In a high-pressure steam distribution system—whether conveying 10 bar saturated steam in a textile plant or 25 bar superheated steam in a power generation facility—the bolted flanged joint is the single most critical containment point in the piping network.
Yet in workshops across Kenya, gasket selection is frequently compromised by cutting corners.
Technicians routinely cut compressed non-asbestos fiber (CNAF) sheet gaskets by hand with a ball-peen hammer, clamp them between Class 150 or Class 300 steam pipe flanges, and wonder why the flange begins hissing and blowing out steam clouds within three months.
At high steam temperatures (exceeding 180°C to 200°C), sheet elastomers bake dry, lose their elastic recovery, and undergo permanent compressive stress relaxation. When thermal cycling occurs or water hammer shocks the line, the embrittled fiber gasket fractures, causing a violent steam blowout that threatens personnel with lethal scalding.
For high-pressure, high-temperature steam lines, the international engineering gold standard under ASME B16.20 is the Spiral Wound Gasket (SWG).
Understanding the anatomy of spiral wound gaskets, decoding ASME color codes, specifying internal inner rings (Style CGI), and applying calibrated star bolt torque is essential for steam safety and zero-leak reliability.
Anatomy of a Spiral Wound Gasket: The Four Engineered Components
A modern ASME B16.20 spiral wound gasket is not a simple metallic washer; it is a high-precision mechanical spring engineered to maintain live elasticity under intense thermal and mechanical cycling:
CROSS-SECTION OF ASME B16.20 STYLE CGI SPIRAL WOUND GASKET:
[ INNER RING ] [ V-SHAPED WINDING + GRAPHITE ] [ OUTER GUIDE RING ]
Solid 316L Steel V-Crimped Stainless Strip + Filler Solid Carbon Steel
┌───────────────┐ ┌───┐ ╱| ╱| ╱| ╱| ╱| ╱| ╱| ╱| ┌───┐ ┌────────────────────┐
│ │ │ │╱ |╱ |╱ |╱ |╱ |╱ |╱ |╱ | │ │ │ │
│ 3.2 mm Thk │ │ │ │ │ │ │ │ │ │ │ │ │ │ 3.2 mm Thk │
│ │ └───┘ │ └───┘ │ │
└───────────────┘ ◄───────── 4.5 mm Thk ──────► └────────────────────┘
▲ ▲ ▲
Prevents Inward Buckling Active Sealing Element Centers on Bolts &
& Turbulent Cavitation Compresses to 3.3 mm Stops Over-Compression- 1The Outer Centering / Guide Ring (3.2 mm Thickness): A solid carbon steel (or stainless steel) outer ring that centers the gasket perfectly inside the flange bolt circle during installation and acts as a mechanical compression stop to prevent over-tightening.
- 2The V-Shaped Metal Winding Strip (4.5 mm Initial Thickness): A continuous strip of V-crimped stainless steel (typically AISI 304 or 316L) wound tightly under precise tension. The V-profile acts like an engineered metal spring, flexing and recovering under pressure spikes and thermal cycling.
- 3The Soft Non-Metallic Sealing Filler: Compressed between the metal winding plies. In steam service, this is flexible exfoliated graphite (99.8% pure), which flows into flange surface imperfections (roughness Ra 3.2 to 6.3 µm) to create a hermetic seal up to 550°C.
- 4The Solid Inner Ring (3.2 mm Thickness): A precision solid metal ring seated on the inside diameter of the windings.
Why Inner Rings (Style CGI) are Mandatory on Steam Lines
In older plants, maintenance teams sometimes purchase Style CG gaskets (which have an outer centering ring but no inner ring) because they cost slightly less.
On steam lines, using a gasket without an inner ring is a dangerous practice:
THE DEADLY PHENOMENON OF INWARD BUCKLING:
Without Inner Ring (Style CG): With Inner Ring (Style CGI):
High Bolt Load Squeezes Windings Solid Inner Ring Backs Windings
[ Flange Face ] [ Flange Face ]
/ \ / \
◄─── [ INWARD BUCKLING ] ───► [ SOLID INNER RING ]
Windings collapse into steam bore Windings remain 100% stable
Graphite shreds into steam traps Zero erosion; smooth laminar flow- 1Inward Radial Buckling: When flange bolts are torqued down, immense radial forces push inward toward the pipe bore. Without an internal solid metal backing ring, the inner wraps of the spiral winding can buckle inward, breaking free and unraveling into the steam stream.
- 2Steam Trap Clogging: Shredded graphite filler and metal winding fragments travel downstream, jamming steam trap orifices, scoring modulating control valve plugs, and causing boiler feed water contamination.
- 3ASME Mandate: ASME B16.20 explicitly mandates solid internal inner rings for all flexible graphite filled spiral wound gaskets in Class 900 and higher, as well as Class 150 to 600 pipelines handling high-velocity steam.
