4 commonly used non-metallic insulating gaskets for pipe flanges

Electrical isolation and tight sealing must coexist at every flanged joint in power generation, refining, chemical plants and marine piping. Non-metallic insulating gaskets deliver both: they block galvanic and stray-current corrosion while holding pressure and temperature. Selecting the right sheet — G10 laminate, fiber-rubber grades such as Klinger C4400/C4430, or expanded PTFE like Gylon 3500 — starts with the duty on the flange gaskets they seal.

How the Four Materials Differ

Four non-metallic grades dominate insulation work on flanged joints in power, refining and marine service. Each solves a different problem, so the shortlist should follow medium, temperature and electrical duty rather than price alone.

Table 1. Typical service window of four non-metallic insulating gasket materials.

G10 Epoxy Glass Laminate

G10 is a thermoset laminate: glass cloth impregnated with epoxy and cured under heat and pressure. The result is a stiff, light-green sheet with exceptional compressive strength and almost no creep relaxation, which suits high-pressure static pipe flanges.

• Strength — very high compressive and flexural performance; it machines cleanly into gaskets, sleeves and complex shapes.
• Insulation — excellent dielectric strength in dry service, the benchmark among non-metallic gasket materials.
• Limits — brittle under sharp impact or flange misalignment; attacked by strong oxidizing acids and hot alkali; limited resilience means thermal cycling may demand retightening.
• Service window — typically -50 to +130°C with air, oils, seawater and mild chemicals; common on transformer flanges, switchgear and marine shaft insulation.

Fiber-Rubber Grades for Steam and Process Media

Aramid and synthetic fibers bound in nitrile rubber (NBR) form the economical workhorses. Klinger C4400 is the soft, conformable grade; Klinger C4430 is the denser high-pressure sheet with a dual-sided 3XA anti-stick coating.

• C4400 — aramid fiber in NBR: excellent compression resilience, puncture resistance and sealing followability on imperfect faces; water, steam, air, hydraulic oil and gases from -40 to +200°C; asbestos-free.
• C4430 — glass and synthetic fiber in NBR at 1.7 g/cm³: liquid service up to 430°C and 100 bar (10 MPa), saturated steam to 250°C, qualified for gas pipelines; the 3XA coating stops flange adhesion and speeds maintenance.
• Limits — moderate resistance to strong acids, bases and polar solvents; C4430 can lose up to 11% thickness when run above 300°C for long periods, so recheck bolt torque; dielectric performance is far below G10 or PTFE, so neither grade suits high-voltage isolation.

Expanded PTFE for Chemical and Cryogenic Duty

Gylon 3500 is unfilled expanded PTFE from Garlock: chemically inert, soft enough to conform to uneven faces and usable from -268 to +260°C. Its dielectric constant is about 2.1 at 1 MHz, with volume resistivity above 1×10¹⁸ Ω·cm.

• Chemistry — resists nearly all media, including strong acids, bases and organic solvents; fits chlor-alkali lines, semiconductor and food service.
• Cryogenics — the widest temperature window of the four; a standard choice for liquefied gas facilities.
• Limits — low mechanical strength and poor creep resistance, so pair it with a limiting ring; cost is the highest; avoid high-wear or high-shear joints.

Match the sheet to the dominant duty: chemical attack favors Gylon 3500, steam and hydrocarbons favor C4430/C4400, and pure electrical isolation under bolt load favors G10. Old or warped flanges seal better with soft PTFE, while budget-critical dry joints lean on G10.

Frequently Asked Questions

Can G10 laminate handle steam service?

No. G10 keeps its stiffness only to about 130°C and turns brittle under thermal cycling. For steam, prefer a fiber-rubber grade or expanded PTFE.

Which material resists strong acids best?

Gylon 3500, an unfilled expanded PTFE, handles almost all chemicals including strong acids, bases and solvents across -268 to +260°C.

Why does C4430 need retorquing above 300°C?

Long service above 300°C can cost the sheet up to 11% of its thickness. Periodic bolt retorquing restores the stress on the gasket.

Are fiber-rubber sheets good for high-voltage isolation?

No. Their dielectric performance is far below G10 or PTFE grades, so they should not be relied on for high-voltage electrical isolation.

Does Gylon 3500 need a retainer ring?

Usually yes. Pure PTFE creeps under sustained load, so a limiting ring keeps the gasket from flowing out of the joint.