Types of Flanges that are considered to be standard
The type of flange that should be used in a piping application is determined primarily by the required strength of the flanged joint. Flanges are used instead of welded connections to make maintenance operations easier (a flanged joint can be dismantled quickly and conveniently).
Let's get started by going over the different types of flanges with illustrations.
WELDING NECK FLANGE
A welding neck flange ("WN") is characterized by a long tapered hub that can be welded to a pipe.
This flange type is typically used in high-pressure and high/low temperature applications where an unrestricted flow of the fluid conveyed by the piping system is required (the bore of the flange matches the bore of the pipe).
The absence of pressure drops prevents negative effects such as turbulence and erosion/corrosion of metals in the vicinity of flanged joints.
The tapered hub allows for a more even distribution of mechanical stress between the pipe and the weld neck flange and makes it easier to conduct radiographic inspections to detect potential leaks and welding defects.
The dimension of the flange (NPS and pipe schedule) must be the same as the dimension of the connecting pipe.
A single full penetration V-shaped butt weld joins a weld neck pipe flanges to a pipe. The dimensions and weights of ASME weld neck flanges are illustrated in this article.
SLIP ON THE FLANGE
A slip-on flange is connected to the pipe or the fittings by two fillet welds, one of which is performed inside and one of which is performed outside the cavity of the flange.
The bore size of a slip-on flange is larger than the outside diameter of the connecting pipe because the pipe must slide inside the flange in order to be connected by means of a fillet weld.
Slip-on flanges are also referred to as "Hubbed Flanges," and they are easily distinguished by their slim and compact design.
The dimensions and weights of slip-on flanges in accordance with ANSI/ASME standards are available on this page.
THREATENED FLANGE
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 the use of seam welds (in some cases, small welds are used to increase the strength of the connection).
Threaded flanges are available in sizes up to 4 inches in diameter and with a variety of pressure ratings; however, they are mostly used in small-diameter piping in low-pressure and low-temperature applications, such as water and air utility services.
Threaded flanges are also a mandatory requirement in explosive environments, such as gas stations and power plants, because the execution of welded connections in such environments would be extremely dangerous.
Consult this article for information on the dimensions of ANSI/ASME threaded flanges.
FLANGE WITH A SOCKET WELDING TECHNIQUE
Socket weld flanges are connected to pipes by a single fillet weld that is performed on the outside of the flange (as opposed to the slip-on flange type, which requires two welds).
According to ASME B31.1, in order to complete a flanged connection using a socket weld flange, the pipe must first be inserted into the socket of the flange until it reaches the bottom of the flange, after which it must be lifted by 1.6 mm and finally welded.
This space must be left to allow for proper positioning of the pipe inside the flange socket after the weld has solidified.
Socket Welding is a method of joining two pieces of metal together. Flanges are used for small-diameter, high-pressure piping that does not transfer highly corrosive fluids.
This is due to the fact that these flange types are susceptible to corrosion in the gap area between the end of the pipe and the shoulder of the socket.
The static strength of socket weld flanges is comparable to that of slip-on flanges, but their fatigue strength is higher due to the presence of a single fillet weld rather than a double fillet weld.
