What is a flange?
Table of Contents
- 1 What is a flange?
- 2 Material of flanges:
- 3 Pressure rate of flanges:
- 4 Types of flanges
- 5 SPECIAL TYPES OF FLANGES
- 6 FLANGE FACE CLASSIFICATION:
- 7 Production process of flange
Flange is a part connecting shaft to shaft, which is used for the connection between pipe ends; it is also used for the connection between two equipment, such as reducer flange, on the inlet and outlet of equipment. Flange connection or flange joint refers to the detachable connection with flange, gasket and bolt as a group of combined sealing structure. Pipeline flange refers to the flange used for piping in the pipeline device, and for equipment, it refers to the inlet and outlet flange of the equipment. There are holes in the flange, and the bolts make the two flanges tightly connected. The flange is sealed with gasket. Flange is divided into threaded connection (threaded connection) flange, welding flange and clamp flange. Flange is used in pairs, low-pressure pipeline can use screw flange, more than four kilograms of pressure use welding flange. Add gasket between two flange plates, and then fasten them with bolts. The flange thickness of different pressure is different, and they use different bolts.
Material of flanges:
Duplex & Super Duplex
20#,A105,A350 LF1/LF2/LF3,MSS SP44 & A694 F42/46/52/60/65/70
Copper-Nickle Alloy,Titanium,Aluminum,Monel,lnconel,Hastelloy and other special material
Manufacturing standard of flanges:
Many of the flanges in each standard are divided into “pressure classes”, depending on the different rates of pressure that are able to endure. The most common flanges pressure classes are #150, #300, #600, #900, #1500, #2500 and #3000 according to ASME designation. To other standards, as DIN, pressure classes are defined by the acronym PN, as for example, PN10, PN16, PN20, PN25, PN40, PN50, PN100, PN150, PN250 or PN420. Flanges from different pressure classes are not usually interchangeable.
A welding neck flange (“WN”)features a long tapered hub that can be welded with a pipe.
Welding Neck Flanges are easy to recognize at the long tapered hub, that goes gradually over to the wall thickness from a pipe or fitting.
The long tapered hub provides an important reinforcement for use in several applications involving high pressure, sub-zero and / or elevated temperatures. The smooth transition from flange thickness to pipe or fitting wall thickness effected by the taper is extremely beneficial, under conditions of repeated bending, caused by line expansion or other variable forces.
These flanges are bored to match the inside diameter of the mating pipe or fitting so there will be no restriction of product flow. This prevents turbulence at the joint and reduces erosion. They also provide excellent stress distribution through the tapered hub and are easily radiographed for flaw detection.
This flange type will be welded to a pipe or fitting with a single full penetration, V weld (Buttweld).
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.
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 calculated strength from a Slip On flange under internal pressure is of the order of two-thirds that of Welding Neck flanges, and their life under fatigue is about one-third that of the latter.
The connection with the pipe is done with 2 fillet welds, as well at the outside as also at the inside of the flange.
The X measure on the image, are approximately:
Wall thickness of pipe + 3 mm.
This space is necessary, to do not damage the flange face, during the welding process.
A disadvantage of the flange is, that principle always firstly a pipe must be welded and then just a fitting. A combination of flange and elbow or flange and tee is not possible, because named fittings have not a straight end, that complete slid in the Slip On flange.
WELD NECK VS SLIP ON FLANGE
Flanged joints made with slip-on flanges are, in the long run, a bit more fragile than connections made with welding neck flanges (in similar service conditions). This seems due to the following facts:
- a welding neck flange features a tapered hub, absent in a socket weld flange, which distributes the mechanical stress between the pipe and the flange more evenly
- a welding neck joint as only one welding area instead of two (socket weld flange).
Another advantage of the welding neck flange is that it can be connected either to pipes and fittings, whereas socket weld flanges suit pipes only.
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 used for special circumstances with their main advantage being that they can be attached to the pipe without welding. Sometimes a seal weld is also used in conjunction with the threaded connection.
Although still available in most sizes and pressure ratings, screwed fittings today are used almost exclusively in smaller pipe sizes.
A threaded flange or fitting is not suitable for a pipe system with thin wall thickness, because cutting thread on a pipe is not possible. Thus, thicker wall thickness must be chosen…what is thicker ?
