Flat Face Flange

Less surface area - more pressure.

Less effect due to surface irregularities (flatness etc)

From a previous question:

Flange Faces

The question:

Why do we use raise face flanges and flat face flange etc.

My answer:

Flat Face Flange - This type of flange face is for all applications where "Cast Iron" (or other brittle material) is used to make equipment and valves. Cast Iron is brittle and is commonly used only in low temperature, low pressure applications. The flat surface allows for full contact across the whole face of the two flanges. Only Full Face Gaskets are to be used for "Flat Face" flanges. This insures full and equal contact between the two mating flanges.

Raised Face Flange - This type of flange face is widely for all forged steel flanges including equipment and valves. The face on these flanges has two levels formed by two "Rings". The inner "Ring" forms the raised gasket contact surface and the outer "Ring" forms a non-contact area. Gaskets for the "Raised Face" flanges are the same size and shape as the inner ring area of this flange. The Bolt holes for these flanges are located in the outer ring area. When a gasket is added and the bolts are installed and torqued the "Step" transition between the heights of the two rings allows for grater force to be applied to the gasket area thus achieving a better seal.

So what happens when you bolt a flat face flange to a raised face flange? That is very simple; the force exerted by the bolts on the fulcrum point (at the edge of the raised face) will cause the weaker cast iron flat face flange to break.

All pipers must learn the problems and preventions related to Flat Face and Raised Face flanges. They are like "Oil and Water", they don't mix. When you are working in a pipe specification (pipe flanges, equipment and valves) with all flat face flanges there is no problem. When you are working in a piping specification (pipe flanges, equipment and valves) with all raised face flanges there is also no problem. But, when you find that you have some flat face flanges (on equipment or valves) mixed in with a raised face specification then you need to take action.

The action to take is to insure that the raised surface of Forged Steel raised face flange is machined off to make it a flat face. Make sure that your piping material Data Base has an out of spec call out for a modified forged flange. Make sure you select that item when required. Make sure the Isometric drawing picture and notes reflects the requirement for a modified flange and make sure the Bill-of-material lists a modified flange.