How to Order Flanges — A Complete Buyer's Guide to Flange Specifications

Introduction

Ordering flanges for an industrial piping project sounds straightforward — but getting the specification wrong can lead to costly delays, installation failures, or worse, a safety incident in the field. Whether you are a procurement professional, a piping engineer, or a project manager sourcing flanges for the first time, understanding what information is required will help you get an accurate quotation, avoid specification errors, and ensure on-time delivery.

This guide walks through every key parameter you need to define before placing a flange order — from pressure class and nominal size to wall thickness, facing type, and material certification requirements.

1. Applicable Standard

The first and most important piece of information is the standard your flanges must comply with. The standard defines everything else — dimensions, tolerances, pressure ratings, and material requirements.

The most widely used flange standards worldwide include:

Standard	Region / Market	Typical Application
ASME B16.5	USA, international	NPS ½ – NPS 24, Class 150–2500
ASME B16.47	USA, international	NPS 26 – NPS 60, large diameter
EN 1092-1	Europe	DN10 – DN4000, PN6–PN400
ГОСТ 33259-2015	Russia, CIS	DN10 – DN1600, PN6–PN400
ГОСТ 12820-80	Russia (legacy)	Flat face flanges, PN1–PN25
ГОСТ 12821-80	Russia (legacy)	Weld neck flanges, PN1–PN200
GB/T 9115	China	DN10 – DN4000
HG/T 20592	China (chemical)	Chemical industry pipelines
JIS B2220	Japan, Asia	DN10 – DN1500

Always state the standard first. Different standards use different dimensions for the same nominal size — a DN100 flange to EN 1092-1 PN16 will have different bolt circles, flange OD, and thickness compared to a Class 150 NPS 4 flange to ASME B16.5.

2. Flange Type

Once the standard is confirmed, specify the flange type. Under ASME B16.5 and most other standards, the main types are:

Code	Flange Type	Typical Use
WN	Weld Neck	High pressure, critical service, butt weld connection
SO	Slip-On	Moderate pressure, easy installation, fillet weld
BL	Blind	Pipeline end closures, isolation
TH	Threaded	No welding required, low to medium pressure
SW	Socket Weld	Small bore, high pressure, fillet weld
LJ	Lap Joint	Frequent disassembly, with stub end

Each type has different dimensional parameters — for example, Weld Neck flanges require a bore diameter matched to the pipe schedule, while Slip-On flanges have a fixed bore that accepts a range of pipe sizes.

3. Nominal Size (DN / NPS)

The nominal size defines the pipe diameter that the flange is designed to connect to. It is expressed as:

• DN (Diameter Nominal) — used in metric systems (EN, ГОСТ, GB/HG, JIS)

• NPS (Nominal Pipe Size) — used in imperial systems (ASME)

Common size ranges:

System	Size Range
ASME B16.5	NPS ½ to NPS 24
ASME B16.47	NPS 26 to NPS 60
EN / ГОСТ / GB	DN10 to DN1600 and above

Important: DN and NPS are not direct millimeter conversions. DN100 corresponds to NPS 4, but both refer to a pipe with an outside diameter of approximately 114.3 mm.

When ordering, always state both the DN/NPS and the pipe outside diameter if you are working with non-standard pipe sizes.

4. Pressure Class / Nominal Pressure Rating

The pressure class or pressure rating defines the maximum allowable working pressure of the flange at a given temperature. This is one of the most critical parameters — undersizing the pressure class is a serious safety risk.

ASME B16.5 pressure classes:

Class	Approx. Max Pressure at 38°C (A105)
Class 150	19.6 bar / 285 psi
Class 300	51.1 bar / 740 psi
Class 400	68.1 bar / 990 psi
Class 600	102.1 bar / 1,480 psi
Class 900	153.2 bar / 2,220 psi
Class 1500	255.3 bar / 3,705 psi
Class 2500	425.5 bar / 6,170 psi

EN / ГОСТ nominal pressure ratings (PN):

PN6 · PN10 · PN16 · PN25 · PN40 · PN63 · PN100 · PN160 · PN250 · PN400

Always check the pressure-temperature (P-T) rating for your specific material and operating temperature — maximum allowable pressure decreases as temperature increases.

