Design and selection of flange

The correct design and selection of flanges is the key to ensure the safe operation of pressure vessels and pipelines. This paper introduces the types of flanges and sealing surface types of flanges, as well as the working principle of flange sealing, and summarizes the design and selection of flanges, which provides reference and help for the design work of technicians.

Flange [1] is widely used in pressure vessels and pipelines. It is mainly used for the connection between equipment, pipelines and equipment and pipelines. It has good strength, convenient disassembly and reliable sealing performance. Flange is one of the important components of pressure vessel and pipeline. It is a prerequisite to ensure the safe use of flange in design and selection. Due to its various structural types and many parameters and external factors involved, the specific implementation process is still complex. Combined with the practical experience in the work, this paper analyzes and summarizes the design and selection of flange.

Flange types and sealing surface types

Types of flanges

From the perspective of calculation, flanges are divided into three types as shown in Figure.1 according to the integrity of their connection to vessels or pipelines.

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Fig.1 types of flanges

  • (1) Loose flange: the flange is not fixed on the container or connecting pipe, or although it is fixed, it cannot be effectively connected as a whole, and the flange and the container or connecting pipe cannot bear the load imposed on the flange together. Such as lap joint flange, flat welding flange connected by general fillet weld, threaded flange, etc. Except that threaded flanges are used in high-pressure vessels and pipelines, loose flanges are often used in working conditions with low pressure (no more than 1MPa) and non flammable, explosive and non-toxic medium.
  • (2) Integral flange: the flange is completely fixed on the container or nozzle and forms an integral structure, which can jointly bear the load imposed on the flange. The fixing methods include forging the flange and the container or nozzle into a whole, or welding connection to ensure full cross-section penetration, such as welding neck flange, long weld neck flange (LWN flange), full penetration flat welding flange, etc. the integral flange can be used It is suitable for high temperature, high pressure, fatigue or impact load, extremely hazardous medium and strong permeable medium.
  • (3) Arbitrary flange: this kind of flange is between the above two flanges. Structurally, the flange is related to the container or connecting pipe, but it does not fully form the overall structure. The most common is the flat welded flange without full cross-section penetration. The strength of this kind of flange is usually calculated according to the overall flange, but for simplicity, the strength can be calculated according to the looper flange when the following conditions are met: The wall thickness of the cylinder or connecting pipe shall not be greater than 15mm; the ratio of the inner diameter of the cylinder or connecting pipe to the wall thickness shall not be greater than 300; the design pressure shall not be greater than 2MPa; the design temperature shall not be greater than 370°C. Any type of flange is commonly used in medium and low pressure occasions with general medium characteristics, and shall not be used in high or low temperature conditions.

Types of flange sealing surface

To ensure the reliability of flange connection sealing, a reasonable sealing surface type must be selected in the design. The commonly used sealing surface types include convex surface, concave convex surface, tenon groove surface and annular surface, as shown in Figure 2.
(1) Raised Face (RF) sealing surface: it is composed of a pair of protruding processing planes, which is easy to process and easy to disassemble the gasket. When non-metallic gasket is used, the gasket is easy to deform and extend to both sides, and the sealing performance is relatively poor. It is often used in occasions where the pressure and temperature are not high and the sealing performance requirements are not strict. When metal + non-metallic composite gasket is used, the overall structure of the gasket is enhanced, the rebound performance is increased and the performance is improved high sealing performance, suitable for occasions with high pressure and temperature or strict sealing requirements.

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Fig.2 types of flange sealing surface
(2) Male-and-Female (M&F) sealing surface: it is composed of a flange face has an area that extends beyond the normal flange face (Male) and other flange or mating flange has a matching depression (Female) machined into it’s face. The gasket is placed on the concave surface. The gasket is easy to be centered and pressed. When pressed, the gasket will not be squeezed out due to the restriction of the concave retaining ring. The sealing performance and scope of application are better than the convex sealing surface under the same conditions, that is, the processing is slightly difficult and the gasket disassembly is not convenient.
(3) Tongue-and-Groove (T&G) sealing surface: it is composed of a flange face has a raised ring (Tongue) machined onto the flange face and the mating flange has a matching depression (Groove) machined into it’s face. The gasket is narrow in width and is placed in the groove. Usually, spiral wound or metal clad gasket is used. The gasket will not be extruded when the bolt is pre tightened, so it is easy to obtain good sealing effect. It is suitable for important occasions with high pressure, high temperature and strict sealing requirements. Its main disadvantage is complex structure, difficult processing and tolerance during assembly The sealing surface is easily damaged, the gasket is not easy to remove, and it is inconvenient to replace the gasket.
(4) Ring-Type Joint (RTJ) sealing surface: Ring type joint flanges are typically used in high pressure (class 600 and higher) and / or high temperature environments above 800 °F (427 °C). Their surfaces are cut with grooves for the installation of steel ring washers. When tightening the bolts, the flange seal compresses the gasket between the flanges into the groove to deform (or stamp) the gasket to make close contact in the groove, so as to form a metal to metal seal. A RTJ flange may have a raised face with a ring groove machined into it. This raised face does not serve as any part of the sealing means. For RTJ flanges that seal with ring gaskets, the raised faces of the connected and tightened flanges may contact each other. In this case the compressed gasket will not bear additional load beyond the bolt tension, vibration and movement cannot further crush the gasket and lessen the connecting tension.

