ASME B16.21 Thickness 3mm 150lb RF FF Non Asbestos Gasket

author: Qishine Industry

2023-09-14

One-Minute Overview

ASME B16.21 sets the dimensions, thickness, and tolerances for non-metallic flat gaskets.
Non-asbestos sheet is fiber (aramid or glass) with a rubber binder; it replaced banned asbestos.
The 3 mm thickness and 150 lb class suit low-pressure water, steam, and general plant service.
Torque to the flange spec and do not over-compress - a crushed fiber gasket leaks.

Non-Asbestos Gaskets to ASME B16.21 at a Glance

A non-asbestos gasket made to ASME B16.21 is the workhorse flat gasket for low- and medium-pressure pipe flanges. The 'non-asbestos' sheet is usually aramid or glass fiber bonded with rubber, and it replaces the old asbestos material that is now banned. This guide explains what the standard covers, what the sheet is made of, how to read a spec like 3 mm thick, 150 lb, raised-face or flat-face, and how to choose and torque the right gasket for a given flange.

What ASME B16.21 Covers

ASME B16.21 is the standard for non-metallic flat gaskets used with pipe flanges from Class 150 to Class 2500. It fixes the inside and outside diameters, the bolt-hole circle, and the thickness so a gasket from any compliant maker fits a standard ASME B16.5 flange. Thickness is typically 1.5 to 3 mm for compressed fiber sheet; the standard also covers spiral-wound and other non-metallic types by reference.

What 'Non-Asbestos' Means

The old sheet was asbestos fiber. Modern non-asbestos sheet uses a different reinforcement:

Compressed fiber (aramid or glass + NBR): general service, water, oil, low-pressure steam, many chemicals.
PTFE sheet: excellent for strong chemicals and pure services; lower temperature limit than fiber.
Flexible graphite: high temperature and steam; needs a jacket or firm seating.
All three avoid the health risk that made asbestos unavailable.

Reading the Spec: 3 mm, 150 lb, RF and FF

A spec line like 'ASME B16.21, 3 mm, 150 lb, RF/FF, non-asbestos' reads as follows:

Field	What it means
Standard	ASME B16.21 (non-metallic flat gasket)
Type	Compressed non-asbestos fiber (or PTFE / graphite)
Thickness	3 mm - typical for low-pressure sheet gaskets
Class	150 lb (PN 20) - low-pressure flange rating
Faces	RF (raised face) or FF (flat face) - match the flange
Service	Water, HVAC, low-pressure steam, oil, general chemicals (by material)

How to Choose the Right Sheet

Pick the sheet by the fluid and the duty, not by price alone:

Match the material to the fluid: PTFE for aggressive chemicals, graphite for hot steam, compressed fiber for water and oil.
Stay within the temperature and pressure limit of the sheet; for hot, high-pressure lines use a spiral-wound gasket instead.
Match the bore and bolt circle to the flange so the gasket seats without bridging the bore.
For potable or food lines, confirm the sheet carries the needed certification.

Installation and Torque

Lay the gasket dry (no paste) on a clean flange face, align the bolt holes, and snug the bolts. Torque to the flange or gasket maker's value in three passes (30, 60, and 100 percent) in a cross pattern. Compressed fiber crushes at high load, so more torque is not better - a crushed gasket loses recovery and weeps. Replace the gasket every time the flange is opened.

Where They Fit

Use non-asbestos sheet gaskets where a soft, low-cost seal is enough:

Building and plant water, heating, and chilled-water loops.
Low-pressure saturated steam and condensate.
General oil and chemical lines within the sheet's rating.
HVAC and utility piping where a soft, cheap seal is enough.

FAQ

What is ASME B16.21?

The standard for non-metallic flat gaskets for pipe flanges: it sets dimensions, thickness, and tolerances from Class 150 to 2500.

What is a non-asbestos gasket made of?

Aramid or glass fiber held by an NBR or SBR rubber binder; some are PTFE or graphite. All replace banned asbestos.

What do 150 lb and 3 mm mean on the spec?

150 lb is a low-pressure flange class (PN 20); 3 mm is the sheet thickness typical for low-pressure service.

RF vs FF - which face?

Raised-face (RF) gaskets sit on the raised ring; flat-face (FF) gaskets cover the full face out to the bolts. Match the flange type.

Can it handle steam?

Compressed fiber handles low-pressure saturated steam. For hotter steam use graphite or PTFE sheet instead.

How tight should I bolt it?

To the flange torque spec in three cross-pattern passes. Do not over-tighten - compressed fiber crushes and leaks.

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