Titanium Flange Pressure Ratings: PN16, PN25 and PN40 with Temperature Derating

Опубликовано Engineering Team, Shaanxi Qianyi Tuoda Technology Co., Ltd от 2026-10-19

A flange rated PN16 is not rated 1.6 MPa at every temperature. That single sentence causes more specification errors than any other in piping design. The nominal pressure printed on the rim applies at 20 °C; as service temperature rises, the allowable working pressure falls, and the fall is steeper for titanium than for carbon steel.

This guide covers the three pressure classes that cover the great majority of industrial duty — PN16, PN25 and PN40 — with the derating behaviour, the bolt and dimensional consequences, a five-step selection procedure and four worked examples.

1. What PN Actually Means

PN is a nominal designation, not a working pressure. It references the pressure-retaining capability of the flange at reference conditions, in a specific material group, in a specific standard. Three consequences follow, and each one has caught out a project:

• PN is temperature-dependent. The rated pressure at operating temperature is lower than the number on the flange.
• PN is material-dependent. Titanium is not in the same material group as carbon steel, so the derating curve differs.
• PN is not Class. PN16 is close to Class 150 and is not identical to it, and the two do not reliably bolt together.

2. Pressure Rating and Temperature Derating

The table below gives the general derating behaviour for Titanium Flanges in these three classes. Use it to understand the trend and to sanity-check a design — not as a substitute for the actual standard table for your material group and standard system.

Class	20 °C	100 °C	200 °C	300 °C	Typical Service
PN16 (low pressure)	1.6 MPa	1.4 MPa	1.1 MPa	0.9 MPa	Low pressure chemical lines, sea water piping, atmospheric tanks; DN50–DN600
PN25 (medium pressure)	2.5 MPa	2.2 MPa	1.75 MPa	1.45 MPa	Heat exchangers, process piping, desalination; the most widely used class; DN50–DN500
PN40 (medium-high)	4.0 MPa	3.5 MPa	2.8 MPa	2.3 MPa	High pressure reactors, steam lines, high pressure gas; DN50–DN400

The number that matters most is the bottom row. A PN16 flange at 300 °C holds 0.9 MPa — a little over half its nameplate rating. Specifying PN16 for a 1.5 MPa line that runs at 300 °C is not a rounding error; it is a design that will fail. This is the single most common pressure-class mistake we see in incoming enquiries.

Why titanium derates the way it does

Titanium keeps its strength to a higher temperature than most metals, but its allowable stress still falls as temperature rises, and the standards apply a material safety factor that is not the same as for steel. The practical consequence: at elevated temperature, the derating is not negligible, and it cannot be approximated by subtracting a fixed percentage. Work from the actual table for your standard and material group.

Choosing between the three

• Up to 1.6 MPa at operating temperature — PN16.
• 1.6 to 2.5 MPa at operating temperature — PN25.
• 2.5 to 4.0 MPa at operating temperature — PN40.
• Duty not yet defined — PN25 covers the widest range and is the class most likely to be in stock when the line is already down.

Note the wording: at operating temperature. Select against the derated value, not the nameplate value.

3. What Changes Between Classes, Dimensionally

Moving up a class does not simply scale the flange. The changes are specific and they affect what will physically bolt together.

Size	PN16 → PN25	PN25 → PN40
DN50 – DN80	Bolt size and bolt count both increase	Bolt size increases again
DN100 – DN300	Bolt size often unchanged; flange body thickness increases	Thickness increases; bolt size typically unchanged
DN400	Bolt size increases	Bolt size steps up again (to M27 in GB/HG tables)

The trap in this table is the DN100–DN300 band, where the bolt diameter stays the same across two classes and only the flange thickness changes. A buyer checking only the bolt size concludes the two flanges are interchangeable. They are not — the thickness difference is the entire pressure capability, and a thinner flange on a higher-class joint is the failure point.

Two further checks that are routinely missed:

• Pipe outside diameter must match the flange bore. A correct flange on a mismatched pipe cannot be installed — the fit-up is wrong before any bolt is tightened.
• Bolt holes from different standard systems cannot be mixed. An ASME Class 150 and an EN PN16 flange of nominal size DN150 have bolt circles that differ by roughly 10 mm. On site this is discovered as "the holes do not line up", and by then the spool is built.

