Weld Neck Titanium Flange: How to Specify Grade, Pressure Class and Standard (8-Step Guide)

Published by Engineering Team, Shaanxi Qianyi Tuoda Technology Co., Ltd on 2026-10-19

A weld neck flange is the connection where a titanium line most often fails — not because the material is wrong, but because the grade, the pressure class or the standard was specified incorrectly, and nobody noticed until the joint was bolted up against a pipe it did not match.

This guide walks through the eight decisions that actually determine whether a titanium weld neck flange works in service: medium, pressure, grade, structure, standard, neck thickness, and the two checks most buyers skip.

Step 1: Collect the service data before you talk to anyone

Most selection errors start here. If the medium and the temperature are vague, the quotation will be vague, and the mismatch shows up months later at the sealing face. Bring this list to the enquiry.

Parameter	Required	Example
Medium composition	Essential	5% dilute hydrochloric acid, 8% dilute sulphuric acid, wet chlorine, acetic acid
Concentration	Essential	5%, 8%, saturated
Operating temperature	Essential	80 °C, 120 °C, 180 °C
Operating pressure	Essential	PN16 (1.6 MPa), Class 150
Thermal cycling range	Recommended	Cycles between 60 °C and 180 °C
Chloride content (Cl⁻)	Essential	Seawater approx. 19,000 ppm; wet chlorine environment
Fluoride content (F⁻)	Recommended	Any hydrofluoric service

The single most important rule: if the service combines chlorides with a reducing acid such as hydrochloric or dilute sulphuric acid, specify Gr7 or Gr12 — not Gr2. This is where most of the field failures originate.

Step 2: Match the grade to the medium

Service	Preferred grade	Alternative	Why
Seawater, wet chlorine	Gr12 (TA10)	Gr7 (TA9)	Ti-0.3Mo-0.8Ni; markedly better crevice corrosion resistance
Dilute HCl below 5%, dilute H₂SO₄ below 10%	Gr7 (TA9)	Gr12 (TA10)	Ti-0.2Pd; palladium addition is what makes reducing acids survivable
Acetic acid, PTA, oxidising service	Gr2 (TA2)	Gr1 (TA1)	The general-purpose grade; good all-round corrosion resistance at moderate cost
High temperature oxidising, structural load	Gr5 (TC4)	Gr2 (TA2)	Ti-6Al-4V, tensile strength up to 895 MPa; for load-bearing parts
High-purity media, deep drawing, no strength demand	Gr1 (TA1)	Gr2 (TA2)	Best ductility, easiest to form

Combinations to avoid:

• Gr2 in 5% HCl at 80 °C — pitting at the sealing face within 3–6 months.
• Gr2 in wet chlorine — crevice corrosion failure.
• Gr1 under repeated thermal cycling at 180 °C — cracking risk at the flange hub.

Step 3: Confirm the flange structure suits the pressure

Weld neck (WN) — for high temperature, high pressure and thermal cycling. The tapered hub carries bending stress, which is why it is the default for severe process lines.
Slip-on (SO) — generally PN ≤ 2.5 MPa, ambient service, lower cost.
Lap joint (LJ) — where frequent dismantling is expected. Not recommended above DN500; the hub stress condition deteriorates.
Integral (IF) — small bore, low pressure, equipment nozzle connections.

Pressure range	Recommended structure	Note
PN0.25 – PN2.5	SO / IF	Low pressure, ambient temperature
PN2.5 – PN10	WN / SO	Medium pressure, routine service
PN10 – PN25	WN (preferred)	Medium-high pressure, thermal cycling
PN25 – PN42	WN with thickened hub	High pressure; butt weld mandatory

Step 4: Lock the standard — and write the version year

Flange standards differ in bolt hole positions and facing dimensions, and parts made to different standards cannot be interchanged. This is the most common assembly error in export orders.