ASME B31.3 Piping Guide says:
Where steel pipe is threaded and used for steam service above 250 psi or for water service above 100 psi with water temperatures above 220° F, the pipe shall be seamless and have a thickness at least equal to schedule 80 of ASME B36.10.
Socket Weld flanges were initially developed for use on small-size high pressure piping. Their static strength is equal to Slip On flanges, but their fatigue strength 50% greater than double-welded Slip On flanges.
The connection with the pipe is done with 1 fillet weld, at the outside of the flange. But before welding, a space must be created between flange or fitting and pipe.
ASME B31.1 1998 127.3 Preparation for Welding (E) Socket Weld Assembly says:
In assembly of the joint before welding, the pipe or tube shall be inserted into the socket to the maximum depth and then withdrawn approximately 1/16″ (1.6 mm) away from contact between the end of the pipe and the shoulder of the socket.
The purpose for the bottoming clearance in a Socket Weld is usually to reduce the residual stress at the root of the weld that could occur during solidification of the weld metal. The image shows you the X measure for the expansion gap.
The disadvantage of this flange is right the gap, that must be made. By corrosive products, and mainly in stainless steel pipe systems, the crack between pipe and flange can give corrosion problems. In some processes this flange is also not allowed. I am not an expert in this matter, but on the internet, you will find a lot of information about forms of corrosion.
Also for this flange counts, that principle always firstly a pipe must be welded and then just a fitting.
Lap joint flanges feature a flat face and are always used in conjunction with a stub end.
Lap Joint Flanges have all the same common dimensions as any other flange named on this page however it does not have a raised face, they used in conjunction with a “Lap Joint Stub End”.
These flanges are nearly identical to a Slip On flange with the exception of a radius at the intersection of the flange face and the bore to accommodate the flanged portion of the Stub End.
Their pressure-holding ability is little, if any, better than that of Slip On flanges and the fatigue life for the assembly is only one tenth that of Welding Neck flanges.
They may be used at all pressures and are available in a full size range. These flanges slip over the pipe, and are not welded or otherwise fastened to it. Bolting pressure is transmitted to the gasket by the pressure of the flange against the back of the pipe lap (Stub End).
Lap Joint flanges have certain special advantages:
- Freedom to swivel around the pipe facilitates the lining up of opposing flange bolt holes.
- Lack of contact with the fluid in the pipe often permits the use of inexpensive carbon steel flanges with corrosion resistant pipe.
- In systems which erode or corrode quickly, the flanges may be salvaged for re-use.
Contrary to all the flange types seen above, blind flanges do not have a center hole, and are used to blind or seal a pipeline, a valve/pressure vessel and block the flow of the fluid.
Blind Flanges are manufactured without a bore and used to blank off the ends of piping, Valves and pressure vessel openings.
From the standpoint of internal pressure and bolt loading, blind flanges, particularly in the larger sizes, are the most highly stressed flange types.
However, most of these stresses are bending types near the center, and since there is no standard inside diameter, these flanges are suitable for higher pressure temperature applications.
A Nipoflange is used for branch pipelines at 90 degrees and is a product manufactured by combining a welding neck flange with a forged Nipolet.
However, a Nipoflange is a solid single piece of forged steel and not two different products welded together.
To install a Nipoflange, the piping staff has to weld the Nipolet part of the device on the run pipe and bolt the flanged part on the flange of the branched pipe.
Nipoflanges are available in different materials, such as carbon steel ASTM A105 (high-temperature service), ASTM A350 (low-temperature carbon steel), ASTM A182 (stainless steel grades, including duplex and super duplex) and nickel alloys (Inconel, Incoloy, Hastelloy, etc).
Nipoflanges are also manufactured in the reinforced variant, which has additional mechanical strength compared to a standard Nipoflange.
A Weldoflange is conceptually similar to a Nipoflange, as that they are a combination of a weld neck flange and a branch fitting connection (a Weldolet in this case). Weldoflanges are made out of a single piece of solid forged steel, not by welding separate parts together.
ELBOFLANGE AND LATROFLANGE
Other less common types of flange Olets is the so-called Elboflange (a combination of a flange and an Elbolet) and “Latroflange” (combination of a flange with a Latrolet). Elboflanges are used to branch a pipeline at 45 degrees.
