Question: Selection of flanges for gas pipelines

　The three commonly used types of sealing surfaces are as follows: 1. Flat sealing surface, as shown in Figures 5–2–5(a) and (b). This type of sealing surface has a simple structure and is easy to manufacture, but the gasket tends to be pushed outward, resulting in poor sealing performance; it is only suitable for applications with low pressure, non-toxic media, and in environments where there is no risk of fire or explosion. 　　2. Concave-convex sealing surface, see Figure (c). The gasket is well centered and not prone to extrusion, making it suitable for slightly higher pressures. 　　3. Mortise-and-tenon type sealing surface, see Figure (d). The gasket is narrow, so the force required to tighten the bolts is relatively low; however, the structure is complex and it is difficult to replace the gasket. It is suitable for use with flammable, explosive, toxic media as well as in applications with high pressures. 　　Steel flanges can be used as finished products or processed according to **standards. http://news.gasshow.com/content_images/2005122211092495.jpg Figure 5-2-4: Various types of flanges http://news.gasshow.com/content_images/20051222110270864.jpg Figure 5-2-5: Types of flange surfaces. Iron cast flanges are made of cast iron, and they are cast together with the cast iron pipes and fittings. Common flanged pipe fittings include flanged pipe supports, flanged inserts, flanged valves, flanged elbows (single-plate and double-plate), flanged tees (single-plate, double-plate, and triple-plate), and flanged reducers (single-plate and double-plate), etc. 　　(II) Processing of steel flanges Flanges are manufactured from A3, 20# steel, or steel as specified in the design requirements. The thickness of the selected steel plate should allow for machining allowances at the outer edge and inner hole of the flange during cutting. Generally, after cutting the material into a ring-shaped blank using an oxygen-acetylene torch, it is machined on a lathe, and then bolt holes are drilled using a drill. When connecting pipes to valves or equipment, it should be done in accordance with the flanges on the valves or equipment. The flange surface shall be flat and smooth, free from any defects such as cracks, radial grooves, spots, burrs, or sand holes that could reduce the strength and sealing performance of the flange. 　　The bolts for connecting flanges are generally made from A3 or A5 steel. Applications with a nominal pressure exceeding 2.5 MPa are processed using grade 35 steel. The hardness of the nut should be lower than that of the bolt; it is generally manufactured from A3 steel. When the nominal pressure exceeds 2.5 MPa, it is manufactured from grade 25 steel. The bolts currently in use are mainly metric ordinary bolts. The threads of metric ordinary bolts are divided into coarse and fine types; those with a pitch of 2.5 mm are considered coarse threads, while those with a pitch less than 2.5 mm are considered fine threads. Thick threads are mainly used for pipe flange connections. The diameter of the bolt should be determined according to the standard flange hole, generally 1–2 mm smaller than the bolt hole. The bolt length should be appropriate, with the exposed length after tightening not exceeding 2 times the pitch. 　　(III) Flange connection 　　1. Threaded connection. Threaded connection refers to the threaded joining of flanges and pipes, and is used for connecting steel pipes with cast iron flanges, or galvanized steel pipes with steel flanges. When machining threads, the thread length of the pipe should be slightly shorter than the internal thread length of the flange. When tightening the threads, care should be taken to align the screw holes of the two flanges. If the holes are not aligned, one can only continue to tighten the flange or remove it and reinstall it. The flange cannot be loosened to realign the holes in order to ensure a tight seal at the interface. 　　2. Flange connection. Flat welding flanges, butt welding flanges, or cast steel flanges are all connected to pipes by welding. During connection, the flange sealing surfaces must remain parallel; the deviation shall not exceed 1.5/1000 of the flange’s outer diameter, nor more than 2 mm. Flange connections should remain coaxial; the center deviation of their bolt holes should generally not exceed 5% of the hole diameter, to ensure that the bolts can pass through freely. The pipe opening should be recessed by 1.3 to 1.5 times the thickness of the pipe wall relative to the flange; it must not be level with the flange’s contact surface. After welding, the weld seam must not rise above the flange contact surface, in order to prevent slag from splashing onto that surface and thus ensure the tightness of the flange. Flange gasket. To ensure a tight connection and prevent leakage, gaskets must be used for flange connections. The thickness of flange gaskets is generally 2 to 3 mm. The material of the gasket is selected based on the properties of the medium transported through the pipe, or under different temperatures and pressures for the same medium; asbestos rubber sheets are commonly used in gas pipelines. Table 5-2-10 Selection Table for Flange Gasket Materials Material Name Applicable Media Maximum Operating Pressure (MPa) Maximum Operating Temperature (°C) Low-pressure Asbestos Rubber Sheet Water, air, gas, steam, inert gases 1.6 200 Medium-pressure Asbestos Rubber Sheet Water, air and other gases, steam, gas, ammonia, dilute solutions of acids and bases 4.0 350 High-pressure Asbestos Rubber Sheet Steam, air, gas 10 450 The inner diameter of the flange gasket must not be smaller than the diameter of the flange hole, while its outer diameter should be less than the distance between the inner edges of the two corresponding bolt holes, so that the gasket does not block the bolt holes ； The edge width of the washers should be consistent, and their edges should be neat. The dimensions of the gaskets must match those of the flange sealing surfaces; the allowable tolerances are shown in Table 5–2–11. Table 5-2-11 Allowable tolerances for washers (mm) Flange sealing surface type: Flat type, Male-female type, Grooved type Nominal diameter Inner diameter Outer diameter Inner diameter Outer diameter Inner diameter Outer diameter

Given a pressure of 40 kilograms, MFM is recommended

4MP still uses concave-convex flanges.

Reply to 3# haojh: What I want to ask is, is it necessary to use a textured surface, or can a smooth surface be used as well? Also, this set of pipelines comes in pressure ratings ranging from 1.6 MPa to 4 MPa – should concave-convex flanges be used for all of them?

Using 4.0RF is fine; choose the inner and outer ring gaskets. Of course, textured surfaces are better, but they are more expensive and harder to install.

Reply to 6# zhy809078: So if RF is chosen, should inner and outer gaskets be used only on the 4MPa pipeline? Do all pipelines, including those with a diameter of 1.6M and 2.5M, require inner and outer rings?

One of our projects uses imperial units: the design pressure for the CO pipelines is 5.0 MPa, the design temperature is room temperature. RF face flanges of 300 LB grade are used, along with wafer gaskets that consist of inner and outer rings made of graphite. If metric flanges are used, their relative stiffness is slightly lower than that of imperial flanges; flanges of grade 4.0 MPa and below use RF faces, which are necked butt-welded flanges. The gasket is a graphite wound gasket with inner and outer rings. If the design pressure is 6.3 MPa or higher, for highly toxic media such as CO, it may be better to use a corrugated surface.

Given the pressure of 4.0 MPa you mentioned, it is essential to use flanges with male and female surfaces, as CO is a toxic, flammable, and explosive gas; any leakage of it poses a serious danger.