Flange Pressure Classes: Class 150 vs PN16

Few mistakes in MEP procurement are as common — or as expensive — as confusing flange pressure classes. A Class 150 flange and a PN16 flange look similar on a pallet. Bolt them together on site and you discover the bolt holes do not line up, the thicknesses differ, and the joint cannot be made up. The flanges go back, the schedule slips, and the variation costs real money.

This guide explains what pressure classes actually mean under ASME B16.5, how PN ratings compare, which facings and gaskets pair with which class, and how to select correctly the first time.

What a Flange Pressure Class Actually Means

A pressure class is not a direct pressure rating stamped in bar or psi. It is a designation within a standard — ASME B16.5 for inch-series flanges — under which the allowable working pressure varies with temperature and material group, per the standard’s pressure–temperature tables.

That nuance matters: a Class 150 carbon steel flange is typically rated around 285 psi at ambient temperature, but its allowable pressure drops as temperature rises. Always check the pressure–temperature rating for your material group and design temperature — never assume the class number alone tells you the working pressure.

Higher classes mean heavier construction: thicker flanges, larger bolts, more of them. Class 150 and 300 cover most MEP building services; 600 and above belong to high-pressure process and power applications.

ASME B16.5 Classes: 150, 300, 600, 900, 1500, 2500

ASME B16.5 standardizes flanges from 1/2″ to 24″ in classes 150 through 2500. For MEP buyers, the practical landscape is:

Class 150 — the default for HVAC, plumbing, chilled water, and low-pressure fire protection. Lightest, most economical, widest availability.
Class 300 — for higher-pressure HVAC, process water, and steam/condensate at moderate pressure. Noticeably heavier bolting than Class 150.
Class 600 and above — high-pressure steam, oil and gas process service. Different facing (usually RTJ) and dramatically heavier.

Within a class, dimensions are standardized regardless of manufacturer — a Class 150 weld neck from any B16.5-compliant source will mate with any other. That interchangeability is the entire value of the standard, and it is why buying to the standard (with MTCs to prove it) matters more than buying to a brand.

PN Ratings (EN 1092-1): PN10, PN16, PN25, PN40

The metric world uses PN (Pression Nominale) designations under EN 1092-1. The PN number is the nominal pressure in bar at 20°C for the reference material — PN16 means 16 bar at ambient, derated with temperature just like class ratings.

PN16 is the metric analogue of everyday MEP service: water, HVAC, and utility piping. PN10 suits low-pressure drainage-type duties; PN25 and PN40 step into higher-pressure process service.

Class 150 vs PN16: Can You Mix Them?

No — never mate a Class flange to a PN flange. Although Class 150 (≈ PN20 equivalent) and PN16 serve similar duty ranges, their drilling patterns differ: bolt circle diameters, number of bolt holes, and flange thicknesses are not the same. A Class 150 flange will not bolt up to a PN16 flange, full stop.

This is one of the most frequent site problems in the Gulf: legacy specifications written in PN, new equipment arriving with Class flanges, and a crew discovering the mismatch at installation. Prevention is simple but non-negotiable:

State the flange standard explicitly on every purchase order: “ASME B16.5 Class 150” or “EN 1092-1 PN16” — never just “150#” or “PN16” without the standard.
When connecting to existing plant, verify the existing flange drilling before ordering.
Transition between systems with a proper spool or adapter — never force a mismatched joint.

Flange Facings: Raised Face, Flat Face, and RTJ

The facing is the sealing surface of the flange, and it must match on both sides of the joint:

Raised Face (RF) — the standard for Class 150 and 300 steel flanges. A small raised ring concentrates bolt load onto the gasket. This is the default facing for MEP and process service.
Flat Face (FF) — full-face contact, used on cast iron flanges and low-pressure applications. Critical rule: never bolt a raised-face flange against a flat-face flange — the raised ring can crack the flat-face flange, particularly in cast iron.
Ring Type Joint (RTJ) — a machined groove holding a metal ring gasket, standard for Class 600 and above and for critical high-pressure/temperature service.

Order check: facing is part of the flange designation (e.g., “WN RF” = weld neck, raised face). If the purchase order does not state the facing, you have not fully specified the flange.

