API 6A and 6BX
The pressure-control standard for wellhead, christmas tree, and surface production equipment in oil and gas service. Where ASME tops out around 2500 PSI, API 6A picks up and runs to 25000 PSI.
Pressure Classes
API 6A flanges are designated by working pressure: 2M (2000 PSI), 3M, 5M, 10M, 15M, 20M, and 25M. The 6B series covers 2M through 5M at standard ring-joint dimensions. The 6BX series covers 10M and above, where the ring gasket geometry differs to handle the higher pressures.
The 2M class is dimensionally close to ASME B16.5 Class 600 but with stricter material and ring-joint requirements. Above 5M, API 6A is the only governing standard for the dimensional family.
Ring Gaskets
Ring-type joint (RTJ) facings are non-negotiable on API 6A flanges. The metal-to-metal seal between flange face groove and ring gasket is what gives the assembly its pressure integrity. R-type gaskets (oval or octagonal) cover 2M through 5M. BX-type gaskets, with self-energizing geometry, cover 10M and above. Mating the wrong gasket type to the flange compromises the seal. The flange and the gasket are ordered together.
Material Specifications
API 6A defines material classes AA, BB, CC, DD, EE, FF, and HH based on operating environment. Standard service is BB or CC carbon and low-alloy. Sour service is DD or EE with NACE MR0175 hardness and chemistry control. Severe sour service is FF or HH with additional restrictions on chemistry and heat treatment. The MTR shows the material class and the supporting test data.
Where API 6A Lives
Wellheads, christmas trees, blowout preventers, choke and kill manifolds, frac trees, and high-pressure flowlines. Drilling, completion, production, and intervention service. The standard does not generally appear on plant process piping or on water service, which fall under ASME or AWWA standards.