In pipeline flange procurement, pipeline design, and on-site installation operations, flange FF and RF are the two most commonly used flange sealing surface forms on the market, and they are also the most easily mixed and mismatched flange types in engineering. Many construction workers experience construction failures such as flange misalignment, gasket crushing, medium leakage, and cracking of cast iron valve bodies due to the structural differences between FF full plane flanges and RF protruding flanges. This article combines HG/T20592 and ASME B16.5 national and American general standards to comprehensively explain the differences between flange FF and RF. It compares them from five dimensions: appearance structure, pressure bearing capacity, gasket adaptation, operating conditions, and price adaptation. It teaches industrial practitioners to quickly select and avoid flange sealing failure problems.

1、 Basic definition: What are FF flange and RF flange?

1. FF flange: Flat Face flange

The full name of FF is Flat Face, also known as Full Flat Flange in Chinese. As the name suggests, the entire end face of the flange is flat without protrusions, and the sealing working face is on the same horizontal plane as the flange bolt hole end face. The end face is processed as a whole and is flat and integrated. It is a special type of low-pressure flange sealing face, with the national standard code indicating FF.

2. RF flange: Raised Face flange

RF, also known as raised face or raised face flange in Chinese, is the most commonly used flange sealing surface in industry. The center sealing area of the flange is higher than the bolt reference plane, forming a circular raised sealing platform. The raised height is fixed and standardized, and the national standard code is RF. RF sealing surface is used for 90% of working conditions in chemical, oil and gas, and steam pipelines.

2、 Six major differences between flange FF and RF core (engineering focus)

1. Differences in appearance and structure

FF full flat flange: The end face is flat, without protrusions or grooves, and the height of the entire flange surface is consistent. The gasket is attached to the entire flange end face, and the stress area is fully covered.

RF protruding flange: The middle sealing area is raised, with a conventional protrusion height of 1.6mm-6.4mm. The bolt hole position sinks, and the tightening force is concentrated in the middle protrusion area, resulting in concentrated sealing force.

2. Difference in pressure rating under pressure

FF flange: limited to low-pressure conditions, suitable for pressures PN6, PN10, PN16, with a maximum pressure not exceeding 1.6MPa. FF flange is strictly prohibited for high-pressure conditions as it is prone to gasket displacement and leakage.

RF flange: Suitable for high and low pressure in full gradient, covering all grades of PN10-PN100, and can be adapted to American Class150-Class600. It has strong adaptability to high temperature and high pressure coupling conditions and far exceeds the versatility of FF flange.

3. Differences in gasket types for adaptation (key selection)

FF special gasket: full-size non-metallic soft pad

Full coverage soft pads must be selected, commonly including rubber pads, PTFE whole pads, asbestos cardboard pads, and non-metallic composite soft pads. The gasket area is the same size as the flange end face, and metal wrapped gaskets or graphite composite gaskets cannot be used. Hard gaskets will fracture brittle cast iron flanges.

RF special gasket: ring-shaped semi metal/metal gasket

Simply adapt to the circular standard gasket, commonly used metal wrapped gaskets, graphite composite gaskets, high-strength PTFE gaskets, and metal tooth shaped gaskets. The gasket size is small, the cost is lower, the sealing pressure is stronger, and it is compatible with high-temperature resistant gasket materials.

4. Differences between adapted media and operating conditions

FF flange applicable working conditions

Only applicable to normal temperature, low pressure, non flammable, non explosive, and non corrosive media: municipal water supply pipelines, sewage discharge pipelines, ventilation and dust removal pipelines, and normal pressure air pipelines in the factory area. It is commonly used for connecting cast iron valves, cast iron water pumps, and PVC/UPVC plastic flanges to evenly distribute bolt pressure and protect brittle cast iron equipment from fracturing.

RF flange applicable working conditions

Universal working conditions for all industries, suitable for high-temperature steam, thermal oil, natural gas, finished oil, chemical weak acid media, circulating hot water, and pressurized process fluids. Widely equipped with centrifugal pumps, complete sets of valves, heat exchangers, reactors, pressure vessels, boiler pipelines, it is a standard sealing surface in the chemical, power, oil and gas, and water treatment industries.

5. Differences in sealing performance

FF flange: The bolt clamping force is dispersed, the surface pressure is low, and the sealing effect is average. It is prone to leakage under vibration and pressure fluctuation conditions, and is only suitable for stable media under normal pressure.

RF flange: The force is concentrated on the central boss, the gasket compression ratio is high, the sealing tightness is high, the resistance to pipeline vibration and compression is strong, and the sealing stability is much better than FF flange.

6. Compatibility differences in docking

Mixing is strictly prohibited: FF flange cannot be directly connected to RF flange, the height of the end face is not matched, and the gasket is subjected to uneven force after tightening, resulting in 100% material leakage, water leakage and air leakage problems; The docking must match the sealing surface of the same model, FF docking FF, RF docking RF.

3、 Brief comparison table between FF flange and RF flange

四、现场选型避坑4条准则

1、铸铁阀体、塑料管道、市政给排水常压管路，统一选用FF全平面法兰，防止螺栓压力压碎铸铁法兰端面；

2、蒸汽、油气、化工工艺管道、钢制阀门、不锈钢管路，默认首选RF突面法兰，行业通用、垫片配件齐全；

3、高低密封面禁止混搭，FF不可对接RF，改造管路更换法兰时，必须统一两端密封面型号；

4、高温、易燃易爆、有毒介质，不选用FF法兰，最低标准选用RF法兰，高压工况升级RTJ环连接面法兰。