Verified Engineering Standard Updated: January 2026 What is a Flange Face? Guide to Flange Face Types and Finishes You are on the final stage of a high-pressure steam line commissioning when a "slight hiss" turns into a full-scale shutdown. The gasket is intact, the bolts are torqued to spec, yet the joint fails. Why? In 90% of cases, the culprit is an overlooked mismatch between the Flange Face Types and the surface finish. Selecting the correct Flange Face Types is not just about matching bolts; it is about creating a leak-proof seal that accounts for internal pressure, temperature fluctuations, and chemical compatibility. This guide breaks down the engineering physics of ASME B16.5 facings to ensure your next piping project remains leak-free. Key Takeaways ✔️ Identification of the 6 core Flange Face Types used in global oil, gas, and power sectors. ✔️ The technical difference between Spiral Serrated and Concentric finishes for gasket seating. ✔️ Critical AARH (Arithmetic Average Roughness Height) standards for ASME compliance. What are the different Flange Face Types? The primary Flange Face Types include Raised Face (RF), Flat Face (FF), Ring Type Joint (RTJ), Lap Joint, Male and Female (M&F), and Tongue and Groove (T&G). Each type is designed to facilitate specific sealing requirements, with RF being the most common for industrial applications and RTJ reserved for high-pressure/high-temperature services. Founder's Engineering Insight "I have seen countless engineers specify a Smooth Finish when they actually needed a Stock Finish for spiral wound gaskets. Remember: the flange face finish is the 'grip' for your gasket. Without the right micro-texture, high-pressure media will find a way out, regardless of bolt tension." — Atul Singla, Founder of Epcland Table of Contents The Critical Role of Flange Face Types Primary Flange Face Types & Applications Surface Integrity: Flange Face Finishes Measuring Roughness: The AARH Metric Flange Face Types Failure Case Study Engineering Insights & Lessons Technical Challenge: Flange Face Engineering Validate your knowledge on ASME B16.5 standards (2026 Edition) Question 1 of 5 Next Question The Critical Role of Flange Face Types in Pressure Containment In the world of piping engineering, a flange is more than a simple connection point; it is a complex sealing system. The selection of Flange Face Types directly dictates the gasket stress distribution and the overall integrity of the piping network. According to ASME B16.5 standards, the face of the flange is the surface that hosts the gasket, and its geometry must be meticulously matched to the operating pressure, temperature, and fluid media. The physics of sealing relies on the "Gasket Seating Stress." When bolts are tightened, the force is transferred through the flange to the gasket. Different Flange Face Types manipulate this force in various ways. For instance, a Raised Face concentrates the load on a smaller area, whereas a Flat Face distributes it across the entire surface to protect brittle materials like cast iron. Understanding these nuances is the difference between a system that lasts 30 years and one that fails during its first thermal cycle in 2026. Primary Flange Face Types and Their Engineering Applications Raised Face (RF) The Raised Face is the most common of all Flange Face Types used in industrial process plant applications. It is easily identified by a small portion of the sealing surface that is "raised" above the bolting circle face. For Class 150 and 300 flanges, this height is standard at 1/16 inch (1.6 mm), while for Class 400 and higher, it increases to 1/4 inch (6.4 mm). The primary advantage of the RF design is that it concentrates more pressure on a smaller gasket area, thereby increasing the pressure containment capability of the joint. Flat Face (FF) Flat Face Flange Face Types feature a sealing surface that is in the same plane as the bolting circle face. FF flanges are typically used in low-pressure applications, such as water treatment or HVAC, and are mandatory when connecting to cast iron equipment. Because cast iron is brittle, using a Raised Face flange would create a bending moment when the bolts are torqued, likely cracking the cast iron flange. The FF design ensures the entire surface is supported, preventing this mechanical failure. Ring Joint