5. Sealing Face Type

The facing type determines how the flange seals against its mating flange and what type of gasket is required. The main facing types are:

Code	Face Type	Description
RF	Raised Face	Most common. Small raised ring concentrates gasket load. Used with spiral wound or sheet gaskets
FF	Flat Face	Entire face is flat. Used with full-face gaskets, typically with cast iron flanges
RTJ	Ring Type Joint	Metal ring gasket in a machined groove. For high-pressure, high-temperature service
MFM	Male / Female	One flange has a raised male face, the other a recessed female face. Precise alignment
TG	Tongue and Groove	Similar to MFM but with a narrower groove. Used in chemical industry

RF is the default for most applications under ASME B16.5. RTJ is specified for high-pressure oil and gas service. If ordering flanges that will mate with existing equipment, confirm the facing type of the mating flange before ordering.

6. Material Grade

The material grade defines the chemical composition and mechanical properties of the flange. Selecting the wrong material can result in corrosion failure, inadequate strength at operating temperature, or hydrogen embrittlement.

Common flange materials and their equivalents:

Carbon Steel:

International	Russian ГОСТ	Chinese GB	Application
A105 / A105N	Ст20	20#	General purpose, ambient to moderate temperature
A350 LF2	09Г2С	16MnDR	Low temperature service, down to -46°C
A181 Gr.70	Ст35	35#	Moderate pressure fittings

Stainless Steel:

International	Russian ГОСТ	EN	Application
AISI 304 / 304L	08X18H10 / 03X18H11	1.4301 / 1.4307	General corrosion resistance
AISI 316 / 316L	10X17H13M2 / 03X17H14M3	1.4401 / 1.4404	Chloride and acid resistance
AISI 321	12X18H10T	1.4541	High temperature, stabilized grade

Alloy Steel:

Grade	ГОСТ	Application
A182 F11	15ХМ	High temperature, up to 560°C
A182 F22	—	Hydrogen service, up to 620°C
A182 F91	—	Ultra-high temperature, up to 650°C
A182 F5	15X5М	Refinery service, sulfur resistance

Special Alloys:

• Duplex 2205 / 2507 — High chloride environments, offshore

• Inconel 625 / 718 — Extreme temperature and corrosion

• Hastelloy C-276 — Highly corrosive chemical service

• Titanium Gr.2 / Gr.5 — Seawater and acid service

7. Wall Thickness / Pipe Schedule (for Weld Neck Flanges)

For Weld Neck flanges, the bore of the flange must match the inside diameter of the connecting pipe. This means you must specify the pipe schedule (wall thickness) so the manufacturer can machine the correct bore.

Common pipe schedules and their relationship to bore diameter:

NPS	Schedule 40 (Std) ID	Schedule 80 (XH) ID	Schedule 160 ID
2"	52.5 mm	49.3 mm	42.9 mm
4"	102.3 mm	97.2 mm	87.3 mm
6"	154.1 mm	146.4 mm	133.4 mm
8"	202.7 mm	193.7 mm	174.6 mm
10"	254.5 mm	242.9 mm	222.3 mm
12"	303.2 mm	288.9 mm	263.7 mm

When ordering Weld Neck flanges, always state:

• Pipe outside diameter (OD)

• Pipe wall thickness (WT) or schedule

• Or simply the bore inside diameter (ID) you require

For Slip-On, Blind, Threaded, and Socket Weld flanges, the bore is standardized — you only need to specify the nominal size.

8. Quantity

State the quantity clearly — both total pieces and, where applicable, how many pieces per size, material, or pressure class if ordering multiple line items.

