In this article I’ll teach you how to design a custom flange according to ASME Section VIII Division 1 Appendix 2 and how to use our spreadsheet to aid you in your journey which, by the way, isn’t that dificult (when you pay attention to details).
First of all, before thinking about designing a custom flange, you should check if it’s really necessary because it’s usually more expensive than a standard flange (see ASME B16.5), since standard flanges are manufactured in large scale by many companies around the world and custom flanges are fabricated according to the customer needs (dimensions, materials, quantity, etc). So, if you have already checked ASME B16.5 (flanges from 1/2″ to 24″) and ASME B16.47 (flanges from 26″ to 60″) and still didn’t find the size or the material that you are looking for, then you can think about designing a custom flange.
Oh! One more thing, it is really important that you have already read the Appendix 2 of the ASME BPVC Section VIII Division 1 (to be honest, it would be a good thing to read the entire code but, if you are in a hurry or felling sleepy, just the appendix would do the job), because it’s the main source of this article and, well, how could you tell if I’m talking nonsense here? As an engineer you shouldn’t believe in everything you see/read, you should always try to understand more about the subject, looking for other sources and point of view.
2. Types of Flange
- Welding Neck (WN): this type of flange is designed to be joined to the pipe by buttwelding, so if you intent to direct weld a flange in an elbow, you need to use this one, as the other types need a straight part to be fitted inside de flange. This flange is especialy utilized when you have cyclic stress acting upon it, because it can, due to the “smooth” transition between the flange and the pipe, better distribute the stresses (rememeber from the strength of material class, where we learned that live corner is one of the worst type of stress intensifier?). The “problem” with this type of flange is that it’s more expensive than the other ones (according to Table 1, it’s not more expensive than the threaded type but, threaded type have a limited use in the industry), as you can see in Table 1, it’s 36% more expensive than the slip-on! Other thing that you need to have in mind is that you need more accuracy when assembly it because it can’t be easily adjusted as the other types and, you also have to chamfer its end (where the pipe will be welded), increasing the overall cost.
- Slip-On (SO): this type of flange is usually prefered by many designers because of its low cost (the cheapest of all type) and easily adjustment, as the pipe that will be welded to it doesn’t need to be cut with great accuracy. It also come with a small hub which helps a little the stress distribution but it’s considered weak compared to the welding neck as its strength ((Brownell, L. E., and Young, E. E. “Design of Flanges”, Chapter 12, in “Process Equipment Design”, John Wiley & Sons, New York (1959).)) compared to the welding neck is only 2/3 and, when fatigue comes to play, it’s only 1/3. It’s worth noting that it’s not advisable to use this type of flange when you have severe conditions (especially involving cyclic conditions) because you have high residual stresses and stress concentrations due to the discontinuity (flange/pipe) which can lead to erosion and corrosion.
- Threaded (TH): this type of flange have a limited use in the industry, which is usually used in pipes that can’t be welded (cast iron, galvanized steel) or non-metallic pipes (plastic). It’s not recomended to use this type of flange with severe conditions (toxic/dangerous fluids, high temperature, high pressure) because it isn’t much realiable, as the threads causes stress concentration and is a permanent you also have to deal with the probability of leakage (you can seal it with weld but wont solve your problems as you may think it would). Others things to be considered is that it is sometimes expensive (see Table 1) and need a threaded pipe to connect to it, so a pipe with small thickness won’t work.
- Socket Weld (SW): this type of flange is very similar to slip-on, the difference is that it have a socket to the pipe, not requiring the inner weld. It is usually used in small-size pipes (up to 1 1/2″). This type of flange isn’t recommended to be used in systems susceptible to cravice corrosion, because of the gap between the flange and the pipe. Another difference between this type of flange and the slip-on is that the socket weld have a fatigue strength 50% greater than the last one.
- Lap Joint (LJ): this type of flange have a main difference compared to the other ones, it isn’t fixed to the pipe (okay, the blind flanges isn’t fixed as well but the purpose of blind flanges is quite diferente from the others, it block the fluid passage instead of let it flow), what you do have to weld to the pipe is something called “stub end”, which is a pipe with a lap. This type of flange have many advantages: freedom to rotate it around the pipe, which helps align the bolt holes; lack of contact with the fluid, which let the use of less expensive materials (carbon steel, iron, etc); can be reused (because it’s not welded). The downside of this type of flange is that it doesn’t have a good fatigue resitence (around 1/10 of the welding neck) not being used in severe conditions systems.
- Blind (BD): this type of flange is used to block the flow, so its most common in equipments, which is used in manway nozzles, rather than pipelines.
3. Flange Facing
- Raised Face (FF): this is the most common type of facing used in the industry, it doesn’t have pressure or temperature limit. According to ASME B16.5, for flange classes 150# and 300# it’s height should be 2 mm and for flange classes 600# and above, it should be 7 mm. Actualy, suppliers use 1,6 mm for 150# and 300# and 6,35 mm for 600# and above. The surface of the facing can be smooth or serrated, if it’s serrated then it can be either concentric or spiral (I’ll not detail the types of facing finishing here because it’s subject for another article). The purpose of the raised face is to decrease the area in contact with the gasket and as a result, increase the contact pressure.
- Flat Face (FF): this type of facing is most used in flanges made of fragile material, as plastics (some) and cast iron, because these materials can’t handle bending very well. The important thing to have in mind is that, it can never be connected to a raised flange facing, because it would generate bending.
