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LS4: Fittings & Seals
For any questions: ahodge@mit.edu or Slack DM
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Thread Type | Description | Illustration |
NPS (national pipe straight) | - Straight variant of the National Pipe Thread standard threading. |
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NPT (national pipe tapered) | - Tapered variant of the National Pipe Thread standard threading. - Used commonly for low to mid level pressure applications. - Tapered attribute allows for increased tightening to prevent leaks. | |
UNF (United National Fine) | - Fine variant of the National Thread Standard. - More threads per length than UNC - Widely used multiple pipe fittings | |
UNC (United National Coarse) | - Coarse Variant of the National Thread Standard. - Widely used for bolts, not so common in pipe fittings |
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Metric | - Not used too often for us | |
BSP (Brittish Standard Pipe) | - Not used too often for us |
There are other types of pipe thread beyond what is in this table, but they aren’t relevant to us. Actually, we’ll really only be using NPT or UNF for the majority of pipe applications. It is also important for us to identify which thread type a certain pipe or fitting has, but since we largely use NPT and UNF, it is pretty easy to eyeball the taper on NPT threads and distinguish the two.
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*note: JIC fittings may also be referred to as AN (Army-Navy). AN fittings have identical design dimensions as JIC, but are manufactured with tighter tolerances.
JIC fittings can also be used to connect piping components to tubing. In this scenario, the non-threaded female component is replaced with a piece of flared tubing and accompanying sleeve.
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Conical Crush SealsFor JIC fitting applications, conical crush seals are often used to repair and prevent any irregularities in the mating surface. These conicals are thin metallic gaskets that are placed over the flared part of a male JIC fitting. Using conicals will greatly increase the reusability of flared fittings |
LubricantsLubricants are often used on pipe threading to ease installation, help with sealing, and reduce wear over time. As a general rule of thumb, lubrication should always be used on metal to metal threading connections if teflon tape is not used. Two commonly used lubricants are Krytox and DC 111. Krytox is used for general piping application, and is necessary when components are exposed to cryogenic temperatures. DC 111 is used for components exposed to vacuum environments (very low pressures) |
Consider a fluid system that is exposed to very high temperatures, and requires frequent untightening and retightening of components for inspection. Answer the following:
What would be the best type of fittings to use and why?
What should we do to ensure that the fittings stay healthy and effective after successive reuse?
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The figure below illustrates the effect of pressure as it pushes the o-ring to seal the clearance gap.
Types of O-Ring Seals
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Seal Type | Illustration |
Face (Static) |
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Radial (Static) Maximum 1500psi recommended | |
Crush (Static) (used for ORB fittings) | |
Piston (Dynamic) Maximum 1500psi recommended | |
Rod (Dynamic) Maximum 1500psi recommended | |
Rotary (Dynamic) Maximum 900psi recommended |
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The pressure difference across an o-ring seal causes the o-ring to be pushed inward or outward, depending on which side has higher pressure. If the pressure is too high, the o-ring will extrude out too much, preventing an effective seal and causing damage. Radial seals specifically are not recommended to exceed around 1500psi of pressure. To prevent damage from overpressurization, you might have to use an o-ring with increased hardness, reduce the clearance length of the seal, or use a second o-ring.
good |
bad |
Effect of Surface Finish
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Okay, so I just told you about a bunch of different factors for o-ring seal design, but how do you actually find the right dimensions for the grooves and gland? There are some basic equations for calculating base dimensions, but the best thing to do is just look them up in a table. These tables are everywhere (they’re in the o-ring manuals), and they’re not too hard to read if you know your o-ring vocabulary. Let’s look at the tables now and see how we can design our glands/grooves, using the below diagrams for dimensional reference.
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Static Axial Seal Illustrations
Static Radial Seal Illustration
If we know the OD of our piping, and which side the pressure is (most often internal) we can first choose our A diameter, which will be the OD of our groove. Our groove ID will then be A - 2G. The “AS-568” number is the standardized size of the o-ring, which you will choose when purchasing from a vendor
Questions
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v. Turns out that the pressure inside the motor due to the burning propellant will exceed 1500psi. Why is this bad, and what can we do to fix/improve our seal?
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