FRP pipe fittings indispensable in FRP pipes - flanges

Flange (flange, transliteration, meaning flange, flange) is a widely used detachable connecting part in pipes and vessels. With the increasingly widespread use of FRP pipes and vessels, the performance and manufacture of FRP flanges have gradually received widespread attention.

Commonly used FRP flanges are mostly used for medium and low pressure pipes and vessels with pressure no more than 3MPa. The main structures include integral flanges, bonded flanges and looper flanges.

The integral flange is generally flat flange with equal wall thickness. The advantage of this structure is that the flange ring and the cylinder are integrally formed, the reinforced glass fiber and fabric are continuous, and can give full play to the characteristics of high strength and easy forming of glass fiber reinforced plastics. Its disadvantage is that the structure with equal wall thickness does not match the stress distribution in the flange, and it is difficult to achieve the design requirements of equal strength and equal stiffness; In use, the joint between flange ring and cylinder is prone to large deformation, even microcracks and cracking damage under the action of longitudinal stress.

The adhesive flange is formed by processing the flange ring and the cylinder separately, and then bonding the two together. This structure is widely used. It gives full play to the advantages of easy forming of FRP. The mold is simple and easy to manufacture. It is suitable for manufacturing large diameter, small batch and special-shaped FRP flanges. However, its major disadvantage is that the glass fiber between the flange ring and the cylinder is discontinuous, so the strength of the joint decreases greatly. Even if glass cloth and chopped felt are used for reinforcement at the corner, the bending moment, shear force and tensile force borne by the flange are mainly borne by the resin matrix on the bonding surface. As a result, the longitudinal stress at the bonding is large and the strength is low, which is easy to be damaged, and the safety is insufficient, and increases with the use temperature, Security needs to be reduced. Relatively speaking, the looper flange not only has the advantages of the integral flange, but also makes full use of the advantages of the large rigidity of the looper l flange ring, which greatly reduces the leakage phenomenon caused by the bending and excessive deflection of the flange ring between the two bolts during the use of the fiberglass flange after fastening. However, as mentioned earlier, the structure with equal wall thickness still does not conform to the stress distribution in the flange. Therefore, it is still necessary to select a more reasonable flange structure according to the stress distribution in the flange, and make full use of the designability and easy forming characteristics of the FRP performance.

According to the stress distribution curve, it is unreasonable for the flange to adopt the same wall thickness structure as the cylinder. A transition cone neck should be set between the flange ring and the cylinder. The wall thickness increases with the increase of stress, and decreases with the decrease of stress. This can greatly reduce the local stress concentration and excessive deformation of flange due to unreasonable structure. It can improve the overall strength and rigidity, reduce the deflection of the flange ring, make the gasket pressure more uniform, and the deformation is basically consistent, which can improve the sealing performance. That is to say, the tapered neck structure can make the FRP flange achieve the design principle of equal strength and equal rigidity, which is a reasonable flange structure form.

From the perspective of molding technology, it is completely feasible to use hand lay-up, cold pressing, pressure injection and other processes to manufacture cone-neck FRP flanges. However, for large, special-shaped and small batch hand-laid FRP flanges, in order to reduce mold investment and shorten the production cycle, the cone-neck structure can also be evolved into column structure. The diameter of the column is equal to the diameter of the large end of the cone, and the height remains unchanged. The structure only slightly increases the consumption of raw materials, does not decrease the performance, and is more convenient to manufacture.

Since the elastic modulus of FRP is only 1/20-1/10 of that of l, the stiffness of FRP flange is far lower than that of steel flange. When the flange is under pressure, it is easy to produce excessive deflection between the two adjacent bolts on the flange ring, which will cause medium penetration and leakage. If the rigidity is increased by increasing the thickness of the FRP flange ring, the cost will rise. The use of the conical neck structure looper flange is an effective way to solve this problem, and its characteristics are: 1 Take the length of the l elastic mold height and the short of the glass fiber reinforced plastic, and the cost will not increase compared with the glass fiber reinforced plastic flange with the same rigidity; 2. Reduce the production and processing procedures of FRP flanges and improve efficiency; 3. It is convenient to disassemble, install and maintain. Looper flange is generally used for occasions with PDn ≤ 50. (where p is the working pressure MPa, and Dn is the flange inner diameter, cm)

At present, the manufacture of vessel flanges and special-shaped pipe flanges generally adopts two methods, namely, cylinder and flange ring bonding and cylinder flanging. Although these methods are simple and easy to implement, strictly speaking, flanges are flat plates. As mentioned above, their structure and stress distribution are not coordinated, and stress concentration, strain concentration, microcracks, cracks and other undesirable phenomena are easy to occur. The process is carried out on the online mold to produce the container and special-shaped pipe with reasonable structure and integrally formed flange.

Mold for forming integral FRP pipe flange. The looper FRP flange mold is basically the same as this one, just omit the guide column part. The mandrel outer diameter of the mold is equal to the flange inner diameter Dn; The outer diameter of the mold is equal to the outer diameter of the flange D; The diameter of the guide post is equal to the bolt hole; The height of the positioning sleeve minus the thickness of the mold equals the thickness of the flange ring; The pressure block is generally 2-4 pieces, and the cylinder with a conical hole is formed after splicing. The inner diameter of the large end of the conical hole is equal to the outer neck of the large end of the conical neck, and the inner diameter of the small end of the conical hole is equal to the outer diameter of the cylinder, that is, the circular conical hole is determined according to the size of the conical neck. The flange barrel, cone neck, sealing line, bolt hole and flange ring can be integrally formed at one time with high geometric accuracy, smooth surface and strong interchangeability. The machining process can be avoided and the production efficiency can be improved.