Bolts and nuts provide for clamping of the flange and gasket components. Bolting is a term that includes stud bolts, nuts, and washers.
Bolting Standards. The following are international standards that pertain to bolting:
ASME B1.1 --> Unified Inch Screw Threads
ASME B18.2.1 --> Square and Hex Bolts and Screws
ASME B18.2.2 --> Square and Hex Nuts
ASME B18.21.1 --> Lock Washers
ASME B18.22.1 --> Plain Washers
ASTM F436 --> Mechanical Properties of Plain Washers
BS 4882 --> Bolting for Flanges and Pressure Containing Purposes
Bolts. A bolt is a fastener with a head integral with the shank and threaded at the opposite end. Bolting for flanges and pressure-containing purposes are usually stud bolts. Stud bolts are fasteners that are threaded at both ends or for the whole length.
The nominal length of an inch-series stud bolt is the overall length, excluding the point at each end. The ends of the stud bolt are finished with a point having an included angle of approximately 90 degrees to a depth slightly exceeding the depth of the thread. Markings indicating the grade of stud bolt are applied to one end of the stud bolt. The minimum length of the stud bolt should ensure full engagement of the nut such that the point protrudes above the face of the nut. For applications that utilize hydraulic stud tensioning tools for tightening, one bolt-diameter is added to this minimum length. Hydraulic stud tensioning and other tightening methods are covered later in this chapter. While there is no maximum length of thread, unnecessarily long studs are avoided due to cost and to prevent corrosion and other damage to exposed threads, which would make subsequent removal difficult.
Heavy Series. Heavy-series nuts are generally used with studs on pressure piping. The nonbearing face of a nut has a 30-degree chamfer, while its bearing face is finished with a washer face.
Lock Nuts. The primary purpose of a self-locking nut is to resist loosening under service conditions experiencing vibration and shock. The self-locking nut produces an interference fit between the bolt threads and the nut threads. Most common self-locking nuts contain a nylon insert. The degree of interference is controlled during manufacture of the nylon-insert minor diameter. The elastic nature of the nylon provides uniform reaction from nut to nut. Generally, in pressure-piping systems, the primary concern is obtaining and maintaining proper stud preload to affect the gasket seal. Vibration is not normally a concern in these applications, and in situations where vibration is prevalent, adequate preload control will prevent nut rotation and loosening.
Flat Washers. Flat washers are used principally to minimize embedment of the nut and to aid torquing. Plain washers are manufactured in accordance with standard ANSI/ASME B18.22.1. Hardened washers are utilized in high-torque applications. Suitable mechanical properties for hardened, stamped, plain washers are covered by ASTM F436. Applicable properties for plain washers rolled from wire shall be AISI 1060 steel or equivalent, heat treated to a hardness of Rockwell C 45–53.
Live Loading. Live loading using belleville springs improves the elasticity of the flange joint. A belleville spring is a washer that is dished in the center to give it a cone shape. The cone shape provides for a very stiff spring, in comparison to coil springs. The cone will deflect and flatten at a specified spring rate (ratio of load to deflection) when subjected to the axial load (Fp) generated in a stud.
Flange assemblies always tend to relax in time, particularly at elevated temperatures. The rate of relaxation is dependent on many factors, including embedment relaxation of studs and nuts, flange rotation, bolt creep, and gasket creep. The relaxation phenomenon is covered more fully in the section ‘‘Behavior of the Flanged Joint System.’’ The high load-deflection or spring rate, characteristics of belleville springs, aid in maintaining bolt preload, compensating for some of the joint relaxation.
The spring rate of a belleville spring depends on geometry, material, and loading conditions. The load-deflection characteristics can be varied by stacking springs in combinations of series and parallel stacks. The hysteresis between increasing and decreasing load (the upper and lower curves, respectively) is caused by the friction between the spring and the loading surfaces. Two springs stacked in parallel doubles the load to flatten the pair with no further increase in deflection. Two springs stacked in series will produce twice the deflection at the same load. #Little_PEng