15 Common Flange Mistakes to Avoid

Flange errors are among the most common causes of piping leaks. These 15 mistakes cover pressure class confusion, face type mismatches, bolting errors, and procurement pitfalls that lead to joint failures.

Technical Mistakes

1. Assuming Class 150 Means 150 PSI

Error: Treating the flange class number as a direct pressure value in psi. Risk: Class 150 carbon steel is actually rated for 285 psi at 100°F (38°C), and this rating drops at higher temperatures. Under-rating or over-rating leads to unsafe designs or unnecessary cost. Fix: Always check ASME B16.5 pressure-temperature rating tables for the specific material and temperature.

2. Mating a Raised Face (RF) Flange with a Flat Face (FF) Flange

Error: Bolting an RF flange to an FF flange in the same joint. Risk: The raised face creates a bending moment on the FF flange, which can crack cast iron flanges or cause uneven gasket compression and leakage. Fix: Machine the raised face down to flat, or replace with matching face types. See flange face types.

3. Mixing ASME B16.5 and EN 1092-1 Flanges

Error: Using an ASME flange on one side and an EN flange on the other side of a joint. Risk: Different bolt circle diameters, number of bolts, and face finishes make alignment impossible. Even if forced together, the joint will not seal safely. Fix: Never mix standards in a flanged joint. Use transition spools if connecting ASME and EN piping systems.

4. Wrong Face Finish for the Gasket Type

Error: Specifying a smooth (30-63 Ra) face finish with a spiral wound gasket, or a rough (>500 AARH) finish with a sheet gasket. Risk: Spiral wound gaskets need 125-250 AARH (3.2-6.3 μm Ra) to grip properly. Too smooth = gasket blows out; too rough = gasket cannot seal. Fix: Match the face finish to the gasket requirements per ASME B16.5 and gasket manufacturer data.

5. Using Slip-On Flanges in High-Pressure or Cyclic Service

Error: Specifying slip-on flanges where weld neck flanges are required. Risk: Slip-on flanges have lower fatigue resistance (2/3 of weld neck) and rely on fillet welds. In high-pressure or vibration service, the fillet weld cracks. Fix: Use weld neck flanges for Class 600+ and all cyclic/high-vibration services.

6. RTJ Gasket Ring Seated on the Wrong Groove

Error: Using an R-number ring gasket that doesn’t match the RX or BX groove, or mixing oval vs octagonal profiles. Risk: Incomplete metal-to-metal seal; the RTJ joint leaks under pressure. Octagonal rings seal better than oval but require matching grooves. Fix: Match the ring number and profile (oval/octagonal) exactly to the flange groove per ASME B16.20.

7. Incorrect Bolt Length for RTJ or Insulation Joints

Error: Ordering bolts sized for RF joints when the actual joint is RTJ (which has a deeper face) or includes insulation kits. Risk: Bolts are too short, resulting in insufficient thread engagement and inadequate gasket compression. Fix: Calculate bolt length based on flange pair thickness + gasket + insulation kit + nut height + 1-3 thread protrusion per ASME PCC-1.

8. Forgetting to Check Bore Compatibility

Error: Not verifying that the flange bore matches the pipe outside diameter and wall thickness. Risk: A flange bored for Sch 40 pipe mated with Sch 80 pipe creates an internal step that causes turbulence, erosion, and weld fit-up problems. Fix: Specify the flange bore to match the actual pipe schedule in the piping class.

Commercial Mistakes

9. Not Specifying the Pipe Schedule in the Flange PO

Error: Ordering flanges by NPS and class only, without specifying the bore/pipe schedule. Risk: Weld neck flanges arrive with standard bore (Sch 40) when the project requires Sch 80 or Sch 160. Rework or reorder required. Fix: Always include the pipe schedule or exact bore diameter in the purchase order.

10. Mixing Imperial and Metric Bolting

Error: Ordering metric bolts for ASME flanges or imperial bolts for EN flanges. Risk: Bolts don’t fit the bolt holes. Even if the diameter is close, thread pitch differs (UNC vs metric), preventing nut engagement. Fix: Use imperial stud bolts for ASME flanges and metric bolts for EN 1092 flanges. Never mix.

11. Missing Heat Number Traceability on Flange MTCs

Error: Accepting flanges without material test certificates traceable to the heat number stamped on the flange. Risk: Untraceable flanges cannot be verified for material composition and mechanical properties. In sour or critical service, this is a code violation. Fix: Require EN 10204 3.1 or 3.2 certificates with heat number traceability for all flanges. See flange materials.

12. Ordering Wrong Flange Standard for the Project

Error: Ordering ASME B16.47 Series A flanges when Series B (API 605) was specified, or vice versa. Risk: Series A and B have different bolt circles, thicknesses, and dimensions for NPS 26-60. Components won’t mate. Fix: Verify Series A vs Series B requirement in the project piping class before ordering.

13. Not Specifying Spectacle Blind Material Class

Error: Ordering spectacle blinds without specifying material grade and pressure class to match the line. Risk: A lower-grade spectacle blind in a high-pressure line becomes the failure point. Fix: Spectacle blinds must match the piping class, material grade, and pressure-temperature rating of the line.

14. Ordering Flanges Without Marking Requirements

Error: Not specifying marking requirements (size, class, material, heat number, manufacturer) per MSS SP-25. Risk: Unmarked flanges are unidentifiable in the warehouse and at site. Quality inspectors will reject them. Fix: Specify marking requirements per MSS SP-25 and the project specification in the PO.

15. Ignoring Lead Time for Large-Bore and High-Class Flanges

Error: Treating NPS 24+ or Class 1500/2500 flanges as standard catalog items. Risk: Large-bore and high-class flanges require custom forging, with lead times of 16-24 weeks. Late ordering delays construction. Fix: Identify long-lead flange items early in the engineering phase and place orders during FEED or early detail design.

Frequently Asked Questions

Does Class 150 mean 150 psi?

No. The class number is a designation, not a pressure value. A Class 150 carbon steel (A105) flange is rated for 285 psi at 100°F per ASME B16.5. The rating decreases with increasing temperature and varies by material.

Can I mate a raised face flange with a flat face flange?

This is strongly discouraged. The raised face concentrates the bolt load on a smaller area, creating a bending moment that can crack flat face flanges (especially cast iron). Either machine the RF down to flat or use matching face types throughout the joint.

Can I use ASME flanges with EN flanges?

No. ASME B16.5 and EN 1092-1 flanges have different bolt circle diameters, bolt quantities, face finishes, and pressure classes. They cannot be bolted together. Use a transition spool with one ASME and one EN flange if systems must connect.

Why does face finish matter for gasket sealing?

The flange face finish determines how well the gasket grips the flange. Spiral wound gaskets require 125-250 AARH (3.2-6.3 μm Ra). A face that’s too smooth lets the gasket blow out under pressure; too rough prevents uniform sealing contact. Check face finish requirements.

What happens if flange bolts are too short?

Short bolts have insufficient thread engagement with the nut, reducing clamping force. Under thermal cycling, vibration, or pressure spikes, the joint loosens and leaks. Per ASME PCC-1, bolts should protrude at least one full thread beyond the nut face.

Can I use slip-on flanges in high-pressure service?

Slip-on flanges are generally limited to Class 300 and below and are not suitable for severe cyclic, high-temperature, or high-vibration services. Their fatigue resistance is about 2/3 that of weld neck flanges, which should be used for all critical applications.