Blinds are flanges without a center bore (hole or opening) and are available in both Raised Face (RF) and Flat Face (FF) styles. While most flange types create a connection point that allows the flow of liquid, gas, or air, blind flanges are used to seal the end of a piping system and prevent flow. Blind flanges may be used when testing pipe pressure, to create an access point in a piping system, to temporarily seal a piping system while modifying or repairing the line, or to create a long-term seal to terminate a piping system.
A blind flange is bolted, rather than welded in place. This allows easier access as needed for pipe system upkeep, inspection, or to allow for future expansion. Pairing with a gasket creates a tight seal: The gasket fills the space between the flange faces, which prevents liquid or gas leaks.
How to Choose the Right Blind Flange
Blind pipe flanges may be produced to match bolt hole measurements for slip-on or welding neck flanges, or if needed, can be custom machined to any other specifications. Either way, when purchasing flanges, choose the appropriate flange features, material, dimensions, and class to meet your application needs.
Find the Best Blind Flange Material for the Job
When choosing blind flanges, you must consider the best flange material and dimensions in relation to the piping standards and requirements of your intended application. Generally, your flange material should match the connecting pipe material. The most common blind flange materials are steel and stainless steel.
In many cases, import material is acceptable to save on cost – in these cases, the flanges will still meet all the standards for pressure applications. In instances where domestic flanges are specified, API International manufactures flanges meeting “Buy American” or meeting American Iron and Steel Institute (AISI) requirements.
Threaded flanges are joined to pipes by screwing the pipe (which has a male thread, generally NPT per ASME B1.20.1) onto the flange, without seam welds (in certain cases, though, small welds are applied to increase the strength of the connection).
Threaded flanges are available in sizes up to 4 inches and multiple pressure ratings, however, they are used, mostly, small size piping in low pressure and low-temperature applications, like water and air utility services. Threaded flanges are also a mandatory requirement in explosive areas, such as gas stations and plants, as the execution of welded connections in such environments would be dangerous.
A slip-on flange is connected to the pipe or the fittings by two fillet welds, one executed inside and one outside the cavity of the flange. The bore size of a slip-on flange is larger than the outside diameter of the connecting pipe, as the pipe has to slide inside the flange to be connected by the execution of a fillet weld. Slip on flanges are also defined “Hubbed Flanges” and they are easy to recognize due to their slim and compact shape.
Long weld neck flanges (“LWN”) are similar to weld neck flanges, with the exception that the neck (tapered hub) is extended and acts like a boring extension. Long neck flanges are generally used on vessels, columns or barrels. These flange types are available also in the heavy barrel (HB) and equal barrel (E) types.
Socket welding flanges
are connected to pipes using a single fillet weld executed on the outer side of the flange (different from the slip-on flange type that requires two welds). According to ASME B31.1, to execute a flanged connection using a socket weld flange, the pipe shall be at first inserted in the socket of the flange until it reaches the bottom of the flange, then it should be lifted by 1.6 mm and finally welded. This gap shall be left to allow proper positioning of the pipe inside the flange socket after the solidification of the weld.
Socket Weld Flanges are used for small-size and high-pressure piping that do not transfer highly corrosive fluids. This due to the fact that these flange types are subject to corrosion in the gap area between the end of the pipe and the shoulder of the socket. Their static strength of socket weld flanges is similar to slip-on flanges’, but their fatigue strength is higher due to the presence of a single, instead of double, fillet weld.
A plate flange is a flat, circular disk that is welded onto the end of a pipe and allows it to be bolted to another pipe. Typically used in fuel and water pipelines, the two flange plates will be bolted together with a gasket in between them. The plate flanges will have bolt holes all around the perimeter and will be used to create junctions, tees and joints.