HG/T 20615 Class Flanges: The Chinese ASME B16.5 Standard Buyers Miss

China's Two Flange Universes

When engineers hear "Chinese flange" they usually think of GB/T 9113. But China actually runs two flange systems in parallel, and confusing them is one of the most common specification errors in cross-border valve procurement:

• GB/T 9113 - the PN (metric) system, dimensionally based on EN 1092-1. This is the default for domestic Chinese and European-pattern projects.
• HG/T 20615 - the Class (imperial) system, modified from ASME B16.5. This is what Chinese suppliers use when the valve or pipe must mate to US-pattern equipment (DuPont, Dow, ExxonMobil specs, etc.).

Both are legitimate, published Chinese standards. The trap is assuming they are interchangeable because "PN16 is about Class 150".

What HG/T 20615 Actually Is

HG/T 20615-2009 "Steel Pipe Flanges (Class Series)" is the chemical-industry standard for Class-rated flanges. Its key facts:

• Pressure classes: Class 150, 300, 400, 600, 900, 1500, 2500
• Nominal size: DN15 - DN600 (NPS 1/2 - 24)
• Dimensions, RTJ (ring-joint) sealing faces and pressure-temperature ratings are essentially identical to ASME B16.5
• Material and inspection requirements are tuned for chemical-industry duty - for example, grades selected for H2S (sour-service) resistance

In practice a valve with HG/T 20615 flanges bolts directly to an ASME B16.5 pipeline. The Chinese committee adopted the US dimensions and converted the inch sizes to metric; the flange geometry is the same.

Large Diameter: HG/T 20623 (ASME B16.47)

Above DN600 the governing Chinese standard is HG/T 20623-2009, based on ASME B16.47. ASME B16.47 keeps two series, and so does HG/T 20623:

• A series - based on MSS SP-44 (the thicker, heavier, more robust flange)
• B series - based on API 605 (more, smaller bolts; smaller bolt circle)

They are not interchangeable with each other: an A-series flange and a B-series flange of the same size and class will not bolt together. The series must be written on the requisition. Many costly field mistakes come from omitting the A/B designation on large-diameter flanges.

The PN vs Class Trap in Concrete Numbers

Even where the pressure classes roughly correspond, the flange geometry does not. A textbook example at DN80:

The outer diameters and bolt circles differ, so a PN16 valve will not seat on a Class 150 flange even though both are "about 150-ish". The same mismatch appears at every size. This is the mechanical reason behind the warning in our PN vs ASME Class guide.

A Decision Rule for Specifying Flanges

1. Identify the mating equipment's flange standard first - that is the one you must match, not a pressure equivalent.
2. If the mating side is US-pattern (ASME B16.5 / B16.47), specify HG/T 20615 / HG/T 20623 on the Chinese valve.
3. If the mating side is European-pattern (EN 1092-1), specify GB/T 9113.
4. For large diameter, always write the A or B series of HG/T 20623 / ASME B16.47.
5. Confirm the sealing face (RF, RTJ, FF, MFM) and the bore schedule (A-series or B-series pipe) explicitly on the purchase order.

Our Chinese-to-international valve standards cross-reference lists where GB/T 9113 and HG/T 20615 sit in the broader GB-to-API/ASME map. Flange standards and bore sizes for more than ninety manufacturers are catalogued in the ValveSpecs Pro database, which makes it straightforward to confirm whether a quoted flange truly mates with your pipeline.