Gasket Engineering: Spiral Wound, Ring (RTJ) and Sheet Gaskets for Flanged Joints

1. Introduction: the engineering challenge of sealing flanged joints

The flange connection is a critical node of any pipeline system. Failure statistics at Ukrainian industrial enterprises show that up to 25% of unplanned stoppages of technological lines are related to leaks through gaskets. At operating pressures from 10 to 420 bar and temperatures from −196°C to +1000°C, the choice of gasket type determines not only tightness, but also personnel safety, environmental compliance and overall production reliability.

The three main types of gaskets for flange joints—Spiral Wound Gaskets (SWG), Ring Type Joint (RTJ), and Sheet Gaskets—have fundamentally different sealing mechanisms, application ranges, and installation requirements. The correct choice between them requires an understanding of the physics of contact deformation, knowledge of standards and real operating conditions.

2. Fundamental principles of sealing

2.1 Sealing mechanism

The sealing of the flange connection is based on the creation of contact pressure (gasket stress), which exceeds the pressure of the working environment. Two key parameters according to ASME PCC-1:

• y is the minimum seating pressure (seating stress), MPa is the pressure required for the initial deformation of the gasket and the filling of micro-uniformities of the flange
• m — maintenance factor — a factor that determines the minimum residual pressure on the gasket under working pressure: σ_min = m × P

For a spirally wound gasket with graphite filler: y = 68.9 MPa, m = 3.0. For a soft rubber sheet gasket: y = 1.4 MPa, m = 1.0. For ring RTJ (soft iron): y = 179.3 MPa, m = 6.5.

2.2 Contact mechanics

The efficiency of compaction depends on the ratio:

σg = Fbolt / Ag ≥ y (when installing)

σg,op = (Fbolt − P × Ai) / Ag ≥ m × P (under pressure)

where F_bolt is the total bolt tightening force, A_g is the effective area of the gasket, A_i is the area on which the internal pressure P acts.

RTJ gaskets implement linear contact (contact width 1.5–3 mm), which gives an extremely high specific pressure at a moderate bolt load. Spiral-wound - planar contact with controlled deformation. Sheet - full planar contact with the lowest specific pressure.

3. Technical specifications and standards

3.1 Regulatory framework

3.2 Classification by type

Spiral Wound (SWG): ASME B16.20 defines types CG (inner and outer ring), CGI (inner ring only), R (no rings). Tape material: AISI 304, 316L, 321, Inconel 600, Monel 400, Hastelloy C-276. Filler: flexible graphite (up to 650°C), PTFE (up to 260°C), mica (up to 850°C).

Annular RTJ: Profiles R, RX, BX according to API 6A. Materials: soft iron, AISI 304/316/321, Inconel 625, duplex. The hardness of the ring should be 20-30 HB lower than the hardness of the flange.

Sheets: Paronite (according to GOST 481 or EN analogues), asbestos-free materials (aramid/NBR, aramid/SBR, graphite reinforced sheets), PTFE, rubber (EPDM, NBR, FKM).

4. Guide to selection and calculation

4.1 Selection criteria

4.2 Selection algorithm

1. Determine the operating parameters: P_rab, T_rab, environment (chemical compatibility).
2. Determine the flange pressure class (PN or Class) according to EN 1092-1 or ASME B16.5.
3. If P > 100 bar or T > 450°C, or medium — H₂, H₂S, acids → consider SWG or RTJ.
4. If the flange has an RTJ groove → use only RTJ of the corresponding profile.
5. For P < 40 bar, T < 200°C, non-toxic environment → sheet gasket is permissible.
6. Check the bolt load calculation according to EN 1591-1 or ASME PCC-1.

4.3 Calculation of the minimum bolt load

According to the simplified ASME methodology:

Wm1 = π × b × G × m × P + (π/4) × G² × P

Wm2 = π × b × G × y

where b is the effective width of the gasket fit (mm), G is the average diameter of the gasket (mm), m and y are the parameters of the gasket.

Required bolt load: W_min = max(W_m1, W_m2).

Tightening moment of one bolt:

T = (K × d × F) / n

where K ≈ 0.2 (lubricated bolt), d is the nominal diameter of the bolt, F = W_min, n is the number of bolts.

5. Installation and commissioning

5.1 Preparation of the flange surface

The roughness of the sealing surface is a critical factor:

• For SWG: Ra 3.2–6.3 μm (ASME B16.5: 125–250 μin)
• For RTJ: Ra 0.8–1.6 μm in the groove
• For sheets: Ra 3.2–12.5 μm

Before installation, it is mandatory to: clean the surfaces of corrosion and remnants of the old gasket, check the absence of radial scratches, measure the parallelism of the flanges (tolerance ≤ 0.15 mm per 100 mm of diameter).