Swivel ring flanges facilitate the alignment of the bolt holes between the two mating flanges, a feature that is helpful in many circumstances, such as the installation of large diameter pipelines, subsea and offshore pipelines, pipe works in shallow waters and similar environments.Swivel flanges suit oil, gas, hydrocarbons, water, chemical and other demanding fluids in petrochemical and water management applications.
In the case of a large diameter pipeline, for instance, the pipe is fitted, at one end, with a standard welding neck flange, and with a swivel flange at the other end: by simply rotating the swivel flange on the pipe, the operators can achieve a perfect alignment of the bolt holes in a way easier and faster way.
The major standards for swivel ring flanges are ASME/ANSI, DIN, BS, EN, ISO, etc. The most common standard for petrochemical application is the ANSI/ASME B16.5 or ASME B16.47.
Swivel flanges are available in all the standard shapes of common flanges, i.e. weld-neck, slip-on, lap-joint, socket weld etc, in all material grades and in a wide dimensional range (sizes can vary from 3/8” to 60” and pressure rating from 150 to 2500).
Swivel flanges can be manufactured in carbon steel (ASTM A105), alloy steel (ASTM A182 F1, A182 F5, A182 F9, A182 F91), and, stainless steel (ASTM A182 F304, A182 F304L, A182 F316, A182 F316L).
EXPANDING FLANGE (“EXPANDER”)
Expanding flanges, or “expander flanges”, are used to increase the bore of the pipeline from a specific point to another or to connect pipes to other mechanical devices such as pumps, compressors, and valves that have different inlets sizes.
The expanding flange represented in the picture is a welding neck flange with a larger bore on the non-flanged end.
Expanding flanges can be used to increase the run pipe bore only by one or maximum two sizes and not more (example: from 2 to 3 or maximum 4 inches).
Expander flanges are a cheaper (and lighter) solution compared to the combination of a buttweld reducer and a standard flange (which is the standard solution for pipe bore increases above 2 sizes).
The most common materials for expanding flanges are A105 (high-temp. carbon steel), A350 (LTCS) and ASTM A182 (stainless steel and above).
Pressure ratings and dimensions of expanding flanges are in accordance with the ANSI/ASME B16.5 specification and are available with raised or flat face (RF, FF).
The drawing of an ASME expanding flange.
REDUCING FLANGE (“REDUCER”)
Reducing flanges, otherwise called reducer flanges, have an opposite function than expander flanges seen above, i.e. they are used to decrease the bore of a pipeline.
The bore of the run pipe can be safely reduced by only 1 or 2 sizes (otherwise a solution based on the combination of a butt weld reducer and a standard flange has to be used).
Reducing flanges are available in most sizes and material grades, and are not generally available from stock.
Reducing flanges follow the same considerations in terms of specifications, sizes and material grades as expander flanges.
FLANGE FACE CLASSIFICATION:
Another important parameter to define a flange is the flange faces. Exist five principal types of flange faces who its possible to see below.
- Flat Face (FF): The Flat Face flange has a gasket surface in the same plane as the bolting circle face. Applications using flat face flanges are frequently those in which the mating flange or flanged fitting is made from a casting.
- Raised Face (RF): The Raised Face flange is the most common type used in process plant applications, and is easily to identify. It is referred to as a raised face because the gasket surfaces are raised above the bolting circle face. This face type allows the use of a wide combination of gasket designs, including flat ring sheet types and metallic composites such as spiral wound and double jacketed types.
- Ring-Type Joint (RTJ): They have grooves cut into their faces which steel ring gaskets. The flanges seal when tightened bolts compress the gasket between the flanges into the grooves, deforming the gasket to make a metal to metal seal.
- Tongue-and-Groove (T&G): The Tongue and Groove faces of this flanges must be matched. One flange face has a raised ring (Tongue) machined onto the flange face while the mating flange has a matching depression (Groove) machined into it’s face.
- Male-and-Female (M&F): With this type the flanges also must be matched. One flange face has an area that extends beyond the normal flange face (Male). The other flange or mating flange has a matching depression (Female) machined into it’s face.
Flange production process is mainly divided into forging, casting, cutting, rolling these four.
