JIS B2220 Steel Flanges: Dimensions, Types & Pressure Ratings

Content

1 Reading the JIS B2220 Standard
2 JIS B2220 Flange Dimensions
3 JIS B2220 Flange Types by Facing and Attachment
4 Carbon Steel Material Specification
5 Pressure Rating of the JIS B2220 10K Flange
6 Difference Between 5K and 10K Flanges

Direct Answer

JIS B2220 Steel Flanges are Japanese Industrial Standard flanges rated at four pressure classes, 5K, 10K, 16K, and 20K, made primarily from carbon steel grades SS400 or S25C, with 10K being the most widely used class for general piping at working pressures up to roughly 1.0 MPa. The number before K indicates the nominal pressure rating in kilogram-force per square centimeter, so a 10K flange is built for higher pressure and uses a larger bolt circle and thicker flange body than a 5K flange of the same pipe size.

Reading the JIS B2220 Standard

JIS B2220 covers steel pipe flanges used across piping systems in Japan and across Asian industrial supply chains. The standard defines dimensions, bolt hole patterns, facing types, and pressure temperature ratings for flanges ranging from 10A to 600A in nominal size. Unlike ANSI flanges, which use pound class ratings such as 150 or 300, JIS flanges use the K rating system tied directly to kilogram-force per square centimeter.

JIS B2220 Flange Dimensions

Dimensions scale with both nominal pipe size and pressure class. The table below shows representative outside diameter and bolt circle figures for a mid-range pipe size across the four common classes.

Class	Nominal Size 100A OD	Bolt Circle Diameter	Number of Bolt Holes
5K	175 mm	140 mm	8
10K	210 mm	165 mm	8
16K	210 mm	165 mm	8
20K	225 mm	175 mm	12

Flange thickness also increases with class, typically moving from around 16 mm at 5K to over 22 mm at 20K for the same 100A pipe size, which accounts for the higher pressure containment requirement.

JIS B2220 Flange Types by Facing and Attachment

Beyond pressure class, flanges are also categorized by how they attach to the pipe and how the sealing face is machined.

Plate Flange (SOP)

Slip-on plate type, the most common and economical option for general piping.

Welding Neck Flange (WN)

Tapered hub design for better stress distribution, used in higher pressure or cyclic load lines.

Blind Flange (BL)

Solid disc used to seal the end of a pipe run or isolate a section for maintenance.

Socket Weld Flange (SW)

Used on smaller diameter pipe where the pipe end inserts into a machined socket before welding.

Threaded Flange (Sc)

Screwed connection for pipe runs where welding is impractical or prohibited.

Carbon Steel Material Specification

Most JIS B2220 flanges are manufactured from one of two carbon steel grades, selected based on service temperature and mechanical strength needs.

SS400	General structural carbon steel, tensile strength around 400 MPa, used for standard temperature and pressure service.
S25C	Medium carbon steel with roughly 0.25 percent carbon content, offering higher strength for demanding piping systems.
SUS304 / SUS316	Stainless variants used when corrosion resistance is required instead of standard carbon steel.

Carbon steel flanges are typically supplied with a black oxide or painted finish, while galvanized coating is specified for outdoor or corrosive environments.

Pressure Rating of the JIS B2220 10K Flange

The 10K class is rated for a maximum working pressure of approximately 1.0 MPa at standard ambient temperature, with the allowable pressure decreasing as service temperature rises, following the standard's pressure temperature derating tables. This makes 10K the default choice for water, air, and general utility piping across most factories and municipal systems, which is why it accounts for the largest share of flanges specified in typical industrial projects.

Difference Between 5K and 10K Flanges

The distinction between these two common classes comes down to three measurable factors.

Factor	5K Flange	10K Flange
Maximum working pressure	Approximately 0.5 MPa	Approximately 1.0 MPa
Typical application	Low pressure drainage and gravity flow lines	General process piping, compressed air, water supply
Flange thickness	Thinner profile	Thicker profile for higher pressure containment
Bolt circle size	Smaller, fewer heavy duty fasteners needed	Larger, matched to higher bolt load requirements

Specifying the wrong class is a common and costly mistake, since a 5K flange installed on a 10K rated system will fail well before reaching the system's intended operating pressure. Always confirm the class marking stamped on the flange face matches the piping system's design pressure before installation.