- Ring Type Joint (RTJ): this type of facing is usually used when you have severe conditions, like high pressure (flanges class 600# and above) and/or high temperature (800°F or 427°C and above). This facing consist of a groove cut into the flange face, where the gasket should be inserted. When the flange is connected and tightened, the compression force generated deforms (plastically, so it can be used only once) the gasket which fills the groove, creating a metal-to-metal seal. The important thing to remind here is that the gasket hardness must be lower than the flange hardness, otherwise you won’t be able to deform the gasket properly.
- Tongue and Groove (TG): this type of facing is generally used when you have corrosive fluid, because the gasket is confined inside its groove, without much contact with the fluid. The downside of this facing is that you need two different flanges to make the joint, one with tongue and other with groove.
- Male and Female (MF): this type of facing is similar to the tongue and groove type, as you need two diffent flanges to make the joint (male and female). The difference between then is that in the TG facing, the gasket is confined and have little contact with the fluid, and in the MF facing, the fluid have contact with its inside diameter.
4. Flange Design
The rules in Appendix 2 apply specifically to the design of bolted flange connections with gaskets that are entirely within the circle enclosed by the bolt holes and with no contact outside this circle, and are to be used in conjunction with the applicable requirements in Subsections A, B, and C of this division.Figure 2 shows an example of what is allowable and what isn’t. I didn’t find any explanation for the reason of why it can’t have contact outside de circle, but I suppose that it may have something to do with the bending generated by the bolt load (if you have any good explanation, please, contact me!). Then, in the item 2-1(b) it says:
The design of a flange involves the selection of the gasket (material, type, and dimensions), flange facing, bolting, hub proportions, flange width, and flange thickness. […] Flange dimensions shall be such that the stresses in the flange, calculated in accordance with 2-7, do not exceed the allowable stresses specified in 2-8. Except as provided for in 2-14(a), flanges designed to the rules of this Appendix shall also meet the rigidity requirements of 2-14. All calculations shall be made on dimensions in corroded condition.We’ll talk about the allowable stresses later on. The excpet provided in 2-14(a) is that you can skip the rigidity calculation if you had successful service experience for fluid services that are nonlethal and nonflammable and designed with the temperature range of -20°F (-29°C) to 366°F (186°C) without exceeding design pressures of 150 psi (1035 kPa).The Code, in the item 2-1(e) also says:
The rules of this Appendix should not be construed to prohibit the use of other types of flanged connections provided they are designed in accordance with good engineering pratice and method of design is acceptable to the Inspector. Some examples of flanged connections which might fall in this category are as follows: (1) flanged covers as shown in Fig. 1-6; (2) bolted flanges using full-face gaskets; (3) flanges using means other than bolting to restrain the flange assembly against pressure and other applied loads.So, it basicly tells you that you can design a flange that isn’t interily according to the Code, but uses good engineering pratices (reliable calculation, for example) and is accpetable by the Inspector. One type of flange that come to my mind when I think about it is the swing bolt type, which you can learn how to design it in this website as well!
5. Flange Materials
- Hubbed flanges may be machined from a hot rolled or forged bar. The axis of the finished flange shall be parallel to the long axis of the original billet or bar. There’s no need to the axis of the flange and the axis of the ballet/bar be concentric.
- Hubbed flanges shall not be machine from plate or bar stock material, unless the material has been formed into a ring, and further provided that: a) in a ring formed from plate, the original surfaces are parallel to the axis of the finished flange. There’s no need to preserve the original surface in the finished flange. b) the joints in the ring are welded butt joints that conform to the requirements of the Code. The thickness to be used to determine postweld heat treatment and radiography shall be the lesser of: t or (A – B)/2. c) the back of the flange and the outer surface of the hub are examined by either the magnetic particle method as per Appendix 6 or the liquid penetrant method as per Appendix 8.
6. Flange Allowable Stress
- longitudinal hub stress SH: not greater than Sf for cast iron and, except as otherwise limited by (a) and (b) below, not greater than 1.5 Sf for materials other than cast iron: a) logitudinal hub stress SH not greater than the smaller of 1.5 Sf or 1.5 Sn for optional type flanges designed as integral, also integral type where the neck material constitute the hub of the flange; b) longitudinal hub stress SH not greater than the smaller of 1.5 Sf or 2.5 Sn for integral type flanges with hub welded to the neck, pipe or vessel wall.
- radial flange stress SR: not greater than Sf;
- tangential flange stress ST: not greater than Sf;
- also (SH+SR)/2 not greater than Sf and (SH+ST)/2 not greater than Sf.
7. Notation List
8. Flange Types (Appendix 2)
This type covers those designs in which the flange has no direct connection to the nozzle neck, vessel, or pipe wall, and designs where the method of attachment is not considered to give the mechanical strength equivalent of integral attachment.Integral Type Flanges:
This type covers designs where the flange is cast or forged integrally with the nozzle neck, vessel or pipe wall, butt welded thereto, or attached by other forms of arc or gas welding of such a nature that the flange and nozzle neck, vessel or pipe wall is considered to be the equivalent of an integral structure. In welded construction, the nozzle neck, vessel, or pipe wall is considered to act as a hub.Optional Type Flanges:
This type covers designs where the attachment of the flange to the nozzle neck, vessel or pipe wall is such that the assembly is considered to act as a unit, which shall be calculated as an integral flange, except that for simplicity the designer may calculate the construction as a loose type flange provided none of the following values is exceeded: g0 = 5/8″ (16 mm), B/g0 = 300, P = 300 psi (2 MPa) and operating temperature = 700°F (370°C).