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You receive a valve with "150-pound flanges," pipe from another supplier with "PN16," and a flange from a third – and the bolt holes do not line up. This is the classic flange-standard trap. The three systems use different sizing, pressure logic and bolt threads, so "compatible" has a precise meaning. Here is how to avoid the costly site rework.
Dimensions look close but are not identical. A 4-inch (DN100) Class 150 flange has an outside diameter of ~229 mm and bolt circle of ~190 mm; a DN100 PN16 flange is 220 mm OD with a 180 mm bolt circle. Both use 8 bolts – yet the holes are off by 9–10 mm, so they will not bolt up. Bolt threads differ too (UNC vs ISO metric). Attempting a forced match causes misaligned bolts, crushed gaskets and leaks. Use a transition flange rated to the lower of the two standards.
The ASME Class is a group designation, not a pressure in pounds. The real allowable pressure comes from ASME pressure-temperature tables by material group and operating temperature. A Class 150 carbon-steel flange is about 19.6 bar (285 psi) at 38°C, but the rating falls as temperature rises. Approximate equivalences: Class 150 ≈ PN20, Class 300 ≈ PN50, Class 600 ≈ PN110. Treat these as guides only – always verify the P-T rating for your material.
The facing decides the gasket. RF (raised face) is the default for most valves and piping. FF (flat face) is used against brittle equipment like cast-iron pumps and PAH (plastic-lined) vessels. RTJ (ring-type joint) is for high-pressure hydrocarbon service with metal ring gaskets. Match the facing to the adjacent flange – an RF valve against an FF pump can over-compress the gasket.
Generally no. Even at a similar nominal size and bolt count, the outside diameter, bolt circle and bolt-hole pattern differ (a 4-inch Class 150 and a DN100 PN16 both use 8 bolts but their OD differs by about 9-10 mm). Use a transition flange with one face per standard, rated to the lower of the two.
PN16 is the closest nominal pressure equivalent (Class 150 carbon steel is rated about 19.6 bar at 38C; PN16 = 16 bar). But equivalent is not interchangeable – the physical dimensions still differ, so the flanges will not bolt together.
The Class is a group designation. The actual allowable pressure comes from ASME pressure-temperature tables by material group and operating temperature. A Class 150 flange in carbon steel is about 285 psi at ambient, but the rating drops as temperature rises.
ASME B16.5 dominates North America, global oil & gas and much of the Middle East. EN 1092-1 (DIN) is standard in Europe, Africa and European-engineered plants. JIS B2220 is used in Japan and Japanese-engineered facilities in Asia. Always specify standard + size + rating + facing on the order.