Flange (RTJ) For high-pressure and high-temperature services (typically Class 600 and above), the Ring Type Joint (RTJ) is the preferred choice among Flange Face Types. These flanges have grooves cut into their faces which accommodate a metallic ring gasket (usually octagonal or oval). When the bolts are tightened, the metal gasket is compressed into the grooves, creating a metal-to-metal seal. This is the most "rugged" sealing method, often found in upstream oil and gas production and high-pressure steam headers. Lap Joint Flange Lap Joint flanges are unique because the flange itself does not have a sealing face. Instead, it is used in conjunction with a "Stub End." The sealing surface is actually on the Stub End, while the flange acts as a backing ring that can rotate freely. This configuration is highly beneficial in systems that require frequent dismantling for inspection or cleaning, as it simplifies bolt hole alignment. Surface Integrity: Flange Face Types Finishes While the geometry of Flange Face Types provides the structural foundation, the surface finish dictates the micro-level interaction with the gasket. ASME B16.5 regulates these finishes to ensure that the friction between the flange and the gasket is sufficient to prevent "blowouts" while being smooth enough to prevent "pathways" for leaks. In 2026, precision machining standards have made it easier to maintain these specific profiles. Stock and Serrated Finishes (Spiral vs. Concentric) The Stock Finish is the most widely utilized surface for Flange Face Types. It features a continuous spiral groove created by a 0.06-inch radius round-nosed tool with a feed rate of 0.03 inch per revolution. This results in a roughness between 125 and 250 micro-inches. Concentric Serrated finishes, though less common, use a similar profile but with closed-loop circles, often specified for gas service where "spiral leakage" is a theoretical concern. Smooth and Cold Water Finishes Smooth finishes are required for metal-to-metal contact or when using thin film gaskets. These typically range from 63 to 125 micro-inches AARH. The Cold Water Finish is an exceptionally smooth surface, almost mirror-like, used in very specific high-vacuum or highly corrosive environments where any surface irregularity could harbor localized crevice corrosion. Measuring Roughness in Flange Face Types: The AARH Metric The Arithmetic Average Roughness Height (AARH) is the gold standard for quantifying the texture of Flange Face Types. It represents the average absolute deviation of the surface profile from the mean line. Miscalculating the required AARH for a specific gasket—such as using a smooth finish with a soft rubber gasket—can lead to the gasket "squishing" out of the joint under pressure. Flange Face Type Standard Finish AARH Range (µ-in) Recommended Gasket Raised Face (RF) Serrated Spiral 125 - 250 Spiral Wound / CNAF Flat Face (FF) Stock Finish 125 - 250 Full Face Elastomer Ring Joint (RTJ) Smooth (Groove) < 63 Metallic Ring (Octagonal) Tongue & Groove Smooth 63 - 125 PTFE / Metallic O-Ring ASME B16.5 Limits for Allowed AARH Imperfections ASME B16.5 provides strict criteria for imperfections on Flange Face Types. For example, deep scratches or "radial streaks" that cross the serrations are generally prohibited if they exceed a certain depth (typically half the serration depth) as they create a direct leak path. In 2026, automated laser scanning is frequently used to verify these tolerances during QA/QC. Don't miss this video related to Flange Face Types Summary: Master Piping Engineering with our complete 125+ hour Certification Course: ...... ✅ 2500+ VIDEOS View Playlists → JOIN EXCLUSIVE EDUCATION SUBSCRIBE 🧮 Gasket Selection & Flange Face Types Compatibility Tool Select your flange parameters below to determine the recommended surface finish and gasket compatibility based on 2026 ASME B16.5 standards. Flange Face Type Raised Face (RF) Flat Face (FF) Ring Type Joint (RTJ) Tongue & Groove (T&G) Pressure Class Class 150 Class 300 Class 600+ Analyze Compatibility Recommended AARH Gasket Choice Flange Face Types Failure Case Study: The High-Pressure Steam Leak In mid-2025, a thermal power plant experienced a recurring leakage on a main steam header operating at 900 PSI. Despite multiple gasket replacements and increasing bolt torque beyond design