There is a running joke about “universal standards” that goes something like this:
“I’m tired of interfacing between the 5 different standards. What if we created a universal standard?”
“Great idea! This is going to make things so much easier!”
………………….
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“I’m tired of interfacing between the 6 different standards. What if we created a universal standard?”
This is often what I think of when working with threaded fluid fittings in liquid cooling systems. Is that a tapered thread or a straight thread? Which type of straight thread is it? Is it a BSPP or a G thread? Or are those the same? Does this thread need Teflon tape?
This post will provide an overview of some threaded fluid fitting types and how they work, as displayed in the graphic below. It will conclude with a handy chart to keep track of fitting compatibility and the different acronyms and terminology seen across the liquid cooling industry.
Tapered Thread
Tapered threads have a constant thread pitch, but the thread diameter varies along the axis of the thread body. Typically, the taper angle is about 1” decrease in diameter per 16” of length, or about 3/4″ per foot. Tapered threads form a seal by compressing the threads of the male and female pieces against each other. In this way, the threads serve both to hold the fittings together and to form a fluid seal. However, due to the helical nature of the threads, a clearance remains between the thread mating points where fluid can escape. For this reason, a thread sealant is often included to minimize the leak, often either a Teflon Tape or a thread-locking additive. NPT and BSP are the two most common tapered threads, each of which uses a different thread pitch.
Straight Thread: Gasket
Straight threads, or parallel threads, have no taper in their threads. This means that the threads only serve to hold the fittings together, and not to provide any sort of sealing action. Instead, a number of different mechanisms are used to provide the seal. Straight threads typically follow either the SAE or BSPP thread conventions, but there are also metric and ANSI/ASME options that frequently arise.
The first sealing mechanism with straight threads is via gaskets. In this method, an o-ring or a gasket insert is included to create the liquid seal. The gasket is often an elastomeric material which compresses in between the male and female fitting bodies when the threads are torqued together, serving to fill in the intervening gap to contain fluid. Gasket seals can be easily replaced for extended seal longevity.
Straight Thread: Flared
Another mechanism of sealing with straight threads is a flared fitting body. In this mechanism, there is a non-threaded, angled (flared) section on the male and female fitting bodies which press against each other to form the fluid seal. The threads are tightened to advance the flared section of the male fitting axially until adequate pressure is achieved on the female flared section to create a robust seal. The flared sections typically have a good surface finish to allow for intimate mating and therefore minimize leak probability.
Straight Thread: Compression
Finally, there is the compression mechanism of forming a seal. Compression fittings are similar in concept to the flared fitting, except instead of the flared section being attached to the fitting body, it manifests itself using a set of ferrules and a compression nut which mate with the system tubing. In the compression fitting, an angled front ferrule is pressed against a flared section on the interior of the fitting body. The compression nut tightens on the straight threads, transmits force onto the front ferrule via the back ferrule, and acts to provide axial pressure between the flared front ferrule and the fitting body flare. Simultaneously, the ferrules act to “grab” the tubing, forming a liquid tight seal on the interior face of the front ferrule and the tubing.
Conclusion
To wrap up, the table below provides a summary and compatibility chart of some different threaded fitting options. This may serve as a quick reference sheet when designing liquid cooling systems, especially when interfacing with other systems that may use different fittings than your organization’s standard. This post does not cover the complete list of thread types and sealing mechanisms, so certainly refer to the helpful resources provided in the References section to fill in with additional information as needed.
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