Monitoring of the actual residual bolt load after tightening is essential to ensure that leak-tightness goals are achieved and becomes an important part of the quality assurance process of achieving flange joint integrity.
All tightening methods provide a degree of stud-preload scatter as a function of their process capability. The only way to be sure that specified stud preload is achieved is to measure it.
There are several methods for performing stud-stretch measuring, including strain gauges, bow micrometers, mechanical extensometers, and ultrasonic extensometers. The most common and versatile is the ultrasonic extensometer.
Theory of Operation
The ultrasonic extensometer operates by placing a high-frequency transducer at one end of the stud. Frequencies used for stud measurement range from 1 to 20 megahertz. At these frequencies a liquid couplant (gel) is used to couple the ultrasound from the transducer to the stud.
An ultrasound wave is generated by the transducer and travels down the body length of the stud. The wave reflects off the opposite end of the stud and travels back to the transducer.
The ultrasonic instrument measures the time of flight of the ultrasound in the stud. Many factors, including material density, stud length, temperatures, and stress are used to convert the time-of-flight measurement into an ultrasonic reference length.
Hooke’s Law and Stud-Stretch Measurement
All studs elongate in their elastic region following Hooke’s law, as outlined in the section ‘‘Function of Bolts’’.
In the relaxed state, a reference length is measured using the ultrasonic extensometer. After the stud has been tightened, an additional reading is made to measure stud stretch. Given the known parameters of effective bolt length (Lb), tensile area of bolt (As), Young’s modulus of elasticity (E), the preload (Fp) can now be directly correlated to stretch. Rearranging equation A7.1 allows the calculation of bolt preload (Fp):
The measured residual preload can then be compared to design preload to ensure it falls within an acceptable tolerance. Alternately the stretch reading ALb actual is compared to ALb design.
Example A7.2 Example Calculation: Application: Heat exchange–Reboiler channel