Flanges are the classic mixed-standards product: the pipe is specified in one system, the flange arrives in another, and the bolt holes do not line up. A five-minute comparison prevents the classic site failure.
Sizes in NPS inches, pressure in Classes (150, 300, 600…), raised face height 2 mm up to Class 300. Bolt count and diameter rise with class. This is the default in oil and gas, and across the Americas.
Sizes in DN, pressure in PN (PN10, PN16, PN25, PN40…). The bolt pattern for a given DN differs from the ASME pattern of the "equivalent" class — a DN100 PN16 flange will not bolt to an NPS 4 Class 150 flange. If your plant is European-built, specify EN and state the PN explicitly.
Its own bolt pattern again, common in Japanese-built plants across Southeast Asia. TheRF face height (3 mm for 2K and above) differs from both ASME and EN.
First, confirm which standard the mating pipework follows — the flange follows the pipe, never the reverse. Second, state the facing type (RF, FF, RTJ) and the gasket type together. Third, for stainless flanges, confirm whether the raised face is integral or a separate welded overlay; the price difference is real.
Every flange should carry the manufacturer's mark, size, class or PN, material grade and heat number. On delivery, check that the heat numbers on the flanges match the mill test certificates in the document pack — this is the check that catches mixed material before it enters your pipeline.
Send the grade, the size and the quantity — you get a costed quotation within two working days.