Flow Control & Piping
Stainless Steel Flanges
Forged stainless steel pipe flanges in 304L and 316L. Weld neck, slip-on, blind, socket weld, threaded, and lap joint types. ASME B16.5 / ASTM A182. Class 150 through 2500. EN 10204 3.1 material certs. Custom NPS and facing accepted.
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Types of Stainless Steel Flanges
Each flange type connects to the pipe differently and is suited to different pressure classes, maintenance requirements, and welding accessibility. Choosing the wrong type creates unnecessary cost and installation problems downstream.
Weld Neck Flange
Class 150–2500
High pressure
The tapered hub transfers stress from the flange face back to the pipe wall, and eliminating the stress concentration point. The bore matches the pipe ID, preventing turbulence and product buildup. The preferred type for high-pressure, high-temperature, and vibration-critical systems. Class 600 and above should nearly always be weld neck.
Slip-On Flange
Class 150–600
Two fillet welds
Lower strength than a weld neck under cyclic stress. But it’s easier to align during installation and significantly lower cost. Widely specified for Class 150 and Class 300 service in water treatment, food processing, and HVAC applications. In these environments, operating pressure remains well within the flange rating, and vibration loading is moderate.
Blind Flange
All classes
Pipe blanking
A blind flange is a solid disc with no bore. It is used to seal the end of a pipe run, close a vessel nozzle, or terminate a pipeline for future extension. Under pressure, it functions as a pressure vessel head. Its pressure rating at a given thickness is determined by the bending stress across the diameter.
Socket Weld Flange
Class 150–2500
NPS ≤ 2″
It is stronger than a slip-on flange because the socket provides mechanical engagement before welding. This design is limited to pipe sizes of 2″ NPS and below. It is commonly used in pharmaceutical, food, and chemical process lines where small-bore high-pressure connections must remain leak-tight without full-penetration butt welding.
Threaded Flange
Class 150–300
High pressure
A tapered pipe thread machined into the bore allows the flange to be screwed directly onto threaded pipe without welding. It is the only flange type suitable for installations where welding is prohibited because of fire or explosion risk, or where no welding capability is available on site.
Pressure Class Reference
ASME B16.5 Pressure – Temperature Ratings
| Pressure Class | MAWP at 38°C (100°F) | NPS Range | Common Application |
|---|---|---|---|
| Class 150 | 19.6 bar (284 psi) | ½″ – 48″ | Water · HVAC · Food · Low-pressure piping |
| Class 300 | 51.1 bar (740 psi) | ½″ – 24″ | General process · Marine · Chemical lines |
| Class 600 | 102.1 bar (1,480 psi) | ½″ – 24″ | Medium-pressure process · Steam lines |
| Class 900 | 153.2 bar (2,220 psi) | ½″ – 24″ | High-pressure process · Oil & gas |
| Class 1500 | 255.3 bar (3,705 psi) | ½″ – 24″ | High-pressure steam · Offshore |
| Class 2500 | 425.5 bar (6,170 psi) | ½″ – 12″ | Extreme pressure · Subsea · Gas injection |
* MAWP values are for ASTM A182 Grade F316L material at 38°C ambient per ASME B16.5 Table 2-1.1.
* Ratings reduce at elevated temperature. Class 150 316L at 200°C drops to ~15.5 bar.
* NPS above B16.5 coverage (>24″ for Class 300+) falls under ASME B16.47 — quote on request.
Flange Types & Selection Guide
Matching Facing to Gasket and Service
The facing type determines which gasket style can be used and directly affects the leak tightness of the assembled joint. Mismatched facings raised face bolted to flat face, for example, are the most common cause of leaks on new flange assemblies.
Facing Type Selector
Raised Face (RF)
Standard for all pressure classes
Flat Face (FF)
Where mating flange is cast iron / plastics
Ring Type Joint (RTJ)
Class 600+ · High-temp · Offshore subsea
Tongue & Groove (T&G)
Heat exchangers · Pump cases · Narrow gasket
Male & Female (M&F)
Paired flanges · Fixed gasket position
RF finish (stock)
125–250 µin AARH (3.2–6.3 µm Ra)
Custom serration
Per drawing · Specify AARH at enquiry
Material Specification
Choosing the Right Grade
The same ASME B16.5 flange in 304L and 316L looks identical. In a CIP washdown environment or offshore installation, one lasts 15 years; the other pits through the flange face in 3. Profab verifies every grade by XRF before machining begins — not after.
304 / 304L Stainless
The standard low-carbon austenitic grade for flanges in non-chloride, non-aggressive environments. “L” suffix limits carbon to 0.03% max, preventing carbide precipitation in the weld heat-affected zone. It’s a critical specification for welded flange assemblies that will not be solution-annealed post-weld. Suitable for water treatment (non-saline), food dry-zone equipment, HVAC, and indoor industrial piping where chloride exposure is absent.
