Bolted Flange Design
Pressure vessels require flanged joints to permit their disassembly, in-spection and cleaning. Bolted joints are also utilized to alleviate stresses at sections where sharp temperature changes occur, such as the joint between tubeside and shellside chambe
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BOLTED FLANGE DESIGN
INTRODUCTION
Pressure vessels require flanged joints to permit their disassembly, inspection and cleaning. Bolted joints are also utilized to alleviate stresses at sections where sharp temperature changes occur, such as the joint between tubeside and shellside chambers in a heat exchanger (Fig. 3.1.1). From a conceptual standpoint, flanged joints may be subdivided into two major categories. (i) Bolted joints (ii) Pressure-actuated joints The so-called "bolted joint" is by far the most common type. Its underlying principle is, in essence, brute force. "Pressure actuated joints" find application in the higher pressure range, typically over 2000 psi. The main difference between these two joint types lies in the manner by which the pressure load is resisted and leak-tightness is achieved. In a bolted joint, pre-load in the bolt provides initial pre-stress in the gasket. Upon introduction of internal pressure, the normal surface load on the gasket drops. The objects in flange design are to ensure that the residual gasket load is sufficient to maintain the joint leak tight, and, to ensure that the stress levels in the flange during bolt pre-load, as well as during pressurization, do not exceed allowable values. Pressure actuated joints, on the other hand, exploit the header pressure force to compress and to seal the gasket. This chapter concentrates on the structural behavior of bolted joints. First, an outline of various types of flanges used in bolted joints is given, followed by a brief discussion on the significance of surface finish of STUB END TUBESHEET
Fig. 3.1.1.
TUBESHEET SKIRT Outside packed floating head heat exchanger. 81
K. P. Singh et al., Mechanical Design of Heat Exchangers © Springer-Verlag Berlin Heidelberg 1984
82
Mechanical Design oj Heat Exchangers
flange facings, and of gasket characteristics on the joint performance. The concepts leading to the characterization of the sealing action of bolted joints are next developed. Conventional design methods and stress analysis procedures are also described. Similar concepts for pressure actuated closures are developed in Chapter 6. 3.2 BOLTED FLANGE TYPES Bolted flanges may be subdivided into three major categories: (i) Ring flange (Fig. 3.2. 1. a) (ii) Tapered hub (or welding neck) flange (Fig. 3.2.l.b) (iii) Lap-joint flange (Fig. 3.2.l.c) The ring flange consists of an annular plate welded to the end of the BACKING (FLANGE) RING
(0) RING FLANGE WITH FLAT FACE
(b) WELDING NECK FLANGE WITH TONGUE FACING
(el} LAP-JOINT (LOOSE) FLANGE (liAISED FACE)
(PRESSURIZED CON.DITION LOADS)
Fig. 3.2.1.
Bolted flange types and typical facing details.
cylindrical shell. A number of equidistant bolt holes (conventionally, a multiple of 4) are drilled on a uniform pitch on a circle (known as the bolt circle). The gasket is confined inside the bolt circle. This joint is utilized in low to moderate pressure applications. If the pressure is quite low (less than 100 psi), wide gaskets which span the entire flange face (within and beyond the b
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