For large orders, also confirm:

• Whether partial shipments are acceptable

• Whether all items must ship together

• Whether a sample or first article inspection is required before full production

9. Heat Treatment Requirements

Depending on the material and service conditions, you may need to specify heat treatment:

Treatment	When Required
Normalizing (N)	Standard for carbon steel forgings
Quench & Temper (QT)	Higher strength or low temperature grades
Solution Annealing	Stainless steel and duplex alloys
PWHT (Post Weld Heat Treatment)	Thick sections and alloy steels
Stress Relief	High-residual-stress applications

Heat treatment condition is often indicated in the material grade designation — for example, A105N indicates normalized.

10. Testing and Inspection Requirements

Specify any additional testing requirements beyond the standard:

Test	Description
PMI	Positive Material Identification — chemical composition verification
UT	Ultrasonic testing — internal defect detection
MT / PT	Magnetic particle / Liquid penetrant — surface defect detection
Hydrostatic	Pressure test to verify integrity
Hardness	Brinell or Rockwell hardness verification
Tensile / Impact	Mechanical property verification
Charpy Impact	Low temperature toughness (required for LF2 / 09Г2С)

11. Material Certification

Specify the required level of material documentation:

Certificate Type	Description
EN 10204 2.1	Declaration of compliance — no test data
EN 10204 2.2	Test report based on non-specific testing
EN 10204 3.1	Inspection certificate — manufacturer's authorized inspector
EN 10204 3.2	Inspection certificate — independent third-party inspector

For most oil and gas and petrochemical applications, EN 10204 3.1 is the minimum requirement. Critical or regulated applications may require 3.2 with a nominated third-party inspector such as Bureau Veritas, SGS, or Lloyd's Register.

12. Surface Treatment and Marking

State any requirements for:

Surface treatment:

• Bare metal (standard)

• Yellow passivation

• Galvanizing

• Painting or epoxy coating

• Anti-rust oil for storage and transit

Marking requirements:

• Heat number

• Material grade

• Pressure class

• Size

• Standard

• Manufacturer's mark

• Additional customer-specific markings

Summary — Complete Flange Order Checklist

Before sending your inquiry, confirm you have the following information ready:

#	Parameter	Example
1	Standard	ASME B16.5
2	Flange type	Weld Neck (WN)
3	Nominal size	NPS 6 / DN150
4	Pressure class	Class 300
5	Facing type	Raised Face (RF)
6	Material grade	A182 F316L
7	Pipe schedule / bore	Schedule 40 / ID 154.1mm
8	Heat treatment	Solution Annealed
9	Quantity	50 pieces
10	Testing requirements	PMI + UT + Hydrostatic
11	Material certification	EN 10204 3.1
12	Surface treatment	Anti-rust oil
13	Delivery requirement	30 days, full shipment

Order Flanges from Fulihua

At Fulihua Pipeline Equipment, our engineering and sales team is ready to support your specification from the first inquiry through to final delivery.

Simply send us your requirements — standard, type, size, pressure class, material, quantity, and any special testing or certification needs — and we will provide a detailed commercial quotation within 12 hours.

Our capabilities:

• Size range: DN15 – DN1500 / NPS ½ – NPS 60

• Pressure class: Class 150 – 2500 / PN6 – PN400

• Standards: ASME B16.5 · B16.47 · EN 1092-1 · ГОСТ 33259-2015 · GB/HG · JIS

• Materials: Carbon steel · Stainless steel · Alloy steel · Special alloys

• Flange types: WN · SO · BL · TH · SW · LJ

• Facing types: RF · FF · RTJ · MFM · TG

• Certifications: EN 10204 3.1 / 3.2 · Third-party inspection available

• Annual capacity: 20,000 metric tons

• Lead time: Standard 15–30 days

Contact Us

Email: sales@flhmetal.com

Website: www.flhmetal.com

Address: Sishui County Eco-Industrial Park, Shandong Province, China

Send us your flange specification and receive a competitive quotation within 24 hours.