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ASME B16.36-2015 — Orifice Flanges
📊 ASME B16.36-2015 establishes the requirements for orifice flanges used in orifice flow measurement (throttling devices), covering pressure-temperature ratings, materials, dimensions, tolerances, marking, flange facing finish, pressure taps, and jack screws. The drive for standardization began with correspondence in August 1956 noting the lack of standards for orifice flanges; the first edition was approved by ANSI on August 15, 1975, with subsequent editions in 1979, 1988, 1996, 2006, and 2009. The 2015 edition was approved by ANSI on February 24, 2015 and runs 30 pages, covering classes 300, 400, 600, 900, 1500, and 2500.
💡 What makes an orifice flange different: besides the standard bolt holes, orifice flanges carry a pressure tap on each side plus jack screw holes to fix the orifice plate. They are purpose-built to clamp orifice plates, nozzles, or venturi throttling elements in a metering run.
1. Scope and General Requirements 📌
Sections 1 and 2 provide the following:
- Flange types: welding neck, slip-on, and threaded orifice flanges;
- Rating classes: 300, 600, 900, 1500, 2500 (Class 400 data in Mandatory Appendix II, U.S. Customary units);
- Nominal sizes: NPS 1 through NPS 24 (per the standard’s tables);
- Units: SI (metric) primary, U.S. Customary in parentheses or separate tables.
| Flange Type | Classes Covered | Standard Table |
|---|---|---|
| Welding neck, threaded, slip-on | Class 300 | Table 1 |
| Welding neck | Class 600 | Table 2 |
| Welding neck | Class 900 | Table 3 |
| Welding neck | Class 1500 | Table 4 |
| Welding neck | Class 2500 | Table 5 |
| Welding neck, etc. | Class 400 | Appendix II (U.S. Customary) |
2. Ratings and Materials 🔥
Sections 3 and 4 specify that orifice flange ratings follow the ASME B16.5 rating tables for the corresponding material group; materials are specified per the listings of B16.5 Appendix III, with allowance for other proven editions of those material specifications. Flange facing finish follows Section 7, and gaskets for raised-face flanges follow Section 8.
⚠️ Orifice flange ratings are “borrowed” from B16.5: at the same size and class, they carry the same pressure-temperature capability as B16.5 flanges. Always check the complete flange-orifice-gasket assembly, not the flange alone.
3. Pressure Taps, Jack Screws, and Dimensions 📏
Pressure taps are what distinguish orifice flanges from ordinary flanges:
- Corner taps: taps located immediately adjacent to each side of the orifice plate — the most common arrangement (Fig. 1);
- Angular meter taps: for ring-joint (RTJ) flanges, taps entering the flange body at an angle (Fig. 2);
- Jack screws: to separate the clamped orifice plate and gasket during inspection (Fig. 3);
- Flange dimensions: Section 11 gives outside diameter, thickness, bolt circle, overall length, etc.; tolerances per Section 13;
- Threads (Section 12): requirements for threaded orifice flanges.
| Feature | Requirement | Function |
|---|---|---|
| Corner taps | One on each side, adjacent to the plate | Measure differential pressure across the plate |
| Angular taps | RTJ flange only, angled entry | Adapt to ring-joint geometry |
| Jack screws | 2–4 per standard configuration | Separate the plate during inspection, protect faces |
| Facing finish | Per Section 7 | Sealing and measurement accuracy |
| Raised-face gaskets | Per Section 8 | Stable plate installation clearance |
4. Orifice Installation and Measurement Notes ⚙️
- Concentricity: the orifice plate must be centered on the pipe; eccentricity shifts the discharge coefficient;
- Straight runs: provide sufficient straight pipe upstream and downstream per flow-measurement standards (e.g., AGA Report No. 3 / ISO 5167);
- Tap lines: keep tap lines short and level to avoid liquid traps and blockages;
- Jack screw use: tighten jack screws evenly when dismantling; never pry against sealing faces or the plate.
🚨 Never disassemble an orifice flange connection under pressure! Depressurize, drain, and confirm zero residual energy before loosening bolts or jack screws. The large differential across the plate can eject high-temperature/high-pressure media and cause serious injury.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection: choose class and type by line pressure and size; below Class 300, wafer-style orifice plates are usually used instead of orifice flanges.
- 🔧 Paired purchase: orifice flanges are bought in pairs; orient taps horizontally or as designed to simplify tap-line routing.
- 📊 Metering verification: perform zero and differential-pressure checks after installation per the flow standard; periodically verify plate thickness and edge sharpness.
- 📋 Certificates: for high-pressure classes, request material certificates and hydrostatic test reports to confirm compatibility with the piping material.
6. Frequently Asked Questions ❓
❓ Q: Can B16.36 orifice flanges interchange with B16.5 flanges?
A: Dimensional bases match, but orifice flanges add pressure taps and jack screw holes with a different functional role. They are not interchangeable in metering service, and ordinary flanges cannot substitute for tapping duties.
A: Dimensional bases match, but orifice flanges add pressure taps and jack screw holes with a different functional role. They are not interchangeable in metering service, and ordinary flanges cannot substitute for tapping duties.
❓ Q: Why must taps sit tight against the plate?
A: Corner taps measure static pressure immediately upstream and downstream of the plate, capturing the differential closest to throttling theory with the smallest uncertainty. Taps farther away include friction-loss error.
A: Corner taps measure static pressure immediately upstream and downstream of the plate, capturing the differential closest to throttling theory with the smallest uncertainty. Taps farther away include friction-loss error.
❓ Q: What are jack screws for?
A: They separate the orifice plate and gasket from the flange faces during inspection, avoiding prying damage and letting the flange be repeatedly opened and closed without harming sealing surfaces.
A: They separate the orifice plate and gasket from the flange faces during inspection, avoiding prying damage and letting the flange be repeatedly opened and closed without harming sealing surfaces.
❓ Q: Why is Class 400 in an appendix?
A: Class 400 is a non-preferred class; dimensional data is provided in U.S. Customary units only in Mandatory Appendix II and is omitted from the metric tables.
A: Class 400 is a non-preferred class; dimensional data is provided in U.S. Customary units only in Mandatory Appendix II and is omitted from the metric tables.
7. Key Takeaways 📌
- Purpose-built geometry: orifice flanges are metering hardware — corner taps, angular RTJ taps, and jack screws distinguish them from ordinary B16.5 flanges of the same class and size.
- Rating inheritance: pressure-temperature ratings follow B16.5 for the material group; verify the complete flange–orifice–gasket assembly rather than the flange in isolation.
- Coverage: welding-neck, slip-on, and threaded orifice flanges, Classes 300, 400, 600, 900, 1500, and 2500, NPS 1 through NPS 24 (per tables); Class 400 data lives in Mandatory Appendix II in U.S. Customary units.
- Measurement accuracy: plate concentricity, straight-run lengths, and short level tap lines dominate flow-measurement uncertainty — follow AGA Report No. 3 / ISO 5167 practice.
- Safety: never break an orifice flange joint under pressure; the differential across the plate can eject media violently. Use jack screws for inspection, never pry bars.