(1). Cast flange and forged flange
The shape and size of the casting blank are accurate, the machining amount is small and the cost is low, but there are casting defects (porosity, crack and inclusion); the internal structure streamline of the casting is poor (if it is a cutting part, the streamline is worse);
Forged flange is generally lower carbon content than cast flange and is not easy to rust. The forging has good streamline, compact structure and better mechanical properties than cast flange;
If the forging process is not proper, there will be large or uneven grains, hardening cracks, and the forging cost is higher than that of casting flange.
Forgings can bear higher shear force and tensile force than castings.
The advantages of casting are that it can produce more complex shape and lower cost;
The forging has the advantages of uniform internal structure and no harmful defects such as pores and inclusions in the casting;
The difference between casting flange and forging flange is different from the production process. For example, centrifugal flange is one of the casting flange.
Centrifugal flange is a kind of flange produced by precision casting. Compared with ordinary sand casting, the structure of centrifugal flange is much finer and its quality is improved. It is not easy to have loose structure, porosity, sand hole and other problems.
First of all, we need to know how the centrifugal flange is produced. The process and products of centrifugal casting to make flat welding flange are characterized by the following process steps:
- ① put the selected raw material steel into medium frequency electric furnace for smelting, so that the temperature of molten steel can reach 1600-1700 ℃;
- ② preheat the metal mould to 800-900 ℃ and keep constant temperature;
- ③ start the centrifuge, inject the steel water in step ① into the metal mold after preheating in step ②;
- ④ the casting is naturally cooled to 800-900 ℃ and kept for 1-10 minutes;
- ⑤ cool with water to near normal temperature, demould and take out the casting.
Let’s learn about the production process of forged flange:
The forging process is generally composed of the following processes: blanking, heating, forming and cooling after forging. The forging process includes free forging, die forging and die forging. In production, different forging methods are selected according to the forging quality and production batch.
Free forging has low productivity and large machining allowance, but the tool is simple and versatile, so it is widely used to forge single piece and small batch forgings with simple shape. The free forging equipment includes air hammer, steam air hammer and hydraulic press, which are suitable for the production of small, medium and large forgings respectively. Die forging has the advantages of high productivity, simple operation, mechanization and automation. Die forgings have high dimensional accuracy, small machining allowance and more reasonable fiber structure distribution, which can further improve the service life of parts.
Basic process of free forging: during free forging, the shape of forgings is gradually forged into blanks through some basic deformation processes. The basic process of free forging includes upsetting, drawing, punching, bending and cutting.
- 1. Upsetting and upsetting is the process of forging the original billet along the axial direction to reduce its height and increase its cross section. This process is often used to forge gear blanks and other disc forgings. Upsetting can be divided into full upsetting and partial upsetting.
- 2. Drawing length is a forging process that increases the length of the blank and reduces the cross-section. It is usually used to produce shaft parts, such as lathe spindle, connecting rod, etc.
- 3. Forging process of punching through hole or through hole on blank with punch.
- 4. Forging process of bending the blank to a certain angle or shape.
- 5. The forging process in which one part of the billet rotates at an angle to the other.
- 6. Forging process of cutting split blank or cutting head.
(2). Die forging
Die forging is called model forging. The heated blank is placed in the forging die fixed on the die forging equipment for forging.
- 1. Basic process of die forging process: blanking, heating, pre forging, final forging, punching and connecting skin, trimming, tempering and shot peening. Common processes include upsetting, drawing, bending, punching and forming.
- 2. Common die forging equipment: die forging hammer, hot die forging press, flat forging machine, friction press, etc.
- Generally speaking, the quality of forged flange is better. Generally, it is produced by die forging, with fine crystal structure, high strength and high price.
Both cast flange and forged flange are common manufacturing methods of flange. According to the strength requirements of components to be used, if the requirements are not high, turning flange can also be selected.
(3). Cutting flange
The inner and outer diameter and thickness of the flange are directly cut out on the middle plate, and then the bolt hole and water line are processed. The flange produced in this way is called cut flange. The maximum diameter of such flange is limited to the width of the middle plate.
(4). Rolled flange
The process of using the middle plate to cut the sliver and then roll it into a circle is called rolling, which is mostly used in the production of some large flanges. After coiling, welding, flattening and processing of waterline and bolt hole are carried out.
Source: Network Arrangement – China Flanges Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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