Gasket Pairing per ASME B16.20

ASME B16.20 governs metallic gaskets for pipe flanges. The pairing rules are straightforward:

Raised-face flanges → spiral-wound gaskets (stainless winding with graphite or PTFE filler, with inner and outer guide rings — typically SS316/graphite for MEP and process service).
RTJ grooves → ring joint gaskets — octagonal or oval metal rings (soft iron, stainless, etc.) seated in the groove. Never substitute a flat gasket into an RTJ groove.
Flat-face flanges → full-face non-metallic gaskets (compressed fiber, PTFE) so load spreads across the whole face.
Never reuse a gasket. Compression sets the gasket permanently; a reused gasket is a future leak.

Match the gasket to the facing, the class, and the service fluid and temperature — and keep gasket certificates with the flange MTCs in your submittal file.

Flange Types at a Glance

Weld Neck (WN) — tapered hub, butt-welded to pipe. The choice for critical, high-pressure, and cyclic service. Most expensive, best performance.
Slip-On (SO) — slides over the pipe, fillet-welded front and back. Economical and easy to align; for lower-pressure service.
Blind — closes a line end or vessel nozzle; essential for test and isolation points.
Socket Weld (SW) — for small-bore high-pressure piping; pipe inserts into the socket.
Threaded — no welding required; for low-pressure utility service where welding is impractical.
Lap Joint + stub end — for stainless or lined systems where you want cheap backing flanges with expensive wetted material.

Selection Guide for Engineers

Define the service — fluid, design pressure, design temperature. This sets the minimum class or PN.
Nominate the standard and class — ASME B16.5 Class 150/300 for inch-series MEP work; EN 1092-1 PN16/25 for metric. Check pressure–temperature tables for your material.
Choose the facing — RF as default; FF for cast-iron/low-pressure; RTJ for Class 600+.
Pair the gasket — per ASME B16.20: spiral-wound for RF, ring joint for RTJ, full-face fiber/PTFE for FF.
Select the material — ASTM A105 carbon steel for general service, A350 LF2 for low temperature, A182 F304/F316 stainless for corrosive service.
Specify bolting and documentation — bolt spec per service, and mill test certificates to EN 10204 3.1 with every heat.

Frequently Asked Questions

Is Class 150 the same as PN16?

No. They serve similar pressure ranges — Class 150 is roughly equivalent to PN20 — but their dimensions differ: bolt circles, hole counts, and thicknesses are not interchangeable. Never bolt a Class flange to a PN flange. Always state the full standard (ASME B16.5 or EN 1092-1) on purchase orders.

What is the difference between raised face and flat face flanges?

A raised-face (RF) flange has a small raised ring around the bore that concentrates bolting force on the gasket — the standard for steel flanges in Class 150/300. A flat-face (FF) flange has a completely flat sealing surface, used on cast iron and low-pressure service. Never mate an RF flange to an FF flange; the raised ring can crack the flat-face side.

Which gasket should I use on a Class 300 raised-face flange?

A spiral-wound metallic gasket to ASME B16.20 with inner and outer rings — typically SS316 winding with graphite filler for general MEP and process service. Match the gasket’s pressure/temperature limits to the service, and never reuse a gasket once compressed.

What information do I need to order flanges correctly?

Six items: (1) standard and class — e.g., ASME B16.5 Class 150; (2) type — weld neck, slip-on, blind, etc.; (3) facing — RF, FF, or RTJ; (4) material — e.g., ASTM A105; (5) size and schedule/bore; (6) quantity, plus MTC EN 10204 3.1 documentation requirements. With those six, any competent supplier can quote accurately.

Request a Quote

Need flanges or gaskets for a project in Saudi Arabia? Ajal Arabia supplies ASME B16.5 flanges — weld neck, slip-on, blind, and more — in carbon and stainless steel, with matched gaskets to ASME B16.20 and mill test certificates for your submittals. Call +966 555 993140 with your flange schedule or BOQ for a prompt quotation and delivery across the Kingdom.

Flange Pressure Classes Explained: Class 150 vs PN16 and How to Choose