads left, male threads right)

Female threads allow installation of compression fittings, barbs, and fitting adapters so tubing can be attached. Examples below show installation of a compression fitting, 90-degree adapter, and tubing.

Male threads screw into major liquid cooling components like CPU & GPU waterblocks, reservoirs, radiators, pumps, manifolds, and distribution plates. Example below shows installation of a male threaded QDC into a manifold.

There are 3 possible thread combinations:

Female to Female threads – Female threads on both ends of a QDC is often used in the middle of a piece of tubing. It is not directly attached to a cooling component like a waterblock or manifold, and requires compression fittings (as shown below) or barbs installed on either end to connect tubing.

Male to Female threads – This combination allows the male end to be secured to a cooling component like a waterblock or manifold (as shown below) and the female end to tubing via a barb or compression fitting (as shown below).

Male to Male threads – This combination is less common but may be useful in applications where connection to tubing is not required.

Female and male threads in PC liquid cooling come in thread sizes G1/4 and G3/8. G1/4 is by far the most common size and is considered "standard". The larger G3/8 threads are reserved for larger systems requiring higher than average flow rates. Compatibility is quite limited with G3/8 thread sizing so keep that in mind when planning. G3/8 to G1/4 reducers are available.

(G3/8 female threads - left, G1/4 female threads - right)

(G3/8 threads with G3/8 to G1/4 reducer sitting on top)

(G3/8 to G1/4 reducer installed on G3/8 quick disconnect)

Materials:

QDCs generally come in either brass or plastics including nylon and acetal.

Brass QDC fittings are durable, handle high pressures and temperatures, and resist corrosion maintaining the cooling loop's integrity. They are also less prone to cracking, and offer better leak resistance than plastic fittings. However, they are heavier, more expensive, and conductive, posing a slight risk in electrical faults though minimal in well-designed systems.

Plastic QDC fittings are lightweight, cost-effective, and non-conductive, reducing the system's overall weight and electrical hazards. However, they are less durable, more prone to cracking, and may wear out faster especially with frequent use. They may also not provide the same level of leak resistance as brass fittings.

Diameter

Quick disconnects, and compression fittings, come in varying diamaters and liquid cooling components, like a GPU waterblock, can come with G1/4 ports sometimes too close together for large diamter quick disconnects to fit side by side. For instance, if a quick disconnect is 28mm in diameter and the G1/4 port spacing is 26mm, then there would be a 2mm overlap. See the diagram below. If you plan to attach a quick disconnect to a component, always confirm the quick disconnect diameter and G1/4 port spacing won't cause interferance. In short, just make sure the diameter of the quick disconnect is 1mm less that the G1/4 port spacing. 

(Narrow nylon QDC - top, wide brass QDC - bottom)

(This diagram shows that two 28mm OD fittings cannot occupy 2 G1/4 ports with 26mm spacing. There would be a 2mm overlap making it impossible for them to fit. A fitting diamter of 25mm and under is required in this situation)

Special Features:

Taking QDC fitting design even further, some are included with more unique features that assist in loop design and cost savings.

Bulkhead – Bulkhead QDCs allow the QDC to be mounted to a panel. This is a great option if for example you want to have a QDC disconnect on the exterior of a case. Drilling a large hole to mount a bulkhead fitting is usually required so keep that in mind when planning. If a hole is needed, consider using a step drill bit with a cordless drill.

Built in Compression Fittings – While tube size options are currently limited, there is significant cost savings in this design because they are no additional fittings needed to secure tubing to the QDC.

Quick disconnect fittings are all about convenience when servicing your liquid cooling loop. Even with a screw type QDC which requires more effort to disconnect, not having to drain your loop is a huge time saver. It’s the only particle option for servicing liquid cooled hardware in servers and HPC workstations. Now armed with the information above, you can make the best QDC selection for your application.

Need assistance? Send us an email at [email protected] and we will be happy to help!