Food · Automation · Indoor
316 / 316L Stainless
The specified grade for flanges in chloride-containing environments, CIP washdown chemical lines, marine piping, and chemical process systems handling acids, halides, or sulphur compounds. The 2–3% molybdenum addition raises the Pitting Resistance Equivalent Number (PREN) to ≥ 24 — the corrosion resistance threshold required by most offshore project specifications. Profab XRF-verifies Mo content on every 316L batch; heat numbers are recorded and available on EN 10204 3.1 certificates.
Marine · Chemical · Pumps
317L · 321 · Duplex 2205
For process environments that exceed 316L’s capability: 317L (3–4% Mo, PREN ≈ 28) for concentrated sulphuric acid service and aggressive chemical processing; 321 (titanium-stabilised) for high-temperature applications above 400°C where 316L is susceptible to sensitisation; Duplex 2205 (PREN ≈ 35) for offshore, subsea, and desalination applications requiring superior stress-corrosion cracking resistance and 30% higher yield strength than standard austenitic grades. All available to ASTM A182 from drawing, with EN 10204 3.1 documentation.
Aerospace · Motorsport · Hi-load
Applications
Where Stainless Steel Flanges Are Used
Stainless steel flanges are specified wherever the piping medium, cleaning regime, or installation environment would corrode carbon steel within the design service life — or where contamination of the process fluid is unacceptable.
01
Chemical & Petrochemical
316L or 317L flanges on acid handling lines, solvent transfer piping, and reactor inlet/outlet nozzles. Class 600–2500 weld neck flanges for high-pressure reactor connections. EN 10204 3.1 certs and positive material identification (PMI) reports required on all flanges entering a permitted process area. Custom orifice flanges for differential pressure flow measurement are common in this sector.
02
Food & Beverage Processing
316L flanges for CIP-capable process lines handling milk, juice, beer, and sauces. Flat face flanges should use full-face PTFE or elastomer gaskets, not raised face with spiral wound gaskets, especially for lines connecting to pumps and other equipment with sensitive seating surfaces. Ra ≤ 0.8 µm internal surface finish on bore-machined flanges for hygienic zone piping. FDA material compliance documentation available.
03
Marine & Offshore
316L or Duplex 2205 flanges for seawater cooling lines, ballast system piping, and offshore platform process connections. Subsea and topside installations require EN 10204 3.1 heat-traceable material certs and, on some projects, DNV or Bureau Veritas third-party inspection of the flange batch. RTJ (Ring Type Joint) facing is standard on Class 600 and above offshore flanges due to its superior sealing performance under vibration and thermal cycling.
04
Water & Wastewater Treatment
304L or 316L slip-on and weld neck flanges for treatment plant piping handling potable water, chlorinated effluent, and chemical dosing lines. Class 150 is the standard pressure class for most water treatment piping. For seawater intake and desalination plant connections, 316L is the minimum acceptable grade; Duplex 2205 is often specified where microbiologically-influenced corrosion (MIC) is a documented facility concern.
05
Pharmaceutical & Biotech
316L flanges with electropolished bore surfaces (Ra ≤ 0.4 µm) for injectable-grade water (WFI) and pure steam lines. Flat face with PTFE-encapsulated gaskets for cleanroom piping. Full material traceability documentation, like ASTM A182 compliance, EN 10204 3.1 certs, and surface finish measurement records, they would be supplied as a documentation package. Custom sanitary-finish flanges to 3-A Sanitary Standards on request.
06
Power Generation & Energy
High-pressure steam piping in power plants uses weld neck flanges to Class 1500 or 2500. 321 stainless for high-temperature steam above 400°C where sensitisation risk eliminates 316L. Weld neck is the only acceptable type for steam flanges above Class 300. The cyclic thermal loading that eliminates slip-on and threaded flanges from this service is well-documented in ASME B31.1 Power Piping. Material certs to ASME SA182 specification available.
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We work closely with your technical team to understand and respond to your unique part needs. Whether it’s a CAD file, material conversion or specific tolerances, we can quickly provide samples and provide expert feedback to accelerate your product development.
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FAQ
Common Questions
Not seeing your question? Email us at [email protected] and we typically reply same day.
When should I use a weld neck flange rather than a slip-on?
Use a weld neck flange for Class 600 and higher under all service conditions. It is also recommended for any pressure class in high-temperature steam service, cyclic-loading applications, and wherever ASME B31.3 or B31.1 governs the piping design. Slip-on flanges are suitable for Class 150 and 300 in stable-pressure, ambient-temperature service with low vibration. Typical applications include water treatment, HVAC, and food processing.
What facing finish should I specify for a spiral wound gasket?
Spiral wound gaskets are designed to seat against a Raised Face with a serration finish in the range of 125–250 µin AARH. It’s a phonographic-style concentric groove pattern. Too smooth (below 63 µin) and the gasket cannot grip the face; too rough (above 500 µin) and the winding may not seal at the peaks.
What is the correct flange type for a system that cannot be welded on site?
Threaded flanges are the only ASME B16.5 type that connects without welding. They are limited to Class 300 and below — above this, thread leakage under cyclic pressure loading is a documented failure mode that
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