An expansion joint is a specially-designed component of compact dimensions that allows differential movement between two adjacent components and maintains the pressure envelope. Sometimes a packed joint is used, (see Mechanical Design of Heat Exchangers, Figure 1, for a packed floating head) but these are limited to low pressure and low severity duty. More common are the metallic expansion bellows used in the shell of fixed tube sheet exchangers or at the floating end of single-pass floating head exchangers of the S or T type ( Figure 1). Specialist suppliers produce three types of metallic bellows, with their typical characteristics given in Table 1. Thick-walled bellows are invariably used in the shell of fixed tube sheet exchangers and are fabricated from a plate of the same material and thickness as the shell barrel; the form is shown in Figure 2. Thin-walled bellows, shown in Figure 3, have convolutions that are cold-formed, either by rolling or hydraulic forming. For high pressures, bellows with multi-ply construction are used. This design of bellows is more flexible than the thick-wall type, although it is more susceptible to damage during exchanger construction and operation. Figure 3 shows essential restraining bolts which keep the unit rigid during handling, but must be removed to allow axial thermal movement. Hot-formed medium wall bellows offer a compromise between thick and thin wall types.
Table 1. Range of bellows classifications
|Bellows||Thin wall||Medium wall||Thick wall|
|Wall thickness, mm||0.5 to 2||2 to 4.5||4 to 13+|
|Convolution height, mm||25 to 75||50 to 65||75 to 150|
|Materials include||Stainless steel||CrMo alloy steel||As shell|
|Manufacture||Cold rolling||Hot rolling||Pressing and welding|
|Comments||Restraining rings often needed||Compromise between thin and thick wall||Rugged but stiff|
|Flexible||Usually a maximum of two or three con volutions because of difficulty of equalizing movement|
|External protection needed|
Tube sheet design standards such as TEMA (1988) allow the fitting of bellows in fixed tube sheet exchangers. They are fitted when tube sheet calculation indicates a tube sheet thickness that is uneconomical. This can occur when the metal temperature difference between shell and tubes is greater than 100°C for carbon steel construction or 50°C when the shell is carbon steel and the tubes are stainless steel. With high temperature differences, it is sometimes not possible to satisfy the tube sheet calculations with any tube sheet thickness; obviously, bellows are then considered.
The bellows themselves are designed and constructed according to a code, the most comprehensive being the Standards of the Expansion Joint Manufacturers Association (1993). The Heat Exchanger Design Handbook (1994) has a chapter giving more information on exchanger bellows and in addition, provides recommendations on specification and operation.
The Standards of the Tubular Exchanger Manufacturers Association (1988). New York.
The Standards of the Expansion Joint Manufacturers Association (1993). New York.
Heat Exchanger Design Handbook (1994). Begell House Inc., New York.