Working principle of flange seal

The flange sealing structure is composed of bolts, gaskets and flanges. It is a sealing structure that can be disassembled and assembled for many times. The containers or pipes are connected together by tightening the bolts, and the gaskets installed on the sealing surface are compressed at the same time. The gaskets are elastically compressed and partially plastic deformed under the action of compression force, and the gap between flange sealing surfaces is fully filled by gaskets to achieve The purpose of preventing media leakage.

Flange design and selection

Flange design

Flange design includes gasket design, bolt design and flange structure design [3], due to many influencing factors, the correct and reasonable optimization design is a very complex work. At present, the flange design methods at home and abroad are constantly updated and developed, and the mechanical calculation model is constantly optimized and improved. Among them, the famous waters method is adopted by most flange design standards, including the domestic common standard GB/T150.3-2011. The specific design points are as follows:

  • (1) According to the design pressure and design temperature of the vessel or pipeline, and considering the toxicity and corrosivity of the medium and the type of flange sealing surface, determine the material, structural form and size of the gasket, and calculate the minimum pressing force required by the gasket. The result is the larger value of both pre tightening state and operating state. The basic requirements of gasket design are to ensure safe and reliable sealing, save cost and increase cost Easy fabrication and disassembly.
  • (2) According to the minimum pressing force required by the gasket and combined with the design working conditions, select reasonable bolt materials, calculate the required bolt area and determine the appropriate bolt specification and quantity. The actually determined bolt area shall be greater than the required calculated area, but it shall not be too large, otherwise it will lead to insufficient flange stiffness deformation or overall yield failure of the gasket or even extrusion.
  • (3) According to the design parameters and medium characteristics, determine the flange material, select the reasonable flange type and sealing surface type, preliminarily draw up the size of each structure, and then carry out repeated trial calculation to continuously adjust the flange structure size until each structure size can meet the corresponding strength check conditions. It should be pointed out that in meeting the minimum spacing between circumferential bolts, wrench radial operation space and sealing On the premise of the surface structure size, the diameter of the bolt center circle should be as small as possible, so as to shorten the force arm and reduce the flange torque; it is also necessary to reasonably adjust the structural size of the flange ring and cone neck to make the flange tend to the full stress state [4], that is, all parts of the flange give full play to the strength performance and ensure sufficient mild stress.

In short, the design of flange is to select appropriate gaskets, set appropriate bolts, determine reasonable flange materials and structural dimensions, and ensure that the flange has compact structure, reasonable stress, light weight and less consumables. Although the flange structure is simple, the design involves many factors, complex stress and cumbersome calculation, such as gasket materials and structures, gasket width and diameter, and bolt materials, specification and center circle diameter, flange material, sealing surface type, thickness, cone neck thickness and height, etc. Therefore, it is difficult to fully optimize the flange design in the actual engineering design. The designer can only determine the relatively reasonable and economic flange structure with the help of design experience and repeated trial calculation.

Selection of flange

Flange is an important component of pressure vessel and pipeline. In order to facilitate use, save economic cost and increase universality and interchangeability, such as domestic commonly used pipe flange standard HG/T20592-20635-2009 steel pipe flange, gasket and fastener [5]. During the specific design, the flange shall be selected according to the standard as far as possible. When selecting the standard flange, first determine the flange material; then determine the appropriate nominal pressure grade according to the flange material to ensure that the design pressure and design temperature are within the allowable range of the standard flange; finally, check the detailed structure in the standard according to the nominal pressure, nominal diameter, type and sealing surface type of the flange Dimension. For standard flange, it is not necessary to consider the requirements of gasket, bolt and flange structure when selecting, because these standards are taken from the experience, test and calculation of relevant industries and have considered the requirements of economy, safety and standardization [6] When the design condition, nominal diameter and other design conditions of the flange exceed the provisions of the standard, the non-standard flange shall be adopted, and detailed design and calculation shall be carried out to meet the requirements of safe use. For the non-standard flange, in order to facilitate the design, the flange type, sealing surface type, gasket material and structure are usually determined first, and then refer to the similar standards in the flange standard The flange size is preliminarily determined by LAN; finally, the strength calculation and parameter adjustment are carried out until the size of each part can meet the verification conditions. At present, the design of domestic flanges is mostly carried out in accordance with the provisions of GB/T150.3-2011 standard. In order to improve work efficiency and facilitate optimization design, we need to use SW6 and other calculation software for auxiliary calculation.

Conclusion

Flange design is the general term of “gasket – bolt – flange” design, which is an important part of pressure vessel and pipeline design. Whether the flange is safe or not is related to the safety of environment, person and property, which must be paid enough attention to in the design. Understanding and mastering the process and key points of flange design, and being able to correctly and reasonably design and select flanges is an important guarantee for safety. The author’s summary of flange design and selection can Help technicians have a more comprehensive understanding of flange design, which is conducive to improving the design process and optimizing the flange structure.
Author: LUO Yong-zhi,WANG Zhi-gang,YANG Bin,LI Yong-hong

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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Reference:

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  • [2] Zheng Jinyang, Dong Qiwu, sang Zhifu. Process equipment design [M]. Beijing: Chemical Industry Press, 2008
  • [3] Professional clerical meeting of design and calculation methods of national boiler and pressure vessel Standardization Technical Committee, Qi Guosheng, Duan Rui. Design guide for Pressure Vessel Engineers [M]. Beijing: Sinopec press, 2013
  • [4] Sang Rubao. Training course for pressure vessel design engineers [M]. Beijing: Xinhua Publishing House, 2005
  • [5] HG/T20592-20635-2009. Steel pipe flanges, gaskets and fasteners [S]. 2009
  • [6] Wang Zhiwen, Cai Renliang. Chemical vessel design [M]. Beijing: Chemical Industry Press, 2005