4. Five-Step Selection Procedure

1. Define the medium. Sea water, dilute sulphuric acid, hydrochloric acid, nitric acid, organic acid — each maps to a different grade. Concentration and temperature matter as much as the acid identity, because the same acid at a different concentration calls for a different grade.
2. Select the grade. Sea water and dilute sulphuric acid — TA10. General chemical — TA2. Reducing acids including hydrochloric — TA9. Nitric acid and high-purity service — TA1. High mechanical load — TC4. Fluoride-bearing media — not titanium at all.
3. Determine the class — after derating. Take the operating pressure and the operating temperature together, read the permitted pressure at that temperature, then choose the class. Never select on the nameplate rating.
4. Select the facing. RF for general media and the widest availability. TG where the gasket must be fully enclosed — toxic or flammable media. MFM for medium pressure with confinement. RJ for high pressure and severe duty.
5. Fix the manufacturing route and the documentation. Forged construction for high pressure and critical duty. Third-party composition and mechanical reports, plus PT on sealing faces and UT on a defined sampling basis. Agree the inspection scope before the order.

5. Four Worked Examples

Duty	Specification	Reasoning
Dilute sulphuric acid reactor 5–10% H₂SO₄, 80 °C, 3.5 MPa	PN40, DN100, TA10, RF	TA10 is required for dilute sulphuric acid — TA2 would pit at the sealing face. 3.5 MPa at 80 °C is within PN40's derated capability.
Sea water heat exchanger 60 °C, 1.8 MPa	PN25, DN150, TA10, RF	1.8 MPa exceeds PN16 and sits inside PN25. TA10 for chloride pitting resistance; TA2 is acceptable in clean flowing sea water but is vulnerable in stagnant or crevice conditions.
High pressure hydrogen 8.0 MPa, ambient	PN100 (≈ Class 600), DN50, RJ	Beyond PN40 entirely. RJ facing mandatory for high pressure flammable gas. Note that hydrogen service also raises a hydrogen-embrittlement question that has to be assessed separately — pressure rating alone does not qualify the joint.
Organic acid storage tank Ambient, atmospheric	PN16, DN200, TA2, RF	Atmospheric duty. Selecting PN40 here buys nothing and costs materially more — the correct class is the lowest that satisfies the duty.

Read the four examples as a pattern, not as four answers. In each case the class was set by the derated pressure, the grade by the medium, and the facing by the consequence of leakage. Those three decisions are independent, and getting one right does not compensate for getting another wrong.

6. Six Selection Errors

Over-specifying low pressure duty. PN40 on an atmospheric tank adds cost, weight and lead time for no benefit. The correct class is the lowest that satisfies the derated duty.

Under-specifying high pressure duty. The mirror error, and the dangerous one. A flange selection that omits the pressure margin is not a cost saving, it is a hazard.

Ignoring derating. Selecting PN16 for 1.5 MPa at 300 °C because the nameplate says 1.6 MPa. At that temperature the flange is rated 0.9 MPa.

Grade mismatched to medium. TA2 specified for dilute sulphuric acid, or TA2 for hot concentrated chloride. Both fail by localised attack at the gasket crevice, invisible from outside until the joint leaks.

Wrong facing for the consequence. RF on a high pressure toxic or flammable service where the gasket needs full confinement. The facing choice is driven by what happens if the joint leaks, not by what is cheapest.

Mixing standard systems in one spool. ASME and EN flanges of nominally the same size do not reliably bolt together. Pick one system and apply it to the whole line.

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We hold Titanium Flanges in TA1, TA2, TA9, TA10 and TC4 from DN10 to DN800, PN0.25 to PN40 (Class 150 to 600), in ASME B16.5, EN 1092-1, HG/T 20592 and GB/T configurations, with RF, FM, TG and RJ facings. Stock items dispatch in 3–5 working days; non-standard sizes to drawing in 7–15 days.

Send us the medium, concentration, operating temperature and operating pressure — we will confirm the derated class, the grade and the facing as one package, with the mill certificate and NDT report included in the quotation.

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