PROCUREMENT STANDARD: Always specify Style CGI (Centering ring + Gasket winding + Inner ring) for steam headers. Never accept Style CG for high-temperature steam lines.
Decoding ASME B16.20 Color Codes on Gasket Flanges
Because a visual inspection of a installed gasket cannot determine whether the winding metal is carbon steel, 304, or 316L, ASME B16.20 establishes a mandatory two-part color coding system painted on the outer edge of the centering ring:
ASME B16.20 OUTER RING COLOR MARKING:
┌────────────────────────────────────────────────────────┐
│ Continuous Outer Ring Paint: WINDING METAL MATERIAL │ <-- E.g. GREEN for 316L SS
│ Intermittent Stripe Paint: FILLER MATERIAL │ <-- E.g. GREY for Graphite
└────────────────────────────────────────────────────────┘1. Outer Ring Base Paint (Winding Strip Material):
- •GREEN: 316L Stainless Steel (Universal standard for industrial steam).
- •YELLOW: 304 Stainless Steel (Standard utility piping).
- •TURQUOISE: Inconel 600 / 625 (Superheated steam > 600°C).
- •SILVER: 321 Stainless Steel.
2. Edge Stripe Paint (Filler Material):
- •GREY STRIPE: Flexible Graphite (Standard for steam; rated -200°C to +550°C).
- •WHITE STRIPE: PTFE / Teflon (Chemical service only; do not use in steam > 200°C due to plastic creep).
- •PINK STRIPE: Mica / Non-Asbestos (High-temperature gas turbines > 650°C).
If an industrial supplier hands you a spiral wound gasket with a White stripe for a 15 bar steam header, reject it immediately. PTFE filler will cold-flow and extrude under high steam temperature, causing joint failure within weeks.
Controlled Compression: The 4.5 mm to 3.3 mm Target
A spiral wound gasket does not seal through brute force; it seals through controlled elastic compression:
- •Uncompressed Thickness: Standard ASME spiral wound gaskets have an initial winding thickness of 4.5 mm ($\pm 0.13\text{ mm}$).
- •Target Compressed Thickness: When properly torqued, the windings compress down until they approach the thickness of the solid metal guide ring: 3.2 mm to 3.4 mm.
- •Spring Back (Resilience): At this 30% compression state, the internal V-shaped metal springs store mechanical energy. If pipe vibration or thermal cycling causes the flange faces to separate by 0.1 mm, the gasket springs back instantly, maintaining continuous compressive contact on the graphite seal.
Bolt Torque Procedure: The Cross-Star Tightening Sequence
More than half of steam flange leaks occur because technicians tightened flange bolts in a circular pattern around the rim with an ordinary spanner. Circular tightening cocks the flange faces, crushing the gasket on one side while leaving the opposite side under-compressed.
CROSS-STAR BOLT TIGHTENING SEQUENCE:
( 1 )
8 ─┐ ┌─ 5
│ │
( 4 )│ * │( 3 )
│ │
6 ─┘ └─ 7
( 2 )The 4-Stage Torquing Protocol:
- 1Lubricate Threads: Apply high-temperature anti-seize paste (copper or nickel based) to bolt threads and nut contact faces. Dry bolts lose up to 50% of torque to thread friction.
- 2Finger Tighten: Snug all bolts hand-tight to verify flange parallelism.
- 3Pass 1: Torque all bolts to 30% of final target torque following the cross-star sequence (1 - 2 - 3 - 4 - 5 - 6 - 7 - 8).
- 4Pass 2: Torque all bolts to 60% of target torque following the same star sequence.
- 5Pass 3: Torque all bolts to 100% of target torque following the star sequence.
- 6Final Circular Pass: Make one final pass clockwise around the bolts at 100% torque to ensure uniform seating across the entire circumference.
HOT TORQUING (RETIGHTENING): Never attempt to torque bolts on a steam line while it is hot and fully pressurized! Always perform initial installation cold. If a flange weeps on initial warm-up, wait for system pressure to bleed off before re-torquing.
Sourcing Spiral Wound Gaskets at STARNEX Nairobi
STARNEX Industrial Spares stocks certified ASME B16.20 spiral wound gaskets from our central Enterprise Road distribution counter in Nairobi.
Our inventory includes:
- •Pressure Classes: Class 150, Class 300, Class 600, and PN16 / PN25 / PN40 ratings.
- •Sizes: 1/2 inch up to 16 inch nominal pipe bore.
- •Configuration: Style CGI (316L SS Inner Ring + 316L/Graphite Windings + Carbon Steel Centering Ring).
- •Flange Stud Bolts: ASTM A193 Grade B7 high-tensile alloy studs with ASTM A194 Grade 2H heavy hex nuts.
For flange torque specifications, gasket sizing, and emergency courier dispatch across Kenya, contact the STARNEX piping engineering desk via WhatsApp at +254 740 459 672.
