Non-Metal Sealing RF, FF ASME B16.21 Gaskets-Reinforcing Safety

A non-metallic ASME B16.21 gasket for an RF (raised-face) or FF (flat-face) flange is the asbestos-free, economical seal for low-to-moderate pressure flanged joints. Compressed fiber and binder conform to the face and give a leak-tight seal without the health risk of old mineral sheets. See our flange gaskets and flange products.

What the B16.21 Non-Metallic Gasket Is

ASME B16.21 sets the sizes for the flat non-metallic gaskets used on NPS 1/2 to 24 flanges. They are cut from compressed non-asbestos fiber, aramid fiber, PTFE, or graphite sheet, bound to hold shape yet stay soft enough to seal. The gasket matches the flange face: RF (raised face) uses a ring gasket that sits inside the bolt circle, while FF (flat face) uses a full-face gasket cut to the full flange OD so the bolt holes land on gasket material.

RF Ring Versus FF Full-Face

• RF ring gasket — for raised-face flanges, sits inside the bolts, standard pipe choice.
• FF full-face gasket — for flat-face flanges, covers the whole face including bolt holes.
• Materials — compressed non-asbestos fiber, aramid, PTFE, or graphite sheet.
• Sized to ASME B16.21 for NPS 1/2 to 24 across the Class 150 to 600 range.

Why Asbestos-Free and Non-Metallic

Chrysotile asbestos gave these sheets their strength, but its fibre is a proven lung hazard and is now banned in most markets. Modern compressed sheets replace it with aramid, glass, or cellulose fiber in a nitrile or SBR binder, so the gasket seals without exposing workers to respirable fibre. For a joint that must also survive heat and pressure, step up to a spiral-wound or metal gasket — but for utility and low-pressure process lines the non-metallic sheet is cheaper and easier to cut.

Key Benefits

• Asbestos-free — no respirable fibre risk for installers and maintenance crews.
• Economical and easy to cut to shape on site.
• Good compressibility — conforms to slightly warped or rough flange faces.
• Handles a wide temperature and moderate pressure range for its class.

Where They Are Used

These gaskets go on water, air, steam, oil, and mild chemical flanged joints in HVAC, marine, water treatment, refineries, and general plant piping. They suit Class 150 and 300 lines where the pressure and temperature stay inside the sheet's rating and the fluid is not a strong solvent.

In the Joint

The gasket sits between the two flange faces and the bolts squeeze it until it fills the small gaps and stops the leak; the flange body carries the pipe and the bolt load. Match the gasket style (ring or full-face) to the face and pair it with bolts torqued evenly in a cross pattern.

Selection and Installation

Pick the sheet for the fluid, temperature, and pressure — nitrile-bound fiber for oil, aramid or graphite for heat, PTFE for chemicals. Check the flange face first: raised face needs a ring gasket, flat face needs full-face. Cut cleanly, keep the faces free of old gasket and dirt, and torque the bolts in stages and in a cross pattern so the sheet compresses evenly. Do not reuse a gasket that has been crushed or has gone hard.

Frequently Asked Questions

RF or FF — how do I know which gasket?

Look at the flange face: a raised ring around the bore is RF and takes a ring gasket; a fully flat face is FF and takes a full-face gasket with bolt holes.

What pressure can a non-metallic gasket take?

It depends on the class and material, but these sheets are for Class 150 to 300 duty; for higher pressure or hot hydrocarbon service move to a spiral-wound or metal gasket.

Compressed fiber or spiral wound?

Compressed non-asbestos fiber is cheaper and easier for low pressure and utilities; spiral wound handles higher pressure, temperature swings, and more demanding media.