Standard	Scope	Typical project
GB/T 21225-2007	Titanium and titanium alloy flanges (Chinese national standard)	Domestic chemical and pharmaceutical projects
HG/T 20592-2009	Chemical raised-face neck flanges (industry standard)	Domestic petrochemical engineering
ASME B16.5	US pipe flanges	Export projects, foreign-owned plants

Facing selection and surface roughness:

Facing	Code	Ra (reference)	Service
Raised face	RF	3.2 – 6.3 μm	Ambient, medium and low pressure
Male-female	FM / M	1.6 μm	Flammable, explosive, toxic media
Tongue and groove	TG	0.8 μm	High pressure hydrogen, aggressive media
Ring joint	RJ	0.4 – 0.8 μm	Class 600 and above

Never mix GB/T 21225 and ASME B16.5 flanges in one joint. The bolt circle diameters differ, and the parts will not align on site. Your drawing must state the standard number and the version year — "machined to standard" is not a specification.

Step 5: Check the neck thickness (worked example)

The minimum neck thickness can be checked against the ASME B16.5 simplified formula:

t = (P × D) / (2 × σ × η − 0.6 × P)

Worked through for a DN150 PN16 weld neck flange in Gr2:

• P — design pressure with a 1.05 safety factor: 1.6 × 1.05 = 1.68 MPa
• D — outside diameter of a DN150 PN16 WN flange: 285 mm
• σ — allowable stress for Gr2 at room temperature: 130 MPa
• η — weld joint efficiency: 0.85

t = (1.68 × 285) / (2 × 130 × 0.85 − 0.6 × 1.68) ≈ 2.18 mm

The theoretical minimum is about 2.18 mm. Add 2 mm corrosion allowance, then round up to the nearest standard wall thickness in GB/T 21225. The DN150 PN16 standard weld neck flange actually uses a 6 mm neck.

Important: the calculation is a reference, not a specification. Always round up to the standard thickness step — no supplier will machine a non-standard wall thickness, and a flange designed below the standard step will not match the mating parts.

Step 6: What a real specification looks like

Recent deliveries from our Baoji workshop show how the grade decision plays out in practice. All customer details are anonymised.

Period	Region	Service	Size / qty	Grade	Type
2024 Q2	Shaanxi	8% dilute H₂SO₄, 70 °C / PN16	DN150, 36 pcs	Gr12	WN
2024 Q3	Shandong	Wet chlorine, 50 °C / PN10	DN100, 24 pcs	Gr12	WN
2025 Q1	Jiangsu	Acetic acid, 120 °C / PN25	DN200, 12 pcs	Gr2	WN
2025 Q2	Inner Mongolia	Chloride-bearing cooling water, 60 °C / PN16	DN250, 18 pcs	Gr12	WN

Note the pattern: three out of four specified Gr12, not Gr2. The Gr2 order was acetic acid — an oxidising medium where the general-purpose grade is exactly right.

Typical lead time: stock sizes 5–7 days; custom sizes with mill certificate and radiographic testing 15–20 days.

Step 7: Three checks before you place the order

1. Forged or cast? Some suppliers cut cost by substituting castings for forgings. Castings can carry shrinkage porosity and slag inclusions, and under high pressure that is a cracking risk. Check the manufacturing method field on the mill test certificate — it must read "forged".

2. Does the heat number trace to the part? The heat number on the mill certificate must match the laser marking or steel stamp on the flange body, and a single batch should carry a single heat number. If a supplier cannot support that verification, treat it as a warning sign.

3. Is the standard fully written on the drawing? Not "machined to standard" — but the full designation with the year, for example GB/T 21225-2007. Bolt circles and facing dimensions differ between standards, and this single line prevents the most common site failure.

Summary

Three things decide whether a weld neck titanium flange works: complete service data, the correct grade for the medium, and a drawing that states the full standard. Everything after that — production, inspection, delivery — is the supplier's job.

Ordering

We hold 1,500 Titanium Flanges in stock in Baoji, China's Titanium Valley, and cut and machine to drawing for non-standard sizes. DN15 to DN600, PN10 to PN40, Gr2, Gr7 and Gr12, to GB/T 21225, HG/T 20592 or ASME B16.5 as specified. Mill test certificates with every shipment; PT and RT reports on request; small batch orders accepted.

For chlor-alkali, PTA, desalination and hydrometallurgy service, send us the service conditions and the drawing and we will assess the grade selection before you commit.

We confirm stock, price and shipping date within 4 hours.

Email sales@sxqytd.com · WhatsApp +86 188 2967 9311

This article is technical reference material based on published industry data and project experience. It is not a product or design commitment. Final selection must be confirmed against the project's actual service conditions and drawings.