ASME B16.36-2020 Explained: Rules for Orifice Flange Assemblies (ASME B16 Series)

What is ASME B16.36-2020?

ASME B16.36-2020 is an American National Standard that establishes the dimensional, material, marking, testing, and pressure-temperature rating requirements for orifice flanges. These are specialized flanges designed to accommodate orifice plates or other primary flow elements for the measurement of fluid flow rate in piping systems. The standard covers Class 300, 600, 900, 1500, and 2500 flanges in sizes from NPS 1/2 to NPS 24. Its core purpose is to ensure dimensional interchangeability, pressure integrity, and functional reliability of these critical measurement components across different manufacturers and projects. It addresses the technical gap between general pipe flanges (covered in standards like ASME B16.5) and the precise needs of flow metering, which require specific bore alignments, pressure tap configurations, and sealing surfaces.

In professional practice, piping designers and instrumentation engineers apply this standard to specify and procure orifice flanges that are compatible with the selected orifice plate and meet the system’s pressure class. Fabricators and manufacturers use it as the definitive guide for production and quality control. During construction and commissioning, inspectors and metering specialists reference B16.36 to verify that installed assemblies conform to the required specifications, ensuring accurate and safe flow measurement.

Problem-Solving and Application Scope

This standard resolves critical challenges in flow measurement system design and safety. It standardizes the interface between the orifice plate, the flange assembly, and the piping system, preventing misalignment that could cause measurement errors or leakage. It defines pressure-temperature ratings that ensure the flange assembly can safely contain the process fluid under operating conditions, mitigating risks of gasket blowout or flange failure. By providing uniform dimensional and marking requirements, it facilitates global procurement and replacement, reducing project delays.

ASME B16.36 is predominantly adopted across North America and is widely referenced in international oil & gas, petrochemical, power generation, and water treatment projects. Its application is specific to piping systems where orifice plates are used for flow measurement, commonly found in feed lines, product streams, utility systems, and custody transfer stations. It is applicable to a broad range of fluids, including hydrocarbons, water, steam, and gases.

Technical and Safety Highlights

Within the ASME B16 series of standards for pipe flanges and fittings, B16.36 holds a unique position as a product standard for a functional assembly rather than a simple connecting component. While B16.5 covers standard flanges, B16.36 specifies flanges that must integrate with a separate, removable metering element (the orifice plate) and include integrated pressure tap connections (e.g., threaded, socket weld, or weld neck outlets).

A core technical principle specific to this standard is the definition and tolerancing of the bore and raised face. The standard meticulously specifies the bore diameter of the flange and its alignment with the orifice plate bore to ensure smooth, predictable fluid flow into the orifice, which is essential for accurate application of flow equations (such as those in ISO 5167 or AGA Report No. 3). The raised face provides a sealing surface for the gasket and orifice plate, with requirements distinct from B16.5 to accommodate the plate.

Key safety frameworks include:
* Pressure-Temperature Ratings: Ratings are established for each flange class and material group, defining the maximum allowable working pressure at a given temperature.
* Mandatory Marking: Flanges must be permanently marked with the manufacturer’s name or trademark, material designation, pressure class, size, and the B16.36 designation to ensure traceability.
* Material Specifications: The standard references ASME material specifications (e.g., SA-105, SA-182) and assigns them to material groups for rating purposes, ensuring known mechanical properties.

Regulatory Context and Comparisons

ASME B16.36 is a consensus standard developed under the procedures of the American Society of Mechanical Engineers (ASME). It is a national standard in the USA and is frequently invoked in engineering specifications, purchase orders, and regulatory frameworks in regions following American engineering practice. While not a law itself, it becomes a mandatory project requirement when specified in contract documents. It is also recognized and referenced by other major codes, such as ASME B31.3 Process Piping Code, which governs the design and construction of piping systems where these flanges are installed.

Conceptually, ASME B16.36 differs from international orifice flange standards, such as those found in the ISO 5167 series on flow measurement. While ISO 5167 provides extensive requirements for the orifice plate itself and the installation conditions (upstream/downstream pipe lengths), it typically references regional flange standards (like EN 1092-1 in Europe or ASME B16.36 in North America) for the flange assembly hardware. The primary difference lies in B16.36’s integration into the comprehensive ASME B16 and B31 code ecosystem, ensuring compatibility with other ASME-rated components. Compared to the general ASME B16.5 flange standard, B16.36 includes all necessary additional features (e.g., pressure tap holes, jack screws) for metering service.

Target Professionals and Practical Risks

This standard is indispensable for:
* Piping Design Engineers: For specifying the correct flange class, type, and pressure tap configuration on piping and instrumentation diagrams (P&IDs) and isometric drawings.
* Instrumentation Engineers: For ensuring the flange assembly is compatible with the orifice plate calculation and meets installation requirements for accurate measurement.
* Procurement Specialists: For preparing technical purchase requisitions and evaluating supplier quotations.
* Quality Assurance/Control Inspectors: For verifying material certifications, dimensions, markings, and pressure test reports (if applicable) upon receipt and before installation.
* Construction Supervisors and Mechanical Inspectors: For confirming proper installation, gasket selection, and bolt tightening.

Engineering Risks of Misinterpretation:
Misapplication of ASME B16.36 can lead to significant technical and financial consequences. Specifying an incorrect pressure class (e.g., Class 150 instead of Class 300 for the service conditions) poses a direct safety hazard, risking flange failure under pressure. Dimensional errors, such as incorrect bore size or pressure tap orientation, will introduce incurable flow measurement errors, impacting process control, custody transfer calculations, and revenue. Non-compliance can lead to rejection of components during site inspections, causing costly project delays. In a post-commissioning audit or after an incident, the use of non-compliant flanges can expose engineering firms and operators to severe liability.

Application in Practice and Common Misconceptions

Real-World Scenario: An engineer designing a natural gas custody transfer station must specify an orifice flange assembly for an NPS 8 line operating at 800 psig and 60°F. Using ASME B16.36, they select a Class 600 weld neck orifice flange with RTJ (Ring-Type Joint) facing for high-pressure sealing. The standard guides them to specify the material (e.g., SA-105), the pressure tap types (e.g., threaded NPT taps), and to ensure the manufacturer’s documentation includes the mandatory B16.36 marking. This specification ensures the procured assembly will mate correctly with the piping, seal effectively, and provide reliable pressure tap connections for the differential pressure transmitter.

Common Misconceptions:
1. Misconception: Orifice flanges to ASME B16.36 include the orifice plate.
Clarification: B16.36 covers only the flange assembly. The orifice plate is a separate component designed per other standards (e.g., ISO 5167, AGA Report No. 3) and must be purchased separately.
2. Misconception: The pressure-temperature ratings in B16.36 are identical to those in ASME B16.5 for the same class and material.
Clarification: While often similar, the ratings are established specifically within B16.36 for the orifice flange product and must be taken directly from its tables, not assumed from B16.5.