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A blind flange is a solid plate used to seal the end of a pipe or vessel. To choose the right blind flange, first define your service conditions (fluid, pressure, temperature), then match the correct ASME standard and pressure class, select the material based on corrosion resistance, confirm the dimensions and facing type for gasket compatibility, and finally verify documentation and installation requirements. This step-by-step approach ensures safety, compliance, and cost-effectiveness.
A blind flange is a type of pipe flange without a bore. It closes off the end of a piping system or vessel nozzle, providing positive isolation during testing, maintenance, or long-term shutdowns. Because it is bolted like any other flange, it can be removed later to reopen the line.
Blind Flange: Permanent or semi-permanent end closure.
Spectacle Blind: Figure-8 device with a solid end and an open end for frequent switching.
Spade & Spacer: Two separate plates (solid and open) swapped during isolation.
Blind flanges are preferred when:
Full positive isolation is required during hydrotesting.
Valves would be more expensive or unnecessary.
Pipeline branches are reserved for future connections.
ASME B16.5: NPS ½″–24″, pressure classes 150–2500.
ASME B16.47 Series A/B: NPS 26″–60″.
Series A: Thicker, heavier, more robust for severe service.
Series B: Thinner, lighter, typically lower cost.
Each material group has a pressure rating that decreases as temperature increases. For example, an ASTM A182 F316 blind flange rated Class 600 holds ~1480 psi at 100 °F but only ~1085 psi at 600 °F. Always:
Identify your material group.
Find your maximum service temperature.
Select the class column (150–2500) that meets or exceeds your design pressure.
AWWA C207: Low-pressure waterworks service.
API 6A: Wellhead and Christmas tree equipment.
API 605: Older large-diameter spec (superseded by B16.47).
Carbon Steel (ASTM A105, A350 LF2): General service, cost-effective.
Alloy Steel (A182 F11/F22): Elevated temperature/pressure.
Stainless Steel (A182 F304/316): Corrosion resistance in chemicals, seawater.For a more detailed breakdown of stainless steel flange types and their typical applications, you can explore our Stainless Steel Flange Types and Applications guide.
Duplex/Super Duplex: Chloride environments, offshore.
Nickel Alloys (Inconel, Monel, Hastelloy): Extreme corrosion or heat.
Low-Temperature Service: ASTM A350 LF2, impact-tested.
Sour Service (H₂S): NACE MR0175 compliant.
Chloride Stress Cracking: Prefer duplex or super duplex.
PTFE lining: Aggressive chemicals.
FBE coating: Buried pipelines.
Hot-dip galvanizing / painting: Atmospheric corrosion protection.
Raised Face (RF): Most common, pairs with spiral wound gaskets.
Flat Face (FF): For cast iron equipment, full-face gaskets.
Ring Type Joint (RTJ): High-pressure, high-temperature, uses metal ring gasket.
Typical serrated finish: 125–500 µin Ra.
Concentric vs. spiral serrations: Both designed to “bite” into gasket.
Proper finish ensures seal integrity under pressure cycles.
Standard Blind: Solid plate, bolted.
Vented/Tapped Blind: Small bore drilled to release trapped pressure.
Orifice Blind: With hole for flow measurement.
Used when frequent isolation is required.
Safer and faster than unbolting heavy blind flanges repeatedly.
Positive isolation for safety and testing.
Cost-effective vs. valves in dead-end service.
Durable under correct class and material selection.
Heavy for large diameters: Requires lifting gear.
Changeover is slow compared to spectacle blinds.
Gasket/bolt sensitive: Incorrect torque = leakage.
Oil & Gas: Isolation of branch connections, pressure testing.
Chemical & Petrochemical: Resistant alloys for corrosive fluids.
Power Generation: Steam lines, high temperature.
Water & Wastewater: AWWA standards for low-pressure.
Pharma & Food: Stainless steel for cleanliness.
Case 1: Refinery hydrotest — Class 600 blind flange prevented backflow during 24-hr pressure test.
Case 2: Offshore platform — super duplex blind flanges installed to combat chloride stress corrosion.
If stainless steel is under consideration for your project, see our dedicated article on Stainless Steel Flange Types and Applications for in-depth insights.
Choosing the right blind flange is not guesswork — it follows a logical, standards-driven process. Below is a practical checklist engineers and buyers can use to avoid costly mistakes and ensure system reliability.