5.2 Tightening procedure

Apply a cross (star) tightening pattern according to ASME PCC-1:

1. First pass: 30% of the target moment
2. Second pass: 60%
3. Third pass: 100%
4. The fourth (control) pass: 100% in the circle

For DN ≥ 600 flanges, it is recommended to use hydraulic bolt tensioners instead of torque wrenches — ±3% accuracy versus ±25%.

5.3 Features of RTJ installation

The ring should lie freely in the groove without forced compression. The gap between the bottom of the groove and the ring (after tightening) is mandatory (metal-metal contact of the flanges, not through the gasket). If the flanges collide along the outer diameter until the ring is completely deformed, the connection is leaky.

6. Failure modes and root cause analysis

6.1 Typical failures of spirally wound gaskets

6.2 Disclaimers by RTJ

• Plastic deformation of the groove: the reason is repeated use or the hardness of the ring is higher than the flange. Solution: API 6A hardness control (difference ≥ 20 HB).
• Corrosive pitting of the groove: the cause is a galvanic pair between dissimilar metals. Solution: choosing a ring from the same alloy or applying a coating.

6.3 Failures of sheet gaskets

• Extrusion (squeezing): when P > permissible for thickness. Thinner spacers (1.5 mm instead of 3 mm) resist extrusion better.
• Cold flow (creep): characteristic of PTFE. Solution: reinforced PTFE gaskets or switch to SWG.
• Break: retightening bolts on thin material.

7. Predictive maintenance and condition monitoring

7.1 Control methods

Ultrasonic leak detection: detectors in the 38-42 kHz range (e.g. SDT 340) detect micro leaks at an early stage. Threshold value for flange connection: > 8 dB above the background level indicates the beginning of leakage.

Bolt load monitoring: ultrasonic bolt elongation meters (accuracy ±1%) or strain gauges at critical connections. A decrease in the load by > 10% from the initial one is a signal for re-tightening.

Thermography: Infrared scanning of flanged joints detects leaks of steam and hot media. A temperature difference of > 5°C between the flange and the leak area is an indicator of a problem.

Emission control (LDAR): according to EN 15446, monitoring of volatile organic compounds (VOC) around the flanges. Permissible level according to Directive 2010/75/EU (IED): < 500 ppm for compounds in volatile media.

7.2 Periodicity of inspections

• Critical connections (H₂, H₂S, HF, steam > 40 bar): monthly UZ control
• Standard technological: quarterly thermography
• Utility (water, air < 10 bar): annual visual inspection

8. Comparative matrix

9. Practical recommendations for Ukrainian enterprises

9.1 Transition from paronite to asbestos-free materials

In accordance with the EU Regulation 1907/2006 (REACH) and the Ukrainian legislation on the restriction of asbestos, enterprises must switch to asbestos-free sheet materials. A direct substitute for PON-B paronite for steam up to 200°C and 16 bar is an asbestos-free sheet based on aramid fiber with an NBR binder (similar to Klingersil C-4400). For tougher conditions (steam 300°C, 40 bar) — a reinforced graphite sheet with a metal insert (similar to Sigraflex).

9.2 Storage of gaskets

According to EN 1514 recommendations:

• SWG: horizontal, no deformation, moisture protection. The shelf life is unlimited for metal components, up to 5 years for graphite filler.
• RTJ: individual packaging, anti-corrosion coating. Hardness check before installation if storage > 2 years.
• Leafy: avoid direct sunlight (degradation of elastomers), storage temperature 5–35°C, humidity < 70%.

10. Summary

The choice of gasket type for a flange connection is an engineering decision that must be based on clear criteria: operating pressure and temperature, chemical compatibility with the environment, flange type, available bolt load and tightness requirements. Spiral wound gaskets provide the optimum balance between reliability and cost for most industrial applications. RTJs are indispensable in high-pressure and high-temperature equipment. Sheet gaskets remain an economical solution for low pressure utility systems.

Correct installation with compliance with the procedure of controlled tightening and regular monitoring of the condition of the joints are mandatory conditions for achieving the design life of the gasket and preventing emergency leaks.

UNITEC-D GmbH supplies a full range of gaskets - spiral wound (all types according to ASME B16.20), ring RTJ (R, RX, BX profiles), asbestos-free sheets and PTFE gaskets. All products have CE certification in accordance with PED 2014/68/EU and are accompanied by material certificates EN 10204 3.1.

Go to UNITEC-D electronic catalog →

Reference list

1. ASME PCC-1-2022 "Guidelines for Pressure Boundary Bolted Flange Joint Assemblies"
2. EN 1591-1:2014 "Flanges and their joints — Design rules for gasketed circular flange connections — Calculation"
3. ASME B16.20-2017 "Metallic Gaskets for Pipe Flanges — Ring-Joint, Spiral-Wound, and Jacketed"
4. DSTU EN 13555:2018 "Flanges and their connections. Parameters of gaskets and test methods for use in calculations of flanged connections according to EN 1591-1»
5. API Standard 6A, 21st Edition "Wellhead and Tree Equipment" — Ring Gasket specifications