Shell-and-tube heat exchangers are widely used in industrial plants. Different structures can be selected according to different operating conditions such as temperature, pressure and medium, such as fixed tubesheet heat exchangers (or fixed heat exchangers), floating-head heat exchangers, filled-tube heat exchangers, U-tube heat exchangers and so on.
Tube sheet is one of the main parts of shell and tube heat exchangers. The reasonable design of tubesheet is of great significance to the correct selection and saving of materials, the reduction of difficulties in manufacturing process, the reduction of cost and the guarantee of safety in use. Therefore, the tube plate strength must be correctly analyzed to determine the thickness of tube sheet reasonably.
GB 151 “Heat Exchanger” provides a complete set of technical requirements for the design, manufacture, inspection and acceptance of shell-and-tube heat exchangers. As a whole, the parameters of the heat exchanger which are applicable to the standard are specified. The calculation method of tube sheet given in the standard (see Chapter 7) can be applied to all pressure and diameter parameters of shell-and-tube heat exchangers with PN < 35 MPa.
GB 151 gives the design and calculation methods of U-tube heat exchanger, floating head heat exchanger and fixed tube-plate heat exchanger. The strength calculation methods of heat exchanger tube sheets with different structure types are different because of different load conditions, support conditions and boundary constraints.
The U-tube heat exchanger has only one tubesheet. Six different connection structures are given in the standard. The calculation model of U-tube heat exchanger is that the tubesheet is a common circular plate which bears uniform load and is weakened uniformly by the holes in the tube. The influence of the non-distributed area around the tubesheet on the tubesheet stress is considered in the calculation method.
For floating head heat exchangers and filled-in heat exchangers, most of the fixed end of a tube plate is clamped between shell flange and tube box flange with bolts and gaskets. The calculation model regards the pipe distribution area of the tube sheet as a circular plate on the elastic foundation and weakened uniformly by the hole, and the non-pipe distribution area around the tube sheet as an annular plate. The whole tube sheet is simply supported and bears uniformly distributed load. For other connection forms, the tube plate is designed according to JB 4732 standard.
In a fixed tubesheet heat exchanger, two tubesheets are fixedly connected with a shell-side cylinder, and the periphery of the fixed tubesheet can be extended as a flange, forming a “tube plate with an extended part as a flange” or directly connected with the shell-side and the tubesheet cylinder to form a “tube plate without flange”. According to the requirement of poor thermal expansion of shell-side cylinder and tube bundle, expansion joint may be set in fixed tube-sheet heat exchanger.
Regardless of the specific structure of the fixed heat exchanger, the size and material properties of almost all the structural elements of the heat exchanger directly or indirectly affect the strength of the tube sheet. Because the strength analysis and calculation of tubesheet are very complicated, so far, most of the national codes have simplified and hypothesized the actual tubesheet to varying degrees. The tubesheet is regarded as an equivalent circular plate which bears uniformly distributed loads, is placed on an elastic foundation and is weakened uniformly by the hole. On the basis of the above simplification, the standard method GB 151 proposed by Professor Huang Keming of Tsinghua University, which considers quantitatively the effects of the following factors on the stress of tubesheet, has been put forward in China.
- (1) the influence of the pipe area (ring plate) on the stress of tube sheet.
- (2) The restraint effect of shell side cylinder (flange), tube box cylinder (flange), bolt and gasket system on the edge corner of tube sheet;
- 3. When the extension part of the tube plate is used as flange, the influence of flange torque on the stress of tube sheet is also discussed.
- (4) When calculating the elastic stress of tube sheet, the primary bending stress caused by pressure load and flange moment and the secondary stress caused by thermal expansion difference between tube and shell side are distinguished, and the allowable values are determined.
Source: China Stainless Steel Tube Sheets Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)