Start with the fundamentals:
Fluid type: Is it clean water, crude oil, sour gas, steam, or a corrosive chemical? Fluids dictate both the material and gasket. For example, seawater requires duplex stainless steel, while hydrocarbons may require carbon steel with internal coating.
Design pressure and temperature: Always use the maximum operating pressure and temperature, not just the average. Blind flanges are stress concentrators — underestimating conditions can lead to catastrophic failure.
Transient loads: Consider pressure surges, water hammer, startup/shutdown cycles, and thermal expansion. Even if the average operating pressure is moderate, transient peaks may demand a higher class rating.
ASME B16.5 covers NPS ½″–24″ with Classes 150, 300, 600, 900, 1500, 2500.
ASME B16.47 covers NPS 26″–60″ with Series A (heavier, more robust) and Series B (lighter, cost-effective).
How to use pressure-temperature tables:
Identify the material group (e.g., ASTM A182 F316 → Group 2.2).
Go to your design temperature (e.g., 600 °F).
Read across the table until you find a class rating that exceeds your design pressure.
Example: A Class 600 stainless blind flange may handle 1085 psi at 600 °F — if your design pressure is 950 psi, Class 600 is acceptable with margin.
Pro tip: Always design with a 10–20% safety margin above expected operating pressure.
Blind flanges must align perfectly with mating flanges:
NPS (Nominal Pipe Size): Determines the bore size compatibility.
OD (Outside Diameter) and Bolt Circle Diameter (BCD): Critical for bolting alignment.
Number & size of bolt holes: Must match the mating flange — even one mismatch means rework.
Thickness: Defined by ASME based on class rating; thicker flanges resist higher bending stresses at the center.
Always cross-check dimensions against the ASME B16.5 or B16.47 dimensional tables. Even a few millimeters off can prevent installation or cause leakage.
Raised Face (RF): Works with spiral wound gaskets — most common.
Flat Face (FF): Used with cast iron equipment, requires full-face gaskets.
Ring Type Joint (RTJ): High-pressure, high-temperature; requires metal ring gaskets.
Surface finish: Ensure the serrated finish (125–500 µin Ra) matches the gasket type.
Real-world tip: Never pair an RTJ flange with a non-metallic gasket; it will leak under load.
Carbon Steel (A105/A350 LF2): Cost-effective for non-corrosive fluids.
Stainless (F304/316): Corrosion resistance, chemical/potable water service.
Duplex & Super Duplex: Offshore, chloride-rich environments.
Nickel Alloys (Inconel, Hastelloy): Extreme heat or acids.
Coatings & Linings: PTFE lining for chemical plants; FBE for buried pipelines; galvanizing for atmospheric exposure.
Also check:
NACE MR0175/ISO 15156 compliance if service involves H₂S (sour gas).
Charpy V-notch testing for low-temperature service.
Vented (tapped) blind: Allows safe release of trapped pressure before removal.
Orifice blind: For flow monitoring while maintaining isolation.
Testing requirements: Radiography, ultrasonic testing, or MPI if specified by project QA.
Certification: Always request EN 10204 3.1/3.2 Material Test Reports (MTRs) for traceability — this proves compliance with ASME/ASTM standards.
Weight: Large-diameter Class 900+ blinds can weigh hundreds of kilos — plan lifting lugs and hoists.
Torque sequence: Use a cross/star pattern tightening in multiple passes. Example: tighten bolts to 30%, then 60%, then 100% of torque value.
Lock-out/Tag-out (LOTO): Ensure line is depressurized before removal. Blind flanges can trap pressure; vented blinds reduce this risk.
Gasket seating: Always ensure gasket is centered — off-center installation leads to blowout.
Upfront cost: Standard blinds are cheaper than spectacle blinds or valves.
Maintenance impact: If isolation is frequent, spectacle blinds or spade & spacers save downtime (no need to unbolt heavy blinds repeatedly).
Lifecycle ROI: Spending more initially on the right material or isolation device often saves far more in reduced leaks, failures, and downtime.
Case insight: A refinery that switched from carbon steel to duplex blinds in seawater service reduced replacement frequency from every 3 years to every 10+ years — saving millions in maintenance.
Choosing the right blind flange is not guesswork — it’s a structured decision. By aligning service conditions with ASME standards, selecting the proper material and facing, and confirming installation practices, engineers and buyers can ensure safe, reliable, and cost-effective piping systems. When in doubt, consult both the ASME pressure–temperature tables and your flange manufacturer’s certification to validate your choice.
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