Combined with practical engineering experience, the selection key points of standard flange of pressure vessel and the design methods of non-standard flange, gasketless welding sealing flange and gasketless welding sealing reverse flange are introduced.
Flange is an important part of pressure vessel. It connects different parts and can be disassembled at any time. It is almost an essential part for equipment that needs to be opened frequently. The design of equipment flange is described in detail in GB150.1-150.4.2011. In NB/T47020-47027-2012, common type A, type B and long neck flanges are standardized for selection in design. However, the flanges encountered in practical work are far beyond this range. It is often necessary to design non-standard flanges, gasketless welding sealing flanges, and even gasketless welding sealing reverse flanges.
Key points of equipment flange selection
The typical structure of gasket sealing flange is shown in Figure 1. See GB150.1-150.4-2011 for the meaning of relevant symbols.
Figure.1 typical structure of gasket sealing flange
Flanges, gaskets and fasteners are a piping system, and matching shall be paid attention to during selection. Simply improving a certain performance can not solve the problem. It is necessary to comprehensively consider the changes of strength, sealing performance and stiffness. Attention shall also be paid to the matching of nut and stud (or bolt) hardness. The nut hardness shall be about 20-40HBW lower than the bolt hardness. Attention shall be paid to the coordination of stud expansion and flange expansion. If necessary, add butterfly spring to compensate for differential expansion and ensure sealing performance.
Maximum allowable working pressure
The maximum allowable working pressure is related to the material and working temperature. See tables 6 and 7 in NB/T47020-2012 for details. The flange standard is prepared based on the allowable stress of 16Mn forgings at room temperature. The maximum allowable working pressure varies greatly with different temperatures and materials. When the temperature is high, the maximum allowable working pressure drops rapidly; When the shell is made of high strength materials, the maximum allowable working pressure increases significantly.
GB150.32011 page 197 puts forward requirements for flange stiffness. Generally, there are few stiffness problems for small-diameter high-pressure flanges. Large diameter and low pressure flanges are prone to stiffness problems. When materials other than the standard are selected (such as austenitic stainless steel), the stiffness shall be re evaluated.
Design of non-standard flange
The design of non-standard flange shall refer to the relevant dimensions and material requirements of standard flange as far as possible.
The structural design of non-standard flange refers to standard flange and shall meet the requirements of GB150.3-2011. The key point is to determine the gasket size and bolt center circle diameter, and consider whether there is tightening space for the nut. If a large diameter (above M48) stud is used, it is necessary to consider whether the extension length of the stud meets the installation requirements of the hydraulic tightening device.
Calculation of coefficient
There are many and complicated coefficients in flange calculation. However, a careful study of GB150.3-2011 shows that almost all coefficients are determined by , the two ratios. Therefore, all coefficients can be calculated by programming without looking up the table. According to the source program of SW6, SW6 is also calculated by programming.
Strength calculation and stiffness check
In terms of strength, the main considerations are: the hardness and size of the gasket; Stud material, quantity and center circle diameter; Flange thickness, size of big end and small end, length of cone neck.
The soft and hard gaskets selected shall be appropriate. Hard gaskets shall be used for high pressure and soft gaskets shall be used for low pressure. The hard gasket will require the size of flange and bolt to be increased, resulting in the huge design of the whole flange system; Too soft gaskets are easy to crush. For the tube box gasket of multi pass heat exchanger, SW6 does not include the gasket area at the intermediate split diaphragm in the calculation of sealing area, but it has been included in ASME standard. SW6 calculation input can be calculated by changing the gasket diameter. Note that the stud diameter is the minimum root diameter of the stressed part. For studs other than the standard, the minimum root diameter shall be entered into the program by itself. If the strength is not enough, pay attention to the value on the calculation sheet (the ratio of small end stress to large end stress of flange). If the value is too large (above 3), it indicates that the flange stress is too concentrated at the small end. At this time, the thickness of the small end shall be increased and the length of the cone neck shall be lengthened; If the factory value is not large, the effect of increasing the flange thickness is more obvious. If the stiffness is not enough, increase the thickness of the small end of the flange or the thickness of the flange. At this time, increasing the length of the cone neck basically does not work.
Design of gasketless welding sealing flange
The typical structure of gasketless welded sealing flange is shown in Figure 2. See GB150.1-150.4-2011 for the meaning of relevant symbols.
Figure.2 typical structure of gasketless welded sealing flange
Essential difference from conventional gasket
The rebound of conventional gasket backing plate forms a seal, and the gasket coefficient is not equal to 0. The welding gasket is sealed by sealing welding, and the gasket coefficient is equal to 0. The difference of gasket coefficient leads to the difference of gasket effective sealing width and flange axial force, which leads to the different stress of the whole flange.
Selection of gasket materials
The gasket is welded and sealed, with good corrosion resistance and leakage resistance. It is used for gasifier, high temperature and high pressure fly ash filter, scrubber, lower end of ammonia synthesis tower and high pressure heat exchanger tube box with large diameter, high temperature and high pressure. The designer shall select UNS S30403, UNS S31603, inconel 600 and other materials as gaskets according to the actual needs of process media.
Structural features and precautions
The structure of gasketless welded sealing flange is shown in Figure 2. During calculation, it refers to the thickness of the flange at the center circle of the stud. After tightening part of the studs, compress the gasket, and then conduct seal welding. The purpose is to ensure that the weld is not stressed and only plays a sealing role. To avoid fracturing the sealing weld when tightening the stud, resulting in leakage.
Strength calculation and stiffness check
The strength is calculated according to HG20582.2011, and SW6 has provided a simple calculation program. Stiffness check is not mentioned in HG20582-201l, but SW6 procedure has been added with reference to GB150.3-2011. The author believes that although the pressure is high at this time and there are few stiffness problems, considering the preciseness of the design and the special circumstances that may be encountered in the future, it is recommended to introduce it in the next upgrade of HG20582-2011.
Calculation of gasketless welded sealed reverse flange
The typical structure of gasketless welded sealed reverse flange is shown in Figure 3. See GB150.1-150.4-2011 for the meaning of relevant symbols. Refer to GB150.3 for the definition and structure of reverse flange. Hg20582.2011 only gives the calculation formula of gasketless welded sealing flange, but does not give the calculation method of gasketless welded sealing reverse flange. The welding sealing reverse flange without gasket is also often encountered in engineering. At this time, the calculation idea is to combine the calculation of welding sealing gasket without gasket in HG20582.2011 with the calculation of reverse flange torque in GB150.3-2011. That is, the gasket coefficient m and sealing specific pressure y are all taken as 0, the sealing width and stud are calculated according to HG20582-2011, and the flange coefficient, flange torque and flange stress are calculated and checked according to GB150.3-2011. Without considering corrosion, the calculation method in reference  can be referred to. It is recommended to introduce this part in the next upgrade of HG20582.
Figure.3 typical structure of gasketless welded sealed reverse flange
The flange design problems often encountered in the actual design of pressure vessels are comprehensively analyzed. It has certain reference value and guiding significance for the design and selection of standard flange, non-standard flange, gasketless welding sealing flange and gasketless welding sealing reverse flange.
Authors: Zhang Jianli, Zhang Yaning
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.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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-  GB150.1-150.4-2011, pressure vessels [S]. Beijing: General Administration of quality supervision, inspection and Quarantine of the people’s Republic of China, 2011
-  NB/T47020-47027 · 2012, pressure vessel flanges, gaskets and fasteners [S]. Beijing: National Energy Administration, 2012
-  Zhao Xiaoping. Strength calculation of gasketless seal welded reverse flange [J]. Chemical fertilizer design, 2010, 48 (2): 22-24