The initial flange design is based on the process conditions. Once the material and type of flange are selected, the standard dimensions for the flange are referenced. However, it's important to note that the flange calculation is just one part of the entire flange connection system. A complete flange connection involves three key components: gasket design, bolt design, and flange body design. These must be carried out in sequence, as a failure at any stage can directly impact the success of the following steps. According to the WA-TERS method, the calculation process can be broken down into two main parts: first, determining the flange torque M0, and second, calculating the various stresses under this moment.
In the optimized flange design, two criteria are simultaneously applied: the minimum load criterion and the full stress criterion. On one hand, the load applied to the flange (i.e., the flange torque) should be as small as possible, meaning that the pre-load and the operating torque should be as close as possible (M1R2tf1R2fUMP). On the other hand, the stresses in the flange should be close to their allowable limits, ensuring that the strength of each component is fully utilized (as outlined in GB150-122). This requires iterative calculations, where certain parameters are adjusted until the flange stresses are below the allowable values and remain balanced.
The design of the gasket, bolts, and flange body will now be discussed individually, following these two optimization principles. Gasket design forms the foundation of the flange joint. Based on the design conditions and the medium used, an appropriate gasket type and material are chosen, along with its size. The flange system is then analyzed in both pre-tightening and operational states, considering the required clamping force and the resulting flange moment, which is expressed in terms of the bolt load. The minimum bolt load required during pre-tightening is given by Wa = 3.14 * D * G * y, while the minimum bolt load during operation is Wp = 0.785 * D² * Gp + 6.28 * D * G * bmp.
Three fundamental parameters define the gasket: y, the gasket specific pressure, which depends on the material and thickness; m, the gasket factor, related to the material and shape; and b, the effective sealing width of the gasket. From the formulas for Wa and Wp, it becomes clear that reducing the value of b can lower the clamping force. Therefore, in principle, a narrower gasket is preferred. However, if the gasket is only subjected to the bolt load during pre-tightening, it may lose its sealing performance due to excessive compression, leading to instability. Hence, the gasket must have an appropriate width. If the gasket width is found to be insufficient during design, the compatibility between the bolts and the gasket must be re-evaluated. Under the condition of ensuring reliable flange connection, the number of bolts could be reduced, their size decreased, or a gasket with a higher y value and harder material could be selected to meet the requirements.
According to the minimum load principle, the clamping force required for the gasket should be minimized, i.e., Wa should be as close as possible to Wp in both pre-tightening and operating conditions. If Wa > Wp, it indicates that the gasket has a high y value and is too rigid. In such cases, a gasket with a lower y value and softer material should be used to reduce the pre-tightening clamping force. Conversely, if the gasket is too soft, a harder and more rigid gasket with a higher y value should be considered instead.
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