In piping engineering, once pressures exceed Class 600 or temperatures hit extremes, standard non-metallic gaskets (like rubber or fiber) are no longer an option. You are left with two heavy-duty contenders: the Spiral Wound Gasket (SWG) et le Joint annulaire (RTJ). Choosing the wrong one isn’t just a maintenance headache—it’s a potential blowout risk.
À Valve JH, we supply valves for some of the harshest environments on earth. We know that the gasket is often the weakest link in any flange assembly. This guide breaks down the engineering differences between these two critical sealing technologies.
The Spiral Wound Gasket: The High-Pressure Workhorse
The Spiral Wound Gasket is the most common high-pressure gasket in the petrochemical world. Its genius lies in its composite construction. It is made by winding a preformed metal strip (usually stainless steel) and a soft filler material (like graphite or PTFE) together under pressure.
This V-shaped metal strip acts like a spring, giving the gasket “resilience” or “recovery.” This means that when the flange moves due to thermal expansion, vibration, or pressure surges, the gasket springs back to maintain the seal. This elasticity is why it’s superior to solid metal gaskets for fluctuating conditions.
The Importance of the Rings
Most industrial SWGs (ASME B16.20) come with rings, known as Style CGI:
- Outer Ring (Centering Ring): Usually carbon steel. It centers the gasket on the bolts and prevents “blowout.”
- Inner Ring: Usually stainless steel. It prevents the windings from buckling inward into the pipe stream and reduces turbulence. Crucial for vacuum or high-pressure service.
The RTJ Gasket: Metal-to-Metal for Extreme Duty
The Ring Type Joint (RTJ) is a different beast entirely. It is a solid ring of metal, precision-machined to fit into a specific groove cut into the flange face.
Unlike the SWG, which relies on compression of a soft filler, the RTJ relies on metal-to-metal deformation. When you torque the bolts, the harder flange material “bites” into the slightly softer metal of the gasket (usually Soft Iron or Stainless Steel). This creates a seal that is incredibly robust against the highest pressures known to the industry (up to 20,000 psi / API 10K and beyond).
Oval vs. Octagonal
You will see two shapes: Oval (the older style) and Octagonal (the modern standard). While oval gaskets can fit in older round-bottom grooves, the octagonal shape provides a better surface seal and is the standard for modern applications pétrolières et gazières.
Head-to-Head: When to Choose Which?
The choice is rarely a toss-up. The application conditions usually dictate the winner.
| Fonctionnalité | Spiral Wound (SWG) | Joint annulaire (RTJ) |
|---|---|---|
| Force principale | Flexibility/Recovery. Handles thermal cycling and vibration excellently. | Extreme Pressure. The only choice for API 10K/15K wellheads. |
| Flange Requirement | Raised Face (RF) or Flat Face. | Ring Joint (RJ) flanges with machined grooves. |
| Installation Difficulty | Moderate. Requires proper torque but is forgiving. | High. Flange alignment must be perfect; grooves must be flawless. |
| Réutilisabilité | Never. Once compressed, it’s done. | Rarely. Technically possible if undeformed, but risky. |
If you are dealing with rapid temperature changes (thermal shock), choose Spiral Wound. If you are dealing with extreme, steady pressure (like a wellhead), choose RTJ.
Need high-pressure valves equipped with the right sealing technology? Check our High Pressure Ball Valves ou contactez notre équipe for custom flange specs.
Field Notes: Avoid These Installation Mistakes
Even the best gasket will leak if installed poorly. Here are the failures we see most often in the field:
1. The RTJ “Mismatched Hardness”
An RTJ gasket works by being *softer* than the flange. If you use a Stainless Steel RTJ on a Carbon Steel flange, the gasket might be harder than the flange groove. When you tighten it, you’ll damage the expensive flange instead of deforming the cheap gasket. Always check the Brinell hardness.
2. Spiral Wound “Unraveling” (Buckling)
This happens when an SWG without an inner ring is used in high-pressure service. The internal pressure can cause the inner windings to “implode” or buckle into the pipe stream. Always specify inner rings for high-pressure classes.
3. Dirty Grooves
For RTJ seals, the surface finish of the groove is everything (typically 63 RMS). A single scratch or a bit of rust in the groove creates a leak path that no amount of bolt torque can fix. Inspect flanges with a flashlight before every install.
Flange Gasket FAQ (Field Notes)
Can I use a Spiral Wound Gasket on an RTJ Flange?
Technically, yes, specialized gaskets exist for this, but it’s a retrofit solution. You shouldn’t design a system this way. Stick to the intended gasket for the flange face.
Why does my Spiral Wound Gasket have different colors?
That’s the ASME B16.20 color code. The rim color tells you the metal (e.g., Yellow = 304SS, Green = 316SS), and the stripe tells you the filler (e.g., Gray = Graphite, Pink = Mica).
What is the “R” number on an RTJ gasket?
That is the size designation (e.g., R-24). It corresponds to the pipe size and pressure class. You must match the R-number on the gasket to the R-number stamped on the flange. They are not interchangeable.
Which is better for steam?
Spiral Wound (with Graphite filler). Steam causes pipes to expand and contract significantly. The “spring-like” recovery of a spiral wound gasket handles this thermal cycling much better than a solid RTJ ring.