limits, the joint continued to weep during thermal cycling. An investigation by the Epcland engineering team revealed a fundamental mismatch between the specified Flange Face Types and the actual onsite machining. The Problem The design called for a Raised Face (RF) with a Smooth Finish (63-125 micro-inches) to accommodate a specific PTFE-filled gasket. However, the replacement flanges installed during a 2024 turnaround were Stock Finish (250+ micro-inches). The Physics of Failure The deep serrations of the Stock Finish acted as micro-channels. Because the PTFE gasket material was too rigid to fully "flow" into the deeper grooves at the operating temperature, steam bypassed the primary seal via the spiral serrations. The 2026 Engineering Resolution: The flanges were re-faced in-situ using portable machining tools to achieve the 125 AARH smooth finish. Additionally, the plant transitioned to a 316SS Spiral Wound Gasket with Graphite filler, which is better suited for the Flange Face Types serrations. This corrected the "pathway" leak and has resulted in zero emissions for over 18 months of operation. Standard: ASME B16.5 Location: Thermal Power Unit 4 Expert Insights: Lessons from 20 years in the field ● Check the "Gramophone" Finish: Always verify if your spiral serrated finish (often called a gramophone finish) is actually continuous. A break in the spiral can create a localized pressure drop leading to premature gasket erosion. ● Never Reuse RTJ Gaskets: While Flange Face Types like RF can sometimes survive a minor gasket swap, Ring Type Joints rely on plastic deformation of the metal ring. Reusing a ring means it will not "bite" into the groove correctly in 2026. ● Alignment over Torque: No amount of torque will compensate for a misaligned flange face. Ensure the faces are parallel within 0.5mm before the first bolt is tightened to prevent uneven gasket compression. Frequently Asked Questions Can I connect a Raised Face (RF) to a Flat Face (FF) flange? ▼ Technically, yes, but it is highly discouraged for brittle materials like Cast Iron. If you must connect them, use a full-face gasket and ensure the bolts are not over-torqued, or ideally, machine off the raised face to match the flat face profile. What is the standard AARH for a Spiral Wound Gasket? ▼ For Spiral Wound Gaskets, ASME B16.5 recommends a flange surface finish of 125 to 250 micro-inches AARH. This provides enough "grip" to prevent the gasket from blowing out under high pressure. Why is my RTJ flange leaking despite the ring being new? ▼ The most common cause is a scratch or pit in the 23-degree groove wall. Even a microscopic defect in these Flange Face Types can prevent the metal-to-metal seal from becoming gas-tight. Is a Tongue & Groove flange better than a Raised Face? ▼ T&G flanges offer better gasket protection and higher sealing pressure, but they are "handed" (Male/Female), making them harder to align and more expensive to maintain than standard RF Flange Face Types. What does "Phonographic Finish" mean in 2026? ▼ It is another term for a Spiral Serrated finish. The name comes from the resemblance to the grooves on a vinyl record. It is the standard finish for most Raised Face flanges. How do I measure AARH in the field? ▼ In the field, engineers use "Surface Roughness Comparators" (S-22 micro-finish scales) for visual/tactile inspection, or electronic profilometers for precise digital readings required by 2026 QA protocols. References & Standards ASME B16.5 - Pipe Flanges and Flanged Fittings ISO 7005-1 - Metallic Flanges Part 1: Steel Flanges API 6A - Specification for Wellhead and Christmas Tree Equipment 📚 Recommended Resources: Flange Face Types Read these Guides 📄 Throttling Valve Guide: Types, Working & Selection (2026) 📄 What is an Access Fitting? Applications, Types & Components (2026) 📄 What are Swivel Joints? Working, Types & Engineering Guide 2026 📄 Industrial Steam Valves: 2026 Engineering Guide to Types & Sizing 🎓 Advanced Training 🏆 Piping Material Specification: All About Flange Group 🏆 Basic Elements of Piping Engineering 🎥 Watch Tutorials What is a Flange? | 60 Seconds Knowledge Booster | EPC Land How to Select the Right Flange in Piping Systems? 🔩 | Flange Rating, Facing, Standards Explained ASME-B16.5 & 16.47 II Series A & B Flanges II What is Flange? II Why flanges are required? II
