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ASME B16.5 Class 900 Slip-On Flange (which governs pipe flanges and flanged fittings from NPS 1/2 through NPS 24), a Class 900 rating signals a component capable of managing severe pressure-temperature combinations. Because of the substantial stresses involved at this rating, Class 900 flanges are physically robust, featuring deep thicknesses, wide bolt circles, and extended hubs compared to lower pressure classes like Class 150 or 300.
To properly integrate these components into a digital CAD model or a physical piping layout, several core dimensions must be evaluated:
Outside Diameter: The total width of the flange disk.
Minimum Thickness: The structural thickness of the flange block, excluding the height of the raised face.
Bore Diameter: The internal machined opening. This is slightly larger than the outside diameter of the matching pipe to allow the pipe to slip inside smoothly.
Hub Diameter at Base: The width of the metal reinforcement ring (hub) where it rises from the flange body.
Length Through Hub: The complete vertical height of the flange, measured from the front sealing face to the back of the hub sleeve.
Critical Engineering Nuances
1. The Class 900 / Class 1500 Dimensional Twin
An essential detail for structural detailing and procurement tracking is the crossover rule embedded within the ASME B16.5 code. For nominal sizes from NPS 1/2 through NPS 2 1/2, Class 900 dimensions are completely identical to Class 1500 dimensions. Physical deviation between the two pressure classes only begins at NPS 3 and larger. Consequently, many manufacturers stamp smaller components with a dual Class 900/1500 rating.
2. Volumetric and Weight Realities
Because of the heavy wall profiles needed to securely contain high-pressure fluids, Class 900 slip-on flanges are exceptionally heavy. For instance, a small NPS 2 Class 900 slip-on flange weighs roughly 25 lbs. However, when scaled up to an NPS 12 configuration, the nominal weight spikes to approximately 325 lbs. Rigging profiles, support spacing, and structural steel calculations must reflect these massive component loads during the initial plant design phase.
3. Welding Clearance Considerations
When using a slip-on style flange in high-pressure services, field installers must ensure the pipe is cut square and inserted into the flange bore so that the end of the pipe is recessed from the flange face. The typical recess depth is roughly equivalent to the nominal wall thickness of the pipe plus 1/16-inch. This clearance protects the precision-machined raised face from weld splatter and leaves adequate room for a clean internal fillet weld without encroaching on the gasket seating surface.
